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Lense Grouting with Fiber Admixture to Reinforce Soil
Reprinted from Grouting: Compaction, Remediation and Testing
Proceedings of sessions sponsored by the Grouting
Committee of the Geo-Institute/ASCE in
conjunction with the Geo-Logan '97 Conference
Held July 16-18, 1987, Logan, Utah
LENSE GROUTING WITH FIBER ADMIXTURE TO REINFORCE SOIL
STEVEN C. CHANDLER¹ Aff. ASCE
Abstract
Lense Grouting is the injection of cement slurry, the consistency of thick cream, that parts the soil. This parting propagates like a hydraulic fracture to a normal design diameter of about 1.5 m. A grout point grid spacing on the soil surface is usually about 2 m center to center. The vertical spacing of each injection is normally 0.31 m. The mechanism of soil reinforcement is based on the friction/bond between the hardened grout lense and the soil. Normally the lenses are found to be horizontal.
Two projects in San Clemente, California were undertaken to reinforce clayey soils which had shown lateral movement. One case was a road that was built on a fill slope. The second was fill behind a crib-type retaining structure. Synthetic fiber reinforcement was added to the lensing grout to maintain tensile strength. Eight years after completing the projects, no additional distress or movement has been reported.
Introduction
Lense grouting is the systematic injection of a cement based slurry into soil. The process has been pursued for, among other purposes, reinforcing cohesive soils (Al-Alusi, 1994 and 1996). A particular aim of this process has been the strengthening of fine grained soil masses that are insufficiently confined laterally (Al-Alusi, 1995; Collin and Mitchell, 1984; Tabbal, 1983). In situations where lateral support by more conventional means is not cost effective, due to logistical or financial constraints, lense grouting has been used to slow and stop soil movement.
The technique of Lense Grouting involves the placement of predetermined volumes of grout at discrete locations throughout the treated soil mass to effect a skeletal network of hard planner grout-filled fractures or ruptures. These grout lenses have been exposed many times in excavations subsequent to grouting and most always are seen horizontal or sub horizontal, at least within a few meters of the soil surface. The purpose of placing given volumes of grout is to maintain control of the grout which otherwise might travel large distances in unknown directions. By placing the grout injections at discrete locations in the soil mass a uniform treatment of the soil mass may be achieved. Normally, up to 0.03 m of lensing grout is injected at each 0.31 m increment of depth as a valve tipped injection pipe is intermittently driven down between pumping stages. Driving the injection pipe maintains a seal between the injection pipe tip and soil. Initiation of fractures is from the end of the valve tipped injection. Often the primary-secondary method of grout point sequencing is used if communication between grouting points occurs.
The constituents of the lensing grout are essentially, cement and water. However, bentonite is a very important admixture to vary the consistency of the slurry and more importantly the bleed characteristics. Fluidifiers and accelerators have also been used to alter the slurry behavior. In the two case histories presented here, a synthetic fiber (polypropylene) reinforcing material was added to the slurry grout at a rate of about 0.5 kg/m3 of grout. The length of the fibers was about 2 cm. The aim was to give grout lenses an apparent tensile strength. The fibers were not expected to increase the grout's ultimate tensile strength. However, they do to some degree maintain it if and when the lense is stressed to failure. Normally lense grouting is done without the addition of fiber reinforcement. However, on these jobs the admixture was offered as further possible enhancement of the system's reinforcing ability. The addition of the fiber reinforcing in Case 1 came as change order to the specifications pursuant to the contractor's suggestion.
A quality control/quality assurance program was maintained throughout the two projects. A colloidal mixer was implemented in the mixing and batching of the lensing grout. The shearing action of the colloidal mixer not only intimately mixed and wetted the cement and bentonite, but most likely helped to pull apart the strands of the reinforcing fibers, maximizing their available surface area. Meticulous records were kept reflecting the batching proportions and the quantities injected at each stage of every injection point. Data was either recorded by the geotechnical engineer or under his direct observation on both projects. The adequacy of the grouting programs have been verified by the review of the grouting records and the soil reaction. There has been no post-testing done since completion of the projects, because the subject soils ceased their movement to the extent that no further concerns have been raised.
The two lensing projects which are the subject of this paper have been in the ground for almost eight years. The engineers who did the distress investigations suggested monitoring the projects' short term and long performance. One of the projects still has the inclinometers in place. However, there has been no engineering follow up on the projects. Over the years the author has made occasional visits to the sites and has had discussions with the project engineers for the City, the maintenance personnel for the City, and the consultants who performed the distress investigations. The maintenance supervisor for the City indicated that he remembered the two projects and has not received any calls concerning distress. Further, he indicated that people are very sensitive to perceived distress and that they would call the City maintenance people with concerns about the slightest movement. The “test of time” is about the only test that has been made on these projects. The work was not engineered as a completely positive and permanent fix. But, it would seem apparent that the reinforcement was sufficient to arrest further movement that would necessitate more drastic and expensive mitigative measures.
Case 1
A. Background
The first lense grouting project is a portion of a street that leads eastward, uphill at a gradient of about 14 percent. The street has asphalt pavement with concrete curbs, gutters, and sidewalks adjacent to a median island. Beyond the sidewalks, the ground surface slopes upward to the south and downward to the north at a ratio of approximately 2:1 (horizontal : vertical). The slopes and the median planters were heavily landscaped and irrigated. Refer to figures 1 and 2 for section and plan view.
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Distress was observed along about 108 m of the north curb, gutter, and sidewalk. Displacements were a maximum of approximately 13 cm laterally and 8 cm vertically. Local residents noticed the changes at the top of the slope and in the roadway. They expressed their concerns to local building officials who in turn had a failure investigation done in October 1988.
Grading of the street was performed between June 1977 and August 1978. A landslide was mapped and it topped out in the project area. Notes on the grading plan called for removal of the slide debris. During the failure investigation confirmation of the slide debris removal could not be made because it did not include deep exploratory drilling. However, there was an understanding that the residents in the immediate vicinity of the study area had not reported damage. There was no visual indication of slope instability. Creep in the cut fill transition and in the landslide debris was not substantiated by the study. Therefore, the observed distress was not seen to be associated with landslide movement.
The generalized stratigraphic profile observed during the field exploration of the failure investigation consisted of high plasticity clay soils underlain by Capistrano Formation siltstone bedrock. The plastic index of the clay soils was 29 to 33. The fill soils were very moist and had a degree of compaction well below 90 percent, based on ASTM D 157-78. Slide debris was found below the fill in some locations.
The conclusions of the failure investigation were: 1. That settlement of the subsoils were the primary cause of distress; 2. The landslide debris if left in place would be subject to settlement under substantial fill loads, and this settlement would be aggravated by the influx of moisture; 3. Settlement of the fill soil was due to low initial placement densities and the influx of water; 4. The cut/fill transition would lead to differential settlements.
One recommendation for solving the soil settlement problem was to remove the suspect soil and replace it with engineered fill. This method, however, would require further studies to investigate the risk of destabilization of the general area, which included completed residential lots. It was also questionable whether this method would be economically feasible. The other recommendation was to lense grout the suspect material to reinforce and strengthen the soil formation and the creep zone.
B. Grouting
In June of 1989, lense grouting was accomplished beneath the traffic lane nearest the top of the northerly descending slope. Three rows of injection points were place along about 55 m of the roadway. This length coincided with the worst distress seen in the roadway, curb, gutter, and sidewalk. The center-to-center spacing along each row between grouting points was about 2.13 m. Between each row the spacing was about 1.82 m. The center row was offset so as to stagger the points with respect to the adjacent rows. The bottom of the treated zone of soil was intended to be at the base of the loose soil as determined by resistance of the soil to penetration by the driven injection pipe. This was between 2 m and 7 m from the surface. Generally, the last injection stage would not accept grout due to the high degree of stiffness in the more competent underlying soil.
C. Summary
The lense-grouting program was planned as a temporary mitigative measure aimed at reducing or stopping fill settlement. A fiber reinforcement admixture was offered as enhancement to the soil grouting system. The grouting campaign was carried out as planned, and fiber reinforcement was used in the mix design. It has been approximately eight years since the job was completed. In that time period it has been reported that no significant creep had occurred. No other mitigative measures have been undertaken at the site since lense grouting.
Case 2
A. Background
The second site is an office and retail complex composed of single story reinforced concrete buildings with paved parking, drives, fire lanes, landscaping and earth retaining structures. The paved parking lot and drive along the north and west perimeter of the complex are supported on engineered fill that is retained by a crib wall. This crib wall varies in height from about 1.5 m to about 5 m. The wall gradually gains height to a maximum at the northwest corner of the development. At the base of the crib wall its northwest corner, the ground surface slopes downward another 9 m at a ratio of 2:1 (horizontal : vertical). The height of this slope lessens going away from the corner to the east and the south. The length of the northern leg of the wall is about 130 m. The west leg is about 85 m long. Similar slopes were cut into the hills that rise to the south and east of the complex. Refer to figures 3 and 4 for section and plan view.
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Cracking in the asphalt concrete pavement appeared a couple of meters behind the top of the crib wall running parallel to it. This distress was brought to the attention of the local building officials and the owners of the property. This was in April 1988 and the original geotechnical engineer for the development was brought in. The engineer determined that the crib wall and the slope were not in danger of collapsing at that time.
The site was rough graded during the summer of 1985. Precise grading, placement of the crib wall which included geogrid reinforced backfill, and the placement of base material for paving was completed in November 1987. In December of 1987, there was a water leak in the northwest area of the complex. About April of 1988, the first signs of distress were investigated and a crack monitoring system was set up.
By March of 1989, the cracks in the asphalt paving had occurred as far back as 10 m from the crib wall. They were hairline to 5 cm in width, with a slight downward offset on the crib wall side of the cracks. Extensive voids were observed below the concrete curb near the sprinkler heads along the landscaped area at the top of the crib wall. At this time, four borings were advanced to a depth of about 11 m where inclinometers were installed. These inclinometers were placed within 12 m behind the crib wall and no further than 36 m from the northwest corner of the crib wall. The inclinometers were monitored for about four months after which a geotechnical investigation of the parking lot distress was completed.
Generally, two material types were encountered in the borings where the inclinometers were placed. They were engineered fill and ancient landslide material consisted primarily of silty clay and clayey silt. The relative compaction, per ASTM D 1557, was found to be as low as 77 percent and as high as 98 percent. The expansion index was medium to high. The soils were very moist to nearly saturated, apparently as a result of either landscape irrigation, water line leakage or both.
The results of the slope inclinometer-monitoring program generally indicated that no significant deep-seated movements were occurring. However, shallow movement appeared to be occurring in the crib wall backfill. The movement appeared to be relatively minor, but over a sufficient time period the accumulated movements had potential to be problematic. In the four months of monitoring, one of the inclinometers deflected 2.92 mm. No obvious signs of crib wall misalignment, down slope bulging or heaving, or any other signs usually associated with slope stability problems were observed at the site.
It was concluded that the distress parallel to the crib wall along the north and west development boundaries was probably caused by a combination of small lateral movement in the upper 4 m of the crib wall backfill. Additional settlements due to the loss through piping of the finer fractions of the backfill were the result of water line breaks and extensive irrigation.
It was recommended that the distress be repaired using lense grouting. It was felt that lense grouting would offer significant advantages in terms of cost and expediency compared to removal and replacement of problem soils.
B. Grouting
In October of 1989, lense grouting was performed in the crib wall backfill along approximately 88 m of the north side and 73 m of the west side. Three rows of injection points ran parallel to the crib wall. Each point was spaced about 1.83 m on center. The rows were spaced the same, 1.83 m apart. The center row of points was staggered with respect to the adjacent rows. The first row closest to the crib wall was about 1.5 m below the outside grade in front of the crib wall. The second row of injection points was advanced to about mid-depth between the first and third row. The third row was driven to a depth of 3 m from the surface.
C. Summary
The lense grouting program was offered as a means of reinforcing the soil that had been moving behind the retaining structure. It was hoped that the reinforcement would be sufficient to reduce or eliminate successive movement. A synthetic fiber additive was mixed into the lensing grout as an added measure of grout reinforcement. Approximately eight years have passed since the work was done and there have been no reports of movement in the grouted soil.
Conclusions
Lense grouting was recommended for reinforcing cohesive soils which were settling and had horizontal movement at the surface towards a descending slope. Lense grouting techniques were utilized with an admixture of fiber reinforcement which was successful in mitigating the soil movement. The application of the lense grouting process is relatively inexpensive and is very expedient compared to conventional remedial methods. The process can be used in many different situations where soils are subject to deformation by static and seismic stresses, and particularly, as in the cases presented, soft, wet, and laterally unconfined cohesive soils.
References
Al-Alusi, H.R. (1994), “Soil improvement to mitigate settlements under existing structures,” Proceedings, Settlement '94, ASCE June 16-18, 1994, College Station, Texas, 1214-1223.
Al-Alusi, H.R. (1995), “Lense grouting in geotechnical engineering,” Proceedings of the Eleventh African Regional Conference on Soil Mechanics and Foundation Engineering, Cairo, December 11-15, 1995, 374-379.
Al-Alusi, H.R. (1996), “Abatement of soil liquefaction under existing structures,” Proceedings of IS-Tokyo '96, The Second International Conference on Ground Improvement Geosystems, Tokyo, May 14-17, 1996, 249-254.
Collin, J.G. and Mitchell, J.K. (1984), “Injection grouting for in-situ earth reinforcement,” Master of Science Thesis, University of California, Berkeley.
Tabbal, M. (1983). The study of cement grout reinforcement in slopes of soft clay, Master of Science Thesis, Stanford University, California.
¹ Branch Manager, Pressure Grout Company, 1330 W. Gaylord Street, Long Beach, CA 90813-1321, (562)432-4100
Soil Improvement to Mitigate Settlements Under Existing Structures
Reprinted from Vertical and Horizontal Deformations
of Foundations and Embankments
Proceedings of Settlement ‘94
Sponsored by the Geotechnical Engineering Div./ASCE
Held June 16-18, 1994, College Station, Texas
SOIL IMPROVEMENT TO MITITGATE SETTLEMENTS
UNDER EXISTING STRUCTURES
H.R. Al-Alusi¹, Member, ASCE
ABSTRACT
Settlements of sites with existing structures are more difficult to mitigate than those of sites without structures. Equipment access, work area, noise, dust, vibrations, and cost are amplified and become more critical.
Following are three case histories involving mitigation of settlements under three different types of structures.
The first case, an office building in the San Francisco Bay Area, involved soil densification under piles to mitigate further settlements. The compaction grout densification process was extended beyond the bottoms of the piles to treat fill materials under the footprint of the building. Additional lense grout reinforcement was required to reinforce the hillside solid to reduce downward movements. Five years after completion of remediation work, the site showed no detectable movement.
The second case concerned a maintenance facility at the June Lakes Ski Resort in the Sierra Mountains, where a structure had been built on top of a fill that was underlain by a layer of gravel and cobbles, Within a year of construction, signs of structural distress were evident. Geotechnical investigations revealed that settlements were caused by at least two factors; the downward migration of the upper fill layer into the large pores of the lower layer, and the possible densification of the upper full under its own weight. The remedial work consisted of providing a barrier between the two layers to allow for an effective compaction grout densification effort of the upper fill layer and to prevent further migrations into the gravel and cobbles layer. No structural distress or any movement has been detected since the remedial work was completed six years ago.
The third case presents the treatment of the old and new footings of the Rose Bowl Stadium in Southern California. A permeation grouting system was selected, designed, and implemented to solidify zones of the sand-gravel-cobbles mixture of the foundation soils to act as pedestals for underpinning the old footings and supporting the new ones.
Introduction
Settlement of structures can be caused by a number of factors. These factors include the settlement of the soil caused by its own weight, loads applied by and through the structure, vibrations, change in groundwater levels or other less known factors such as plant root moisture extraction, erosion of a soil layer into a courser particle layer, chemical reactions, thermal exchange, mineral dissolution, underground erosion due to migration of smaller soil particles caused by groundwater gradients, and many others.
Loads applied by and through a structure may include its own dead load, live loads, wind, seismic, impact and other functional loads. A frequently encountered settlement problem is the inadequacy of soil density/strength resulting in soils consolidating or compacting under their own weight. A soil improvement can be affected by simply densifying the soil mass in-situ without removing the soil or affecting the structure.
Mitigation of soil settlement under existing structures by in-situ pseudo-static densification has been used for more than forty years in the U.S.A. These solutions are achieved by compaction grouting (further detailed in case history No.1). Other lesser known methods include soil solidification, soil reinforcement, soil sealing, and other methods of soil treatment. Each one of these approaches has several critical details that demand the engineer's and contractor's full attention to achieve successful completion. The three cases presented in this paper represent soil improvements to mitigate settlements caused by several factors. Each case involves an existing structure where on-going settlements needed to be halted.
Case 1
A two-story office building, measuring 24.4 X 76.3 meters, exhibited continuous settlements within five years of construction completion. When the differential settlement reached 100 mm it became evident that a remedial work program was necessary.
The site was resting on a two-stage graded fill (Fig.1). Fill thicknesses (wedges) of less than 1.5 meters and up to 6.1 meters underlaid the footprint of the subject structure. Upon completion of construction the longest side of the building was parallel to a heavily vegetated slope of about 1:1, with a height of 4.8 to 6.1 meters. The building was resting on drilled piles of varying depths from 2.8 to 4.9 meters and with diameters of 0.46 to 0.61 meters. The piles were connected by grade beams, with the floor slabs doweled to the beams.
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Twenty years before completion of the building, rough grading had been completed; the final grading and building construction were completed about twelve years before this remedial work started.
Settlement monuments on the grade beams showed a maximum differential settlement of 100 mm across the building, (Fig 1). Observation of the soil surface in comparison to the grade beam showed a difference of an additional 200 mm of soil movement downward relative to the grade beams. Slabs were exhibiting sagging of up to 70mm between the grade beams.
Continuous monitoring showed that there were two types of movement. The first was downward, which was attributed to the compaction and consolidation of the fill and native soils. This movement was detected by settlement monuments and the generation of voids below slabs. The second was a hillside creep movement caused by the seasonal drying and wetting of the near surface soils of the slope. A typical soil profile of this site is shown in Fig. 2. The pile settlements were attributed to the additional loading imposed by the negative skin friction generated by the downward movement of the fill materials.
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Approach Concept
A number of solutions were considered, among them were:
A. Removal and preservation of building, excavating and re-compacting soils, and resetting building back on same location.
B. Re-supporting building on additional and deeper piles.
C. Improvement of soils by in-situ densification and mitigation of downhill soil movement, by soil reinforcement.
Solution (a) was quickly discarded because of its prohibitive cost and time requirements. Solution (b) was estimated to be many times higher in cost and time requirements than Solution (c).
The remedial work consisted of two major items, namely:
1. Densification of soils below the bottom of the piles and under the rest of the building using compaction grouting, and
2. In-situ soil reinforcement using deep lense grouting under the hillside area
Fig. 2 represents a conceptual sketch of the remedial work undertaken for this building.
Compaction grouting is the injection of a highly viscous sand-cement mixture designed to volumetrically displace and densify the soils around the point of injection. Compaction grout by definition (Committee on Grouting 1980) is a grout with 50 mm or less slump per ASTM C143-78. Grout materials, pressures and rate of injection were designed to prevent the permeation of the grout into the soil mass and to prevent the fracturing of the soil itself. The strength of the grout material is irrelevant in the compaction grouting process. The amount of densification and the extent of the densification process are the crucial elements in this operation. Soils closest to the grout bulb will exhibit highest densification with a diminishing effect away from the point of injection.
Compaction Grout Materials
Materials used in compaction grouting have a wide range of properties. Theoretically, any materials that will not permeate, spread, or fracture the soil when injected in an acceptable material. For cost considerations, local materials for a given project site are usually given first priority. Additives can be used to improve the grout material pumpability. As an additive, portland cement is widely used with sufficient water to effect a workable mix. The use of cement is strictly for the workability and pumpability of the material and does not affect the required degree of densification. Compaction grout materials with no cement content or other additives have been reported (Stoker and Wardwell 1987). A set of particle size distributions of materials used in compaction grouting, compiled by the author, is given in Fig. 3. A cement content of five to fifteen percent has been used with these materials.
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Soil Densification
The remedial work included only a portion of the building as seen in Fig. 2. This portion represents approximately two-thirds of the total area of the building.
Based on the available settlement records, 63 piles were found to be in need of re-support. A single injection point was used for each pile. Injection points were designed so that the tip of each grout casing was between the center of the pile bottom and the top of the competent soil layer. At each injection point grouting continued until pile upheave or maximum grout pressure of 4100 kPa at the point of injection was detected.
The aim of this portion of the treatment was to create a grout bulb (footing) as large as possible under the pile until refusal criteria, as given above, were met. The grout take was largely dependent upon the consistency of the material below the bottom of the pile and the distance between it and the competent soil layer below it. Grout takes ranged between 0.17 to 4.73 cubic meters with an effective spherical bulb diameter of 0.67 to 2.11 meters.
For the remaining soil mass (Fig. 2), a grid pattern with a spacing of 1.83 X 1.83 meters was established. The sequence of injections was designed to first create a confinement of the soil mass to be densified, then to proceed with the remaining densification process. Each injection point was driven to the target depth. Grout extrusion started in stages of 0.61 meters in the vertical direction without stopping until a maximum pressure of 4100 kPa was reached or a ground upheave was detected. A total grout take of five to seven percent of the volume of the treated soil was accomplished resulting in four to eight percent increase of the soil dry density.
Soil Reinforcement
The deep soil reinforcement included injections of gout lenses to a maximum depth of 11 meters. Lenses were installed at 0.31 meters intervals vertically. Each lense was designed to fracture the soil and install grout to create a lense of 3 meters in diameter with a thickness of 3 to 6 mm.. Injections were installed in a grid of 1.83 X 1.83 meters, Fig. 2. The over-lapping of these lenses provided a continuation of the reinforcement to resist the small but on-going creep movement. A slurry grout mix of water/cement of 2 was used together with additives, as needed, to provide for the pumpability of grout and to facilitate fracture initiation.
The mechanism of soil reinforcement is based on the friction/bond between the hardened grout lense and the soil, much the same as metal strips in reinforced earth applications (Tabbal 1983).
Performance
No detectable settlements have been observed in the four years since completion of the work.
The hillside showed minor movement for a few months after work completion, but even those movements were greatly reduced to hardly detectable amounts since then.
Case 2
This case presents remediation of a condition of downward migration of a finer grained fill soil into a layer of gravel and cobbles. A concrete-block building of 18.3 X 39.7 meters with a slab-on-grade floor was constructed in 1986. A cut-fill approach was used to create the original level pad. The gravel and cobble layer was covered with additional fill of silty san 4.58 meters thick, Fig. 4. Within a year after construction of this building, cracks in the walls and the concrete slab appeared. A maximum differential settlement of more than 76 mm across the building was measured before the initiation of the remedial work. A trench excavated adjacent to one of the footings, just before undertaking the remedial work, revealed a substantial void between the bottom of the footing and the soils below it. Four borings drilled around the building showed evidence of extensive intrusion of the silty sand layer into the gravel and cobble layer.
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Approach Concept
The lateral and vertical extension of the gravel and cobbles zone, coupled with its very high permeability, necessitated the installation of a grout blanket (barrier) just under the silty sand full to block the silt and sand intrusion into the gravel and cobbles layer. Cemchem, a controlled fast-gel grout, was selected for this situation. This proprietary system can be controlled to set between twenty seconds and one hour after mixing. With proper mix design, length of grout pipe, depth of soils to be injected and equipment arrangement, the grout can be designed to set within a few seconds after it leaves the tip of the grout pipe.
The remedial work consisted of installing this barrier, then densifying the soils above it using compaction grouting. This was followed by void filling, and structural lifting using compaction-grouting techniques to lift and level the structure and slabs.
Remedial Work
Using CemChem grout a blanket with a nominal thickness of 0.3 to 0.6 meters was established in the areas that needed it. By probing in a grid of 0.92 X 1.22 meters, it was first determined whether or not the sand intrusion had reached a point where it had already established a barrier. If the grout permeated the soil, it was assumed that a barrier had not yet been created. If, on the other hand, it did not permeate the soil, the assumption was that the sand had already created a barrier within the gravel and cobble layer, thus no further work would be required in the vicinity of that injection point. The establishment of the blanket required more than 30,300 liters of CemChem grout. This procedure was then followed by compaction grouting to densify the loose soils above the barrier. A grid of 3.66 X 4.27 meters for casing injections was first driven and pumped, followed by splitting this spacing into 1.83 X 2.14 meters. Total compaction grout injected was about 140 cubic meters, which resulted in an improvement of the bulk density of the treated soils of between 15 and 21 percent. The compaction grout was injected in stages of 0.3 to 0.6 meters starting at the top of the blanket and moving upwards to the bottom of the floor slabs or footings. Grout injections were terminated when the grout pressure reached 3445 kPa or the surface lifted to an unacceptable level.
Grout Materials
For the CemChem system, portland cement is the base material. Portland cement Types I, II or V have been successfully used together with bentonite and additives to produce the required gel time.
For the compaction grouting, locally available silty sand was used. This material was found to satisfy the criteria given in Fig. 3. Portland cement was added at the rate of ten percent by weight of grout.
Performance
More than six years after undertaking this remedial action, no distress or movement has been reported
Case 3
Loose to medium granular soils undergo volumetric changes (settlements) under additionally imposed loads, vibrations, and seismic activities. If such soils contain larger particles of gravel and boulders in a heterogeneous formation, settlement predictions become highly complicated.
This case history involves the 1915 Rose Bowl Stadium in Southern California (listed as a historical monument). The stadium was undergoing an expansion project involving the press box and new executive suites which resulted in additional loading on the footings. Portions of the new expansion would be supported by some of the old stadium footings and others by the new footings. It was determined that the old footings themselves rested on loose uncompacted fill, making it impossible to underpin the old stadium footings without damage to the structure unless and until the soils below these footings were given additional support.
Approach Concept, Remedial Work and Materials
The solution to this condition was through a permanent solidification system using permeating grouting with chemicals. Injections designed to create “solidified pedestals” of about 1.22 meters in diameter were used. For permanency, strength, and environmental considerations, an ultrafine cement grout was selected. Before finalizing the designs, a pilot test program was undertaken at the subject site. The results revealed satisfactory grout permeation into the soils with an unconfined compressive strength exceeding 1380kPa. The geotechnical design proceeded with 1,000 pedestals (injections) under the old and new footings.
The site soils immediately below the footings had a graduation that ranged between silt particles and cobbles. Less than five percent of the particles passed the 200 U.S. Standard Sieve (0.075 mm) while the largest particles were up to 100 mm. In ultrafine cement more than 80 percent of its particles are smaller than 6 microns. A water/cement ratio of 4:1 was used. Each injection required 170.5 liters. Nominal pressure used for these injections was 345 kPa.
Performance
The program proved to be successful in terms of being able to affect the required solidification. More than a year has passed since the completion of this work. Full loads have been imposed with no signs of any settlement. It is fair to assume that the designs and remedial work will perform successfully based on the excellent grout take that was recorded at the site and the strength of the obtained samples.
CONCLUSIONS
Mitigation of settlements of existing structures involves stringent requirements to satisfy the site, soil, and structural specifics. The three case histories presented in this paper show how such specialized methods can be used to halt the settlement of structures in a cost-effective way. Replacing structures, re-excavating or re-supporting existing structures on piles are not the only solutions available to the geotechnical engineer today.
APPENDIX – REFERENCES
Committee on Grouting of the Geotechnical Engineering Division (1980). “Preliminary glossary of terms relating to grouting.” J.Geotech. Eng.Div., Proc. ASCE, 106(FT7), 803-815.
Stoker, G. G. and Wardwell, S. R. (1987) “Compaction grouting of the Phoenix drain tunnels.” Proc. 1987 Rapid Excavation and Tunneling Conf., New Orleans, 1, 575-582.
Tabbal, M. A. (1983). “The study of cement grout reinforcement in slopes of soft clay,” Engineer thesis, Department of Civil Engineering, Stanford University, Stanford, California.
¹President, Pressure Grout Co., 1975 National Avenue, Hayward, CA 94545-1709.
Compaction Grouting: From Practice to Theory
Reprinted from Grouting: Compaction, Remediation and Testing
Proceedings of sessions sponsored by the Grouting
Committee of The Geo-Institute/ASCE in
Conjunction with the Geo-Logan '97 Conference
Held July 16-18, 1997 Logan, Utah
COMPACTION GROUTING: FROM PRACTICE TO THEORY
H.R. Al-Alusi, Member ASCE
Abstract
Compaction grouting is a method of in-situ soil densification by grout injection under pressure. With almost no theoretical consideration, compaction grouting emerged from “mudjacking” applications, through deep slurry consolidation to evolve into a compaction idea that was perpetually perfected using available theory to reach today's state-of-the-art technology.
This paper presents the requirements for successfully implementing a compaction grout densification program (CGD Program). It presents what the geotechnical engineer should look for during the investigation campaign and how to develop findings into design parameters. Mathematical and physical models of the CGD method are reviewed. Improvement in bearing capacity, reduction of settlements, or density considerations are given with applications. Further research ideas and directions are discussed.
Introduction
Conceptually, compaction grout is injected under pressure to displace the soils and produce high in-situ density. With today's state-of-the-art technology, compaction grouting requires a minimum of three main components: suitable knowledge of site soils, proper equipment, and the know-how to apply the technique to achieve the target results.
Although knowledge of the site soils and proper equipment are essential for a compaction grout densification program (CGD), this article will not be dealing with theses items specifically. This article's main focus will be on the understanding and the successful application of a CGD program.
Mudjacking (slabjacking) has been known to involve the use of cement slurry of “mud”, which is a mixture of sand and cement. Because of its high mobility, once the slurry grout fractured the soil, it was possible to control its installations. Sanded mixes offered better control depending upon their sand content and viscosity. However, the use of sanded mixes (mud) was not without problems.
In the 1940's and 1950's, equipment was not available that could deliver sanded grout at high pressures. Although the grout pressure did not need to be higher than a fraction of a N/mm^2 to allow for the weight of the slab, along with whatever local resistance existed, the technology at that time lacked two main aspects; namely, availability of equipment and an understanding of the behavior of the grout being used.
As equipment improved, providing better delivery, higher pressures, and more accurate monitoring, it was realized that for a heavy slab with additional local resistance, the grout was pressing and densifying the underlying soil before lifting the slab to the required level. The next step was to extend the pipe into the soil to obtain better densification, and then extend the pipe to a lower depth to densify the lower soils. This became known as top-to-bottom (stage-down) compaction.
The relationship between grout viscosity and soil consistency was not clear enough to secure controlling the placement of grout without fracturing the soil and losing control over it. It took some time and exposure to understand this relationship.
Before the geotechnical engineer's involvement, this approach was hit-or-miss, depending upon what depth the operator could reach, how much effort was available, and what pressures the equipment could deliver. The geotechnical engineers, inside and outside the specialty-contracting field, provided the necessary soil information and the anticipated objectives of the grouting program.
However, in stage-down compaction, it is tedious to densify an upper zone, then drill through the compacted grout, reinstall the injection pipe, and then reseal it. From a practical point of view, this procedure is not efficient. Though it could be argued that the added grout helps by introducing additional weight to the soil for improved confinement, in reality an added weight on only five to ten percent has a negligible effect on the densification process. It was quickly realized that whatever, if any, benefits were derived from this procedure; they were outweighed by the cumbersome impracticality.
Stage-up compaction (bottom-to-top) is a more efficient technique. In this process, a grout pipe is installed to the maximum depth required (usually a competent soil/rock layer) and compaction starts on top of it, proceeding upwards in increments of about 60 centimeters or less. In terms of drilling and pipe installation, a stage-up process is superior and does not involve resealing, redrilling, or reinstallation of the injection pipe. As a result, stage-up compaction is typically employed as more efficient and cost-effective.
Relationship Between Soil Type, Grout Viscosity, Pressure, and Required Volume of Grout
With time and extensive practical use of “compaction grouts,” it became clear that the viscosity of the injected grout must be limited. In 1980, the Committee on Grouting of the Geotechnical Engineering Division of ASCE defined compaction grout as not more than one-inch (25mm) slump. Slump value is per ASTM C143. Although this requirement can be used for the majority of encountered soils, it is too restrictive for low consistency soils, such as peat, very soft clays and silty clays. On the other hand, it is not conservative for high friction soils such as sands/silty sands, where a slump of less than 25mm is needed. Figure 1.a represents the qualitative relationship between the type of soil and the required slump value.
Grout pressures are related in an opposite way to the soil type from grout viscosity. Figure 1.b shows pressure requirements for various soils. The rate of pumping is high for low consistency soils and, low to very low for high friction soils, see Fig.1.c.
Because of their ease in accepting grout under pseudo-static conditions, low friction soils accept more grout volumes than high friction soils, as qualitatively shown in Fig.1.d. This results in higher degrees of improvement for soil density, as depicted in Fig.1.e.
Each of the figures (Figures 1.a through 1.e) is for one variable versus the type of soil, assuming all other variables and equipment capabilities are the same.
Theoretical Considerations:
For a homogeneous, isotropic material, the grout pressures within the soil mass dissipate at a spherical boundary, centered at the tip of the injection pipe. At this boundary stresses and strains caused by the grouting process are nil. For the purpose of this discussion, call this boundary the neutral boundary. The following state of stress shall exist:
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For a homogeneous, linear, elastic, and isotropic material, volumetric strain is the volume of grout divided by the soil volume within the neutral boundary, or:
ev = Vg / Vnb ……………………………………….[1]
where: Vg = volume of grout
Vnb = volume of soil within the neutral boundary
If we define a soil bulk modulus as Eb = Pg / ev
Or, ev = Pg / Eb ……………………………………….[2]
Substituting equation [1] into equation [2], gives
Vg / Vnb = Pg / Eb ………………………….[3]
The increase in density of the soil mass (Dg) can be represented as :
Dg = Dm / Dnb where Dm is the “introduced mass,”
substituting for Vnb from equation [3], gives
Dg = (Dm / Vg) * (Pg / Eb)
The “introduced mass” is not the mass of the injected gout. The mass introduced into volume Vnb that effectively raises the density of the soil within Vnb is the volume of the introduced grout multiplied by the density of the soil itself. To better understand the effect of this mass, let us assume that the grout is injected in a balloon and that air is used instead of grout. What the air and balloon would displace is a mass with volume of grout Vg and a density of soil Ys. On the other hand, if we assume that we inject a grout of an extremely high density, such as (Yg = ¥ ), then the same argument holds, i.e., the effect of introducing such a grout on the soil mass surrounding it is also equal to the volume of he injected grout times the density of the soil itself. Hence, the effect of the grout is irrelevant, and
Dm = Vg * gs
where: gs = unit weight of the soil at the point of injection
Therefore,
Dg = gs * (Pg / Eb) or
Eb = gs * (pg / Dg)…………………………..[4]
For a given set, Ys, can be taken as a constant for all practical purposes. Eb, by definition, represents the relationship between the volume and the pressure of the grout. Eb is a property of the soil.
Collected pressures and improvements of the soil density by the author are presented in Table 1. The soils bulk modulus is determined according to equation [4]. Plotting these values against soil type gives us Fig. 2, which is in qualitative agreement with Fig.1b.
Soil may be, to a practical degree, homogeneous enough to allow same properties from one point to another within a soil mass. Thick soil formations are expected to exhibit more isotropical behavior than thin zones and layers. Most soils, specifically man-made fills and residual soils, tend to be orthotropic rather than isotropic. Again, soil deposits of relatively high thicknesses are expected to have some degree of isotropy.
Linearity and elasticity are the least two assumptions that can be satisfied by everyday soils. At low strain level, soils may exhibit certain linearity and elasticity aspects. However, at higher strain levels soils are neither linear nor elastic.
Practical Considerations
Grout introduced in the ground forms a bulb (Al-Alusi, 1994 & 1996) without permeating the soil or fracturing it. The formation of such a bulb is directly connected to the equipment's capabilities to deliver the proper viscosity grout and at sufficient pressures. In order to pump a 25-50 mm slump grout at a pressure of 3000-7000 kPa, it takes specialized pumps and proper material. Grout is introduced into the soil mass, displacing air, solids, and water. In this regard, two main categories of soils need to be recognized: an unsaturated soil and a saturated soil.
In an unsaturated soil, there is no need to impose a limit on the rate of grout pumping, while in a saturated soil it is limited by the ability of the soil mass to dissipate the generated pore water pressure. Otherwise, the pore water pressure dissipation can be expedited by way of wick drains, sand drains, or perforated standpipes. Other considerations are:
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1. Thickness of grouting zone:
The grouting procedure should be applied to the full thickness of a soft/loose soil layer when undertaking the eradication of settlements. Improving only a portion of the settlement-contributing zone may aggravate the problem. The whole zone should be densified unless a detailed settlement calculation including the added mass of grout, shows that future settlements are acceptable. When there is a bearing capacity insufficiency for a concentrated load, compaction can be carried out to improve the soils in the vicinity below the foundation element without necessarily reaching a competent soil layer. However, such a load transfer does not necessarily eliminate a settlement problem.
2. Spacing of Injection Points:
A primary, secondary, tertiary system should always be used as much as possible. An equilateral triangular spacing is the most efficient for a grid pattern. Common primary spacing is 2.5, 3.0 or 3.7 meters depending on the thickness of the layer that needs to be densified. Further space splitting is possible by introducing injections within the triangles.
3. Staging:
For best result, vertical staging should be reduced to the shortest practical distance. A 60-cm (two-foot) staging is usually used with a tendency to limit it to 30-cm only (one-foot).
4. Sequencing:
Sequencing is a tool to confine the effects of grout and limit it to a pre-defined area. In a large open area that needs to be densified, compaction injections should be place around the whole area (i.e., around the perimeter). The interior of this area is then densified. One way this can be accomplished is by isolating the overall area into suitable cells, then dandifying the perimeter of each cell, followed by densification of the interiors.
5. Effects of soil stratification:
For soil profiles where a relative soft/loose (low Eb) layer is next to a firmer (high Eb) layer, the possibility of grout puncturing through the high Eb layer to the low Eb layer exists. For such a condition, the lower Eb layer should be densified first to a point where its density matches the high Eb layer.
6. Effects of a CGD program on groundwater movement:
The introduction of discrete bulbs of grout together with the densification effect around them, have a negligible influence on the permeability of silts, clays and their combinations. Theoretically, in sands, silty sand and their combinations, an effect exists, although such effects are hardly measurable. First, unlike piles or large diameter shafts, compaction grout bulbs are discrete, and second, the target of the CGD Program is to raise the soil density by a small percentage (5% to 15%), which in turn produces practically immeasurable permeability changes in subsoil's.
7. Stoppage Criteria:
a. Grouting performance, such as the amount of predetermined grout consumed (grout take), maximum pressure reached, or given rate of pumping at a given pressure.
b. Grouting procedure failure, such as a sudden drop in pressure, indication soil fracture (10 kPa or more).
c. Grouting site limitations, such as the maximum grout pressure that can be reached, pressure within the soil as measured by a pressure cell embedded in the soil, vertical or lateral movement of an inclinometer, pile top, retaining wall, footing, etc.
8. Effects of Grout Strength:
The strength of grout material has absolutely no effect on compaction performance whatsoever. Grout bulbs are discrete without interconnections. Therefore, the strength of the grout material is irrelevant. This relates very well to the same finding of irrelevance for the density of the grout material as presented above.
Future Research and Directions
The relationship between grout viscosity and soil consistency is probably of prime importance for the successful achievement of a CGD Program. Fig.1.a will need to be quantified through fully controlled CGD Programs, followed by verifications. At present, practitioners tend to be conservative by lowering the required slump value.
The amount of grout needed to raise a soil's density to a prescribed value, such as the relationship given in Fig. 1.e, will need to be verified assuming the soil mass to be homogeneous after a CGD Program.
Values for the volume of grout will follow almost directly into Fig. 1.d and hence will qualify both Figs. 1.d and 1.e.
A factor that can help tremendously in establishing Figures 1.b, 1.c, 1.d and 1.e is Eb. Although Table 1 is based on actual grouting case histories, Eb values and relationships to other established soil parameters can be improved using existing engineering data.
Conclusions
Soil improvement by in-situ densification using pseudo-static compaction techniques has been successfully used with limited theoretical understanding. Although not much theoretical understanding has been behind the development of the CGD systems, such an understanding is prerequisite to solidifying and crystallizing the main concepts of the technique.
Another integral part of this understanding is the information derived from controlled performance data and subsequent verifications. The need is still high for results of controlled densification projects. The density of the grout material itself is found to be irrelevant to the performance of the technique. A new term, Bulk Modulus of Soil, is introduced which relates the pressure of grout to the volume of grout, and hence the degree of soil improvement. The irrelevance of both the strength and density of the compaction grout is proven.
References
Committee on Grouting of the Geotechnical Engineering Division (1980). “Preliminary glossary of terms relating to grouting.” J. Geotech. Eng. Div., Proc. ASCE, 106(GT7), pp. 803-815.
Al-Alusi, H. R. (1994), “Soil improvement to mitigate settlements under existing structures.” Proceedings of Settlement '94. Vertical and Horizontal Deformations of Foundations and Embankments, Geotechnical Engineering Division, American Society of Civil Engineers, June 16-18, College Station, Texas, pp. 1214-1223.
Al-Alusi, H. R. (1996), “Abatement of soil liquefaction under existing structures.” Proceedings of IS-Tokyo ‘96/The Second International Conference of Ground Improvement Geosystems, May 14-17, Tokyo, Japan, pp. 249-254.
¹President, Pressure Grout Company, 1975 National Avenue, Hayward, CA 94545-1709, (510) 887-2244.
Lense Grouting in Geotechnical Engineering
ELEVENTH
AFRICAN REGIONAL CONFERENCE ON
SOIL MECHANICS AND FOUNDATION ENGINEERING
CAIRO/ 11-15 DECEMBER, 1995
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H.R. Al-Alusi*, Member, ASCE
SYNOPSIS
This paper presents the concept of Lense Grouting, its histroy and development, the equipment and the materials employed, and various applications. Lense grouting intentionally fractures the soil mass and introduces grout. Applications in alluvial deposits, man-made fills, and other soil masses that lend themselves to near horizontal fracturing have been successfully used to mitigate movements caused by expansive soil conditions, reinforce hillside slopes, and mitigate soil settlements/liquefaction in areas where there are buried features which are sensitive to pressures. This method of soil grouting was developed and has been used mostly on the West Coast of the United States. Three case histories using this application are discussed. This new pressure grouting application adds another remedial tool to the arsenal of the geotechnical engineer and engineering geologist to assist in implementing his solutions in a cost-effective way.
Introduction
Pressure grouting is the technique of introducing a grout material in a soil or a rock mass to improve certain physical properties.
In rock formations the grout is introduced into existing cracks, fissures, joints and crevices by drilling through the rock to intercept these features and fill them with grout without fracturing the rock. If a soil material occupies a joint or a fissure, the grout may displace the soft material to fill the available space with the more competent material of the grout. The technique most often employed for the majority of rock grouting jobs is void filling, using a pressurized system. Depending on the size of the cracks or joints, the grout material may range from non-particulate chemicals and particulated ultrafine cement slurries (with near water viscosity), to sanded mixes of portland cement with large aggregates.
For soils the interparticle relations together with the presence of water and air make the physical conditions within the soil mass different from those of rock. There are seldom any cracks, joints, or fissures except possibly near ground surface. Pressure grouting in soil involves one of the following:
Displacement, as in compaction grouting (Al-Alusi, 1994 and 1995);
Permeation, as in chemical or ultrafine cement grouting (Al-Alusi 1994); or Fracturing, as in lense grouting, which is the subject of this paper.
In compaction grouting the grout is introduced in the soil mass to volumetrically displace the soil particles without permeation or shear (fracture). In permeation grouting, the grout permeates the pores of the soils by migrating through the pores without displacing the soil particles. Lense Grouting is a shearing system that fractures the soil mass without displacing the soil articles or permeating the soil pores. Upon hardening, the grout lenses become an integral part of the soil mass.
HISTORY AND DEVELOPMENT
From its inception the Pressure Grout Company pursued development of this system with three main objectives:
To introduce grout lenses within a soil mass designed to impede/eliminate moisture exchanges within the soil mass for the mitigation of expansive soil movements.
To mechanically reinforce the soils by introducing a material which possesses a shear strength higher than that of the existing soils.
To introduce grout lenses within a soil mass to lend structural rigidity for the mitigation of slumping in cases of soil liquefaction or settlement of low consistency soils which are necessitated in areas where a compaction or a permeation system cannot be used (i.e. in liquefiable soils near buried utility lines or adjacent to a retaining wall).
THE MECHANICS OF LENSE GROUTING
Theoretically, in a homogeneous, linear, elastic and isotropic body of soil, which is virtually a non-existent material, and without the effects of boundary conditions, soil fracturing is anticipated to be radiating from the point of injection. In the oil drilling industry vertical fractures are usually observed. These fractures and others are usually governed by the properties of the rock present and their jointing and layering, rather than by the mechanics of ideal materials. In soils, beside the effects of layering, rather than by the mechanics of ideal materials. In soils, beside the effects of layering and other soil properties, the closeness of the injections to the grouting surface (i.e. boundary conditions) tremendously affects the mechanics of fracturing.
The lense grouting technique was developed to seek planes of least resistance to enable fracture development parallel to these planes. Alluvial deposits lend themselves to such fracturing. Man-made fills being laid down in horizontal layers offer a very good field for this application. Generally any formation that has any layering, jointing or weakness in a horizontal or near-horizontal direction, are excellent candidates for lense grouting. To assist in the initiation of horizontal fracturing, specially designed tips at the end of the driven pipe are used. A high-pressure pulsation, 7,000 to 30,000 kPa, is employed with the tip tool to start the fracture, then the pressures are reduced. The radial or vertical extension of a single lense is determined by the volume of grout that is pre-designed to be injected.
MATERIALS AND EQUIPMENT
The grout material must be relatively thin with a consistency that will not plug small cracks and joints. A special mix(es) of portland cement, bentonite clay, and additives have been successfully used for the initiation of soil fractures. The use of special additive is discussed below. Sand can be added if required, but usually it has an impeding effect, plugging up the paths of the lenses. Upon hardening the lense grout material is optimized to produce a very low water permeability, high tensile strength, high modulus of elasticity or other preferred characteristics, all in comparison to that of the existing soil. To be able to proportion, mix, hold, transfer, and inject the lense grout material the following items are needed:
Graduated tanks (two to five may be needed). Each tank shall be properly graduated, labeled and instrumented with pedals and mixers.
Transfer pumps between tanks.
A colloidal (centrifugal) pump is needed to break down cement lumps.
In-line screens, and/or tank screens with vibrators to intercept cement and bentonite clay lumps and break them if possible.
Duplex or triplex pumps capable of producing 7,000 to 30,000 kPa. Advanced cavity pumps can be used, but because of the lack of pulsation they do not produce soil fracturing as desired.
Pressure gauges, flow meters, recorders, and monitors as needed.
Drilling/driving equipment as appropriate.
ADDITIVES
To modify the consistency of the grout to perform more efficiently for a given soil or for conditions at a given site, additives are used. Common additives include those that will increase the strength of the grout material, increase or decrease its set time, or will help in improving or maintaining the tensile strength of the lenses, such as synthetic fibers, see Case History No. C.
APPLICATIONS
I. Abatement of Expansive Soil Conditions
Lense grouting has been successfully used to eliminate or significantly reduce moisture exchanges between the soil and the environment.
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Expansive soils movement mainly affect low-load to medium-load intensity structures. Typically, residential structures and warehouses fall in these categories. The interior of such structures are normally paved and protected from moisture exchanges. However, the soils at the perimeter of the structure, if not specifically isolated from the environmental moisture exchange, will be susceptible to the movement of expansive soils.
An ideal approach to the mitigation of this problem is to have the subject soil saturated or at a near-saturation condition (such as during the end of the wet season), then install lenses to lock in the moisture, see Figure 1. To have the soils saturated is also important because in a dry condition the surfaces of expansive soils will be cracked with sizable fissures, adversely impacting the installation of grout. In a typical installation the lenses extend to about seven feet below ground surface, which approximates the depth of seasonal moisture changes.
II. Hillside Reinforcement
Hillside creep and minor slope instability has been successfully abated using lense grouting, see Figure 2.
In the case of hillside creep, the grout lenses serve as reinforcing elements, much like using reinforcing steel bars in concrete (Collin and Mitchell 1984, and Tabbal 1983).
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Load transfer is achieved through the friction between soil and fiber. The reinforcing effects can be enhanced by overlapping the lenses, or making them vertically closer to each other by shortening the grout shot distance in the injection.
III. Abatement of Liquefiable Soils Conditions
In areas adjacent to utility lines, retaining or basement walls, and other pressure sensitive elements where a compaction or a permeation system is not applicable, the use of lense grouting has been used. The concept for such an approach is to install random lenses to lend a near liquefied soil mass support until it regains its strength upon dissipation of excess pore water pressure.
IV. Thwarting Settlements Near Pressure-Sensitive Structures
The concept of this application is similar to that of (III) above. However, the objective of this application is to thwart future settlements using a lense grouting approach. Figure 3 shows an application for lense grouting in San Francisco, California for the mitigation of pile settlements in soft bay mud.
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CASE HISTORIES
A. Expansive Soil Conditions
The author is familiar with and has records of at least 150 structures throughout Caliofrnia, Nevada, Arizona, Texas and New Mexico, where lense grouting was applied for the mitigation of expansive soil conditions. These applications have been judged to have been successful, as illustrated by the fact that no further grouting or other mitigative measures have been needed at these sites. Generally, monitoring consists of visual observations of the structure for a number of years after completion of work and comparisons to its behavior before grouting.
B. Slope Reinforcement, San Francisco, California
Lense grouting was used to reduce downward movement near the surface of a 15 meter high slope adjacent to an office building in San Francisco, California. The deep soil reinforcement included injections of grout lenses to a maximum depth of 11 meters. Lenses were installed vertically at 0.31 meter intervals. Each lense was designed to fracture the soil and install grout to create a lense of 3 meters in diameter with a thickness of 3 to 6 mm. Injections were installed on a grid of 1.83 x 1.83 meters. Overlapping the lenses provided a continuation of the reinforcement to resist the small but on-going creep movement (Al-Alusi 1994).
No detectable movement has been observed in the six years since the work was completed. Movements are monitored by settlement points and inclinometers.
C. Slope-Hillside Reinforcement, San Clemente, California
Within a few years of its development, the streets and sidewalks in a hillside area in the Southern California City of San Clemente showed signs of distress. The native siltstone was overlain by a clay fill which ranged in thickness from 0.3 to 4.3 meters. The superficial distress observed was primarily manifested as cracking and displacement in a 55-meter stretch of street gutters, curbs and sidewalks. Displacements were a maximum of 12.7 centimeters laterally and 7.6 centimeters vertically. To remove and replace the fill was ruled out as economically unfeasible. Alternately, lense grouting was considered and designed for this site.
The objective of the lense grout design was to reinforce the fill material itself and to improve its bonding to the native soils. Lense grout injections were spaced on a grid of 2.15 x 1.83 meters and extended to a depth of 5.80 meters below street level.
Synthetic fibers were used in this grout at the rate of 0.5 to 1.0 kilograms per cubic meter of grout.
CONCLUSIONS
Unconventional grouting methods such as lense grouting are beneficial and sometimes the only method available for a given site.
Lense grouting has proven successful in the mitigation of expansive soil conditions, reinforcement of slopes and hillsides, and reinforcement of sites where permeation or compaction grouting are not feasible.
REFERENCES
Al-Alusi, H. R. 1994. Soil improvement to mitigate settlements under existing structures, Proc. Settlement '94, sponsored by the Geotechnical Engineering Division/ ASCE June 16-18, 1994, College Station, Texas, 1214-1223.
Al-Alusi, H. R. 1996. Abatement of soil liquefaction under existing structures. To be presented and published for IS Tokyo '96, sponsored by the Second International Conference on Ground Improvement Geosystems, May 14-17, 1996.
Collin, J. G., and Mitchell, J.K. 1984. Injection grouting for in-situ earth reinforcement. Master of Science Thesis, University of California, Berkeley.
Tabbal, M. 1983. The study of cement grout reinforcement in slopes of soft clay, Master of Science Thesis, Stanford University, California.
*President, Pressure Grout Company, 1975 National Avenue, Hayward, CA USA 94545-1709.
Grouting and Deep Mixing
PROCEEDINGS OF IS-TOKYO '96
THE SECOND INTERNATIONAL CONFERENCE ON GROUND IMPROVEMENT GEOSYSTEMS
TOKYO
14-17 MAY 1996
GROUTING AND DEEP MIXING
Edited by
RYOZO YONEKURA, Tokyo University, Japan
MASAAKI TERASHI, Nikken Sekkei Nakase Geotechnical Institute, Japan
MITSUHIRO SHIBAZAKI, Japan Chemical Grouting Association,
Abatement of soil liquefaction under existing structures
H.R. Al-Alusi
Pressure Grout Company, Hayward, Calif., USA
ABSTRACT
Liquefaction mitigation under a functioning structure represents a challenge to the engineer and the geotechnical contractor and accentuates cost, time, and disturbance of the facility's use. Access is the most important aspect of such a project. Two case histories where soil liquefaction was required to be abated under existing structures are presented.
In Case One, a filter building was underlain by a 3.7 meter liquefiable sand layer. The approach taken to reach the target soil was to drill horizontally about 28 meters, and extrude compaction grout in 1.5-meter stages to densify the soil.
In Case Two, a liquefiable sand layer and a silty/clay fill layer were identified under an existing multi-story building. The building was supported on old wood piles. Two systems were selected for this project; permeation grouting or sands and lense grouting for silty/clay layer.
In conclusion, it was possible, using cost-effective methods, to mitigate the risk of liquefaction under existing structures without interrupting the use of the facilities.
1 INTRODUCTION
Improvement of liquefiable soils follows a number of methods and techniques that are well established in the industry. These available methods and techniques become very limited and restricted for locations where a structure is in place but underlain by liquefiable soils. In a situation where a structure must continue to function, the restrictions multiply. Liquefaction mitigation of a soil layer under a functioning facility represents a challenge to the engineer and the geotechnical constructor.
Almost every project for abatement is unique in its approach. Consultation and close coordination between the owner of the facility, the geotechnical engineer, and the specialty contractor are of tantamount importance for these projects.
This paper presents two case histories where grouting methods were successfully used under existing structures for the abatement of soil liquefaction in a cost-effective way without interfering with the operation of the facilities.
2 CASE ONE: COMPACTION GROUTING FOR SAND DENSIFICATION
Compaction grouting has been successfully used for sand densification for liquefaction mitigation (Mitchell & Wentz 1991). The process involves the controlled injection of a stiff sand/cement mixture to volumetrically displace the soils to increase its density. Grout stiffness is related to the slump value, ASTM C143-78. For a controllable grouting process, this value should be maintained to less than 5 centimeters (Al-Alusi 1994). The success of this process is hinged on having the ability to maintain the grout near the point of injection by volumetrically displacing the soils without fracturing them. Theoretically, compaction grouting loses its significance upon fracturing the soils. Ground surface monitoring for vertical and/or horizontal displacements (in cases involving slopes) is a must for every application.
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2.1 Site conditions and liquefaction potential
An exploration to ascertain the soil conditions for expanding a wastewater reclamation plant in Los Angeles, California (see Figure 1), identified a liquefiable soil layer approximately 3.7 meters thick. The expansion program included adding more filters to the existing filter building, which was 25 X 30 meters in plan. Although the new filters could be founded in a deep engineering fill, the liquefiable soils under the existing filters building needed to be approached differently.
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Bordering the structure on threee sides were other facilities in such close proximity that made it possible to excavate for a horizontal drilling operation. The remaining side of the structure that was available for horizontal drilling was a short side. An excavation of about 10 meters was made with conventional soldier beams and lagging. The ground water level was dropped using a traditional pump and sump method.
The project site was located in a seismically active are of Southern California. The dominating fault, located approximately 4.8 kilometers miles from the site, was considered to be capable of generating a magnitude 7.5 earthquake. Probabilistic seismic risk analyses estimated the Peak Ground Acceleration at 0.35g and 0.6g for a design life of 100 years. The densification process became critical due to the building's sensitivity to settlement (Harding Lawson 1990).
Soil borings and Cone Penetrometer Test (CPT) soundings encountered a 2.5-meter layer of relatively loose sand. The geotechnical engineers' site seismicity evaluation and cyclic shear strength study indicated that this layer was potentially liquefiable when subjected to the ground accellerations of design-earthquakes, see section in Figure 2.
Seismically-induced settlements were found to be likely to occur in the loose to medium dense layer (3.7 meters thick). These settlements were calculated to be on the order of 5 to 13 centimeters using methods proposed by Tokimatsu and Seed (1987).
2.2 Approach, Drilling, and Grouting
The project specifications called for extending the densification process of a 1.5--meter strip around three sides of the building, see Figure 2. For this strip a single row of vertical injections, spaced at 1.5 meters, was used. The installation of these injections served to confine the grout under the building and to provide correlations between test results and the amount of the injected grout (grout take) under building for quality assurance. The same termination criteria that were used for horizontal grouting, as discussed below, were used.
Two rows of horizontal injections were installed 1.8 meters apart, and centralized in the middle of the liquefiable zone, see Figure 3. The spacing between injections was about two meters. Each injection was extended horizontally for the full length of the existing filters, reaching a maximum of 26 meters. A partial drilling of 3.5 centimeter hole was first made with the aid of drilling foam. Upon completion of the hole a nominal five centimeter close-ended pipe was driven in.
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Compaction grout extrusions started at the far end of the excavation side. A staging of 1.5 meters was used. The sequence of grouting was by the primary-secondary injection method, alternating injections between the two rows.
At each stage, grout was injected until one of the following criteria was met:
1. A maximum pressure of 5500 kPa is reached at the point.
2. Inception of ground or structural uplift.
3. A quantity of 400 liters is pumped at a given stage.
Throughout this operation the high pressure criterion controlled the process. Grout takes ranged between 11 and 37 liters. A total of 19 horizontal injections were completed.
Throughout the grouting operation, horizontal and vertical, a laser survey system was employed to monitor any uplift in the structure or ground surface. None was detected at any stage.
2.3 Test results and discussions
Standard Penetration Tests were utilized to evaluate the effectiveness of the grout densification process around the perimeter of the existing building. A comparison of test results conducted before and after the densification process is shown on Figure 4. Since the SPT results were erratic, a correlation between the SPT increase and the calculated density increase caused by the added grout mass in a given volume of soil was made using published correlations (Holeyman & Wallays 1984, Winterkorn and Fang 1991, Bowles 1982). These calculations revealed that the relative density of the soil was raised from about 45 to 70 percent, and that the SPT values were raised from a range of 10 to 20 blows to a range of 30 to 40 blows. These results indicated that the targeted soils were improved to well above the critical penetration values required for a magnitude 6 earthquake event, and at or near that of a magnitude 7 earthquake event, see Figure 5.
It was not possible to perform blow count tests or any sounding tests, such as static cone penetrations, under the structure. Although other sounding tests, such as ultrasound and cross-hole geophysical tests, were available, it was concluded that such methods would not reveal sufficient useful results, especially in zones where known gravel existed. The effectiveness of the grouting program in this zone was evaluated by comparing the amount of grout injected in various stages, which was calculated to be about three percent of the total volume of soil, to the theoretical density improvement.
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3 CASE TWO: CHEMICAL AND LENSE GROUTING AROUND EXISTING PILES
Liquefiable soils, due to their particle size distribution and resulting high permeability, are usually amenable to chemical/permeation grouting. Chemical grouting, simply put, is a pure permeation grouting, which utilizes two or more material components whose chemical reaction results in a hardened matrix within the soil mass. In saturated soil masses, the grout is expected to displace water from the soil pores. In partially saturated soils it displaces air and water. Confinement and control of the grout are two key elements of a successful chemical grouting job. They can be achieved by working in a designed pattern, with or without a prescribed gel time (i.e., time required after mixing to start hardening).
Lense grouting is a soil fracturing technique where a cement slurry grout is injected at an initial high pressure of 700 to 3000 kPa, then reduced until a predetermined amount of grout is injected. In man-made fills and alluvial deposits, near horizontal fractures are achieved using engineered tips at the bottom of the injection pipe to facilitate fracture initiation (Al-Alusi 1994).
3.1 Building and Foundation Conditions
A seismic upgrade program was to be implemented for an eight-story concrete and masonry structure measuring 25 X 25 meters at the basement level. The building was located on a corner in the downtown area of San Francisco, California. It was erected in 1907 in the area that had experienced ground failure during the April 18, 1906 earthquake. During the Loma Prieta earthquake of October 17, 1989, the building sustained structural damage. The foundation consisted of pile groups, as shown on Figure 6. Timber piles, 46 centimeters in diameter and 10+ meters long, are believed to have supported this structure. The pile caps and floor were of reinforced concrete construction. Below the basement bottom, the soils were predominantly sandy silt with clay layer started, see Figure 7. The blow count (standard penetration test) was between 1 and 6 for layer 1, and between 2 and 13 for layer 2. Based on the very low blow count and high ground water level at which was at about the basement level, it was determined that liquefaction was most likely to occur in these sand formations during an earthquake comparable to the design event of magnitude 7.
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3.2 Restrictions and criteria of treatment methods
Compaction grouting could eliminate the potential for liquefaction of the soils below the basement floor slab. However, because of the presence of the timber piles and the likelihood that some of them were in a partially deteriorated condition, coupled with the high pressures inherently associated with compaction grouting, this method was deemed to be unacceptable even though it would have been the most cost effective.
In order not to affect the timber piles by high grouting pressures, a chemical system was selected that would solidify the sands of layer 2 with a low strength grout to render the material non-liquefiable. A sodium silicate based grout was used with a gel time of ten minutes and an ultimate unconfined compressive strength of the grouted sand of 190 kPa was selected and installed. Because layer 1 was not susceptible to chemical grouting, this method was limited to layer 2 only. The exact location of the piles within each group (cap) was unknown. Few attempts were made to define the pile locations, which revealed that the actual locations were not as shown on the available plans. Consequently, the design of the chemical grout system was made to confine each group in an isolated cell, then inject more grout within the confined cell to refusal, see figure 6. Refusal is defined as a grout pressure of 1300 kPa or a predetermined amount of grout based on the theoretical volume of voids within the cell using a porosity of 0.35.
The procedure followed was that of a closed end pipe vertically jetted into the ground using water. At the proper depth, grout was injected in stages of 30 centimeters in the vertical direction for the full 2.8 meter depth which was required (between elevation -5.5 and elevation -8.3). By following a primary and secondary injection pattern, a wall of chemically grouted soils was installed around each group of piles (See Figure 6). Voids, caused by the difference in rigidity between slabs, soils, pile caps, and the generation of settlements of the underlying mud of the San Francisco Bay, were suspected to be in this area. Before injecting the chemical grout, a probing program was adopted to look for voids immediately below both the basement floor slab and the pile caps. Encountered voids were filled up and the soils were tightened.
After completing the chemical injections, a lensing program for layer 1 was initiated. Lense grout injections were spaced on a 1.5 X 1.5-meter grid, covering the space between the pile caps. These injections were extended vertically between the bottom of the basement floor slab and the top of layer 1, elevation -2.8 to elevation -5.5. A vertical staging of 30 centimeters was used. In each stage 28 liters of 12-sack cement slurry was injected (one sack = 42.7 kilograms). The initiation pressure was between 700 and 3000 kPa, then dropped to 200 to 450 kPa. There were few instances where these pressures were not achieved until several tens of liters of grout were emplaced, indicating the presence of a void.
3.3 Results and Discussions
Laboratory prepared samples of site sand with chemical grout, indicated an unconfined compressive strength well above the required 190 kPa at 28 days, see Table 1. Grout takes and pressures for each injection were checked to assure the proper installation of each stage. During the primary chemical injections, the feedback pressure at the point ranged between 70 and 200 kPa. The secondary injections ranged between 70 and 500 kPa, indicated grout presence within the nearby soils.
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For the lense grouting program, no testing of the grouted soils can be practically made. Quality assurance was achieved by monitoring the grout amounts and pressures at the point of injection.
4 CONCLUSIONS
The risk of potential liquefaction can be eliminated or at least reduced in certain cases using cost-effective methods without interrupting the functions of the facility. The increased awareness of owners, engineers, and public agencies to the soil liquefaction potential beneath their projects usually makes them search and explore available methods for an acceptable solution.
Geotechnical grouting techniques have been available for quite some time (chemical grouting for about 150 years, compaction grouting for about 30 years, and lense grouting for about 15 years), but their adaption and use for the abatement of soil liquefaction and has been limited to the last 10 to 15 years. The use of compaction grouting in horizontally driven casings proved to be a workable solution. At least as far as the author is aware, such an approach has never been tried before to the extent used in Case 1 above.
In Case 2, the use of a combination of grouting methods otehr than compaction grouting was dictated by the presence and condition of grouting methods other than compaction grouting was dictated by the presence and condition of the timber piles and soil conditions. The goals of both projects were successfully achieved to the point where the subject soils were made non-liquefiable under design-earthquake events.
REFERENCES
Al-Alusi, H.R. 1994. Soil improvement to mitigate settlements under existing structures. Proceedings of Settlement '94. Vertical and Horizontal Deformations of Foundations and Embankments, Geotechnical Engineering Division, American Society of Civil Engineers, June 16-18, College Station, Texas
Bowles, J.E. 1982. Foundation Analysis and Design. McGraw-Hill Publishers, Inc.
Harding Lawson Associates 1990 and 1992. Geotechnical Investigation Reports for Tapia Wastewater Reclamation Plant Expansion.
Holeyman, A. & Wallays, M. 1984. Deep Compaction by Ramming (in French). Proceedings of International Conference In-Situ Soil and Rock Reinforcement, Paris, 367-372.
Mitchell, J.K. & Wentz, F.J.,Jr. 1991. Performance of improved ground during the Loma Prieta Earthquake. Report No. UCB/EERC-91/12, Earthquake Engineering Research Center, University of California at Berkeley.
Tokimatsu, K. & Seed, H.B. 1987. Evaluation of settlements in sands due to earthquake shaking. Journal of the Geotechnical Engineering Division, Proceedings of the ASCE, Vol. 113, No. 8, August, 861-878.
Winterkorn & Fang 1991. Foundation Engineering Handbook. Van Nostrand Reinhold.