Resources

• Lense Grouting with Fiber Admixture to Reinforce Soil
• Soil Improvement to Mitigate Settlements Under Existing Structures
• Compaction Grouting: From Practice to Theory
• Lense Grouting in Geotechnical Engineering
• Grouting and Deep Mixing

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. CHANDLER* 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.

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.

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.

* Branch Manager, Pressure Grout Company, 1330 W. Gaylord Street, Long Beach, CA 90813-1321, (562)432-4100