1.3 COMPACTION GROUTINGINTRODUCTION

Compaction grouting technology has grown steadily over the last 30 years and is a well-established and accepted practice Initially developed for structural settlement control and re-Ieveling, the technique has since been used to solve a number of other geotechnical problems.During underground construction, particularly soft-ground tunneling for subway systems, compaction grouting has been used to control surface settlements by pregrouting to densify and stress the soils to the point of heave. Thus, the injection of grout (through preplaced grout pipes) just after the tunnel boring machine passes is much more effective in reestablishing arching over the excavation and controlling surface settler.lents (Baker et al 1983). The stabilization of karst formations by compaction grouting to remediate existing or potential sinkhole problems has also become widely accepted. In recent years, compaction grouting to depths on the order of 120 m (400 ft) has been accomplished in karstic areas (Anonymous 1996). Compaction grouting is also now widely accepted as a site improvement technique, both for the mitigation of liquefaction potential and for soil densification to increase bearing capacity and reduce settlements (Chastanet and Blakita 1992).Among the more recent innovative uses of compaction grouting is the formation of a continuous foundation element by constructing a pressure-injected footing at depth to support a column of compaction grout up to design elevation The compaction grout 'pile' thus formed generally incorporates a continuous reinforcing member. As this emerging concept is developed to its full potential, better quality controls will be required to verify pile shape and continuity.Advances have been made with regard to equipment, monitoring and testing The evaluation of geophysical methods and other in situ tests to image the compaction grout in the ground and provide profiles of site improvement for bearing capacity and, more recently, liquefaction control is ongoing (Byle et al1991, Boulanger and Hayden 1995).Although compaction grouting has been in use in the United States since the early 1950's, there has been little fundamental research on this technique. Published technical papers have been primarily observations of successful performance and empirically derived theory (Graf 1992). Research was performed at the University of Florida in 1992 to develop a new theory and design methodology for compaction grouting to protect construction against local zones of sinkhole settlement (Schmertmann and Hcnry 1992). Research is ongoing at several universities to meet the need for the establishment of mixed material criteria versus grout pressure used, effective radius of compaction versus volume of grout and pressure, and effective pumping rates versus soil characteristics. This current research is summarized in this section.

FUNDAMENTAL CONCEPTSWelsh (1992) reports that the ASCE's Committee on Grouting defines compaction grouting as the injection of less than 50 mm (2 in) slump grout (normally a soil-cement with sufficient silt sizes to provide plasticity, together with sufficient sand sizes to develop internal friction). The grout does not enter soil pores but remains in a homogeneous mass that gives controlled displacement to compact loose soils, gives controlled displacement for lifting of structures, or both.The primary purpose of compaction grouting is to increase the density of soft, loose or disturbed soil, typically for settlement control, structural re-Ieveling, increasing the soil's bearing capacity and decreasing settlement for shallow footing construction, and for the mitigation of liquefaction potential Casing is installed to the maximum treatment depth and very stiff grout is injected at high pump pressure as the casing is withdrawn incrementally, thus forming a column of interconnected grout bulbs, as shown in Fig. 1.3-1

Fig. 13-1. Compaction grout bulb construction.

Compaction grouting can either be performed 'stage down,' fIom the upper to the lower limit of the treatment zone or, more commonly, in a 'stage-up' process fIom the lower limit upwards. The 'stage-up' mode typically involves the following procedure (Warner and Brown 1974).

1. Advancing a casing (either drilled or driven) to the bottom of the zone to be stabilized. The casing should be a snug fit to ensure full pressure is applied at the design depth

2. Injecting the grout at a specific depth until refusal criteria are achieved, generally based on injected grout volume, injection pressure or ground surface heave.

3. Extracting the casing to the next depth interval4. Repeating steps (2) and (3) until the upper limit of the treatment zone is reached.

Compaction grout bulbs are seldom neatly spherical and can be more cylindrical in shape, with the shape depending on the stage length, particularly for the stage-up method (Byle 1992).Key to the compaction grout program is the controlled placement of the grout. Many factors influence grout placement, particularly the composition of the grout mix but also pumping rate, injection pressure, stage length and grout hole spacing. Graf (1992) discusses grout composition, pressures and grout hole spacing. Pumping rates and pump pressures are discussed by Rubright and Welsh (1993) and Warner and Brown(1974).The differing approaches and conclusions in these and other published articles confirms that, although compaction grouting has been in use in the United States since the early 1950's and is a well-established and accepted practice, there is as yet no consensus of professional opinion. Typically each practitioner and engineer establishes his or her own rules as to the materials and methods to be employed based on experience and project-specific considerations The research programs summarized in this section may help to coalesce professional opinion.NEW DEVELOPMENTSOver the last ten years, several advances have been made in compaction groutingequipment, and in monitoring and testing of the compaction grouting program.Equipment and MaterialsContinuing advancements in pumps, protective pressure sensors (gage savers) and flowmeters are improving the application control of the technique. The smoother flow paths incorporated in new pump design, and better sealing, are allowing harsher grout mixes to be injected and higher pressures to be applied For densification and releveling applications, these improvements may allow compaction grout to be injected without the addition of cement to the grout mix. This would allow regrouting at the same locations at a later time. Also, the use of harsher mixes can reduce the tendency for hydro-fracturing.

MonitoringNon clogging, full-flow pressure sensors allow more reliable monitoring of pressure, and allow the use of transducers to replace analog pressure gages. These transducers, combined with electronic flowmeters, provide the ability to use data acquisition systems for data collection, storage and analysis This enhances retrieval, sorting and interpretation of the data. The use of electronic hardware and software will allow various parameters to be evaluated against and correlated with other site geotechnical data.

TestingAccurate verification of anticipated improvement is a common problem with compaction grouting Until quite recently, engineers relied on localized methods such as performance testing or standard penetration, cone penetration and dilatometer testing. (Byle et al. 1991). Direct transmission seismic testing, however, provides data from which average improvement values may be calculated with some accuracy. Byle et al. (1991) report that seismic methods are based on the transmission of seismic waves through the soQ The velocity of the seismic waves is a function of basic soil properties such as modulus, density and Poisson's ratio Measurement of the seismic wave speed between any two points provides an average of the wave speeds for the materials between those points. Seismic methods such as illustrated in Fig 1.3-2 may be used to evaluate the average improvement of the soil mass as distinct from more traditional methods of obtaining localized values.

Cross Hole Seismic Testing Down Hole Seismic TestingFig. 1.3-2.

RESEARCH PROGRAMSAcademia has taken an interest in compaction grouting and helpful research is currently underway. The mechanics of the expanding grout bulb and the expanding cavity in the soil are being analyzed and modeled. Also, centrifuge testing is helping to confirm the behavior of the modified soil. In addition, more and more projects are being performed with pre- and post-testing, providing much needed information for establishing an empirical data base. Thus, all of the above will help in the design and predictability of compaction grouting in future projects.The need exists for future research on the reinforcing aspects of the compaction grout bulbs or columns formed in the ground with regard to the composite properties of the treated soil mass, allowing even more applications of this technique in solvinggeotechnical problems.Test Program at the National Geotechnical Experimentation Site, Kersey, COThis test program has been undertaken to demonstrate the bearing capacity of compaction piles and to investigate the ability of verification methods to identify in situ injected shapes. The scope of the program includes the following1. Perform further site investigation of soil conditions within the site area.2. Construction of 12 compaction piles.3 Use cross-ho 1 e seismic testing to identify the shear wave velocities of test site soils before, during and

after grouting, including a 30-day after-grouting test to allow for grout hydration. It is hoped that this will give better identification of the location of the grout column boundary and show the columns shape and continuity in situ.

4. Load test four of the compaction piles to bearing capacity failure5. Lab-test grout mix design for sieve analysis and 30-day strength6. Excavate columns to determine actual injected shape and evaluate ability of cross hole seismic testing

to predict actual shape.

It is anticipated that this test program will provide additional insight intopotential Q/A and Q/C procedures that could enhance the use of injected grout columns as structural components As previously noted, current applications indicate compaction grout piles provide high load capacities in difficult

soil conditions and can be constructed on a cost effective basis to virtually any depth, size, shape or angle in "low" load bearing soil types.

At the time of this publication, the initial seismic survey, grout column injection and load tests have been completed. Excavation of the grouted columns and analysis of the resulting data will be completed as soon as the post-injection seismic survey is performed (anticipated by the summer of 1997). The current schedule is to publish the findings of this program in 1998.

North Carolina State UniversityIn 1995, research to study fundamental aspects of the compaction grouting process was funded by the U.S. National Science Foundation. The focus of the program at North Carolina State is to characterize the in situ densification that occurs around a grout bulb, to investigate the influence of grout constituents on the behavior of the injected grout, and to develop a rational and simple field test to characterize compaction grout properties relevant to injected behavior. To date, the following aspects of this work have bccn accomplished:

1. Based on the development of a cavity expansion model for grout injection, it has been shown that thelimit soil pressure at the grout/soil interface, a function of in situ soil conditions, will be reached when the grout bulb expands to about 2.5 times the original diameter of the borehole. Increasing grout pressures beyond this point are either the result of increasing shear resistance in the grout due to drainage or the interaction with previously injected grout zones Due to elastic compression that occurs at the boundary of the zone of influence, densification adjacent to the injected grout, as measured by volume displacement, was shown to be equal to only 50% of that calculated based on the injected grout volume.

2. Due to the increase in lateral stress that results from expanding a compaction grout bulb, interpretation of SPT or CPT soundings after grouting cannot be done using conventional charts relating penetration resistance to density. A model incorporating the lateral stress from cavity expansion theory with Skempton's expression for predicting relative density based on overconsolidation ratio was investigated. The analysis shows that the increase in lateral stress caused by grouting results in over-prediction of the post-injection density unless one explicitly accounts for this effect New design charts relating measured N-values, existing effective overburden stress and increase in density as a function of grout bulb spacing have been proposed

3. The influence of compaction grout properties and rate of injection have also been investigated as they relate to the potential for hydrofracture. The analyses performed have resulted in the development of a "hydrofracture index," defined as the ratio of the rate of injection to the permeability of the adjacent ground, V ,/k. For the assumption of no bleeding from the grout, analyses suggest that hydro fracture will occur when V /k > 50 m2 ( 60 yd 2) The verification of this concept will be addressed in injection tests in 1997.

4. Finally, the undrained and drained shear strength, compressibility and permeability of a representative compaction grout have been determined The addition of clay to the grout mix was shown to have a much more significant effect on permeability than on the drained shear strength (i.e., the frictional characteristics). Furthermore, the mineralogy of the added clay was shown to be of great significance, with the effect of one and two percent bentonite being approximately the same as the addition of five and ten percent kaolinite, respectively. The modeling of pore pressure dissipation within the grout bulb and the corresponding increase in shear strength suggest that the addition of clay to a grout mix need not be considered undesirable as long as the resulting permeability of the grout does not become low relative to the desired rate of injection It has been shown that grouts containing relatively clean sand and a few percent bentonite do not have permeabilities significantly lower than conventional grouts containing significant silt contents. Accordingly, these grouts would not be expected to have significantly different behavior during injection These findings will also be investigated with full-scale injections during 1997 (Borden and Ivanetich 1997)

University of MarylandResearch into compaction grouting began in 1996 at the University of Maryland, funded by the U.S National Science Foundation and the U.S Army Waterways Experiment Station The principal thrust of the work is in small scale physical modeling of the whole compaction grouting system.

Preliminary small Ig models have been conducted principally to develop technique In dissecting those models, however, grout bulbs were found to be distinctly non-spherical, and the effect on the in situ soil was a

combination of compaction, shear distortion, and displacement. The major shortcoming in those models is the absence of confinement of the soil undergoing compaction, confinement that arises from the soil overburden This will be remedied by repeating grouting in small models but while they are rotating on a geotechnical centrifuge.

The role of the geotechnical centrifuge is to increase the self-weight of all components in the model. By doing this, self-weight stresses at all depths in a soil model are made to equal the self-weight stresses at all geometrically corresponding points in a full scale soil profile A series of centrifuge models is planned to conduct a study of the influences of grout mix, soil type, injection pressure, depth of grouting, and the presence or absence of surface surcharge, on the effects of grouting on the in situ soil. The models allow for full control over pre-grouting soil conditions, and for systematic post-grouting dissection to a degree generally not possible in the field. This will provide invaluable information to numerical models, to other laboratory researchers who work to simulate small portions of the grouting process, and to practitioners who will recognize their field observations in the model results.

CASE HISTORIESAmong the advances in the state of the practice of compaction grouting are the ability to grout to greater depths and the use of the compaction grouting technique to densify soils for liquefaction mitigatioQ A project summary for each of these applications is presented here. Additional papers on compaction grouting are presented in the ASCE Grouting Committee's the Conference Proceedings for Geologan '97.Mission Valley, San Diego, CA

In the Mission Valley area of

San Diego, three new light rail transit bridges through the San Diego river flood plain are supported on

individual piers at typical intervals of37 m (120 ft). The piers bear on large diameter caissons, up to 30 m (100 ft) deep, that are founded in dense sands and gravels underlying potentially liquefiable soils (Fig. 1.3-3).

Fig. 1.3-3. Profile of compaction grouting for Mission Valley light rail bridges

Although the caissons are founded below the zone of potential liquefaction, they nevertheless rely on lateral support from the surrounding ground for stability. Compaction grouting was specified by the geotechnical engineer to provide long-term protection for the caissons and the bridge superstructure in the event of seismic-induced liquefaction. Prior to bridge construction, the compaction grouting program was performed to depth of between 24 m and 35 m (80 and 115 ft) around the six abutments and 68 caissons to densify the soil beyond the threshold where liquefaction would occur.

Petronas Twin Towers, Kuala Lumpur, MalaysiaOfficially the world's tallest building, the Petronas Twin Towers structure is founded on concrete-barrette friction piles extending to a maximum depth of 125 m ( 410 ft) below grade ( Fig. 1.3-4). The piles terminate in the Kennyhill formation, composed of residual soil and weathered limestone over limestone bedrock containing numerous solution cavities as well as soft slump (raveled) zones at the soil/limestone interface. Concern for potential structural settlements due to these subsurface conditions beneath the Twin Towers' footprints led to a compaction grouting program at depths of up to 160 m (525 ft) to fill the voids and improve the soft zones. Measurements taken during superstructure construction showed both total and differential settlements less than the maximum predicted, indicating successful deep ground improvement.

Fig. 1.3-4. Generalized subsurface profile for Petronas Twin Towers.

CONCLUSIONSCompaction grouting is becoming more widely accepted as a site improvement densification technique for bearing capacity and the reduction of settlements, and for liquefaction control. Grouting to greater depths than previously achievable has been realized. The technique of the compaction grout pile is emerging as a viable foundation option. Current university research, supported by national and federal agencies and specialty contractors, is ongoing in an effort to establish criteria for the design and predictability of future compaction grouting projects

Possible future innovations will incorporate the use of drains intermediate to points of injection to enhance drainage during injection and prior to a secondary injection sequence. Development of a nux design that would revert to a permeable sandy matrix would allow drainage to take place at the points of primary injections, such that secondary injections would benefit from improved drainage. A further advancement in compacting grouting would be the ability to induce vibrations into the pumped grout and/or adjacent soil to enhance densification of certain soil gradations.

ACKNOWLEDGMENTSResearch at North Carolina State University is being conducted under the direction ofProfessor Roy Borden, and supported by Hayward Baker Inc. Professor Deborah Goodings is directing the research at the University of Maryland. Professor Gooding's research program, as well as the program at Kersey, Colorado, is supported by Denver Grouting Services.BIBLIOGRAPHYIn addition to those publications referenced in the preceding section, the followingpublications on compaction grouting are recommended.Baker, W.H. (1985) "Embankment Foundation Densification by Compaction Grouting." Issues in Dam Grouting,

Baker, W.H., ed., pp. 104-122.Byle, M. I. (1992). "Limited Compaction Grouting for Retaining Wall Repairs." Grouting, Soil Improvement

and Geosynthetics, ASCE Geoteclll1ical Special Publication No.30, pp. 288-300Berry, R. M and Buhrow, R. P (1992). "Settlement, Structural Failure and In-place Repair of Above Ground

Storage Tanks." Grouting, Soil Improvement and Geosynthetics, ASCE Geotechnical Special Publication No.30, pp.240-251.

Brill, G. T. and Darnell, K. E. (1992) "Retention System Using Compaction Grouting in Clay Soils" Grouting, Soil Improvement and Geosynthetics, ASCE Geotechnical Special Publication No 30, pp 791-802

Dai, X. and Borden, R.H (1996). The Change of In S'itu Soil Conditions Induced by Compaction Grouting, Research Report, North Carolina State University, 125 pp.

Daugherty, C. W., Stirbys, A. F. and Gould, I. P (1995) "Compaction Grouting Effectiveness, A146, Los Angeles Metro Rail." Verification of Geotechnical Grouting, ASCE, Geotechnical Special Publication No.57, pp. 153-163.

Francescon, M. and Twine, D. (1992). "Treatment of Solution Features in Upper Chalk by Compaction Grouting." Grouting in the Ground, Bell A. L., Ed., Thomas Telford, London, pp 327-348.

Graf, E. D. (1992). "Compaction Grout, 1992." Grouting, Soil Improvement and Geosynthetics, ASCE Geotechnical Special Publication No.30, pp. 275-287.

Ivanetich, K. B. and Borden, R.H. (1996). Shear Strength and Consolidation Properties of Compaction Grouting, Research Report, North Carolina State University, 213 pp.

Lamb, R. T. and Hourihan, D T (1995). "Compaction Grouting in a Canyon Fill." Verification of Geotechnical Grouting, ASCE Geotechnical Special Publication No.57, pp. 127-141.

Salley, J.R., Foreman, B., Henry, J. and Baker, W. H. (1987). "Compaction Grouting Test Program in Pinopolis West Dam." Soil Improvement -A Ten Year Update, ASCE Geotechnical Special Publication No.12, pp. 92-97.

Warner, J. (1992). "Compaction Grout: Rheology vs. Effectiveness," Grouting, Soil Improvement and Geosynthetics, ASCE Geotechnical Special Publication No.30, pp.229-239.

Warner, J., et al. (1992). "Recent Advances in Compaction GroutingTechnology." Grouting, Soil Improvement and Geosynthetics, ASCE Geotechnical Special Publication No.30, pp. 252-264.

Warner, J. (1982), "Compaction Grouting -The First Thirty Years." Grouting in Geotechnical Engineering, ASCE, pp. 694-707.

REFERENCESAnonymous. (1996). "Dynamic Duo." Civil Engineering, ASCE, Vol. 66, No.7, July, pp.40-43.Baker, WH, Cording, E.J., and MacPherson, H.W. (1983). "Compaction Grouting To Control Ground Movement

During Tunneling." Underground Space, Permagon Press Ltd., Vol. 7,205-212.Borden, R. H. and Ivanetich, K. B. (1997). "Influence of Fines Content on the Behavior of Compaction Grout."

Grouting Proceedings, Geo-Logan 97, 14 pp.Boulanger, R.W. and Hayden, R.F., (1995). "Aspects of Compaction Grouting of Liquefiable Soils." Journal

Of Geotechnical Engineering, Vol. 121, No.12, 844-855.Byle, M.J., Blakita, P.M. and Winter, E. (1991). "Seismic Testing Methods for Evaluation of Deep Foundation

Improvement by Compaction Grouting." Deep Foundation Improvements: Design, Construction and Testing, Esrig, M.I. and Bachus, R.C.,eds. ASTM STP 1089, American Society for Testing and Materials, Philadelphia, PA, pp. 234-247.

Chastanet, J. D. and Blakita, P. M. (1992). "Wanaque Filtration Plant Subgrade Stabilization. A Case History" Grouting, Soil Improvement and Geosynthetics, ASCE Geotechnical Special Publication No.30, pp. 265-274.

Graf, E. D. (1992). "Earthquake Support Grouting in Sands." Grouting, Soil Improvement and Geosynthetics, ASCE Geotechnical Special Publication No 30, pp. 879-888.

Rubright, R. and Welsh, J. (1993). "Compaction Grouting." Ground Improvement, Moseley, M. P.,Ed., Blackie Academic & Professional, pp. 131-148.

Schmertmann, J. H. and Henry, J. F. (1992). "A Design Theory for Compaction Grouting." Grouting, Soil Improvement and Geosynthetics, ASCE Geotechnical Special Publication No.30, pp. 215-228.

Warner, J. and Brown, D. R. (1974). "Planning and Perfonning Compaction Grouting." Proceedings, Journal of Soil Mechanics amd Foundation Division, ASCE, Val.100, No. GT6, pp. 653-666.

Welsh, J. P. (1992). "Grouting Techniques for Excavation Support." Excavation Supportfor the Urban Infrastructure, ASCE, New York, N.Y., pp. 240-261.