C CI Comp Action Grouting by Geraci

8/6/2019 C CI Comp Action Grouting by Geraci

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Compaction grouting is a well-established method of in-situsoil improvement, and can also be used to lift and level structuresfounded on poor foundation soils. It is primarily used todensify weak foundation soils, although is often used for manyother treatment objectives, including lifting and relevelingof structures, filling sinkholes and solution cavities, increasingcapacity of piles, mitigation of soil piping, and mitigationof soil liquefaction potential. Inherent in the compactiongrouting process is the ability to work in and around existingstructures and infrastructure elements, allowing for subsurfacesoil improvement without the need for structure demolition.

MechanicsCompaction grouting is a mechanical process that involves

injection of stiff soil/cement grout below grade under high

pressure in order to displace nearby surrounding soil. A bulb-shaped grout inclusion (Figure 1) is typically formed at eachpoint of injection. Injection casing is usually set to the base ofthe interval to be treated, and then withdrawn a short distance(typically 1 to 5 feet) to open the first pumping interval orstage. Once one or more terminal pumping criteria are met,pumping at that stage will cease, the casing is withdrawn to thenext stage, and the process repeated. This sequence continuesuntil the target soil profile has been treated, or lack of overburd-en pressure at shallow treatment depths (3 to 5 feet below theground surface) results in rapid surface uplift. Figure 2shows atypical compaction grouting treatment profile.

The grouting process makes use of available confining pres-sure in the target soil mass, which accommodates the injectedgrout by consolidation until the force exerted by continuedbulb expansion exceeds the local confinement. At that time, theground surface begins to bulge above the grout bulb, and anyfurther grout injection is translated into uplift of the overlyingsoil and structures.

Compaction grout applies a very large load to a small area fora short time, but in doing so, rapidly accelerates the consolida-tion rate of the soil near the injection. Improvement in a large

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Compaction Grouting forIn-Situ Ground ImprovementBy Jeffrey Geraci, P.G., C.E.G.

Figure 1:Grout bulbexcavated duringa 1991 study ofcompaction groutrheology. The studyis documented in a

paper by Warner, et.al. in ASCEs GSPNo. 120.

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Figure 2: Generalized compaction grouting injection profile

Figure 3: Compaction grouting of an historic military barracks buiin Hawaii at the edge of a dormant volcanic crater. The building wlifted off of the ground to provide construction access for various trabut not a prerequisite for compaction grouting.

area is achieved by making a series of injections, commoa uniformly spaced array. Effective injection spacing canthe order of 8 to 10 feet, although may be somewhat tighthe event of shallow treatment. Temporarily, the groutinssure can exceed the soil overburden weight because thetion force confining the grout is the result of both the weigthe shear resistance of the soil displaced above the injectio

A variety of ground conditions may be treated tcompaction grouting, granular soils, silts and some claycritical that the grouting contractor pump at a relatively sloto allow dissipation of pore water in the surrounding soto limit the grounds tendency to fracture. Pumping ratecubic feet per minute or less are generally acceptable. Grotreatment of saturated highly plastic or organic soils (pearesult in local soil displacement, however, little or no actu


2/4STRUCTURE magazine December 2005

Figure 4: Groutinjection forliquefactionmitigationbeneath asmokestack at SanDiego, Californiashistoric Hotel delCoronado. Suchlimited-access

project constraintsare common inthe compaction

grouting industry.

Figure 5:Comparison of

pre- and post-treatment N1(60) values froma residentialliquefactionmitigation projectin Laguna Beach,California. Soilimprovementin the targettreatmentinterval wasconsiderable.

improvement will likely be achieved. Site ground conditions should bewell understood prior to beginning any production grouting program.Often times it is desirable to conduct a pilot grouting program as ameans of gathering additional site performance data.

Grout MaterialsIn 1980, the Grouting Committee of the American Society of Civil

Engineers (ASCE) defined the term compaction grouting as:Grout injected with less than one inch slump. Normally a soil cement

with sufficient silt sizes to provide plasticity together with sufficientsand sizes to develop internal friction. The grout generally does notenter soil pores, but remains in a homogenous mass that gives controlleddisplacement to compact loose soils, gives controlled displacement forlifting of structures, or both.

For the purpose of this article, the term will include a broader-based class of limited-mobility displacement grouting, as this typeof grout can be effectively emplaced at slightly higher slump values.

A compaction grout mix is designed to be a blend that will flow inthe injection pump and hoses, but resist the tendency to flowupon exiting the grout casing at the point of injection through loss ofbleed water. It is typically a mixture of silty sand, eight to ten percentportland cement or fly ash, and sufficient water to develop the desiredconsistency. Compressive strength of the grout material is typically onthe order of 150 to 300 psi, although adjustments in cement content,aggregate quality and use of additives can yield slightly strongervalues. However, for most compaction grouting applications, groutcompressive strength would only need to be equal or greater than thesurrounding treated soil.

Classic ApplicationsCompaction grouting was recently used to address differential

ment potential beneath an important historical military site atHarbor, Hawaii. Reported byBuilding Industry Hawaiito be thcompaction grouting project done on the islands, this project invgrout treatment of differential fill beneath a barracks buioriginally built in 1943 as part of the United States Navy PacifiRadio Unit. The barracks building had been constructed interior margin of Makalapa Crater, a dormant volcanic featur

had been partially filled with earth material.Approximately 4,500 cubic feet of low-mobility sand/cement

was used to strengthen the problematic soils. The project also invreplacement of the foundation system, and extensive interior aterior renovations. Grout treatment was strictly used to strengthtreated soil profile, as all grouting and foundation work was perf

while the building was suspended in place (Figure 3).Displacement grouting has been used to fill subsurface so

cavities or dolines at the University of California, Santa Crupre-construction site improvement measure. The process was rly performed at the campus to stabilize foundation subgrades foconstruction, including the universitys new science and enginbuildings, and a student housing structure.

Dolines are sinkholes that develop in areas underlain by limor marble bedrock, resulting from local chemical erosion. Surfater will pick up a small quantity of carbonic acid from the natucay of organic matter. The water eventually percolates under-grallowing small quantities of this weak acid to slowly dissolve poof the rock, usually along existing fractures or joints. Calcium caate is carried away in solution, leaving behind the insoluble poof the rock. The result is a doline, typically filled with weak clasilt unsuitable for modern structural support.

Mitigation measures at the Santa Cruz, California campus indrilling and grouting on a widely-spaced injection array. The genical engineer was constantly on-hand to adjust the grout injectiray to conform to empirical grouting performance data as the

progressed. Grout design considerations included a minimum stcriterion of 500 psi, and a zero- to one-inch slump. Fine aggfor the grout mix design conformed to a specific gradation tolin order to limit the grouts mobility when pumped. This gradabasically silty sand with between 10 and 30 percent passing thesieve, and containing little or no clay.

Figure 6:Grout injectionbeing performedat one of the thranchor stanchion

for a televisionbroadcast tower.

continued on nex



Figure 7: Stiffening of the supporting soils for the lower tier of this retainwall was accomplished by carefully-controlled and closely-spaced compac

grout injection. Grouting in this case was performed in preparation forseries of tieback anchors that were installed once grouting was complete.

Figure 8: Grout consumption at each injection stage was contoured toproduce this diagram. Soil piping along the basal bedrock surface hastenlocal soil loss in overlying intervals, and is reflected in the sporadic relativtakes in the hydraulic fill.

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The grouting process involved pumping through a small diametersteelcasing, initially set to the base of the solution cavity, and withdrawnin short intervals or stages as grout injection progressed. Groutingtermination criteria were based on achievement of a maximum pressure,or a measured degree of ground uplift. A total of 31,500 cubic feet ofgrout was pumped at a total of 29 locations to stabilize the subgrade forthe student housing building, and maximum treatment depth rangedfrom approximately 60 to 90 feet below the surface.

Compaction grouting is also routinely used to raise and relevel

structures and foundation elements that have settled due to compress-ion of supporting soils. This type of releveling requires a great deal ofexperience on the part of the practitioner, and careful monitoring ofchanges in relative floor elevation throughout the entire project. Thedegree of lift achieved depends on several factors, including treatmentdepth, grout consumption, the structures response, and the clientsobjectives. Lifting structures with displacement grouting methods canhave an advantage over conventional underpinning techniques, sincegrouting can actually address cause of settlement and provide supportfor slabs and other appurtenances.

Liquefaction MitigationMitigation of liquefaction potential has become an important goal

for many ground improvement projects. Compaction grouting tech-niques have been used in this capacity at many locations, includingthe Monterey Bay Aquarium Research Institute in Moss Landing, Cali-fornia; a portion of the historic Hotel Del Coronado in San Diego,California (Figure 4), and as part of a retro-fit of a former Montgom-ery Wards building for a new Target retail center, also in San Diego.This was one of the largest projects of its kind, involving injection of165,000 cubic feet of low-mobility grout at 2,762 injection points.

Compaction grouting treatment for liquefaction mitigation was recent-ly conducted for a proposed residential site in Laguna Beach, Californiato densify sandy alluvial soils. Project planning called for a new hometo be built on a lot where the engineer had identified a potentialliquefaction hazard. Site stratigraphy consisted of approximately 20 feet

of thinly-bedded soft to medium-stiff silts and clays, overlying loose tomedium-dense sandy alluvium. A sharp near-horizontal contact withlocal conglomeratic bedrock was encountered at approximately 60 feetbelow the ground surface (BGS). Ground improvement was primarilydirected at treating saturated granular alluvium.

Plan-view grout injection point spacing of 8-feet on-center was se-lected for the Laguna Beach project, based on results of a preliminarytest program. The graph in Figure 5shows considerable improvementin SPT N1 (60) values, as determined through cone penetrometer(CPT) testing. Results were compiled from two pre-treatment sound-ings, and seven post-treatment tests. A total of about 22,000 cubic feetof grout was emplaced at this site, with an equivalent displacement ratioof 5.6 percent.

Soil StiffeningCompaction grouting was used to strengthen the soil reaction

zone of a failing anchor structure for a television broadcast towernear Portland, Oregon. An unusual wind storm with sustained gustsof up to 50 mph had induced random motions in the supportingcables for the 1,100-foot high, 680 ton broadcast tower. Observedamplitudes in the 2-inch diameter support cables were on the orderof 20 feet. Subsequent geotechnical studies revealed that two of thetowers anchors were failing with respect to uplift.

Compaction grouting and a series of micropiles were applied as aremedial construction measure to stabilize the anchors prior to re-ten-sioning of the support cables. Each anchor had to be able to comfort-

ably resist approximately 300 tons of uplift. Subsequent monof the structures instrumentation has demonstrated that towanchor motions are within expected parameters. The accompaphoto (Figure 6) shows grout being injected near one of the t

anchor stanchions to stiffen soils in the foundation reaction zone Another unusual structure to benefit from compaction gr

included a failing earth-retaining wall in Orange County, Cal(Figure 7). Compaction grouting was performed in the supportinbeneath and behind the lower-tier wall to address weakened band to stiffen the inbound reaction zone for a series of tieback anthat were ultimately installed. Careful monitoring of injectedquantities, surface uplift and wall deflection were critical tprojects success.

Compensation for Soil PipingDisplacement grouting was used to repair a breach beneath

wall and revetment during construction of the Ocean Institute

Ocean Education Center at Dana Point Harbor, Californiaharbor had locally breached a 16-foot deep construction excaduring a high-tide event, and grouting was used to restore reve



ResourcesAlthough this article is intended as a broad-brush overview of the subject, there are

many grouting-related resources available for design professionals, both in print and onthe internet. One of the most recent publications to compile a comprehensive bodyof knowledge on the subject is a text by James Warner, titled Practical Handbook of Grouting: Soil, Rock and Structures, published by John Wiley & Sons, Inc., 2004. Manyother publications and proceedings are also available from the ASCE Bookstore and otherpublishers. The Geo-Institute Grouting Committees website is another good resourcefor grouting information, and can be found at: www.grouters.org. The committee

will soon be releasing an updated Glossary of Grouting Terms, and a set of CompactionGrouting Guidelines.

support once the breach was sealed. Local pipingof fill soils along a basalbedrock surface contrib-uted to a weakened con-dition in the area of thebreach, and grouting wasperformed along the re-mainder of the seawall to

identify and heal otherpotential zones of soilloss. A grout mix with aslump of approximately6 inches was used for thisapplication, allowing thegrout to travel slightlyfarther from the point ofinjection.

Figure 8depicts anomalous zones of relatively high grout consumption, in profile view.It is believed that those zones represent areas of chimney effect soil loss resulting frompiping. This early stage of sinkhole formation was effectively healed through the displace-ment grouting process. This is also an example of how grouting can be used as an effectivediagnostic tool.

Reporting and MonitoringCompaction grouting results are carefully recorded in the field by the foreman or

responsible technician. Records of casing installation and grout consumption per stageare maintained, as well as injection pressure and production rate data. The degreeto which the ground accepted grout treatment is typically expressed as a volumetricdisplacement ratio of grout injected to soil treated; either based on a hypothetical radiusof influence surrounding each injection point, or by considering the total area treated.

Although compaction grouting is typically considered to be a low-tech process, itsapplication has begun to benefit from many of the high-tech gadgets that are becomingavailable. The use of computers has given the industry a means of measuring grout flow,pressure and consumption in real time, allowing engineers and practitioners to identify

ground shears in the very early stages. The in-line flow meter and pressure transducer shownin Figure 9provides metering of very-stiff grout mixes with wireless telemetry.Simple manometer surveys are nearly always used to quantify pre- and post-grouting

changes in relative floor elevation, although laser scanners have great potential as a meansof documenting minute structural deformations with little added expense. There are alsomany types of automatic devices that can be applied to measure surface heave or movementof critical structures, including tiltmeters, electronic manometers, and autonomous totalstation arrays. However, conventional manometers and optical levels are consideredsufficient effort for most applications and project budgets.

Figure 9: Computertechnology is becomingmore common in the

grouting industry. Themanifold shown abovecontains a pressuretransducer and wirelesstransmitter that worksin conjunction with anin-line flow meter. This

configuration allowsreal-time monitoringof pressure and flow forvery stiff (zero-slump)

grout mixtures.

Jeffrey Geraci is a project manager with Moore&Taber, a grouting and ground improvementcontractor serving the western United States. He is a Professional Geologist and CertifiedEngineering Geologist in the State of California. Jeff is a member of the ASCE Geo-InstituteGrouting Committee, and ACI Committee 552 - Geotechnical Cement Grouting.

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C CI Comp Action Grouting by Geraci