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  18 Grouting for Strength 18.1 INTRODUCTION There are many occasions when it would be desirable to be able to addstrength to a soil formation. For clays this can be done by preconsolidating(overloading). For granular materials, other procedures are available,including densification and grouting. If the fluids (air or water) in the soilvoids are replaced by a solid, it becomes more difficult for the individual soilgrains to undergo relative displacements, thus adding shear resistance (orstrength) to the soil mass. This additional strength may be useful inpreventing the movement of material from under loaded zones, in increasingthe bearing capacity and the slope stability of grouted formations, and inreducing settlements in zones adjacent to or above excavations. Figures10.2, 10.3, and 10.4 illustrate some of these applications of grouting. 18.2 STRENGTH OF GROUTED SOILS The shear strength of a granular soil is due primarily to the nesting (orinterlocking) of grains and the consequent resistance of the grains to rollingor sliding over each other. Conditions which cause the grains to interlockmore strongly increase the soil shear strength. Thus, dense sands are Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  stronger than loose sands, and loaded sand masses are stronger thanunloaded sand masses. Since granular materials weigh 100lb/ft 3 or more(dry), soil deposits are loaded by their own weight, and a sand stratumextending 10ft below ground surface is considerably stronger at its bottom(where the grains are confined by vertical and lateral pressures) than it is atthe top, where the grains are relatively free.The shear strength of a granular material is represented by the solidline in Fig. 18.1 (See the portion of Sec. 10.4, discussing strength. Thefriction angle shown (30 8 ) is typical. From the figure, assuming the soilweighs 100lb/ft 3 , at 5ft below grade (equivalent to a normal stress of 500lb/ft 2 ) the shear strength is about 290lb/ft 2 (about 2psi). Ten feet belowground surface, the shear strength is 4psi, etc.Granular soils have no unconfined strength and cannot be subjected tounconfined compression testing. To determine the shear strength of granular F IGURE  18.1  Representation of shear strength of a granular soil. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  materials, artificial confining pressures must be applied, as in triaxial anddirect shear testing. See Chap. 9 of Ref. [1]. Grouted soils, however, can betested in unconfined compression. The unconfined strength added by thegrout appears as an intercept on the vertical axis. In clays and clayey soils,this intercept is called cohesion, and grouting may indeed be considered toadd cohesion to the soil. Grouting generally has little effect on the frictionangle, so the strength of a grouted soil can be represented by the dashed linein Fig. 18.1. In the figure, the grout strength is shown as 150lb/ft 2 or about1psi. As small as this value may be, it still more than doubles the ungroutedsoil strength at a 2-ft depth.When chemical grouting  does  affect the friction angle, the affect ismostly a small decrease. Thus, the upper line of Figure 18.1 would no longerbe parallel to the lower line, and would in fact intersect the lower lineeventually, as shown in Figure 10.7a. At that point, representing a specificdepth for a specific soil, the grouted and ungrouted strengths are equal. Atgreater depths, the grouted soil would be weaker than the ungrouted soil.It is hard to conceive of a grout that does not provide at least 10psi of cohesive strength. Thus, any grout will make a significant, if not major,increase in the ungrouted strength of granular deposits within a shortdistance of ground or exposed surface. (This statement does not apply torelatively shallow soils heavily loaded by foundations or otherwiseconfined.) It is only at great depths (for example, 500ft, where the verticalpressure due to the weight of the soil generates shear strengths of the orderof 200psi) that the contribution of most chemical grouts to soil shearstrength will become negligible or even negative. (These comments, of course, do not apply to grouting in rock formations.)Cement grouts are generally thought of as adding significant strengthto a grouted formation, while clays (when used as grouts) are generallythought of as adding no strength. All chemical grouts fall between these twoextremes. Neat cement grouts can have unconfined compressive strengths of 1000 to 1500psi. Only some of the resorcinol-formaldehydes approach orexceed these values. (This does not account for epoxies and polyesters,which have been used for rock grouting but due to their high viscosities andcosts are not applicable to soils.) Other chemical grouts, including thosethought of as ‘‘strong,’’ fall far below neat cement in strength. For strengthapplications, the grouts considered strong, and generally used, include thehigh-concentration silicates, the aminoplasts, and some of the phenoplasts.The grouts generally not used for strength applications include theacrylamides and acrylates, the lignosulfonates, the low-concentrationsilicates, and some of the phenoplasts.The technical literature contains a wealth of information related to thestrength of stabilized soils. Most of these data are the result of quick, Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  unconfined compression tests on samples of various ages (curing times),various curing conditions, and various length to diameter ratios. It isobviously difficult to compare data from different sources, or to usepublished data for design purposes, without knowing the test parameters. If the purpose of tests at any one agency is to compare various materials andformulations, the test parameters are not very important, as long as theyremain consistent. If the purpose is to obtain values for design, the testparameters  are  very important, and must be established to conform asclosely as possible to the anticipated field conditions.It is actually the creep strength of a grouted soil (either in unconfinedcompression or triaxial compression, depending on the specific application)that should be used for design purposes, not the unconfined compressivestrength. In the absence of specific creep data, the value of one-fourth toone-half of the unconfined strength may be used for applications. A suitablesafety factor must be applied to these suggested values.In selecting a grout to be used for any application, it is obviouslynecessary to select one with sufficiently low viscosity to be able to penetratethe formation. In strength applications, this criterion may eliminate all thegrouts with adequate strength. In terms of usable strength (including asafety factor of 2), a range of values for preliminary design use is given asfollows (these values must be verified by actual tests prior to final use indesign of a grouting operation):Lignosulfonates 5 to 10psiLow-concentration silicates 5 to 15psiAcrylamides, acrylates 5 to 20psiPhenoplasts 5 to 30psiAminoplasts 10 to 50psiHigh-concentration silicates 20 to 50psiIn the case histories that follow, it will be noted that values specified forgrout strength are much higher than the conservative values listed above.This is because the job specified values are confirmed by quick, unconfinedcompression tests (and are most probably ultimate values), while the valuesrecommended above account for creep and embody a safety factor. 18.3 GROUTING FOR STABILITY New construction in the vicinity of existing structures often causes concernabout the possible reduction in bearing capacity under existing footings orfoundations. Such was the case illustrated by Fig. 10.2, where a structurewas to be placed occupying all the space between two adjacent buildings. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.

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