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96
CHAPTER 8
Conclusions
Based on the results of the literature review, full-scale field analyses suggest that soilcrete block depths greater than
testing, numerical analyses, and--finally--the simplified analy- about 10 ft will provide limited increased benefit for a lat-
ses, the following conclusions have been developed: eral deflection limit of 1.5 in. at the pile cap.
5. A cemented block also can be efficiently created by excavat-
1. Significant increases in the lateral resistance of bridge ing soft clay and replacing it with flowable fill. The flowable
foundations can be achieved by soil improvement tech- fill can be placed below a pile cap prior to pile driving or
niques. The greatest benefits will typically be achieved around the periphery of the pile group after driving. In
when improving soft clays; however, significant improve- comparison with in-situ treatments, it is necessary to main-
ment is also possible with loose sands. tain a stable excavation after excavation, which may be dif-
2. Excavating soft clay and replacing it with compacted gran- ficult in soft clay. In this study, problems were encountered
ular fill increases the lateral pile-soil resistance as well as the in obtaining a consistent compressive strength of the flow-
lateral passive resistance on the pile cap. Typical increases able fill. In addition, tests performed 2 years after treatment
in lateral resistance are 10% to 50%, with the highest showed strength degradation in test specimens.
increases occurring when the contrast in strength is the 6. Numerical analyses suggest that the lateral resistance of the
greatest. The compacted granular fill should extend 5 pile soilcrete block is relatively insensitive to the strength of the
diameters below the ground surface and 10 pile diameters soilcrete. Therefore, soil improvement techniques that can
beyond the face of the piles to obtain the full lateral resis- produce a compressive strength greater than 100 psi may be
tance of the granular soil. sufficient for practical purposes. Shear calculations can be
3. Ground improvement techniques such as soil mixing and used to check the minimum strength requirement.
jet grouting can create a cemented volume of "soilcrete" 7. Full-scale field tests and FEM analyses indicate that place-
in-situ with compressive strengths of 100 to 600 psi. This ment of a narrow dense compacted granular zone adjacent
soilcrete block is most effective when it encompasses the to a pile cap or abutment in loose sand can significantly
entire pile group below the cap, although significant im- increase the lateral passive resistance provided by the cap.
provement also can be obtained with soilcrete walls around Typically, when the width of the dense zone is equal to the
the periphery of the pile group. During full-scale lateral cap height, the passive resistance is increased to about 60%
load testing, jet grouting below a pile cap increased lateral of that which would be obtained for a homogenous dense
resistance by 500 kips or (160%) relative to the 280 kip lat- backfill extending about four times the height of the cap.
eral resistance in untreated clay. Soilcrete walls produced A generalized equation can be used to compute the per-
by jet grouting and soil mixing adjacent to a pile group centage of the passive force as a function of backfill width,
produced increases of 400 kips (185%) and 170 kips (60%), dense sand friction angle, and loose sand friction angle.
respectively, relative to untreated conditions. 8. Simple cost comparisons indicate that ground improvement
4. Under lateral loading, the soilcrete zone tends to move as a techniques have the potential to produce increased lateral
block and develop increased lateral resistance from passive pile group resistance at significantly less cost than would
force on the back of the block and adhesion on the sides be obtained by simply driving more piles and extending the
of the block, rather than increased pile-soil resistance. This pile cap. Although costs are expected to vary with locality,
lateral resistance can be computed using the principles of these results make it clear that engineers should investigate
basic soil mechanics for passive force and side shear under this alternative as part of their overall effort to produce a
undrained conditions with some adjustments. Numerical cost-effective foundation solution.
