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Suggested Citation:"Chapter 8 - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2011. Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils. Washington, DC: The National Academies Press. doi: 10.17226/14574.
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96 Based on the results of the literature review, full-scale field testing, numerical analyses, and—finally—the simplified analy- ses, the following conclusions have been developed: 1. Significant increases in the lateral resistance of bridge foundations can be achieved by soil improvement tech- niques. The greatest benefits will typically be achieved when improving soft clays; however, significant improve- ment is also possible with loose sands. 2. Excavating soft clay and replacing it with compacted gran- ular fill increases the lateral pile-soil resistance as well as the lateral passive resistance on the pile cap. Typical increases in lateral resistance are 10% to 50%, with the highest increases occurring when the contrast in strength is the greatest. The compacted granular fill should extend 5 pile diameters below the ground surface and 10 pile diameters beyond the face of the piles to obtain the full lateral resis- tance of the granular soil. 3. Ground improvement techniques such as soil mixing and jet grouting can create a cemented volume of “soilcrete” in-situ with compressive strengths of 100 to 600 psi. This soilcrete block is most effective when it encompasses the entire pile group below the cap, although significant im- provement also can be obtained with soilcrete walls around the periphery of the pile group. During full-scale lateral load testing, jet grouting below a pile cap increased lateral resistance by 500 kips or (160%) relative to the 280 kip lat- eral resistance in untreated clay. Soilcrete walls produced by jet grouting and soil mixing adjacent to a pile group produced increases of 400 kips (185%) and 170 kips (60%), respectively, relative to untreated conditions. 4. Under lateral loading, the soilcrete zone tends to move as a block and develop increased lateral resistance from passive force on the back of the block and adhesion on the sides of the block, rather than increased pile-soil resistance. This lateral resistance can be computed using the principles of basic soil mechanics for passive force and side shear under undrained conditions with some adjustments. Numerical analyses suggest that soilcrete block depths greater than about 10 ft will provide limited increased benefit for a lat- eral deflection limit of 1.5 in. at the pile cap. 5. A cemented block also can be efficiently created by excavat- ing soft clay and replacing it with flowable fill. The flowable fill can be placed below a pile cap prior to pile driving or around the periphery of the pile group after driving. In comparison with in-situ treatments, it is necessary to main- tain a stable excavation after excavation, which may be dif- ficult in soft clay. In this study, problems were encountered in obtaining a consistent compressive strength of the flow- able fill. In addition, tests performed 2 years after treatment showed strength degradation in test specimens. 6. Numerical analyses suggest that the lateral resistance of the soilcrete block is relatively insensitive to the strength of the soilcrete. Therefore, soil improvement techniques that can produce a compressive strength greater than 100 psi may be sufficient for practical purposes. Shear calculations can be used to check the minimum strength requirement. 7. Full-scale field tests and FEM analyses indicate that place- ment of a narrow dense compacted granular zone adjacent to a pile cap or abutment in loose sand can significantly increase the lateral passive resistance provided by the cap. Typically, when the width of the dense zone is equal to the cap height, the passive resistance is increased to about 60% of that which would be obtained for a homogenous dense backfill extending about four times the height of the cap. A generalized equation can be used to compute the per- centage of the passive force as a function of backfill width, dense sand friction angle, and loose sand friction angle. 8. Simple cost comparisons indicate that ground improvement techniques have the potential to produce increased lateral pile group resistance at significantly less cost than would be obtained by simply driving more piles and extending the pile cap. Although costs are expected to vary with locality, these results make it clear that engineers should investigate this alternative as part of their overall effort to produce a cost-effective foundation solution. C H A P T E R 8 Conclusions

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 697: Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils examines guidance for strengthening of soils to resist lateral forces on bridge pile foundations.

The report presents computational methods for assessing soil-strengthening options using finite-element analysis of single piles and pile groups and a simplified approach employing commercially available software.

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