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1 SUMMARY Design Guidelines for Increasing the Lateral Resistance of Highway-Bridge Pile Foundations by Improving Weak Soils The objective of this study was to determine the viability of ground improvement methods for increasing lateral resistance of bridge foundations and to develop simple design approaches for predicting the increased resistance. The lateral resistance of bridge foundations is often a critical component in the design of highway bridges. Lateral loads can be produced by earth- quakes, wind, wave action, ship impact, and traffic. The foundation designer must verify that the lateral capacity of a foundation exceeds the lateral demand transmitted by the column. When the lateral capacity of a new foundation is inadequate, the designer typically increases the number or diameter of piles. For existing bridge foundations, additional piles, drilled shafts, or micro-piles are added to increase the lateral resistance. Furthermore, an expanded pile cap or connecting beam often is required to structurally connect the new piles to the existing pile group. Although these structural approaches provide the required lateral resistance, they may also be relatively expensive and time consuming. An alternative approach is to use soil improvement techniques to increase the strength and stiffness of the soil surrounding the foundation and thereby increase the lateral resistance of the pile group. For new construction, the improvement could readily be performed on the entire block of soil below the pile cap footprint and extending laterally several pile diameters from the perimeter pile. For retrofit of the existing foundations, soil improvement would be easier to accomplish around the perimeter of the pile group, but some techniques such as jet grout- ing also could treat the zone below the footprint of the pile cap. Typically, soil improvement would be only needed to extend to relatively shallow depths in the range from 10 to 20 ft. To evaluate the ability of ground improvement to increase lateral pile group resistance, 16 full-scale lateral load tests were performed on pile groups in soft clay after using ground improvement methods that included soil mixing, jet grouting, replacement with compacted sand, replacement with flowable fill, and replacement with rammed aggregate piers. The tests clearly demonstrated that significant increases in the lateral resistance of bridge foundations can be achieved by soil improvement techniques with the potential for cost savings. The greatest benefits typically will be achieved when improving soft clays; however, significant improvement is also possible with loose sands. Excavating soft clay and replacing it with compacted granular 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 resist- ance of the granular soil. 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
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2 significant improvement 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 lateral resistance in untreated clay. Soil- crete 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. 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 basic soil mechanics principles for passive force and side shear under undrained conditions with some adjustments to account for limits on depth. Numerical analyses suggest that soil- crete block depths greater than about 10 ft will provide limited increased benefit for a lateral deflection limit of 1.5 in. at the pile cap. They also indicate that the lateral resistance of the soilcrete block is relatively insensitive to the strength of the soilcrete. Therefore, soil improve- ment techniques that can produce a compressive strength greater than 100 psi may be suffi- cient for practical purposes. Shear calculations can be used to check the minimum strength requirement. A cemented block also can be efficiently created by excavating 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 maintain a stable excavation after excavation, which may be difficult in soft clay. In this study, problems were also encountered in obtaining a consistent compressive strength of the flowable fill. In addition, tests performed 2 years after treatment showed strength degradation in test specimens. Full-scale field tests and finite-element methods (FEM) analyses indicate that placement 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 percentage of the passive force as a function of backfill width, dense sand friction angle, and loose sand friction angle. 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 alter- native as part of their overall effort to produce a cost-effective foundation solution.