Lateral Resistance of Deep Foundations in Liquefied Sand

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The lateral load capacity of deep foundations is critically important in the design of bridges, buildings, and other structures in seismically active regions. Although fairly reliable methods have been developed for predicting the lateral resistance of piles in non-liquefied soils, there is very little information to guide engineers in the design of piles that are surrounded by liquefiable soils. An accurate assessment of the resistance-displacement relationship for piles in liquefied soil is needed to determine whether additional piles are required for a foundation in liquefied sand, or whether soil improvement must be undertaken to prevent liquefaction altogether. These issues become even more important as the engineering profession attempts to move to performance-based design codes where estimates of displacements are required.A series of full-scale lateral pile load tests were performed at Treasure Island in San Francisco Bay after a surface layer was liquefied using controlled blasting techniques. These tests demonstrated that controlled blasting can induce liquefaction in a well-defined volume of soil for full-scale experimentation. Because of the success of that test program, another series of full-scale liquefaction load tests were undertaken in connection with the design of a new bridge in Charleston, South Carolina. Funding for the testing program was provided by the South Carolina Dept. of Transportation; this action is to support a detailed analysis required to develop lateral load-deflection (p-y) curves for the liquefied sand and to generalize the load test results.These analyses take full advantage of over $250,000 invested in the construction of the foundations, instrumenting the structure, and conducting the testing. The experimental results provide time histories of bending moment at regular intervals along the length of the shafts; the excess pore pressures at corresponding depths; and load, deflection and rotation measured at the pile head. Using these results, p-y curves are being computed as a function of excess water pressure. Several techniques for developing these curves are being used and compared. The influence of fines content and sand density on the p-y curves is also being evaluated. For the dynamic testing, accelerometers were placed at regular intervals along the length of the shaft. The results from the dynamic and static load tests are being used to evaluate the effect of loading rate on the measured resistance, and to evaluate the influence of damping on resistance.As a supplement to the load-testing program, shear wave velocity measurements were made during the blasting to define the change in velocity as a function of the water pressure change. In addition, CPT (cone penetration test) soundings were made at several intervals after blasting to determine the rate of change in penetration resistance with time. Information of this type often becomes very important in assessing the quality of soil improvement techniques where it is not possible to wait for long-term test results.