Lateral Resistance of Deep Foundations in Liquefied Sand

深基础的横向承载能力在地震活跃地区的桥梁、建筑物和其他结构的设计中是至关重要的。虽然已经开发了相当可靠的方法来预测非液化土壤中桩的侧阻力，但很少有信息可以指导工程师设计被液化土壤包围的桩。需要对液化土中桩的阻力-位移关系进行准确的评估，以确定液化砂中的基础是否需要额外的桩，或者是否必须进行土壤改良以完全防止液化。随着工程专业试图转向基于性能的设计规范，这些问题变得更加重要，因为这些设计规范需要对位移进行估计。在旧金山湾金银岛采用控制爆破技术液化表层后，进行了一系列全尺寸横向桩荷载试验。这些试验表明，控制爆破可以在一定体积的土壤中引起液化。由于该试验方案的成功，在南卡罗来纳州查尔斯顿的一座新桥的设计中进行了另一系列全尺寸液化负荷试验。测试项目的资金由南卡罗来纳州交通部提供；该操作是为了支持对液化砂进行横向载荷-挠度（p-y）曲线所需的详细分析，并推广载荷测试结果。这些分析充分利用了在基础建设、结构检测和进行测试方面投资的25万美元。实验结果提供了沿轴长有规则间隔的弯矩时程；对应深度处的超孔隙压力；并在桩顶处测量荷载、挠度和旋转。利用这些结果，p-y曲线被计算为超水压的函数。正在使用和比较几种绘制这些曲线的技术。细粒含量和砂密度对p-y曲线的影响也进行了评价。在动态测试中，加速度计沿着轴的长度按规则间隔放置。动态和静态负载试验的结果用于评估加载速率对测量电阻的影响，以及评估阻尼对电阻的影响。作为荷载试验程序的补充，在爆破过程中进行了剪切波速测量，以确定速度变化作为水压变化的函数。此外，在爆破后的几个间隔时间内进行了CPT（锥形侵彻试验）测深，以确定侵彻阻力随时间的变化率。在不可能等待长期试验结果的情况下，这类信息在评估土壤改良技术的质量时往往变得非常重要。

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.