Lateral Resistance of Deep Foundations in Liquefied Sand

液化砂中深基础的侧向阻力

• 批准号: 0085353
• 负责人: Kyle Rollins
• 金额: $ 9.74万
• 依托单位: Brigham Young University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-09-01 至 2004-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0085353&HistoricalAwards=false
• 关键词: Lateral; Resistance; Deep; Foundations; Liquefied

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.

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至关重要。 虽然已经开发出相当可靠的方法来预测桩在非液化土壤中的横向阻力，但很少有信息来指导工程师设计被液化土壤包围的桩。 需要准确评估液化土中桩的阻力-位移关系，以确定液化砂地基是否需要额外的桩，或者是否必须进行土壤改良以完全防止液化。 这些问题变得更加重要，因为工程专业试图转移到基于性能的设计规范，其中需要估计的位移。一系列全尺寸的横向桩载荷测试进行后，在旧金山弗朗西斯科湾金银岛表层液化控制爆破技术。 这些试验表明，控制爆破可以诱导液化在一个明确的土壤体积的全面规模的实验。 由于该测试计划的成功，在南卡罗来纳州查尔斯顿的一座新桥设计中进行了另一系列全尺寸液化载荷测试。 测试项目的资金由南卡罗来纳州教育部提供。运输;这一行动是为了支持详细的分析，以得出液化砂的横向荷载-挠度（p-y）曲线，并概括荷载试验结果。2这些分析充分利用了投资于基础建设、结构仪表和进行试验的250，000美元。 实验结果提供了弯矩的时间历程，在定期沿着轴的长度;在相应的深度的超孔隙压力;和负载，挠度和旋转测得的桩头。 利用这些结果，p-y曲线被计算为超压的函数。 正在使用和比较几种绘制这些曲线的技术。 还评估了细粒含量和砂密度对p-y曲线的影响。 对于动态测试，沿轴的长度沿着以规则间隔放置加速度计。 动、静载荷试验的结果被用来评估加载速率对测得的阻力的影响，以及评估阻尼对阻力的影响。作为载荷试验程序的补充，在爆破过程中进行剪切波速度测量，以确定速度的变化作为水压变化的函数。 此外，在爆破后的几个时间间隔进行CPT（圆锥贯入试验）测深，以确定贯入阻力随时间的变化率。 在无法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方面往往变得非常重要。