Full-Scale Static and Dynamic Lateral Pile Group Testing

许多重要的建筑和桥梁是由打入地面的钢管或桩组成的基础来支撑的。这些桩组提供的水平阻力是决定结构能否在地震中幸存下来而不造成重大破坏的关键。虽然已经发展了相当可靠的方法来预测缓慢施加荷载下单桩的水平阻力，但指导工程师设计密集分布的群桩，特别是在快速施加荷载下的信息很少。然而，这些有限的现场试验数据表明，在给定的每根桩荷载下，群桩将比单个孤立桩承受更大的位移和更高的应力。在设计中，通常使用折减系数来考虑这些群桩效应，以减小土体对群桩提供的阻力，但对于适当的折减系数，存在相当大的不确定性。建议的研究具有以下目标：(1)评估桩间距对测量的群桩折减系数的影响，并为这些系数开发作为桩间距的函数的设计曲线；(2)确定大(5排)群桩的折减系数概念的有效性，并确定折减系数是否在第三排之后保持不变；(3)确定桩周围的重复加载和间隙形成对测量的群桩折减系数的影响；(4)检查加载速率对桩周围土壤提供的阻力的影响，以及(5)提供记录良好的病历，用于评估和校准计算机和物理模型。这些目标将通过对全尺寸桩和群桩进行一系列水平载荷测试来实现。这些测试将补充已经在现场进行的群桩测试，桩间距为中心3英尺。首先，将对一根直径为12英寸的钢管桩进行水平载荷测试，以供比较。载荷将以10个增量施加，每个增量15个循环，以模拟大地震的重复载荷。接下来，将对中心距4英尺的五排桩群进行水平荷载试验。将采用相同的循环加载程序，并测量每根桩所承载的荷载。然后，加载框架将被移至相邻的群桩进行测试，间隔6英尺。除了缓慢施加的载荷外，还将使用火箭雪橇(统计装置)快速施加载荷，该装置将模拟地震载荷施加的速度。在数据整理之后，将使用现有的计算机模型分析试验结果，并确定适当的群桩折减系数作为桩间距的函数。此次和之前现场工作的负荷测试结果也将提供给日本的几名研究人员，以使用复杂的2-D和3-D计算机模型进行详细分析。这些分析应该有助于量化在快速施加载荷期间相对于缓慢施加载荷时所看到的增加的阻力。潜在的日本研究人员将包括金泽大学的松本教授、日本港口研究所的黄井秀树博士，以及其他表示有兴趣分析结果的研究人员。

Many important buildings and bridges are supported by foundations consisting of groups of steel pipes or piles driven into the ground. The horizontal resistance provided by these groups of piles is critical in determining whether or not the structures will survive an earthquake without significant damage. Although fairly reliable methods have been developed for predicting the horizontal resistance of single piles under slowly applied loads, there is very little information to guide engineers in the design of closely spaced pile groups, particularly under rapidly applied loads. Nevertheless, the data from these limited field tests indicate that piles in groups will undergo much more movement and higher stresses for a given load per pile than will a single isolated pile. These pile group effects are commonly accounted for in design by using reduction factors to reduce the resistance provided by the soil on the piles in the group, but there is considerable uncertainty regarding appropriate reduction factors. The proposed research study has the following objectives: (1) Evaluate the effect of pile spacing on measured group reduction factors and develop a design curve for these factors as a function of pile spacing, (2) Determine the validity of the reduction factor concept for a large (5-row) pile group and determine if the reduction factors remain constant beyond the third row, (3) Determine the effect of repeated loading and gap formation around the piles on the measured group reduction factors, (4) Examine the effect of loading rate on the resistance provide by the soil around the pile, and (5) Provide a well-documented case history for use in evaluating and calibrating computer and physical models. These objectives will be accomplished by conducting a series of horizontal load tests on a full-scale pile and pile groups. These tests will supplement pile group testing already conducted at the site with pile spacing of three feet on centers. First, horizontal load testing will be performed on a single 12-inch diameter steel pipe pile for comparison purposes. Load will be applied in 10 increments with 15 cycles per increment to simulate the repeated loading from a large earthquake. Next, horizontal load tests will be conducted on a five-row pile group spaced at four feet on centers. The same cyclic loading procedure will be applied and the load carried by each pile will be measured. The load frame will then be moved to an adjacent pile group for testing at six feet spacing. In addition to the slowly applied loads, load will be applied rapidly using a rocket sled (Statnamic device) which will simulate the speed at which earthquake loads are applied. Following data reduction, the test results will be analyzed using available computer models and appropriate pile group reduction factors will be determined as a function of pile spacing. The results of the load tests from this and previous work at the site will also be provided to several researchers in Japan for detailed analysis using sophisticated 2-D and 3-D computer models. These analyses should help quantify the increased resistance seen during rapidly applied loads relative to slowly applied loads. Potential Japanese researchers will include Prof. Matsumoto of Kanazawa University, Dr. Susumu Iai of the Japanese Port and Harbor Research Institute, and other researchers expressing interest in analyzing the results.