Full-Scale Static and Dynamic Lateral Pile Group Testing

许多重要的建筑物和桥梁都是由一组钢管或桩基组成的基础支撑的。这些桩群提供的水平阻力是决定结构能否在地震中幸存而不造成重大破坏的关键。虽然已经发展出相当可靠的方法来预测单桩在缓慢荷载作用下的水平阻力，但很少有信息可以指导工程师设计紧密间隔的桩群，特别是在快速荷载作用下。然而，这些有限的现场试验数据表明，在给定的荷载下，群桩比单桩承受更大的运动和更高的应力。这些群桩效应通常在设计中通过使用减缩系数来减少群桩上土体提供的阻力来考虑，但是对于适当的减缩系数存在相当大的不确定性。建议的研究有以下目标：(1)评价桩距对实测群折减系数的影响，并绘制出这些系数随桩距变化的设计曲线；(2)确定大桩（5排）群折减系数概念的有效性，并确定第三排以上的折减系数是否保持不变；(3)确定反复荷载和桩周间隙形成对实测群折减系数的影响；(4)检查加载速率对桩周围土壤提供的阻力的影响，(5)提供一个充分记录的案例历史，用于评估和校准计算机和物理模型。这些目标将通过在全尺寸桩和桩群上进行一系列水平荷载试验来实现。这些测试将补充已经在现场进行的桩群测试，中心桩间距为3英尺。首先，水平荷载试验将在单个直径12英寸的钢管桩上进行比较。荷载将以10个增量施加，每个增量15个循环，以模拟大地震的重复荷载。接下来，水平荷载试验将在五排桩组上进行，每排桩中心间距为四英尺。将采用相同的循环加载程序，并测量每根桩所承受的载荷。然后将荷载框架移动到相邻的桩组进行6英尺间距的测试。除了缓慢施加的载荷外，还将使用火箭雪橇（静态装置）快速施加载荷，这将模拟地震载荷的施加速度。数据缩减后，将使用现有的计算机模型分析测试结果，并确定适当的桩群缩减系数作为桩间距的函数。这次和以前在现场进行的负荷测试的结果也将提供给日本的几位研究人员，以便使用复杂的二维和三维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所看到的增加的电阻。潜在的日本研究人员将包括金泽大学的松本教授、日本港口研究所的井井进（Susumu Iai）博士，以及其他对分析结果感兴趣的研究人员。

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.