A field study on the behavior of static drill rooted nodular piles with caps under compression

The static drill rooted nodular (SDRN) pile is a new type of composite pile that consists of a precast pile and surrounding cemented soil. Its cost advantages and environmentally friendly construction have been proven in applications in Southeast China. Moreover, the composition of pipe pile and nodular pile is based on the load transfer mechanisms of a pile foundation, which is effective in optimal design. This paper presents a field study on the behavior of SDRN piles under compression. The load-displacement response, axial force, mobilized load of pile cap, skin friction, and tip resistance of the composite pile are discussed. Here, the bilinear base load-displacement model was adopted to analyze the test results. It is found that providing caps on the static drill rooted piles takes full advantage of the static drill rooted method, and drilling and grouting into the soil beneath the cap, which can be considered a type of ground improvement treatment, can increase the bearing capacity of the pile cap; thus, setting a pile cap for this type of piles is recommended. The existence of the caps in the field tests decreased the skin friction of the upper part of pile shaft because of the additional settlement of the surrounding soil, which developed owing to the pressure from the caps. The frictional capacity of the concrete-cemented soil interface was much higher than that of the cemented soil-soil interface. The skin friction of the lower part of the pile shaft was about 1.25 times in clayey soil and 2.0 times in sandy soil compared with the bored pile. It can be concluded that the cemented soil-soil interface of the SDRN pile was probably better than the concrete-soil interface of the bored pile. The test results fitted the first stage curve of the bilinear model well, and it can be supposed that the base soil was strengthened because of the permeation of the cement paste.中文概要目 的研究带承台静钻根植竹节桩的抗压承载性能, 承台在荷载传递过程中所起作用, 带承台静钻根植竹节桩的桩侧摩擦性能和桩端承载性能。创新点通过带承台静钻根植竹节桩的现场静载试验对其抗压承载性能进行分析。方 法在预制桩制作过程中在桩身安置钢筋应力计(图1), 通过含有钢筋应力计的试桩的静载试验对其抗压承载性能(图7 和8)、各级荷载作用下的桩身轴力(图9 和10)、承台承载性能(图11和12)、桩侧摩擦性能(图18)以及桩端承载性能(图19 和20)进行分析与研究。结 论1. 静钻根植工法通过搅拌注浆能够改善承台下部土体性质, 在静钻根植桩中设置承台能充分发挥静钻根植工法的优势, 在该种桩基中设置承台比较合适;2. 承台的存在使桩周土体产生沉降, 使得试桩上部桩土相对位移较小, 从而使试桩上部桩侧摩阻力减小, 试桩下部土体所提供的侧摩阻力相比灌注桩都有所提高;在粘性土中试桩桩侧摩阻力值为灌注桩的1.25 倍, 在砂性土中达到2倍, 说明试桩中水泥土-土体接触面摩擦性能优于灌注桩混凝土-桩周土体接触面的摩擦性能, 且在砂性土中提高程度更大;试桩桩端荷载位移曲线基本呈线性, 与双折线模型相似;由于有水泥土的渗入, 桩端土的强度得到提高, 使桩端承载性能也得到提高。