基于透明土的管桩贯入特性模型试验研究

由于受土塞效应等因素影响,开口管桩和闭口管桩在贯入过程中的挤土效应存在明显的差异;然而针对该两者贯入特性的研究相对较少。基于透明土和PIV（Particle Image Velocimetry）技术,开发了相应的桩基贯入模型试验系统,用于桩基贯入过程中桩周土体变形的非插入式测量。试验选用的透明土由玻璃砂和具有相同折射率的孔隙液体制成。该试验系统主要包括：线性激光光源、CCD（Charge-Coupled Device）相机、自动沉桩加载仪和计算机控制系统等。激光射入透明土,和透明土之间的相互作用产生了独特的散斑场,通过CCD相机摄取贯入过程中各个时刻散斑场变形的图片,然后通过PIV技术对这些变形前后的图片进行处理,即可得到整个土体位移场。分别进行了开口管桩和闭口管桩的桩基贯入试验,得到了对应的土体位移场,并对桩基贯入引起的桩周土体径向位移试验结果与圆孔扩张法、应变路径法等理论计算结果进行对比分析;研究结果表明,对于闭口管桩,由于竖向位移的影响,试验值比圆孔扩张法结果小,其与应变路径法更为接近;对于开口管桩,由于沉桩过程中土塞作用,试验值比圆孔扩张法大,其影响不可忽略。

Model tests on pipe pile penetration by using transparent soils

The squeezing effects of open-ended and closed-ended pipe piles are obviously different because of “soil plug”. However, the researches focused on this problem are relatively little. One small-scale model test system is developed to study the non-intrusive measurement of the soil deformation during pile penetration using the transparent soils and PIV (particle image velocimetry) technique. The transparent soils are manufactured by baked quartz with a pore fluid which has the same refraction index. The test system contains a line laser light, a CCD (charge-coupled device) camera, a loading machine and a computer. The whole displacement field during the pile penetration can be obtained as follows: a distinctive laser speckle pattern can be generated by the interaction between the laser and the transparent soils when the light goes through the transparent soils. Then a sequence of speckle pattern images are captured consecutively during the pile penetration by the CCD camera. Finally these images are analyzed by using PIV. The model tests on the penetration of an open-ended pipe pile and a closed-ended pipe pile are both carried out, and the data are compared with the results obtained by the cavity expansion method and strain path method. The results show that, for the closed-ended pipe pile, the radical displacements obtained by the model tests are smaller than those by the cavity expansion method, and are close to those obtained by the strain path method; for the open-ended pipe pile, the “soil plug” obtained by the model tests is larger than that obtained by the cavity expansion method.