Strain Distribution Measurement on a Surface of Soil Sample by Laser Speckle Method and Limit State Design of Soil Structures

利用激光散斑法研制了一种高精度的土样表面局部应变分布测量系统。该原理是基于斑点随固体表面的变形而线性移动。该系统由CCD摄像机、个人计算机DSP图像处理单元和相关系数的快速计算方法组成。图像采集显示应变分布所需时间约为4秒。首先，我们测量了三轴试验试样的局部应变分布。将应变路径控制三轴试验结果与常规排水三轴试验结果进行了比较。应变路径控制试验表现出以下特点：在大应变域中，应变分布更为均匀；膨胀时的体积变化较大；峰值偏差应力减小10%。然后将该系统应用于离心机模型试验，实现了局部应变分布的实时、精确测量。这一实现具有以下重要意义：通过实时测量应变分布，可以准确地确定失效时的加速度；与以往的带标记的照片测量相比，大大节省了测量时间，提高了测量精度；理解从小应变到大应变的分布规律，将大大促进大坝等重要土质结构的设计。此外，该系统使离心模型试验成为检验数值结果以及土的本构方程和参数有效性的工具。

We developed a highly precise measuring system of local strain distribution on a surface of soil sample by the laser speckle method. The principle is based on that speckles move linearly depending on deformation of solid surface. The system is constituted of a CCD camera, a DSP image processing unit using a personal computer and a fast calculation method of correlation coefficients. The time required for image acquisition to display of strain distribution is about 4 seconds.Firstly, we measured local strain distributions on specimens for triaxial test. The results of strain-path controlled triaxial test was compared with the one of conventional drained triaxial test. The following characteristics for the strain-path controlled test are shown: the strain distribution is more uniform even in a large strain domain; the volume change in dilation is larger; the peak deviator stress is smaller by 10%.Then we applied this system to centrifuge model test to realize the real time and precise measurement of local strain distribution. This realization has the following valuable meanings: an acceleration at failure can be accurately determined by the real time measurement of strain distribution; the drastic saving time and improving accuracy can be obtained compared with the previous measurement by photos with markers; the design of important soil structures such as dams will be significantly progressed by comprehension of strain distribution from small to large strain. Furthermore, this system makes the centrifuge model test use for a tool of investigating the validity of numerical results as well as constitutive equations and parameters of soils.