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

研制了一套高精度的激光散斑法土样表面局部应变分布测量系统。其原理是基于散斑随固体表面变形而线性移动。该系统由CCD摄像机、基于PC机的DSP图像处理单元和相关系数快速计算方法组成。图像采集到应变分布显示所需时间约为4秒。首先，我们测量了三轴试验试件的局部应变分布。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。结果表明，应变路径控制试验具有应变分布均匀、剪胀体积变化大、偏应力峰值小10%等特点，并将该系统应用于离心模型试验，实现了局部应变分布的真实的实时、精确测量。这一认识具有以下重要意义：通过对应变分布的真实的时间测量，可以准确地确定破坏加速度;与以前的带标记的照片测量相比，可以大大节省时间，提高精度;通过了解应变分布，从小应变到大应变，对大坝等重要土结构的设计将有重大的进步。该系统使离心模型试验成为检验数值计算结果、土的本构方程和参数的有效性的工具。

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.