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

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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.