A deformation measurement system based on particle image velocimetry (PIV) and close-range photogrammetry has been developed for use in geotechnical testing. In this paper, the theory underlying this system is described, and the performance is validated. Digital photography is used to capture images of planar soil deformation. Using PIV, the movement of a fine mesh of soil patches is measured to a high precision. Since PIV operates on the image texture, intrusive target markers need not be installed in the observed soil. The resulting displacement vectors are converted from image space to object space using a photogrammetric transformation. A series of validation experiments are reported. These demonstrate that the precision, accuracy and resolution of the system are an order of magnitude higher than previous image-based deformation methods, and are comparable to local instrumentation used in element testing. This performance is achieved concurrent with an order of magnitude increase in the number of meas...

Soil deformation measurement using particle image velocimetry (PIV) and photogrammetry

Soil stress-strain behaviour is highly non-linear and this has an important influence on the selection of design parameters for simple routine geotechnical calculations. Non-linear behaviour can be characterized by rigidity and degree of non-linearity and these can be determined from measurements of very small strain-stiffness, peak strength and failure strain. Very small strain-stiffness can be found from measurements of shear wave velocity in situ or in laboratory tests. Peak strength and failure strain can be measured in routine laboratory tests but are strongly influenced by initiation of shear bands. The important non-linear stiffness parameters for soil are related to its composition and to its current state. Back-analyses of the load settlement behaviour of full-scale and model foundations demonstrate the influence of non-linear soil behaviour. Variations of stiffness with settlement calculated from full-scale and model foundations agree well with non-linear soil stiffnesses based on rigidity and d...

https://www.icevirtuallibrary.com/doi/pdf/10.1680/geot.2000.50.5.487

Non-linear soil stiffness in routine design

Small strain stiffness and stiffness degradation curve of Bangkok Clays

Although the small-strain shear modulus of saturated soils is known to be significantly affected by stress history, consisting of the overconsolidation ratio (OCR) and recent stress history, the ef...

Effects of current suction ratio and recent suction history on small-strain behaviour of an unsaturated soil

Coral sand has individual characteristics that differ from silica sand, such as creep behavior that is always attributed to particle crushing under high stress states. To understand the creep behav...

Comparative study of coral sand and silica sand in creep under general stress states1

Conventional procedures for determining deformations during triaxial tests are based on measurements made externally to the cell. This practice is inadequate, particularly if the small strain stiffness of the soil is being investigated, because errors are introduced which limit the accuracy and resolution of the measurements. The errors can only be eliminated if axial strains are determined internally, within the cell, and locally over the central one third of the specimen. Likewise, the radial dimensions should be monitored at specimen midheight. The last ten years has seen the introduction of a diverse range of small-strain measuring devices. This has been largely in response to recognition of the importance of achieving strain measurement accuracy of at least 10−3% for small-strain stiffness evaluation. This paper summarizes the types of instrumentation currently available and their modes of operation, benefits, capabilities, and limitations. Typical results comparing external and internal strain measurements are presented. A system for classifying internal-strain measuring devices is presented. It is anticipated that test results based on small strain determinations will be a more frequent requirement in the future. Accordingly, the paper is intended to assist prospective users in becoming familiar with the various techniques that have been used and presents information that should enable selection of appropriate equipment for particular applications. Given that improvements in existing systems are warranted, it is hoped that the paper will also provide stimulus for further research and development.

JD Frost

Papers

A review of instrumentation for measuring small strains during triaxial testing of soil specimens