Volume 35, Issue 1 (January 2012)
Image Analysis of Strains in Soils Subjected to Wetting and Drying
A novel image based technique has been developed to measure strains of soil specimens subjected to wetting-drying by using vapor equilibrium with closed-loop ventilation. A digital SLR camera, fitted with a macro telephoto lens, and an advanced photo system type-C (APS-C) sensor have been used to take images of specimens during wetting-drying. By using the simple radial model of optical distortion, it has been shown that macro fixed focal length lenses introduce extremely small strain errors, which can be neglected. It has also been shown that APS-C sensors are preferable to large Full Frame sensors when the specimen image does not fit the entire sensor area and pixel density, rather than pixel resolution, becomes the key parameter controlling measurement precision. Images of specimens have been acquired at different times during wetting-drying of the soil and processed by using the Adobe Photoshop software to assess evolution of strains. In particular, the planar strain field has been measured by means of a virtual “optical” strain gage rosette tracking the same three points on the specimen image throughout equalization. Multiple optical strain gage rosettes at different scales and positions have been employed to assess the uniformity of the strain field. The method was validated by imposing six different values of suction on six distinct specimens of the same isotropically compacted bentonite, with five values producing soil shrinkage (drying) and one value producing soil swelling (wetting). An excellent repeatability of results was observed in terms of both water content and strains. In addition, for each suction level, almost identical strain equalization curves are obtained from the analysis of multiple optical strain gage rosettes of different sizes placed at different locations over the specimen image. This confirms uniformity of the strain field and excludes the presence of friction at the specimen base.