An Improved Method for Measuring Soil Shrinkage Curves with Consideration of Crack Formation.

Soil cracking during drying is an important phenomena of vertisol at dehydrating, but is usually ignored during the measurement of soil shrinkage curves. We introduced an improved method that combined the image technology and fine needle approach for measuring soil volume change during cracking formation under a drying process in the laboratory. By using image analysis, we calculated the crack area. The crack depth was obtained by inserting a needle to the crack and measuring the portion of needle that located in the crack with a caliper with an accuracy of 0.01mm. The subsidence depth of the sample was measured with the caliper too. Meanwhile, soil water content was determined gravimetrically. The improved method was applied to measure the shrinkage characteristics of a lime concreted black soil at three thickness: 4.5 mm, 9.7 mm, and 16.5 mm. The results showed that  (1) the propagation of cracking process differed with the thickness of soil samples, as indicated by the various  final crack numbers and nodes; (2) the cracking area and subsidence depth increased with decreasing soil water content; (3) including soil-specific cracking characteristics improved the performance of soil shrinkage curve models (with R² greater than 0.95), and the best results appeared in the Logistic model, which had an R²above 0.99 for all the thicknesses.

key words:method, Soil Shrinkage Curve, cracks, thickness