Water sensitivity of shear strength of purple paddy soils

Abstract: Soil shear strength is one of the most important indicators in engineering design and soil erosion, also an essential parameter for soil precision tillage and design of agricultural machinery and equipment. In the previous work, although the shear strength of several types of soils has been investigated, few of these researches include that of purple paddy soils. Compared with purple soils, purple paddy soils have long been submerged in natural conditions; this special characteristic must have far-reaching influence on the mechanical properties of soils. Therefore, to investigate the water sensitivity of shear strength of purple paddy soils which are widely distributed over the Territorial Management Project in the Dazhu village, Qiantang town, Hechuan district in Chongqing, their shear strength was measured through triaxial test by considering the change of moisture directly, and accordingly some suggestions were put forward in dealing with purple paddy soil for engineering construction. In batch experiment, soil samples collected from the roadbed in the project were first air-dried, and after removal of visible pieces of plant debris and stones, all of the samples were ground to pass a 2-mm sieve for subsamples. According to test requirements, the water content of the subsamples was pre-adjusted to the required values carefully in preparation for triaxial compression test. Then unconsolidated undrained (UU) triaxial compression test was conducted under four different confining pressure (100, 200, 300, 400 kPa) using groups of samples at eight distinctive soil water contents (9%, 11%, 13%, 15%, 17%, 20%, 21% and 23%). The results indicated that: 1) The principal stress of purple paddy soils increases with the decrease of moisture under the same confining pressure, and the principal stress increases with the increasing confining pressure under the same water content, in which the increments become smaller with increasing soil water content. 2) When the water content of purple paddy soil is around a critical value, the variation of shear strength is slight; while the water content is above the critical value, the shear strength of purple paddy soils decrease with increasing water content. 3) The water content of purple paddy soil is of great significance on its cohesion. The number of cohesive strength peaks at 176.2 kPa at 13% and ends at 80.2 kPa at 23%. 4) The internal friction angle is negatively linearly correlated with soil water content (from 30.2° at 9% to 4.9° at 23%). This information confirms that purple paddy soils are water- sensitive, which is also beneficial to the reasonable design of local agricultural infrastructure.