Shear Strength Of Purple Soil Of Sloping Farmland In Chongqing

Purple soil is a kind of special soil with shorter forming time, whose parent material mainly comes from Mesozoic rocks of Triassic, Jurassic, Cretaceous and Tertiary. It normally appears with strong physical weathering and weak chemical weathering, which are basically consistent with parent material. Purple soil is widely distributed in the Sichuan Basin, with the biggest distribution area in Chongqing especially. Shear strength, the ability to resist shear failure, is an important mechanical properties of the soil and is often regarded as a pivotal parameter for engineering design specifications. In recent years, with the development of agricultural engineering projects, a large number of projects will be built on sloping land. However, the shear strength values of sloping farmland soils vary with different spatial locations. So it is necessary to research the shear strength of sloping farmland soil of purple soil area, with important guiding significance for the design and construction of agricultural engineering.Taking Red brown purple soil of Jurassic Suining Formation rock formation and dark purple soil of Triassic Feixianguan Formation as the research object, this paper discussed the shear strength characteristics of Chongqing purple soil slope cropland area with the method of direct shear tests, comparative analysis and linear fit. Conclusions of this study are asfollows:(1) The changing characteristics of gravel content, particle composition, micro-aggregates and other physical parameters.The gravel content and the soil bulk density of Red-brown-purple soil and dark-purple muddy in slope crest is greater than the foot of hill. Red brown purple clay particles and micro-aggregates content from the top of the hill to the foot of the slope presents significantly different, with no significant differences in dark purple mud. Red Brown purple liquid limit, plastic limit and plasticity index appear different, less than that in slop crest. However, dark purple liquid mud limit, plastic limit and plasticity index difference is small.(2) The shear strength characteristics of slope farmland soil.Shear stress varies with shear displacement, increasing rapidly at first and then increasing with a gradually decreasing amplification. The phenomenon of cut hardening occurs in ost of the soil samples during shearing process. Softening phenomenon occurs only in dark purple clay with 9% soil moisture content coupled with 100kPa vertical pressure, and corresponding shear displacement of shear stress or shear during peak 4mm for shear strength.The change of shear strength and vertical pressure presented remarkable linear correlation. The greater the vertical pressure was, the greater shear strength would be. Rate of change of Shear strength from vertical pressure was related moisture. When the other conditions were fixed, the cohesion and the internal friction angle of dark purple mud is greater than red-brown purple soil. One or two peaks of cohesion appear when moisture content was changing. Additionally, peaks of soil from slop toe occur later than soil from the slop crest. The maximum cohesion of red-brown purple soil S-1～S-5 from crest to toe is 26.07kPa,25.62kPa,28.43kPa,46.53kPa and 53.75kPa respectively, and the corresponding moisture content is 15%,11%,17%,19% and 19% respectively. The maximum cohesion of dark purple soil F-1～F-5 from crest to toe is 62.30kPa,57.49kPa, 54.73kPa,52.93kPa and 49.94kPa respectively, and the corresponding moisture content is 13%,15%, 13%,17% and 19%, respectively. Internal friction angle varies with the moisture content with a significant negative correlation. The changing rate of the internal friction of red-brown purple soil in slop crest from soil moisture content is greater than that of the toe, while the dark purple soil follow the opposite rule. Cohesive force and internal friction angle is related to soil spatial location. The cohesion of red-brown purple soil from top slop is smaller than that in the foot of slop. The internal friction angle of red-brown purple changes slightly with spatial position in low water content, but changes is relatively greater with high water content. On the contrary, for dark purple mud, little difference of the cohesion and internal friction angle of soil emerges between slop crest and slop toe.(3) The factors of shear strength on slope farmland soil.The difference of clay content in red-brown purple soil from the top to foot of slope is greater than that of dark purple soil, so as the difference of particle size distribution (PSD) and rock fragment. Hence, as for determining the effects of factors on soil shear strength, the results of red-brown purple soil are better than dark purple soil’s in this paper. The clay content mainly influences the cohesion and this effect is much greater than that on internal friction angle. The results shows that the clay content is greater when the cohesion is the greater, especially for red-brown purple soil. Meanwhile, the particle size distribution (PSD) has some influence on the cohesion and internal friction angle. For the effects of PSD on cohesion under different soil water content, the relationships of red-brown purple soil and dark purple soil are shown as 21%>17%>25%>9%>13% and 9%>17%>13%> 25%>21%, respectively. For the effects of PSD on internal friction angle under different soil water content, the relationships of red-brown purple soil and dark purple soil are shown as 25%>13%> 21%>17%>9% and 25%>13%>9%>17%>21%, respectively. Besides, the rock fragment can also affect the soil shear strength. Moreover, the effect would be different with different soil under different soil water content. In this paper, the effects relationships on cohesion of these two soils mentioned above are shown as 21%>17%>25%>13%>9% and 13%>9%>25%>21%>17%, respectively, and the effects relationships on internal friction angle are shown as 13%>25%>21%>9%> 17% and 9%>13%>17%>25%>21%, respectively. Overall, the influence of rock fragment on cohesion is more obvious than that on internal friction angle.