反复剪切作用下砒砂岩土壤力学性能试验

Experiment on mechanical properties of Pisha-sandstone at recurrent shear

为分析砒砂岩土壤的力学性能,该文根据砒砂岩土壤的粒径分布特点,对比分析3种不同粒径（≥0.5~1 mm、≥0.25~0.5 mm、〈0.25 mm）,4种不同含水率（5%、8%、11%、14%）土壤试样在法向压力50~300 k Pa作用下反复直剪的力学性能,得出以下结论：1）不同粒径的砒砂岩在第1次剪切中剪应力出现峰值,第2、3、4次的剪应力与剪切位移关系曲线呈微硬化型。粒径范围为≥0.25~0.5 mm的砒砂岩峰值强度和残余强度均最低;2）砒砂岩随含水率增加,峰值强度和残余强度均有所降低。粒径范围为≥0.5~1 mm的峰值强度和残余强度受含水率影响最显著;3）进一步拟合得到了砒砂岩法向位移和剪切位移的函数关系,可以较好地反应砒砂岩剪切过程的剪胀规律。该研究为砒砂岩区水土流失及边坡稳定问题的防治提供力学依据。

Pisha-sandstone is a kind of loose rock stratum. Since the rock stratum is thin and low in pressure, it is hard to form rock, poor to glue the sandstone and low in structural strength. In nature, the stratum is apt to become muddy when encountering water and to form sands when encountering wind, consequently resulting in severe erosion. Geological disasters happen continually due to unsteady margins of Pisha-sandstone area, and its peak strength and residual strength are the important indexes in the assessment of the stability of landslides. The strength is in close relation with its density, particle diameter, contact pressure between particles and moisture contents. According to China＇s general scheme of soil fraction, the Pisha-sandstone is graded into coarse sand（≥0.5-1 mm in diameter）, medium sand（≥0.25-0.5 mm in diameter）, and fine sand（0.25 mm in diameter）. Under four moisture contents（5%, 8%, 11%, 14%）, recurrent direct shear tests were performed on Pisha-sandstone by using the Digital Direct/Residual Shear Apparatus to investigate its changing laws of peak strength and residual strength. Based on the particle diameter of Pisha-sandstone soil, the recurrent direct shear tests were performed on three ranges of particle diameters（≥0.5-1 mm, ≥0.25-0.5 mm, 0.25 mm） under the pressure at 50, 100, 200 and 300 k Pa. We found that among the four recurrent direct shear tests of Pisha-sandstone at the same particle diameter, the peak value of shearing stress appeared at the first shear in different ranges of particle diameter; the peak value of the fourth shear was very similar to that of the third shear, and the shear strength of the fourth shear was actually the residual strength. When the particle diameter was between ≥0.5-1 mm and moisture content at 5%, the peak value appeared at the first shear, which tended to be strain softening. With the increase of normal pressure, the strain softening increased. At the same moisture content and normal pressure, the shear strength and residual strength of the Pisha-sandstone differed from its particle diameters, which were lowest in the diameters ≥0.25-0.5 mm. For the Pisha-sandstone of same particle diameter, the peak strength and residual strength were increased with the decrease of moisture content. In the particles at the diameter ≥0.5-1 mm, the peak strength varied with the moisture content. In the particles at the diameters ≥0.25-0.5 mm and 0.25 mm, the shear peak slightly reduced with the moisture content. When the moisture content was lower than 11%, the residual strength of Pisha-sandstone did not change significantly. When the moisture content was over 14%, the residual strength of Pisha-sandstone in three diameters were significantly reduced, especially in the Pisha-sandstone in the diameters ≥0.5-1 mm. Therefore, 11%-14% were the critical values of moisture content, during which the residual strength of Pisha-sandstone was apt to change dramatically. Moreover, the functional relationship between normal displacement and shearing displacement was achieved and analyzed by SAS software, which can better reflect the dilatancy law of Pisha-sandstone in the shearing process.