Effect of Coarse Grain Content on Shear Strength of Mixed Soil in Western Sichuan
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摘要: 混合土在川西地区广泛分布，由其构成的山区公路路基的稳定性是实际工程中亟需关注与研究的问题. 针对混合土抗剪强度受粗颗粒含量变化影响较大的特征，选取川西山区路基填筑用的天然混合土，分别进行小型与大型直剪试验，系统研究了粗颗粒含量对混合土剪应力-位移曲线和内摩擦角的影响规律. 试验结果表明：粗颗粒含量差异导致混合土抗剪强度发生明显变化，且在不同的粗颗粒含量区间，抗剪强度会呈现不同的变化趋势；当粗颗粒含量从0增至30%时，内摩擦角增长速率较慢；在30%～60%之间递增时，内摩擦角增长速率变快；超过60%时，增长速率回缓，即有两个界限值；通过颗粒骨架理论解释了界限值产生原因，并从宏微观角度分析了抗剪强度在不同区间内的主要影响因素差异，可为混合土结构物稳定性评估提供取值与理论分析依据.Abstract: Mixed soil is widely distributed in the west of Sichuan, and the stability of the highway subgrade made up of mixed soil in mountainous areas is an issue needing urgent attention and research in engineering practices. Considering that mixed soil shear strength is affected greatly by the content of coarse particles, the natural mixed soil used for subgrade in mountain areas of Western Sichuan was chosen and used in small and large direct shear tests to systematically study the influence of the coarse grain content on the shear stress-displacement curve and internal friction angle of the mixed soil. Experimental results show that the shear strength of mixed soils changes significantly with the coarse grain content, and presents different variation tendencies in different coarse grain content ranges. When the coarse grain content increases from 0 to 30%, the internal friction angle grows slowly. With the coarse grain content increasing from 30% to 60%, the growth rate of the internal friction angle becomes faster; and then it slows down again beyond the content of 60%. Therefore, there are two threshold values formed, which can be explained from skeleton theory. Differences in the main influencing factors of shear strength in different ranges are finally analyzed from macro and micro aspects, which can provide the reference value and theoretical basis for mixed soil structure stability evaluation.
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图 2 不同竖向应力下的剪应力-位移曲线（dmax = 5 mm）
Figure 2. Relation curvesbetween shear stress and displacement under different vertical stresses (the maximum grain diameter dmax = 5 mm)
图 3 不同竖向应力下的剪应力-位移曲线（dmax = 20 mm）
Figure 3. Relation curves between shear stress and displacement under different vertical stresses (dmax = 20 mm)
图 4 不同竖向应力下的剪应力-位移曲线（dmax = 60 mm）
Figure 4. Relation curves between shear stress and displacement under different vertical stresses (dmax = 60 mm)
图 5 内摩擦角与粗颗粒含量关系曲线
Figure 5. Relation curve between internal friction angle and percentage of coarse grain content
表 1 矿物成分测试结果
Table 1. Mineral constituents test results of soils
土编号 蒙托石 伊利石 高岭石 石英 斜长石 方解石 赤铁矿 1号 0 0 84 12 0 3 1 2号 3 31 14 31 0 12 9 3号 5 31 12 44 8 0 0 
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表 2 试验方案
Table 2. Test program
试验种类 土编号 粒径/mm 粗颗粒含量/% 试验直径/mm 小型直剪 1号 < 5 0 63.5 2号 < 5 0 63.5 3号 < 5 0 63.5 大型直剪 1号 < 20 28.2 504.6 1号 < 60 44.0 504.6 2号 < 20 40.3 504.6 2号 < 60 47.7 504.6 3号 < 20 61.7 504.6 3号 < 60 73.5 504.6
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表 3 不同粗颗粒含量下混合土的粘聚力和内摩擦角
Table 3. Cohesions and internal friction angles of mixed soil with different coarse grain contents
粗颗粒含量/% 粘聚力/kPa 内摩擦角/（°） 0 30.00 36.45 28.2 56.84 37.39 40.3 48.97 39.09 44.0 35.58 41.29 47.7 31.16 42.20 61.7 39.50 46.42 73.5 35.33 47.50
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