Analysis of shear strength characteristics of saturated soft soil under self-weight preload consolidation load conditions
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摘要: 宁波沿海平原区浅部饱和软土厚度一般30～40 m，不同深度土体自重应力具有一定差异，而室内直剪固结快剪试验采用统一标准的固结预压荷载，势必会造成固结压力与自重应力不匹配的现象，由此导致各试样因固结程度不同而对其抗剪强度产生影响。针对饱和软土实际应力条件，开展了规范法、自重预压荷载作用下的室内对比试验。结果表明：与规范法相比，自重预压固结法得到的饱和软土黏聚力提高幅度为48%，内摩擦角降低13%。当预压固结荷载小于其自重应力时，试样未充分固结变形，其颗粒间的胶结作用和摩擦、嵌入咬合作用减弱，导致其黏聚力及内摩擦角均相应降低；预压固结荷载大于土体自重应力时，孔隙水的排出使试样产生较大的固结变形，土颗粒重新排列，胶结作用产生的固化黏聚力减小，而咬合作用产生的内摩擦角则相应提高。最终呈现出自重预压荷载作用下饱和软土出现黏聚力增大，内摩擦角降低的抗剪强度特性。Abstract: In the Ningbo coastal plain area, the thickness of shallow saturated soft soil is generally 30~40 meters, the self-weight stress of soil at different depths has certain differences. Using a unified standard consolidation preload in the indoor consolidated quick shear test would inevitably lead to a mismatch between the consolidation and the self-weight pressure, thus affecting the shear strength of the samples due to different degrees of consolidation. Focusing on the actual stress conditions of saturated soft soil, indoor comparison tests under standard method and self-weight preload were carried out in this research. The results show that compared to the standard method, the cohesion of saturated soil increased by 48% and the internal friction angle decreased by 13% after consolidation with self-weight preload. When the pre-consolidation load is less than its self-weight stress, the sample is not fully consolidated. The cementation between particles and the friction, interlocking and biting action are weakened, leading to a decrease in cohesion and internal friction angle. When the preloading consolidation load is greater than the self-weight stress of the soil, the drainage of pore water causes the sample to undergo greater consolidation, and the soil particles are rearranged. The cohesive force generated by the cementation effect is reduced, and the internal friction angle generated by the interlocking effect increased correspondingly. The shear strength characteristics of saturated soft soil under self-weight preload show an increase in cohesion and a decrease in the internal friction angle.
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图 1 不同固结压力固结后土样在各级垂直压力作用下的剪应力–剪切位移曲线
Figure 1. Shear stress–displacement curves under different vertical pressures for soil samples consolidated at different pressures
图 2 试样抗剪强度与剪切垂直压力关系曲线
Figure 2. Curve of the relationship between sample shear strength and vertical pressure
图 3 饱和软土抗剪强度指标对比曲线
Figure 3. Comparison curves of shear strength parameters for saturated soft soils
表 1 取样浅部土层分布及物理力学性质表
Table 1. Distribution of shallow soil layers and their physical and mechanical properties
编号 土层名称 分布深度/m 密度/(g·cm−3) 含水量/% 孔隙比 液限/% 饱和度/% 塑性指数 液性指数 固结状态 ①2 黏土 0～1.2 1.79 38.4 1.09 47.5 95.2 21.6 0.71 正常固结 ②2 淤泥 1.2～10.0 1.68 58.1 1.59 44.5 100 21.2 1.64 ③2 淤泥质黏土 10.0～15.0 1.74 44.9 1.23 39.4 100 21.2 1.64 ④1 淤泥质粉质黏土 15.0～28.0 1.78 37.2 1.04 34.3 100 16.4 1.18 ④2 淤泥质黏土 28.0～40.0 1.77 45 1.25 43.3 100 18.6 1.09 
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表 2 室内试验方案
Table 2. Indoor test plan
组号 样号 土样中心点
深度/m中点自重
压力/kPa预压固结
荷载/kPa剪切垂直
荷载/kPaK1，K2 1 15.85 约150 K1：150
K2：50，100，
200，30050，100，
200，3002 16.85 3 17.85 4 18.85 5 19.85 6 20.85 7 21.85 8 22.85 9 23.85 10 24.85
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表 3 土样抗剪强度指标取值
Table 3. Values of shear strength parameters for soil samples
样号 抗剪强度指标 样号 抗剪强度指标 黏聚力
c/kPa内摩擦角
φ/（°）黏聚力
c/kPa内摩擦角
φ/（°）K1-1 18.0 6.5 K2-1 13.6 7.5 K1-2 18.5 6.9 K2-2 14.3 7.6 K1-3 19.3 7.0 K2-3 14.0 7.5 K1-4 20.9 6.8 K2-4 12.1 8.1 K1-5 21.2 6.9 K2-5 13.5 8.2 K1-6 22.5 6.8 K2-6 12.5 8.4 K1-7 23.4 7.5 K2-7 14.7 8.8 K1-8 22.0 8.2 K2-8 16.3 9.3 K1-9 22.5 8.0 K2-9 15.2 9.0 K1-10 23.2 8.2 K2-10 16.0 9.5 均值 21.1 7.3 均值 14.2 8.4
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