考虑净正应力影响的膨胀土土–水特征曲线试验研究

工程建设中,具有胀缩性、裂隙性和超固结性等特殊性质的膨胀土多处于非饱和状态和复杂变化的外加应力环境下,对膨胀土进行考虑应力状态的土–水特征曲线(soil-water characteristic curve,SWCC)试验研究,定性描述和定量表征膨胀土在一定应力状态下的土水特征,具有工程实践意义。采用GEO-Experts高级型应力相关土–水特征曲线压力板仪(SDSWCC-H)测定了河南南阳膨胀土在4个不同净正应力下(0,40,99,199 kPa)的土–水特征曲线,并利用试验数据对提出的能明确考虑净正应力影响的膨胀土SWCC表征方程进行验证。结果表明:(1)由于土体的间断级配,南阳膨胀土呈现出双峰土–水特征曲线(bimodal soil-water characteristic curve);(2)净正应力会对膨胀土的持水能力与进气值产生影响,具体表现为在10~100 kPa基质吸力范围内,净正应力会显著影响膨胀土在脱湿过程中水分排出的速度;净正应力也改变了双峰SWCC第二个波峰的进气值;(3)已提出的SWCC模型曲线拟合精度较高,拟合结果良好,模型可以很好地表征南阳膨胀土在恒定净正应力下基质吸力与持水量的关系。

Experimental study on soil-water characteristic curve of expensive soil considering net normal stress

The expansive soil is a kind of special soil with swell-shrinking, fractured and over-consolidated properties. In engineering construction, the expansive soil is generally under unsaturated state and complex applied stress environment. It is practical to describe qualitatively and quantitatively the soil-water characteristic curve (SWCC) of the expansive soil under a certain stress state. Based on this, the SWCC of Nanyang expansive soil in Henan Province are measured under four different net normal stresses (0, 40, 99, 199 kPa) using the GEO-Experts high-type pressure plate apparatus for stress-dependent SWCC (SDSWCC-H). Furthermore, a proposed SWCC equation which can clearly consider the effect of the net normal stress is validated using the experimental data. The results suggest that: (1) Nanyang expansive soil presents bimodal SWCCs because of gap-graded soil; (2) The net normal stress can significantly affect moisture realease speed of soil in the matrix suction range from 10 to 100 kPa and also change the air-entry vaule of expansive soil; (3) The accuracy of the SWCC model based on measured data of bimodal SWCC for Nanyang expansive soil is verified with high fitting precision. The proposed SWCC model can well reflect the relationship between matrix suction and water retention capacity under constant net normal stress.