Experimental and numerical studies on gas pressure–induced deformation and failure of unsaturated soil
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摘要: 覆土填埋是處理城市垃圾的主要手段之一，但垃圾中的有機質在降解時會生成大量氣體，若產氣速率過快或導排不順，內部將產生過高的氣壓，導致土質覆蓋層發生變形破壞，進而影響填埋場的穩定性. 基于此，開展氣壓誘導非飽和土破壞的二維模型試驗和數值模擬，研究不同土體厚度、氣壓條件下的土體變形破壞機制. 結果表明，氣壓誘發土體破壞的過程可分為水氣運移、局部微裂縫、貫穿主裂縫和內部空洞四個階段. 土體破環主要發生在充氣孔上方與表層之間的倒三角區域內，其破壞形態根據是否有前期氣壓作用可分為“劈裂型”與“爆裂型”兩種. 土體的破壞壓力隨覆土厚度的增加近似線性增大，適當的氣壓有利于增加土體的穩定性，在此基礎上提出臨界穩定氣壓的概念. 通過提出的數值模擬方法研究了土體內部的滲流變化規律，發現氣壓會增加土體內部的孔隙壓力，同時會驅使水向四周擴散，導致周圍土體的飽和度發生變化；最后，研究結果表明有效應力增量的區域性變化可能是臨界穩定氣壓產生的原因，可為實際工程提供參考.Abstract: Earth-cover landfills are one of the primary means of treating urban garbage. However, the organic matter in the garbage generates a large amount of gas upon degradation. The internal gas pressure will be substantially high if the gas generation rate is considerably high or if the gas drainage is not smooth, resulting in the deformation and destruction of the soil cover, thus affecting the stability of the landfill. Accordingly, a plane model test of gas pressure-induced failure of unsaturated soil was performed using a self-designed test device, and the deformation and failure mechanisms of soil under different soil thicknesses and gas pressures were comparably studied through numerical simulation. The results revealed that the process of soil damage induced by gas pressure can be divided into four stages—water and gas migration, local micro crack generation, main crack penetration, and internal cavity formation; Soil damage mainly occurs in the inverted triangle area between the top of the inflatable hole and the surface layer. The soil failure modes can be divided into two types—splitting and burst failure—depending on whether there was a previous gas-pressure effect. The failure pressure of soil increases in an approximately linear fashion with an increase in the thickness of the overlying soil. Accordingly, the concept of failure stress ratio was proposed, and it was observed that the failure stress ratio of each soil layer thickness can be approximately regarded as a constant, which has a certain importance for monitoring the landfill overburden in practical engineering. Additionally, the test results revealed that appropriate gas pressure is conducive to increasing the stability of soil mass; the soil mass will gradually become unstable if gas pressure exceeds a certain value, based on which the concept of critical stable gas pressure was proposed. Furthermore, the proposed numerical simulation method was used to establish a corresponding two-dimensional numerical model with reference to the model test. The numerical results, including the fracture propagation pattern and failure pressure results, were consistent with the model test results. On this basis, the seepage variation law within the soil mass was deeply studied. It was observed that the gas pressure increases the pore pressure inside the soil while driving the water to diffuse around, causing changes in the saturation of the surrounding soil. Finally, the simulation results revealed that regional change of effective stress increment may be the cause of critical stable gas pressure, providing a reference for practical engineering.
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Key words:
- model test /
- gas pressure /
- unsaturated soil /
- water & gas migration /
- two-phase flow
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圖 1 模型試驗裝置. (a) 試驗裝置示意圖; (b) 試驗裝置實物圖
Figure 1. Model test device: (a) schematic of the test device; (b) physical diagram of the test device
圖 3 模型試驗破壞過程. (a) 水氣運移; (b)微裂縫產生; (c)主裂縫產生; (d)內部空洞
Figure 3. Failure process of the model test: (a) water and gas migration; (b) microcrack generation; (c) main cracks penetration; (d) internal cavity
圖 4 模型破壞機理. (a) 劈裂擴展方向力學分析; (b) 土體破壞模式
Figure 4. Model failure mechanism: (a) mechanical analysis of splitting propagation direction; (b) soil failure mode
圖 5 不同試驗條件下土體的最終破壞形態. (a) A組; (b) B組
Figure 5. Final failure mode of soil under different test conditions: (a) group A; (b) group B
圖 6 A組試驗條件下的土體破壞壓力. (a)破壞氣壓; (b)破壞應力比
Figure 6. Soil failure pressure under the group A test conditions: (a) failure gas pressure; (b) failure stress ratio
圖 9 不同土層厚度條件下破壞壓力試驗值與模擬值
Figure 9. Test and simulation values of failure pressure under different soil layer thicknesses
圖 10 不同土層厚度條件土體破壞形態數值結果
Figure 10. Numerical results of soil failure patterns under different soil thickness conditions
圖 11 模型滲流場演化過程. (a)孔隙水壓力; (b)孔隙氣壓力; (c)飽和度
Figure 11. Evolution of model pore pressure: (a) pore water pressure; (b) pore gas pressure; (c) saturation
圖 12 監測點處的壓力變化規律. (a) 孔隙氣壓力;(b) 孔隙水壓力
Figure 12. Variation in the pressure at monitoring points: (a) pore gas pressure; (b) pore water pressure
圖 13 監測點處的飽和度變化規律
Figure 13. Variation in the saturation change law at monitoring points
圖 14 觀察區內的有效應力增量對比
Figure 14. Comparison of effective stress increment in the observation area
表 1 土體材料參數
Table 1. Soil material parameters
Parameter Value Bulk modulus /Pa 1.33×107 Shear modulus /Pa 8×106 Porosity 0.4 Soil density /(kg·m?3) 1600 Cohesion /Pa 0 Friction angle / (°) 31.88 
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表 2 模型試驗方案
Table 2. Model test scheme
Test groups Number d/cm Loading form of gas pressure
A1 10 Gradually increase 2 15 Gradually increase 3 20 Gradually increase
B4 20 After 20 minutes of 10 kPa gas pressure, then gradually increase 5 20 After 20 minutes of 20 kPa gas pressure, then gradually increase 6 20 After 20 minutes of 30 kPa gas pressure, then gradually increase 7 20 After 20 minutes of 40 kPa gas pressure, then gradually increase 8 20 After 20 minutes of 50 kPa gas pressure, then gradually increase 9 20 After 20 minutes of 60 kPa gas pressure, then gradually increase
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表 3 模型流體參數
Table 3. Model fluid parameters
Parameter Value Permeability coefficient, ks/(m·s?1) 1×10?5 Viscosity ratio 56 m 0.445 n 0.5 Air entry value /Pa 1500 Saturation 0.4 Residual saturation 0.01 Gas density /(kg·m?3) 1.25 Water density /(kg·m?3) 1000
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