Investigation Of The Mechanism For Intercepting Groundwater Of The Unsaturated Zone Formed By Air Injection Method In A Slope

The rising of groundwater level induced by intense rainfall has significant influence on the slope stability.Rainfall causes the rising of the groundwater level by infiltration on the slope surface and subsurface water movement from a higher elevation,and the latter is the major replenishment of groundwater resources for potential landslides where a large catchment area exists upstream.Slope drainage is crucial to landslide mitigation,yet current drainage technologies still encounter difficulties in achieving fast drainage.Therefore,the exploration of efficient and rapid drainage options is urgently required for landslide mitigation.The air injection method is to construct an artificial unsaturated zone with low permeability to intercept groundwater by injecting compressed air into the slope,which provides a fresh perspective on slope drainage.The paper performs a series of physical model tests based on a large-scale slope model and numerical simulations to investigate the mechanism for intercepting groundwater of the unsaturated zone formed by air injection method in a slope.The variations of the groundwater levels during the air injection under different injection pressures are studied.The formation process of the low-permeability unsaturated zone formed by the air injection method,and the features of the air-water flow in the unsaturated zone are analysed.Besides,the influence of the features of the unsaturated zone on the effectiveness of the air injection method in intercepting groundwater is investigated.The main conclusions are as follows:(1)The air injection divides the slope into three zones:zone I,where the groundwater levels increase,is located upstream of the injection point;zone II,the unsaturated zone,is located near the injection point;zone III,where the groundwater levels decrease,is located downstream of the injection point.Two phases are identified following initiation of air injection:a formation phase of the unsaturated zone,in which the compressed air drives groundwater away to form an unsaturated zone,and a steady-state phase of the unsaturated zone,in which the unsaturated zone prevents the groundwater in the upstream slope flowing to the downstream slope due to its low permeability.The groundwater levels have a rapid and significant decrease in a closer position downstream from the injection point.The average drop in the groundwater level downstream of the slope can exceed 13.2%for one injection point test.Besides,the air injection method continues to intercept the upstream groundwater flow within a certain period of time after air injection is ceased due to a large amount of compressed air still sealed in the slope.(2)The injection process undergoes the following five phases sequentially based on the characteristics of high pressure gas diffusion and migration:Phase 1,the injected air is dissolved in the groundwater and migrated by diffusion and advection.Phase 2,a small amount of immobile gas phase consisting of closed bubbles appeares near the injection point.Phase 3,the gas phase becomes continuous and begins to flow in a small fraction of the larger and interconnected pores in the soil.Phase 4,the gas breakthrough occurres and the injected air flows along the preferential flow paths.Phase 5,the air flow is no longer concentrated in several preferential flow paths,the number of air flow paths increases,and the pores expand in the original pore air flow paths.The air-water configuration in the unsaturated zone gradually transitions from a closed-air system to a bi-opened system,and finally becomes close to a closed-water system.(3)The effectiveness of the air injection method in intercepting groundwater is significantly affected by injection pressure.When the injection pressure is lower than the entry pressure,the air injection method has little effectiveness in water interception.When the injection pressure is within the range of the entry pressure to a level higher than the breakthrough pressure,the effectiveness in water interception increases as the injection pressure increases.However,the injection pressure should not exceed the maximum injection pressure to avoid unexpected deformation and damage of the slope.The average drop in the groundwater level downstream of the slope can exceed 27.3%for multiple injection points test.(4)Based on the two-dimensional design sketch of the air injection method,the paper establishes the simplified theoretical relationship between the interception ratio,which is defined to evaluate the effectiveness of the air injection method in intercepting groundwater,and the characteristics of the unsaturated zone.The interception ratio is affected by the permeability,the length and the width of the unsaturated zone.The interception ratio is linear with the length of the unsaturated zone,and the linear coefficient depends on the decrease in hydraulic conductivity of the unsaturated zone The interception ratio increases quickly with the rise in the width of the unsaturated zone in the initial stage,and then gradually stabilizes,and the value of the stable interception ratio depends on the decrease in hydraulic conductivity of the unsaturated zone.(5)With the increase in the injection pressure,the two-phase flow in the expanding unsaturated zone gradually changes from being water-flow-to air-flow-dominant,and the efficiency of the unsaturated zone in intercepting groundwater enhances accordingly.The shape of the unsaturated zone is approximately oval in the two-dimensional plane.The length of the unsaturated zone formed by multiple injection points is not a simple superposition of the unsaturated zone formed by one single injection point,and the expansion of the length of unsaturated zone is not symmetrical but tends to be in the direction of the downstream slope owing to the influence of groundwater flow.