针对珠江口重大工程建设中浅层气砂土渗流灾害防控的迫切需求,旨在建立基于物理机制的非饱和水力特性高效预测方法,以解决传统试验周期长、成本高的技术瓶颈,基于珠江口含浅层气的典型储气层砂土,开展压力板仪SWCC测试以及非饱和渗透试验测试。基于等效毛细管模型理论,将土体抽象为多级连通孔隙网络,在由小到大的基质吸力作用下,孔隙通道由大孔径到小孔径依次排水,然后假设孔隙半径与基质吸力满足Young-Laplace 方程,建立砂土粒径分布数据与土水特征曲线、非饱和渗透系数之间的关系,提出基于土体粒径分布的砂土土水特征曲线与非饱和渗透系数的预测方法。最后将非饱和渗透系数预测结果与实测数据进行对比,初步验证了预测方法的合理性。研究成果可为滨海工程防灾减灾提供技术支撑。

[Objective] In response to the urgent need for preventing and controlling seepage disasters in shallow gassy sandy soil during major engineering construction projects in the Pearl River Estuary, this study aims to develop an efficient prediction method for unsaturated hydraulic properties based on physical mechanisms, thereby overcoming the technical bottlenecks of long testing cycles and high costs in traditional experiments. [Methods] Using typical gassy sandy soil from shallow gas reservoirs in the Pearl River Estuary, soil-water characteristic curve (SWCC) tests using a pressure plate apparatus and unsaturated permeability tests were conducted. According to the abstract conceptualization of the AP model, the soil internal structure was assumed to consist of a vast number of interconnected pore channels with different pore sizes. Based on the equivalent capillary tube model theory, under matric suction increasing from low to high, pore channels drained sequentially from larger to smaller sizes. Then, by assuming that the pore radius and matric suction satisfied the Young-Laplace equation, the relationships between sandy soil particle size distribution data, SWCC, and unsaturated permeability coefficient were established. Consequently, a prediction method for the SWCC and unsaturated permeability coefficient of sandy soil based on particle size distribution was proposed. [Results] Experimental results showed that the particle size distribution of the sandy soil was closely related to its water retention characteristics. Its air entry value (AEV) was generally below 10 kPa, and the residual volumetric water content was typically higher than that of pure quartz sand, approximately 8%. The residual matric suction corresponding to residual volumetric water content ranged from 10 to 20 kPa and was influenced by the fine particle content. The scaling factor α had a critical impact on the predictive performance of the AP model and showed a better correlation with the normalized volumetric water content (θ/θ_s). The optimal fitting equation was α=1.243 1+0.369 4exp(-4.9θ/θ_s). [Conclusions] The pore channels in the soil are categorized into n grades (r₁, r₂, r₃, …, r_n) from largest to smallest pore radius. Under unsaturated conditions, water in the larger pore channels is drained, and only channels from grades m to n (r_m, …, r_n, where m<n) remain water-filled, serving as the main pathways for unsaturated seepage. Based on the particle size distribution curves of sandy soil samples, the mass fraction of the i-th component (with a spherical particle radius of R_i) in the samples is w_i, from which the relative permeability coefficient of the sandy soil can be directly calculated without conversion via the SWCC. A comparison with a typical statistical model—the Jackson model—is conducted, which verifies that the method proposed in this study for predicting the unsaturated relative permeability coefficient of sandy soil based on the particle size distribution curve is reasonable and feasible.