The use of a geotextile to treat subgrade subsidence after subsidence has occurred is investigated in this paper. To optimize the anchorage length and buried depth of the geotextile and evaluate the influences of the two factors on subgrade subsidence treatment, finite element analysis is performed and validated with existing model tests. The soil pressure, displacement, tensile force and deformation of the geotextile are studied. The results showed that the geotextile prevented an upward development of subsidence and stabilized the upper soil. The increase of the anchorage length of the geotextile transferred greater soil pressure from the subsidence to a stable area, induced a greater tensile force in the geotextile, and resulted in less soil displacement. As the anchorage length of the geotextile increased from 375 mm to 1500 mm, the surface settlement was effectively reduced from 1.05% to 34.18% when comparing to the situation without a geotextile. As the buried depth of the geotextile increased from 2 m to 6 m, the percentage of surface settlement was effectively reduced from 29.14% to 65.91% when comparing with the settlement corresponding to a buried depth of 2 m. It is suggested that the anchorage length of a geotextile should be the length of the subsidence with respect to width and that the buried depth of the geotextile should be 3–4 m for subsidence treatment. This provides insight into the treatment of sinkholes using geosynthetic approaches in karst areas.

A new in-soil tensile test apparatus was developed to simulate the in-soil stress condition of geosynthetics in a real working state. An in-soil tensile test with or without friction was conducted, along with in-air tensile tests of eight types of geosynthetics, to evaluate tensile properties, including tensile load and secant tensile stiffness under different confining stresses using various methods. Under confinement, the tensile properties of geosynthetics increased with growing confinement, which were greater than those for the in-air tensile test, and the tensile properties were similar for tests with or without friction. The tensile load improvement factors F_T−σ and f_T−σ were introduced to quantify the confinement and reinforcement–soil friction differences between different tests, and the improvement ratios γ₁ and γ₂ were proposed to quantify the improvement of tensile load under confinement for the test with and without friction, respectively. Two formulas were proposed to predict the confined tensile properties of geosynthetics. The results showed that confinement was the major factor that improved the tensile properties of geosynthetics compared to reinforcement–soil friction. When the improvement ratio is not less than 1.5, the confined tensile load should replace the in-air tensile load with the tensile strength design of geosynthetics, which provides an insight into the design of reinforced soil structures.

The geotextile can be used to treat a subgrade collapse in karst areas. The subsidence width is an important factor affecting the geotextile to treat subgrade collapses. However, the available studies on the influence of the subsidence width on geotextile treatment settlement are limited. To study the effect of the subsidence width on the geotextile control of subgrade collapses, the half-symmetric model test had been established. To make up for the deficiencies of the model test, the optimized subsidence width was probed through a numerical calculation under ten different situations conducted by the finite element analyses. Previous full-section model test results were used to verify the rationality of the half-symmetric model and calibrate the input parameters of the numerical models. The influence of the subsidence width on soil pressure, tensile force and deformation of the geotextile, and soil settlement was analyzed. With the increase of the subsidence width, more loads of the subsidence area were transferred to a stable area via the geotextile, the vertical normal stress at the edge increased rapidly, the tensile force of the geotextile and vertical soil displacement in the subsidence area increased noticeably. When the anchorage ratio of L ≤ 2.0B, the geotextile fracture or soil failure occurred during the model test which indicated the geotextile treatment of the subgrade collapse was not suitable for projects with an anchorage ratio of L ≤ 2.0B. The geotextile might be reaching the limit of its tensile stiffness when the anchorage ratio of L = 2.22B. This is providing an insight into the treatment of a subgrade collapse in karst areas using geotextile.