Weakened zones: Let's consider the case where the karstic rocks are overlain by an impermeable layer. In this case, two scenarios of weakening zone formation are possible. To illustrate, we assume that the karst cavity is circular in plan.
1.1) D ≤ h: The diameter of the cavity's contact area with the overlying impermeable layer is smaller than or comparable to the thickness of the layer. In this case, two forces act on the impermeable layer. On the one hand, there is the mass of the overlying soil pressing directly downward, and on the other hand, there is the upward buoyant force of karstic waters, directed opposite to gravity. When this system is in equilibrium, the structure of the overburden remains largely unchanged. Changes begin when the pressure from the soil outweighs the buoyant force of the water. Causes of such an imbalance may include:
changes in the hydraulic regime of underground karstic waters;
the additional load on the overburden due to the appearance or rise of a groundwater horizon;
pressure from newly constructed buildings;
detachment of impermeable layer particles from its base due to karstic water;
enlargement of the karst cavity.
As a result, the volume of soil above the cavity (which can be described as an inverted elliptical paraboloid) tends to move downward under the influence of gravity. This changes the stress-strain state of the entire soil mass around the cavity. The most significant changes occur near the sides of the paraboloid, where complex deformations (tensile, shear, etc.) take place, with little increase in soil porosity but with a decrease in strength properties.
1.2) D ≥ h: The diameter of the cavity's contact area with the impermeable layer is larger than its thickness but comparable to it. In this case, the impermeable layer, typically possessing plastic properties, bends inward into the cavity under the pressure of the overlying layers, altering the stress field distribution [1] and reducing soil strength. As in the first scenario, deformations occur without increasing pore volume. This situation is characterized by distinctive bowl-shaped subsidence of the impermeable layer, which is well-tracked in paleo landscape surface maps [2]. In some cases, the subsidence of the impermeable layer also affects the overlying layers, sometimes extending to the surface, where it manifests as surface subsidence.
Loosening zones: The increase in pore volume in the soil is primarily associated with the mechanical displacement of soil particles under the influence of water flow and gravitational forces. In karst regions, the activation of such transfer from the overlying layers into the karstic rocks requires the absence or breach of an impermeable layer, as well as the presence of a receiving cavity.
2.1) In karstic rocks like carbonates and sulfates, voids can range from large cavities to cracks. If karstic rocks are in direct contact with sandy soils and are not overlain by an impermeable layer, suffosion — the removal of fine particles from the overburden into open cracks and cavities by groundwater flow — is likely. This process leaves behind voids filled with liquid or gas, increasing soil porosity and decreasing density.
2.2) Often, large karst cavities are located near the roof of soluble rocks. As the cavity grows, there is a high risk of ceiling collapse due to the weight of the overburden. In the absence or insufficiency of an impermeable layer, the overlying soil mass breaks into the cavity, forming collapse voids. These voids may grow upward, eventually reaching the surface and forming sinkholes.
3) Loosening and weakening zone (mixed-type anomaly): An example would be a geological section where karstic rocks are overlain by a layer of loose sandy soil, with a clay or loam layer above it. Karstic voids primarily interact with the sandy soil. In the initial phase, loosening begins through either suffosion or collapse of the karst cavity, forming collapse voids. The loosened zone then reaches the cohesive clay soil. Depending on the ratio of the loosening zone's diameter to the thickness of the clay layer, the latter may either bend inward into the loosening zone (similar to point 1.2) or undergo stress that reduces its strength without changing its shape, as described in point 1.1.
Based on the above observations and conclusions, the author plans to develop a classification of anomalously weakened zones in the overburden soils. This classification will help to identify and predict potential karst manifestations depending on the geological structure.
