Surface subsidence causes damage such as the failure and deterioration of buildings, infrastructures, dams, underground utility lines, ground water regimes, etc., resulting in severe economic loss and environmental hazards. The major cause of subsidence is underground mining activities. In order to minimize or prevent subsidence damage, it is necessary to understand subsidence phenomena. It is difficult to simulate or predict subsidence development because of the complexity in physical characteristics such as rock failure and yield behavior, dimensional variations and time dependent behavior. In this paper a new physical subsidence modeling technique is introduced. The method utilizes laser optical triangulation distance measurement devices, which can scan the surface of any material, including granular or viscous materials, and digitally measure vertical distances with an extremely high accuracy and resolution. With this new technique, the effect of cavity shape and size, depth, and material parameters can be analyzed. Using this unique technology and method of analysis, significant results were produced. Subsidence profiles, subsidence factors, and angles of draw were analyzed. Further research is being continued using the same technique for simulating subsidence with different model materials for various underground cavity dimensions, tunneling, and time dependent subsidence phenomena.
When underground excavation is performed over a significant area, the overlying rock mass subsides into underground cavities.. The majority of subsidence results from underground coal mining that employs longwall and room-and-pillar mining methods. Subsidence may also be caused by failures of tunnels and underground cavities naturally created in limestone rich formations. Subsidence is also caused by withdrawal of fluid, such as ground water or oil. However, the most significant subsidence problems result from underground mining. Surface subsidence damages surface structures such as foundations, utility lines, infrastructures, ground water regimes, etc. In order to avoid or reduce subsidence damages, it is imperative to know the subsidence characteristics of the particular site, and proper design work has to be performed to prevent or minimize subsidence hazards. There are two types of subsidence: (1) pit, also called sinkhole or pot hole, and (2) trough or sag. 1]. Pit subsidence is characterized by an abrupt sinking of the surface, resulting in circular steepsided, crater-like features. Trough subsidence is a gentle, gradual depression of the surface. Subsidence is controlled by many factors, including width of unsupported cavity, height of cavity, thickness of overburden, strength and fracture system of rock, hydrology, and time. For the last several decades, various methods of analysis have been utilized for interpreting and predicting subsidence phenomena. The principal methods of interpreting and predicting subsidence can be grouped as follows [2,3].
Empirically derived relationships
The empirically derived relationships are determined based on large numbers of observations and case studies. The profile functions method and the influence functions methods use equations derived from field measurement data. Analytical modeling consists of theoretical and numerical models that treat subsidence as a problem involving the laws of elasticity, plasticity, and visco-elasticity . Finite element methods are the most commonly used in this group [3-6].