Rotation Of Strata Research Articles

Physical simulation and theoretical evolution for ground fissures triggered by underground coal mining.

Underground coal mining activities are prone to cause movement and breakage in geological strata and also lead to mining subsidence and even ground fissures. Along the direction working panel advancing, ground fissures may occur in roof in front and/or behind working panel. However, the investigations of previous similarity tests in lab only emphasize on the region behind working panel. By improving strata material property in construction and mounting artificial pressure devices, two physical simulation tests were conducted and successfully investigated the simulated results. Then, the mechanical model of “cantilever beam and elastic foundation beam” was proposed to calculate the stress distribution and the crack initiation angle in overlying strata and it well explains the mechanisms of ground fissures generation and propagation. Results show that, the maximum internal force in roof always occurred in front of working panel. However, because the void space in gob due to excavation is large enough to cause the bend and rotation of roof strata, compare to the triaxially compressed region in front of working panel, the roof always broke off at some positions above gob since the stress concentration resulting from such bend and rotation of strata could easily reach the limit strength of strata rocks. Also, the length of cantilever beam changed dynamically as respect to the panel advancing and the breakage intervals. Thus, the breakage position where the internal force first reached the limit tensile strength is not fixed and there will be two different kinds of relative positions between the crack initiation point and the working panel. The crack initiation direction is always perpendicular to the internal force, and the crack propagation direction is affected by the initiation angle, overburden-separation degree and the position of the hydraulic shields. If there is no overburden-separation or less, the roofs will break off as a composite beam and the propagation direction will be roughly along the central line between the initial broken point and the support position. Otherwise, the roof strata will bend with the support shields moving forward, then the fracture angle will be close to the initiation angle and the fault surface will be stepped.

Structural Development of the Williams Range Thrust, Colorado

An area in southwestern Middle Park, Colorado, has been studied in the field. Mapped principal structural features include the Williams Range thrust and a major normal fault east of the thrust. Direct observation of a cross section of the thrust surface, data on its elevation and its trace, an observation of its dip, and an observation on rotation of strata in the overblock show that the thrust surface is curved, being nearly horizontal in its westernmost exposures with progressively steeper eastward dips to the east. The mapped structural features postdate deposition of the Pierre Formation, and the thrust is known to have experienced recurrent movement. This paper explains the development of the mapped principal structural features in terms of a system of stresses acting on the boundaries of the area. Because east-west cross sections are similar, an explanation is offered for an average cross section of, specifically, height equal to 6500 feet and length equal to 32,500 feet. Analysis is treated as a two-dimensional problem. The system presented consists of a combination of two systems of stresses. One is characterized by a step displacement of the midpoint of the lower boundary, the other by a cosine displacement of the lower boundary. The analysis assumes that stress within the body is describable by methods of theory of elasticity and that faults and stress are related by the Mohr criterion. The analysis also assumes that faulting does not affect the orientation of principal stresses as described at the instant prior to inception of faulting and that it tends to increase the magnitudes of stress near its tip and decrease them where faulting has transpired. The system accounts for the attitude of faulting in the average cross section and predicts fault development where faulting has occurred. The explanation is compatible with lesser structural features of the overblock of the area, including high-angle faulting at the north and synclinal development in the youngest Tertiary rocks between the thrust and major normal fault. The explanation is compatible with the regional structural geology and is geologically practicable.