Guimin Zhang

Failure mechanism of bedded salt formations surrounding salt caverns for underground gas storage

Understanding the failure mechanism of bedded salt formations surrounding salt caverns is of great importance for underground gas storage. However, laboratory mechanical experiments of cores alone are insufficient to determine the mechanical properties of bedded salt formations, because the stress state of the cores varies spatially, and man-made damage might have occurred during the coring process, especially at the interfaces. Therefore, both physical simulation experiments and numerical analyses are needed to better understand the failure mechanism of bedded salt formations surrounding salt caverns. According to the physical simulation tests, the uniaxial and triaxial compressive strength curves of bedded salt rocks appear to be U-shaped as the dip angle changes, implying that shear failure may occur more easily at the top and bottom haunches of the cavern than elsewhere. Numerical analyses show that plastic zones occur initially at the top and bottom haunches of the cavern, which is accordance with the physical test results and theoretical analyses. For the two simulated models, the plastic zones in the interlayers tend to expand towards the model boundary to induce the instability of the salt cavern, particularly at the middle of the cavern with soft and weak interlayers after years of creep. Conversely, the plastic zones in the rock salt begin to occur at the top and bottom haunches of the cavern and then expand gradually to other places, albeit with a limited scope. The results suggest that the creep of rock salt can lead to the failure of interlayers in bedded salt formations, thereby affecting the stability of salt caverns.

Rock Drillability Assessment and Lithology Classification Based on the Operating Parameters of a Drifter: Case Study in a Coal Mine in China

Groundwater inrush and gas outburst pose serious threats in the coal industry of China (Bai et al. 2013). Among the many measures taken to prevent such risks, borehole drilling is the most widely used preventive measure. Many cases have shown that the accidents frequently occur near the zones of the geological conditions undergoing great change, such as faults and lithological interfaces. In practice, there are currently two methods used to determine these geological conditions. The first method is by using a drilling core, and the other is by an estimation according to the stone dust produced during the drilling of the boreholes. Since the first method has been found to be inefficient, the second method is the most widely used. However, the results of the second can be heavily influenced by the method operators. With regard to different rock types, the operating parameters of a drifter, such as the rotation and penetration velocities, may vary. Therefore, it may be possible to assess the rock drillability and geological conditions based on the operating parameters of a drifter. This idea is the motivation for the present research study. The previous rock drillability indices (Zare and Bruland 2013) were indirect measures of the rock drillability, and determined based on the laboratory tests of rock cores. Only a few research studies have focused on the determination of the instantaneous rock drillability according to the operating conditions of a drifter, which is an engineering requirement in coal mines (see aforementioned statement), and this is the subject of the present study.

Collapse mechanism of the overlying strata above a salt cavern by solution mining with double-well convection

In solution mining of salt formations, unreasonable salt cavities formed may lead to surface collapse hazards. In this paper, a mathematical model was proposed to analyze the collapse mechanism of the overlying strata above a salt cavern induced by solution mining with double-well convection. In the proposed model, the collapses of the overlying strata were supposed to occur layer by layer, and a thin plate with four edges clamped was introduced to calculate the critical collapse span of each layer. The limit breaking distance of the thin plate can be solved by setting the corresponding surrounding condition. According to the solution, the limit breaking distance is related to the dimensions, the mechanical properties of the rock, the buried depth, and the force status. For the convenience of calculation, a span criterion was introduced to distinguish the limit breaking distance. To keep the immediate roof more stable, the span criterion should be larger. As a case study, the collapse incidents at Dongxing Salt Mine were analyzed by the proposed model, and the collapses were verified to be inevitable under its mining and geological conditions. Discussions were finally carried out to study the influences of the thickness of the immediate roof, tension strength, Poisson ratio, and buried depth on the collapses. Above all, the collapses will occur more easily with the decrease of the thickness, tension strength, and Poisson ratio of each stratum. Especially, the collapse depth will not increase linearly with the buried depth, because of the bulking effect of the overlying strata.

The Load Capacity Model and Experimental Tests of a New Yielding Steel Prop

As the mining depth increases year by year, the deformation and failure of deep roadway become more and more serious, and new support equipment with high supporting force and yieldable character is quite necessary for mining safety. In this research, a new yielding steel prop with high stable load capacity was introduced, which features sustaining large deformation in the field. Based on principle stress method and elastic-plastic theory, a mathematical model of load capacity was proposed for the new prop. The results show that the stable load capacity of the prop increases linearly with the increase of the effective number of the steel balls. Meanwhile, the stable load capacity of the prop increases initially and decreases afterwards with the increase of the radius of the steel ball. Under the fixed radius of the steel ball, the stable load capacity will increase with the decrease of the gap between the inner tube and the outer tube. The stable load capacity of the prop calculated using the theoretical model quantitatively agrees with that of the experimental tests, with only an error within 5%.

Mechanism of collapse sinkholes induced by solution mining of salt formations and measures for prediction and prevention

In salt mining, the salt caverns formed by solution mining may lead to collapse sinkhole disasters. Predicting and preventing this type of sinkholes is a real and urgent problem. In this paper, three collapse sinkholes at the Dongxing Salt Mine were taken as examples to investigate the collapse mechanism of overlying strata above salt caverns induced by solution mining. Geophysical exploration was firstly carried out, with results showing that the overlying strata dropped down layer by layer; the prediction measures concluded that whether the anhydrite roof is destroyed or not can be treated as a marker for the identification of a salt cavity anomaly. Then, a mechanical model for the anhydrite roof was established. According to the model, major factors that may affect the stability of the anhydrite roof were identified and qualitatively analyzed, including the pressure decrease of the brine, the strength decrease of the rock in the roof under the immersion of brine, and the increase of horizontal stress in the roof. Numerical simulation was carried out to further analyze the collapse process and the influencing factors. Some prevention technologies were drawn as follows: (1) Mining of salt formations should be prohibited within the impact scope of the weak structural zone; (2) As regards the protective layer, a certain thickness of salt formation above the salt cavity should not be mined to prevent the dissolution upward; (3) An oil pad should be adopted to reduce or even avoid the contact of brine and roof; (4) Appropriate brine pressure should be maintained to improve the stability of the salt cavity when it is abandoned. The above prediction and prevention measures are not only applicable for halite but also suitable for other minerals obtained by solution mining, such as natural soda, Glauber’s salt, and others.