Distribution Measurement and Mechanism Research on Deformation due to Losing Water of Overburden Layer

Abstract Based on FBG fiber Bragg grating technology and BOTDA distributed optical fiber sensing technology, this study used fine sand to simulate overburden layer in vertical shaft model equipment. The placing technique and test method for optical fiber sensors in the overburden layer were studied, combined with MODFLOW software to simulate the change of the water head value when the overburden layer was losing water, and obtained the deformation features of overburden layer. The results showed that at the beginning of water loss, the vertical deformation increased due to larger hydraulic pressure drop, while the deformation decreased gradually and tended to be stable with the hydraulic pressure drop reducing. The circumferential deformation was closely related to the distance between the drainage holes, variations of water head value and the method of drainage. The monitoring result based on optical fiber sensing technology was consistent to the characteristics of water loss in overburden layer simulated by MODFLOW software, which showed that the optical fiber sensing technology applied to monitor shaft overburden layer was feasible

A DOFS-Based Approach to Calculate the Height of Water-Flowing Fractured Zone in Overlying Strata under Mining

The distributed optical fiber sensing (DOFS) is a technique that can obtain full spatial and temporal information concerning the behavior of a large range of measurand fields along a fiber path and realize the distributed monitoring of the overburden section under mining. To calculate the height of water-flowing fractured zone caused by the exploitation of coal, this study employed distributed optical fiber sensors with OSI-C-S optical frequency domain reflectometry (OFDR) technology and designed a similar-material model test based on the engineering geological conditions of Daliuta Coal Mine. Through the test, deformation characteristics of overlying strata were studied, the linear relationship was summarized between the strain gradient and the shear stress measured by fiber sensors when the rock layer cracks, and a method was proposed of using the measured strain to measure the height of the water-flowing fractured zone in overlying strata. The test results show that there are several locations where the sign of the shear stress changes (positive to negative or vice versa) in the overlying strata during the initial stage of coal seam mining. As the working face advanced, the change locations gradually concentrated at the place where the rock cracks. By identifying the breakpoints of the rock and the locations where the sign of the shear stress measured by fiber sensors changes, this paper calculated the height of the water-flowing fractured zone in Daliuta Coal Mine. After comparing the height with the abscission layer position in the model test and the predicted height by the empirical formulas in the specification, it has been found that the three results are basically consistent, which in turn verifies the accuracy of this method

A Study on Distribution Measurement and Mechanism of Deformation due to Water Loss of Overburden Layer in Vertical Shaft

Based on FBG fiber Bragg grating technology and BOTDA distributed optical fiber sensing technology, this study uses fine sand to simulate overburden layer in vertical shaft model equipment. It studies the placing technique and test method for optical fiber sensors in the overburden layer, combined with MODFLOW software to simulate the change of the water head value when the overburden layer is losing water, and obtains the deformation features of overburden layer. The results show, at the beginning of water loss, the vertical deformation increases due to larger hydraulic pressure drop, while the deformation decreases gradually and tends to be stable with the hydraulic pressure drop reducing. The circumferential deformation is closely related to such factors as the distance between each drainage outlet, the variations of water head value, and the method of drainage. The monitoring result based on optical fiber sensing technology is consistent with the characteristics of water loss in overburden layer simulated by MODFLOW software, which shows that the optical fiber sensing technology applied to monitor shaft overburden layer is feasible

Experimental and Numerical Investigation of Sinkhole Development and Collapse in Central Florida

The mechanisms of sinkhole formation, development, and collapse are investigated in this study using experimental and numerical methods. Sandbox experiments are conducted to understand how excessive groundwater pumping triggers sinkholes formation. The experimental results indicate that the change of hydrologic conditions is critical to sinkhole development. When seepage force increases due to increase of hydraulic gradient, clay and sand particles start moving downward to form a cavity. The confining unit is of particular importance because the cavity is first formed in this layer. Based on the conceptual model developed from the sandbox experiments, the Fast Lagrangian Analysis of Continua (FLAC) code and Particle Flow Code (PFC) are coupled to simulate the sandbox experiments. PFC was used to simulate particle movement in the sinkhole area, and FLAC is used for other areas. While the current numerical simulation can simulate the experiment results such as the sizes of the cavity and the sinkhole, the simulation capability is limited by the computing cost of PFC. More effort of model development is necessary in the future study

Experimental and Numerical Investigation of Sinkhole Development and Collapse in Central Florida

The mechanisms of sinkhole formation, development, and collapse are investigated in this study using experimental and numerical methods. Sandbox experiments are conducted to understand how excessive groundwater pumping triggers sinkholes formation. The experimental results indicate that the change of hydrologic conditions is critical to sinkhole development. When seepage force increases due to increase of hydraulic gradient, clay and sand particles start moving downward to form a cavity. The confining unit is of particular importance because the cavity is first formed in this layer. Based on the conceptual model developed from the sandbox experiments, the Fast Lagrangian Analysis of Continua (FLAC) code and Particle Flow Code (PFC) are coupled to simulate the sandbox experiments. PFC was used to simulate particle movement in the sinkhole area, and FLAC is used for other areas. While the current numerical simulation can simulate the experiment results such as the sizes of the cavity and the sinkhole, the simulation capability is limited by the computing cost of PFC. More effort of model development is necessary in the future study

A GIS-Based Probabilistic Spatial Multicriteria Roof Water Inrush Risk Evaluation Method Considering Decision Makers’ Risk-Coping Attitude

A combination of geographic information system (GIS) and spatial multicriteria decision making (MCDA) in mine water inrush risk evaluation is widely used, but the randomness in the process of index weight determination and the risk-coping attitude of decision makers are not considered in the decision making process. Therefore, this paper proposes a probability-based roof water inrush risk evaluation method (GIS-MCDA) by combining the Monte Carlo analytic hierarchy process (MAHP) and ordered weighted averaging (OWA) operator. This method uses MAHP to determine the weight of the evaluation indicators, reducing the randomness of the analytic hierarchy process (AHP) to determine the weight of the evaluation indicators using the OWA operator to quantify the five risk-coping attitudes of decision makers and incorporate the risk attitude of decision makers into the evaluation process. Taking the Liangshuijing Coal Mine in northern Shaanxi as an example, the application of the GIS-MCDA method showed that the method makes the risk results of roof water inrush more objective and comprehensive and reduces or avoids the risk of decision making due to human subjective tendency change