Catastrophic mechanism and its prevention and control of seepage deformation and failure of mining rock mass Ⅲ: a review of water retention bulkhead

The types of seepage instability of water retention bulkhead are divided according to the analysis of typical cases of leakage instability of water retention bulkhead in mines in China. The seepage instability criterion of water retention bulkhead is reviewed, and an expression of the critical hydraulic gradient against seepage failure of water retention bulkhead and surrounding rocks is recommended. The scientific and technological issues that need to be further researched, and the contents and ideas that need to be tackled urgently are discussed. The results show that the seepage instability of water retention bulkhead can be divided into three categories, namely, the seepage instability of the surrounding rock fracture and the interface between bulkhead and surrounding rock due to seepage, the deformation and instability of surrounding rock and water retention bulkhead caused by water pressure, and the deterioration and instability of surrounding rock caused by water immersion. The parameter sensitivity analysis was carried out for the safety factor of the seepage stability in bulkhead and surrounding rock, and the results showed that the rock mass integrity coefficient and the water-conducting pathway penetration coefficient of the surrounding rock were the main influencing factors. In the design and construction, enhancing the integrity of the surrounding rock mass and improving the impermeability of the surrounding rock water pathway is the key to ensuring its anti-seepage stability. For the calculation of the critical hydraulic gradient against seepage failure of water retention bulkhead, the integrity of the rock mass, the adhesion strength between the surrounding rock and the bulkhead, the fault fracture zone, the ground stress and hydraulic environment, and the deterioration performance of the surrounding rock immersion should comprehensively be considered. The parameter sensitivity analysis shows that these factors are all highly sensitive factors, and the key parameters are the water immersion damage factor and the water-conducting pathway penetration coefficient of the rock mass; therefore, when determining the critical hydraulic gradient, it is necessary to focus on finding out the important basic data such as the hydraulic properties, the strength of structural surface strength of the surrounding rock and the water pressure applied to the retention bulkhead. The evolution mechanism of seepage field under the multi-field coupling conditions of the water retention bulkhead and surrounding rock combination, the stabilization mechanism of seepage failure at the intersection of the bulkhead-surrounding rock and the discontinuities of the surrounding rock structure are two key scientific problems that need to be solved urgently. In the next step, the geological model of the bulkhead -surrounding rock combination and its geological environment evolution, the seepage mechanism and instability criterion with high water pressure, the life-cycle safety and stability control methods should be studied

Sensitivity Analysis of Anchored Slopes under Water Level Fluctuations: A Case Study of Cangjiang Bridge—Yingpan Slope in China

This paper presents an improved slope stability sensitivity analysis (ISSSA) model that takes anchoring factors into consideration in umbrella-anchored sand and clay slopes under reservoir water level fluctuation. The results of the ISSSA model show that the slope inclination and the layout density of anchors are the main controlling factors for sand slope stability under fluctuation of the water level, while the slope inclination and water head height are the main controlling factors for slope stability in the Cangjiang bridge—Yingpan slope of Yunnan province in China. Moreover, there is an optimum anchorage angle, in the range of 25–45 degrees, which has the greatest influence on slope stability. The fluctuation of the reservoir water level is an important factor that triggers slope instability; in particular, a sudden drop in the surface water level can easily lead to landslides; therefore, corresponding measures should be implemented in a timely manner in order to mitigate landslide disasters

Experimental Investigation on Pouring Aggregate to Plug Horizontal Tunnel with Flow Water

This paper presents an experimental investigation on the main factors that influence the effects of pouring aggregate to plug a tunnel that has been inundated by groundwater to reduce the flow velocity. Moreover, a criterion for plugging the tunnel under infiltrating water to resist flow is proposed. A range analysis and analysis of variance both show that the influencing factors on the efficiency of plugging in descending order is the aggregate particle size, followed by initial velocity of the water flow, and then the water–solid mass ratio. The sedimentation process of the aggregate is likened to the deposition of solid particles into slurry in which the particles settle under gravitational force, thus accumulating at the bottom of the tunnel model due to the forces of the water flow and gravity. The critical velocity of the water that will transport the aggregate without settling can be used as a criterion to determine whether there has been a successful plug of the resistance to flow in the tunnel. The experimental results show that the critical velocity of fine aggregate is less than that of coarse aggregate, and the section with smaller sized aggregate or fine aggregate that resists water flow is flatter. In addition, the required minimum space between two pouring boreholes for a successful resistance to flow is discussed

Hydrogeological Investigation for the Assessment of Spring Pollution Due to Abandoned Mines in a Karst Area

This paper presents a hydrogeological investigation case study for assessing the sources and pathways of spring pollution in a karst area in Guizhou Province, Southern China. Our methods included hydrogeological surveys, geophysical surveys, cave detection, tracer tests, and borehole drilling. The results showed that the pollution of the Longdong Spring is controlled by the amount of rainfall, which also determines the duration of pollution. The concentration of Fe and Mn ions in the spring was found to exceed that in the code for drinking water quality (China) by more than 300 times. The investigations showed that the source of the pollution is the mine water drainage from the abandoned Feilong coal mine, which is located to the northwest of the Longdong Spring. The drainage to the spring was found to occur 24 h after the groundwater level in the goaf reached approximately +908 m above sea level. This indicates a recharge pathway between the abandoned mine and the Longdong Spring. Recommendations for grouting around the spring and sinkhole for pollution control are put forth

On-Site Monitoring for the Stability Evaluation of a Highway Tunnel above Goaves of Multi-Layer Coal Seams

This paper presents the on-site monitoring of a medium–long highway tunnel constructed above a goaf of a multi-layer coal seam, in order to evaluate and maintain safety during operation. The case study of the Tianzimiao medium–long highway tunnel in Shanxi province was conducted above a goaf of a multi-layer coal seam with typical geological and engineering conditions in China, where a total of four coal seams (seam no. 3, 8, 12, and 15) were mined out with a total thickness of up to 11 m. Methods including data collection, engineering geological survey, drilling, geophysical prospecting, testing, and on-site monitoring were adopted, and a geo-mechanical model was established to conduct the research. Stratified monitoring was applied to investigate the individual settlement and deformation of the four layers of the goaf below, and a prediction of the possible deformation in tunnel floor ground was made based on the stratified measurements. The settlement of the tunnel sidewall, the internal stress in the fractured zone of the surrounding rock, and the deformation above the tunnel entrance were also monitored, and the monitoring data were compared with the numerical simulation results for the safety evaluation of the tunnel. The results show that the current tunnel deformation values and trend are both within the safety scope of the evaluation and prediction. The stability evaluation method for the multi-layer goaf used in this paper and the long-term on-site monitoring and timely feedback during operation is helpful to ensure the safe use of the tunnels above the goaves of multi-layer thick coal seams

Influence of the Aggregate-Pouring Sequence on the Efficiency of Plugging Inundated Tunnels through Drilling Ground Boreholes

This paper presents an experimental and field investigation on the efficiency of plugging by pouring aggregate in different sequences through multiple boreholes in a tunnel with flowing water. There have been controversies surrounding the selection of the pouring order for different particle sizes of aggregates and the order in different boreholes. A visualized experimental setup is used to investigate the influence of the pouring orders on the efficiency of plugging through multiple boreholes under the flowing-water condition. A case study of the salvage of a flooded mine using ground directional boreholes was investigated and compared with the experimental results. The water-pressure difference at the aggregate-capping moment, when fine aggregate was poured first and coarse aggregate later, was relatively small, compared to that when fine aggregate was poured upstream and coarse aggregate, downstream. The result implies that the efficiency of plugging with the order of pouring fine aggregate first and coarse aggregate later in different boreholes is better than that with the order of pouring fine aggregate upstream and coarse aggregate downstream. When the poured aggregate is about to be capped, increasing the pouring intensity with the same or a larger particle size is more conducive to capping. The case study shows that pouring fine materials in the early stage reduced the cross-sectional area; in the later stage, the aggregate particle size was gradually increased, which can be helpful in forming an effective water-barrier section in the tunnel. The pouring of aggregate provided a base for cement grouting to form a water-plug section with a length of 106 m, resulting in a sealing efficiency of 100% for the case

Propagation and sealing efficiency of chemical grouting in a two-dimensional fracture network with flowing water

In this study, an orthogonal array experiment is conducted by using a transparent fracture network replica. Image processing and theoretical analysis are performed to investigate the model sealing efficiency (SE), factors influencing SE, and the effect of flowing water on propagation. The results show that grout propagation can be classified into three patterns in the fracture network: sealing off, partial sealing, and major erosion. The factors controlling the SE in a descending order of the amount of influence are the initial water flow speed, fracture aperture, grout take, and gel time. An optimal value for the combination of the gel time and grout take (artificial factors) can result in a good SE. The grouting and seepage pressures are measured, and the results reveal that their variations can indicate the SE to some extent. The SE is good when the seepage pressure at each point increases overall; the frequent fluctuations in the seepage pressure indicate a moderately poor SE, and an overall decline in the seepage pressure indicates a major erosion type. The deflection effect of grouting shows an approximately elliptical propagation with the long axis expanding along the wider fracture opening, demonstrating further application in grouting design

Changes in Particle Size Composition under Seepage Conditions of Reclaimed Soil in Xinjiang, China

The distribution of reclaimed soil particle size under seepage conditions after the management period will directly determine the success or failure of reclamation work. The geotechnical experimental method was used in this paper to study the changes in the granulometric composition of soil. The results show that the granulometric composition of the reclaimed soil varied obviously at different depths. The granulometric composition of the soil at a depth of 10 cm was not much different from undisturbed reclaimed soil (URS). At a depth of 30 cm, as the sharp decrease of the content of fine particles resulted in coarser reclaimed soil, the soil became more uniform, with an increase in porosity and water content. At a depth of 50 cm, the fine particle content was generally slightly lower than that of URS. At a depth of 70 cm, the fine particle content of the soil greatly exceeded that of the URS, with the finest soil particles and lowest porosity. The main reason for the above-mentioned changes of granulometric composition in the reclaimed soil was the seepage in soil caused by irrigation during the management period. The research results can provide a reference for management after land reclamation at non-metallic mines in Xinjiang, China