Showing papers on "Groundwater-related subsidence published in 2016"

Water availability and land subsidence in the Central Valley, California, USA

Abstract: The Central Valley in California (USA) covers about 52,000 km2 and is one of the most productive agricultural regions in the world. This agriculture relies heavily on surface-water diversions and groundwater pumpage to meet irrigation water demand. Because the valley is semi-arid and surface-water availability varies substantially, agriculture relies heavily on local groundwater. In the southern two thirds of the valley, the San Joaquin Valley, historic and recent groundwater pumpage has caused significant and extensive drawdowns, aquifer-system compaction and subsidence. During recent drought periods (2007–2009 and 2012-present), groundwater pumping has increased owing to a combination of decreased surface-water availability and land-use changes. Declining groundwater levels, approaching or surpassing historical low levels, have caused accelerated and renewed compaction and subsidence that likely is mostly permanent. The subsidence has caused operational, maintenance, and construction-design problems for water-delivery and flood-control canals in the San Joaquin Valley. Planning for the effects of continued subsidence in the area is important for water agencies. As land use, managed aquifer recharge, and surface-water availability continue to vary, long-term groundwater-level and subsidence monitoring and modelling are critical to understanding the dynamics of historical and continued groundwater use resulting in additional water-level and groundwater storage declines, and associated subsidence. Modeling tools such as the Central Valley Hydrologic Model, can be used in the evaluation of management strategies to mitigate adverse impacts due to subsidence while also optimizing water availability. This knowledge will be critical for successful implementation of recent legislation aimed toward sustainable groundwater use.

Analysis of Factors in Land Subsidence in Shanghai: A View Based on a Strategic Environmental Assessment

Abstract: It has been observed that in the urban center of Shanghai, land subsidence has accelerated, and the groundwater level has continued to drop even though the net withdrawn volume (NWV) of groundwater has remained unchanged since 1980. An analysis of monitoring data shows that drawdown of the groundwater level is one of the factors that have influenced land subsidence since 1980. The NWV of groundwater in urban areas, however, is not the critical factor controlling the drawdown of the groundwater level. Since the 1980s, there have been many underground works constructed in the unique strata of Shanghai, which has an interlayered structure known as a multi-aquifer-aquitard system (MAAS). Investigation into land subsidence caused by urban construction is now receiving much attention. Based on the principle of a strategic environmental assessment (SEA) for sustainable urban development, this paper presents a discussion and analysis of the factors which can influence the development of land subsidence during continued urbanization in Shanghai. The main factors include the additional loading caused by the construction of structures, the cut-off effect due to construction in aquifers, the drawdown of groundwater level caused by leakage into underground structures, and the decrease of groundwater recharge from neighboring zones. SEA is recommended for the future development of Shanghai.

An overview of integrated surface subsidence-reducing technology in mining areas of China

Abstract: Surface subsidence-reducing technologies are currently being used within coal mining areas in China according to the requirements of harmonious development of coal mining and environment protection. Here, these technologies were overviewed, including the strip mining, backfill mining, overburden-separation grouting and vacant space grouting of caving rocks, and the problems in their applications were analyzed. It is proposed that integrated surface subsidence-reducing technology can rapidly develop in China, while aiding the safe development of coal mining areas. The theories and application principles of five types of integrated surface subsidence-reducing technologies were introduced, including strip mining combined with overburden-separation grouting, strip mining combined with vacant space grouting of caving rocks, strip mining combined with overburden-separation grouting and vacant space grouting of caving rocks, overburden-separation grouting combined with vacant space grouting of caving rocks, and strip mining combined with backfill mining. Also an engineering application of strip mining combined with backfill mining in Daizhuang coal mine was introduced, and the application results show that the surface subsidence coefficient was 0.08 and more than 48.8 Mt of coal were recovered, which confirmed that integrated technologies could effectively reduce surface subsidence and waste of coal resources. Finally, the prospects for the development of integrated technologies were discussed, and the shortcomings of theory, effective evaluations and practical applications of these integrated technologies were examined.

Progression and mitigation of land subsidence in China

Abstract: Land subsidence was first observed in Shanghai nearly a century ago, in 1921. Land subsidence attributed to groundwater extraction has been severe in China and is still occurring. Recent subsidence and associated earth fissures occurring since 2000 in three principal regions—the North China Plain, Fenwei Basin and Yangtze Delta—are introduced, and historical subsidence in these areas is briefly summarized. The subsidence-affected area in these regions with cumulative subsidence greater than or equal to 200 mm is more than 90,000 km2 and covers 22 provinces (cities), which include intensively developed and densely populated areas. Earth fissures accompanying the subsidence create significant geohazards; more than 1,000 earth fissures have been identified in the North China Plain, Fenwei Basin and Yangtze Delta. Effective land-subsidence-monitoring networks, that include continuous global positioning system (GPS) stations, repeat GPS and leveling surveys of geodetic benchmarks, InSAR, borehole extensometers, and groundwater observation wells, have been established in these three subsidence-affected areas. Mitigation measures and administrative means have been implemented in some areas, with good results in the Yangtze Delta area.

Research on the surface movement in a mountain mining area: a case study of Sujiagou Mountain, China

Abstract: Landslides caused by underground mining are very common in mountain mining areas, and thus threaten human lives and assets. Under the superposed influence of underground mining and landslides, surface movement in mountain areas is different from that in flat areas. Further understanding of the law of surface movement in mountain mining areas can protect the locals and their assets. In this study, an improved survey technique called spherical fitting real-time kinematic was employed to monitor surface movement regularly, and similar material simulation was used as a supplement to reveal the process of strata failure. Results showed that the ground would uplift during underground mining in the valley bottom areas, an action that caused the subsidence velocity curve to appear as a skewed distribution instead of a Gaussian distribution. To improve the prediction accuracy, the prediction model of surface subsidence in mountain mining areas was updated. The mountain surface subsidence prediction system was developed using Visual Basic. Finally, the mechanism of mining landslide was studied. This work is of general interest to the professional public and it will contribute to a more objective knowledge of understanding the surface movement in mountain mining areas.

Anthropogenic and geologic influences on subsidence in the vicinity of New Orleans, Louisiana

Abstract: New measurements of ongoing subsidence of land proximal to the city of New Orleans, Louisiana, and including areas around the communities of Norco and Lutcher upriver along the Mississippi are reported. The rates of vertical motion are derived from interferometric synthetic aperture radar (InSAR) applied to Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) data acquired on 16 June 2009 and 2 July 2012. The subsidence trends are similar to those reported for 2002–2004 in parts of New Orleans where observations overlap, in particular in Michoud, the 9th Ward, and Chalmette, but are measured at much higher spatial resolution (6 m). The spatial associations of cumulative surface movements suggest that the most likely drivers of subsidence are groundwater withdrawal and surficial drainage/dewatering activities. High subsidence rates are observed localized around some major industrial facilities and can affect nearby flood control infrastructure. Substantial subsidence is observed to occur rapidly from shallow compaction in highly localized areas, which is why it could be missed in subsidence surveys relying on point measurements at limited locations.

Land subsidence and earth fissures in south-central and southern Arizona, USA

Abstract: Land subsidence due to groundwater overdraft has been an ongoing problem in south-central and southern Arizona (USA) since the 1940s. The first earth fissure attributed to excessive groundwater withdrawal was discovered in the early 1950s near Picacho. In some areas of the state, groundwater-level declines of more than 150 m have resulted in extensive land subsidence and earth fissuring. Land subsidence in excess of 5.7 m has been documented in both western metropolitan Phoenix and Eloy. The Arizona Department of Water Resources (ADWR) has been monitoring land subsidence since 2002 using interferometric synthetic aperture radar (InSAR) and since 1998 using a global navigation satellite system (GNSS). The ADWR InSAR program has identified more than 25 individual land subsidence features that cover an area of more than 7,300 km2. Using InSAR data in conjunction with groundwater-level datasets, ADWR is able to monitor land subsidence areas as well as identify areas that may require additional monitoring. One area of particular concern is the Willcox groundwater basin in southeastern Arizona, which is the focus of this paper. The area is experiencing rapid groundwater declines, as much as 32.1 m during 2005–2014 (the largest land subsidence rate in Arizona State—up to 12 cm/year), and a large number of earth fissures. The declining groundwater levels in Arizona are a challenge for both future groundwater availability and mitigating land subsidence associated with these declines. ADWR’s InSAR program will continue to be a critical tool for monitoring land subsidence due to excessive groundwater withdrawal.

Characteristics of land subsidence, earth fissures and related disaster chain effects with respect to urban hazards in Xi’an, China

Abstract: The ancient capital of Xi’an is a typical city in China with a shortage of water resources. With an urban population of 7 million, its domestic and industrial use of water depends almost entirely on groundwater. Large-scale long-term extraction of groundwater has caused severe land subsidence and triggered the reactivation of active normal faults in the Quaternary under-consolidated sediments underlying the city. This has further led to ground ruptures that have gradually evolved into earth fissure zones. During this process, there has been a close spatiotemporal relationship between land subsidence and the formation of earth fissures. From field monitoring data, the subsidence bowls are typically located in loess depressions between earth fissure zones within structural blocks of sediment cut by active Quaternary faults and preexisting fault planes. The subsidence bowls are generally elliptical in shape, with the long axes consistent with the preferred orientation of the earth fissures. The underlying Quaternary active faults and preexisting fault planes predispose the intervening sediment blocks to formation of earth fissures, and intense groundwater extraction has led to accelerated subsidence and earth fissuring during the past 50 years. Mining groundwater, land subsidence and earth fissures occur in a sequence and constitute a disaster chain with respect to urban hazards in Xi’an. Thus, it is important to understand the relationships between these events. This paper summarizes the characteristics of land subsidence and earth fissures in Xi’an and discusses the chain of relationships that connects them.

Permeability variations within mining-induced fractured rock mass and its influence on groundwater inrush

Abstract: This paper is concerned with the evaluation of permeability of fractured rock mass due to the cover stress re-establishment, which is a major factor in controlling water and gas flow rate induced by mining operations in fractured rock. The case study considered in this paper is based on the results of observations of groundwater inrush and a decrease in water inflow from the fractured roof strata due to mining advancing in the Taiping Coalmine, Shandong Province, China. A conceptual model of an effective porous media was used to assess the permeability distribution in the fractured zone induced by coal mining. The cover stress re-establishment in gob fractured rock mass was evaluated using an empirical formula based on the surface subsidence. A simplified conceptual model of the fractured zone was used to evaluate the deformation of fractured zone along with the evaluation of changes in the rock permeability above the gob due to the cover stress re-establishment. These data were then used to calculate the water inflow rate into the panel. Predicted water inflow rates have been found to be in good agreement with those from monitoring data. This study improved the understanding of the mechanisms of the post-mining cover stress re-establishment on permeability change of the overburden fracture rock strata. These results can then be applied for numerical simulations of the process of overburden failure and consequent groundwater inrush due to coal mining.

Geomechanics of subsidence above single and multi-seam coal mining

Abstract: Accurate prediction of surface subsidence due to the extraction of underground coal seams is a significant challenge in geotechnical engineering. This task is further compounded by the growing trend for coal to be extracted from seams either above or below previously extracted coal seams, a practice known as multi-seam mining. In order to accurately predict the subsidence above single and multi-seam longwall panels using numerical methods, constitutive laws need to appropriately represent the mechanical behaviour of coal measure strata. The choice of the most appropriate model is not always straightforward. This paper compares predictions of surface subsidence obtained using the finite element method, considering a range of well-known constitutive models. The results show that more sophisticated and numerically taxing constitutive laws do not necessarily lead to more accurate predictions of subsidence when compared to field measurements. The advantages and limitations of using each particular constitutive law are discussed. A comparison of the numerical predictions and field measurements of surface subsidence is also provided.

Strip Coal Pillar Design Based on Estimated Surface Subsidence in Eastern China

Abstract: Pillar design based on estimated surface (PDBES) is and its application in Daizhuang Coal Mine in eastern China is discussed. The purpose of strip coal pillar design is to use modern scientific and engineering practices to develop a safe underground mine in which the ground surface movement and deformation will be controlled to protect the surface buildings. This pillar design method is based on pre-mining estimations of surface subsidence. Surface subsidence is the main cause of potential damage to surface structures and must be estimated before coal mining.

Surface dynamic subsidence prediction model of solid backfill mining

Abstract: Solid backfill mining is a filling mining method that integrates subsidence control and solid waste disposal and is an effective way to extract coal under buildings, railways and water bodies. In this study, we propose a prediction model to accurately predict the surface dynamic subsidence process of solid backfill mining and assess mining damage. First, a dynamic subsidence function of roof in solid backfill mining was established, whose validity was verified using in situ measured subsidence data from the roof of two backfill working faces. Then, this function and the Knothe time function were combined to create a dynamic surface subsidence prediction model of solid backfilling mining, which accumulates surface subsidence caused by backfilling materials’ compression deformation at different times. Finally, an engineering case study demonstrates that the model is effective. This prediction model provides scientific reference for guidance in the design of solid backfill mining and the reduction in mining damage.

Soil and water conservation in mining area based on ground surface subsidence control: development of a high-water swelling material and its application in backfilling mining

Abstract: It has always been a crucial issue in the mining industry that the surface subsidence induced by underground coal mining can lead to the severe deterioration or loss of soil and water resources in mining area. Backfilling has become one of the most frequently used mining method to solve this puzzle, as it can effectively control the movement of overlying strata and therefore soil and water resources will not be destroyed by mining activities. A new type of high-water swelling material (HWSM) was developed to address the key issues of current backfilling materials used in collieries, such as the high cost, inadequate strength and low expansion rate. Based on the principle that the strength of backfilling body satisfies the requirement, a critical ratio of water to solid material (a mixture of fly ash which was used as an aggregate, lime, anhydrite, cement and additive ingredients) was determined which varied from 0.8:1 to 0.9:1. Meanwhile, the mass ratio of fly ash, anhydrite, lime, cement and additives was determined to be 24.6:0.8:5.1:1.8:1. Laboratory tests showed that strength of samples that met the required setting time for practical use could reach 5.39–5.52 MPa after 63 days. The expansion rate after 3 h varied between 9.9 and 10.8 % and the bleeding rate ranged from 0.3 to 0.33 %. Industrial application has been conducted in Wangtaipu colliery and results showed that the proposed HWSM had an outstanding mechanical performance with good expansion rates and fluidity. These properties indicate that the developed material is suitable for use as a good backfilling material in mining so as to effectively reduce the ground surface subsidence and to conserve the soil and water.

Surface Subsidence Analysis by Multi-Temporal InSAR and GRACE: A Case Study in Beijing

Abstract: The aim of this study was to investigate the relationship between surface subsidence and groundwater changes. To investigate this relationship, we first analyzed surface subsidence. This paper presents the results of a case study of surface subsidence in Beijing from 1 August 2007 to 29 September 2010. The Multi-temporal Interferometric Synthetic Aperture Radar (multi-temporal InSAR) technique, which can simultaneously detect point-like stable reflectors (PSs) and distributed scatterers (DSs), was used to retrieve the subsidence magnitude and distribution in Beijing using 18 ENVISAT ASAR images. The multi-temporal InSAR-derived subsidence was verified by leveling at an accuracy better than 5 mm/year. Based on the verified multi-temporal InSAR results, a prominent uneven subsidence was identified in Beijing. Specifically, most of the subsidence velocities in the downtown area were within 10 mm/year, and the largest subsidence was detected in Tongzhou, with velocities exceeding 140 mm/year. Furthermore, Gravity Recovery and Climate Experiment (GRACE) data were used to derive the groundwater change series and trend. By comparison with the multi-temporal InSAR-derived subsidence results, the long-term decreasing trend between groundwater changes and surface subsidence showed a relatively high consistency, and a significant impact of groundwater changes on the surface subsidence was identified. Additionally, the spatial distribution of the subsidence funnel was partially consistent with that of groundwater depression, i.e., the former possessed a wider range than the latter. Finally, the relationship between surface subsidence and groundwater changes was determined.

Coal pillar safety and surface deformation characteristics of wide strip pillar mining in deep mine

Abstract: The wide strip pillar mining of Tangkou coal mine in Eastern China is the engineering background in the paper. Coal pillar safety and surface deformation characteristics of wide strip pillar mining in deep mine are studied by using in situ measurement, three-dimensional similar material simulation test, and theoretical analysis. Results show that in deep mine, the plastic zone’s width of coal pillar becomes larger obviously and the vertical stress of coal pillar presents a saddle-shaped distribution. The lateral deformation of coal pillar mainly concentrates on the area 9.5 m from the edge of coal pillar, which is discontinuous, stepped, and mutational. In addition, if only one working face with strip pillar mining is mined, the influence extent caused by coal mining is weaker and the surface subsidence and curvature are smaller. If multiple working faces with strip pillar mining are mined, the surface subsidence is obvious, the subsidence factor is 0.22, the subsidence basin is larger but gentler and uniform, and the surface deformation is continuous and long term. On the basis of the characteristics of wide strip pillar mining in deep mine, the safety of wide coal pillar in deep mine is checked.

A Unified Model for Soil Shrinkage, Subsidence,and Cracking

Abstract: Many clayey soils shrink as they dry, causing a shift of porosity from inside to outside the soil aggregates and leading to the formation of shrinkage cracks and/or surface subsidence. During swelling, shrinkage cracks begin to seal and/or the soil surface rises. Previous models have focused on describing shrinkage at the aggregate level, with little success in predicting soil cracking and subsidence. To remedy this shortcoming, we provide a unified, physically based set of governing equations for these three pore domains (aggregates, cracks, and subsidence) and predict the porosity distribution among domains as a function of soil water content and minimal (up to six) additional parameters. Examples collected from a variety of soils show how these functions describe shrinkage of soil samples in the laboratory; quantify the relationships among soil suction, soil shrinkage, and water content using the same set of parameters; and predict sealing of soil cracks in the field. This approach provides the framework for accurate and unified hydromechanical modeling of swelling soils.

Research and application of roadway backfill coal mining technology in western coal mining area

Abstract: In China’s western coal mining area, the traditional room mining technology is facing coal pillar instability, mine earthquake, large-area roof subsidence in the goaf, surface subsidence, water and soil loss, vegetation deterioration, and other environmental problems. To solve the aforementioned problems and to improve coal recovery, the roadway backfill coal mining (RBCM) method was proposed as a solution and its technical principle and key equipment were presented in this paper. In addition, the microstructure and mechanical behavior (strain-stress relation in confined compressive test) of aeolian sand and loess backfill materials were studied for a rational backfill design for underground mines. Further, coal pillar stress, plastic zone change, and surface deformation of the RBCM schemes were studied using the FLAC3D numerical simulation software, and a reasonable mining scheme of “mining 7 m and leaving 3 m” was determined. The engineering application in Changxing Coal Mine shows that the RBCM method with loess and aeolian sand as backfill materials allows a stable recovery of coal pillars with a recovery ratio of more than 70 %. The maximum accumulated surface subsidence and the maximum horizontal deformation were measured to be 15 mm and 0.8 mm/m respectively, indicating that the targeted backfilling effect can help protect the environment and also control surface subsidence.

Combining differential SAR interferometry and the probability integral method for three-dimensional deformation monitoring of mining areas

Abstract: With the exploitation of coal resources, ground surface subsidence continues to occur in mining areas, destroying the ecological environment and significantly affecting the daily productivity and life of humans. The differential synthetic aperture radar interferometry D-InSAR technique is widely used to monitor ground surface deformation because of its unique advantages such as high accuracy and wide coverage. However, conventional D-InSAR technology provides only one-dimensional 1D displacement monitoring along the radar line of sight LOS. This article proposes a method based on an analysis of the mining subsidence law for true three-dimensional 3D displacement monitoring by combining D-InSAR and a subsidence prediction model based on the probability integral method. In this approach, 1D displacement, obtained using D-InSAR, is then combined with the prediction model to obtain the 3D displacement of ground surface target points. Here, 3D displacement curves were obtained for the Fengfeng mining area China using RadarSat-2 images obtained on 9 January and 2 February 2011. True ground surface displacement was measured simultaneously by levelling when the 152under31 s working face was being exploited in Jiulong mine. Vertical displacement and inclined deformation calculated using the proposed method were compared with levelling survey data and the results showed average differences of 3.2 mm and 0.1 mm m−1, respectively; the calculated maximum displacement in the east–west and south–north directions were 106 and 73 mm, respectively. The spatial distribution of the displacements was in accordance with the mining subsidence law. Thus, the new method can retrieve highly accurate 3D displacements caused by mining subsidence.

Strata movement and surface subsidence prediction model of dense solid backfilling mining

Abstract: Surface subsidence is the most common disaster in coal mining areas, which threatens human lives and assets. In this study, a prediction model is proposed to predict the surface subsidence caused by dense solid backfilling mining. Firstly, the surface subsidence is monitored by high-resolution survey and its static and dynamic characteristics are analyzed. Then, the universal law of strata movement and load distribution on the main roof are analyzed by similar material simulation and numerical simulation, respectively. The strata movement and deformation of dense solid backfilling mining are totally different from caving mining. There is no caving zone but a limited height of fractured zone developed in the strata. The load distribution on main roof follows the piecewise function. Finally, the prediction model is created based on the combination of elastic foundation beams and volume invariant transfer principle, and the prediction values basically coincide with measured values. This prediction model can provide reference data for the evaluation and management of coal mining hazards.

A new method of surface subsidence prediction for natural gas storage cavern in bedded rock salts

Abstract: The surface subsidence above underground natural gas caverns in salt formations is mainly induced by the volume shrinkage of the cavern. In this paper a new method, based on the Mogi model, to predict the subsidence above bedded rock salts gas storage cavern is proposed. Firstly, the equivalent elastic boundary deformation of the cavern volume decrease is assumed to be the first-order approximation of the real subsidence induced by creep shrinkage. Secondly, the cavern shape is simplified to be a spherical one with the same depth and volume, subjected to the pressure change of hydrostatic pressure in the sphere. And then, solving for the subsidence above, the storage is analyzed as the boundary displacement problem of the spherical cavern subjected to pressure change of hydrostatic pressure in the sphere in semi-infinite space. Based on the equivalent elastic model, the Mogi model, which has been developed for subsidence prediction in volcano seismology, is introduced to solve the boundary displacement problem. Then the expressions for subsidence and for horizontal displacement are obtained. The advantage of the new method is that the ground displacements can be calculated directly from the cavern volume shrinkage, thus the calculation has much been simplified. The validity of the method is verified by comparing to numerical simulation results, which show that the Mogi model has high validity for subsidence prediction for the gas cavern in salt formations. In the end, some constructive discussions on the application and improvement of the Mogi model are presented for its further applications.

Subsidence prediction and susceptibility zonation for collapse above goaf with thick alluvial cover: a case study of the Yongcheng coalfield, Henan Province, China

Abstract: Mine collapse is a common geological hazard associated with mining areas. This study analyzes mine collapse above the Yongcheng mining area located on the Huanghuai Plain in eastern Henan Province, China. The aim is to predict surface subsidence and evaluate associated disaster risks in goaf with thick alluvial cover. The surface deformation above seven mined coalfields and six unmined coalfields was calculated using the probability integration method. The results showed that the final maximum ground subsidence would be 7.25 m for the Suburban mine and 5.3 m for the Xinzhuang mine. As part of a broader study, land subsidence was also measured over a 1-year period in 2012–2013 by interferometric point target analysis using Radarsat-2 satellite synthetic aperture radar. Interferometric displacement maps were validated with leveling data. Based on the principles of fuzzy mathematics and the analytical hierarchy process, a susceptibility assessment system was developed to define the risk from mine collapse for the coalfields across the mining area. A hazard-zoning map was also produced using the spatial analysis function of ArcGIS. These research results can serve as a reference for farmland reclamation, town planning, and the restoration of the natural environment in this area.

Square-form structure failure model of mining-affected hard rock strata: theoretical derivation, application and verification

Abstract: The coal-bearing strata consist of rock and coal strata with diverse thickness or mechanical strength, and thus they, especially the thick–hard rock strata, are distinguishing in movement and failure laws during coal mining. The failure of the thick–hard rock strata may affect the strata stability and cause serious coupled dynamic hazards and environmental problems. In the paper, the failure model of thick–hard rock strata was derived theoretically, which was applied to calculate the failure laws of the thick–hard igneous sill, and then, the model was verified by the methods of numerical simulations and surface subsidence measurements. The results indicate that: (1) the failure laws of the hard roof strata were analyzed based on the four-edge-clamped thin-plate assumption, and thus, the square-form structure failure of the hard roof stratum, which case is defined as the SSF model, occurs most possibly with the square gob for larger normal stress components; (2) the SSF model was applied to calculate the failure laws of the thick–hard igneous sill, and it would be broken with the approximate square-form gob of 310 m long and 350 m wide in Yangliu Coal Mine, Huaibei Coalfield, China; and (3) both the simulation results and the surface subsidence measurements indicate that the initial failure length of the thick–hard igneous sill was about 320 m with the square-form gob, verifying the correctness of the SSF model. The results could provide the theoretical guide for mining rock strata control and dynamic hazards prevention.

Surface subsidence characteristics of grout injection into overburden: case study of Yuandian No. 2 coalmine, China

Abstract: A study on the surface subsidence characteristics is essential for evaluating the effects of subsidence control technologies and improving such technologies. One such technology is grout injection into overburden (GIO), which is used in coal mining areas. In the past, the subsidence reduction ratio—a single index—was often used to evaluate the final amount of subsidence control achieved by GIO; however, little research has been conducted on the dynamic process of surface subsidence. In this study, the surface subsidence characteristics of GIO on the side of a stopping line (SSL) of a longwall were examined through in situ monitoring, and the characteristics of areas with grouting were compared with those without. The final maximum subsidence, horizontal displacement, and subsidence rate decreased considerably (70.2, 80.4, and 77.5 %, respectively) with the use of GIO. However, 4.6 % of the mining height subsided at a certain surface point on the SSL before GIO controlled subsidence at that point. Compared with the duration of the active period without grouting, that with grouting and the corresponding subsidence decreased considerably (44.2 and 87.1 %, respectively). Generally, it is quite difficult to control the surface subsidence when GIO is implemented on the SSL of a longwall because the surface and subsurface are already affected by the coal extraction, without grouting on the side of the open-off cut. Thus, the success of GIO technology in the case study described in this paper demonstrates its effectiveness in controlling the surface subsidence in coal mining areas.

Numerical Study of Mine Site Specific Multiseam Mining and Its Impact on Surface Subsidence and Chain Pillar Stress

Abstract: This paper investigates various multiseam mining related parameters using mine site specific data and numerical simulations. Two important mining effects—subsidence and stress—are analysed for different possible mining layouts. A geological mine dataset has been used to generate a numerical model. The predicted surface subsidence magnitude and surface profile have been compared under different scenarios to assess potential options in multiseam mining strategies. The effects that seam separation distances, mining offset, panel layout and panel orientation each have on surface subsidence and chain pillar stress magnitude have been investigated. The numerical simulation shows that ascending or descending mining directions have little impact on controlling the surface subsidence in multiseam mining and predicted an almost identical maximum stress development at the chain pillars. Numerical simulations infer that the orientation of the top panels control the subsidence profile.

Deficiencies in 2D Simulation: A Comparative Study of 2D Versus 3D Simulation of Multi-seam Longwall Mining

Abstract: The reliable prediction and management of mining-induced surface subsidence is one of the environmentally challenging issues for the coal mining industry. Because coal mining companies operate under strict environmental accountability, the absence of robust and reliable analysis tools may significantly affect the industry’s ability to gain approval and licenses when significant surface subsidence issues are involved. This issue becomes even more critical in multi-seam mining conditions, where high-stress concentration and large amounts of surface subsidence are expected to generate during multi-seam mining, hence could affect the feasibility and safety of all seams being mined. To obtain mining approval, it is, therefore, imperative to understand the geomechanical effect of mining in one seam on the mining of the underlying/overlying seams, and to accurately predict the magnitude and profile of surface subsidence. Various computer programs using empirical or numerical approaches have been developed to estimate the stresses at pillars and coal seams during multi-seam mining (Bigby et al. 2007; Ellenberger et al. 2003; Mark et al. 2007; SCT 2010; Sears and Heasley 2013). However, empirical-based models have severe limitations, which often make them inapplicable for assessing the feasibility of multi-seam mining at green field sites. Instead, numerical simulations are widely employed for this purpose. Due to the complexity of the problems and the computational times, researchers and engineers generally resort to twodimensional (2D) simulations. The present study assesses the performance of 2D and 3D numerical simulations and presents comparisons of subsidence profiles and stresses in pillars obtained during multi-seam mining. We modeled two different seams, each with four mining panels, using an in-house, 3D, finite element code called COSFLOW (Adhikary et al. 1996; Adhikary and Guo 2002). A unique feature of COSFLOW is the incorporation of Cosserat continuum theory in its formulation (Cosserat and Cosserat 1909). In the Cosserat model, interlayer interfaces (i.e., joints, bedding planes) are considered to be smeared across the mass. In other words, the effects of the interfaces are incorporated implicitly in the choice of stress–strain model formulation. The Cosserat model incorporates the bending rigidity of individual layers in its formulation, unlike other conventional implicit models. COSFLOW produced very accurate results when simulating surface subsidence due to longwall mining at Appin Colliery in New South Wales in Australia (Guo et al. 2004).