Floor Aquifer Research Articles

A two-dimensional model test system for floor failure during automatic roadway formation mining without pillars above confined water

The coal mining in North China is seriously threatened by floor water inrush from the Ordovician limestone, so it is particularly important to explore failure characteristics. Automatic roadway formation mining without pillars (ARFMP) provides an effective method for reducing floor failure. To explore failure characteristics above confined water, a two-dimensional model test system is built up. Such a system can be applied to compare the difference of floor failure between ARFMP above confined water and traditional retained coal pillar mining, and to intuitively analyze the lifting law of confined water under dynamic disturbance. The core features of this system include the customized spring sets and water bag adopted as materials to simulate the floor aquifer, the guide lift hose and branch conduit used as the lifting channel of confined water, and the flow rate and cumulative volume monitored in real time. The customized “cutting-support-blocking” integrated device is the key equipment to simulate ARFMP. Then, the model test is conducted on a typical engineering case. By comparing stress, displacement, and water inrush characteristics of floor in different areas, failure characteristics of the roof-cutting side, the middle of working face, the collapse column area, and the coal pillar side were obtained, and it was clarified that ARFMP can effectively prevent floor water inrush. According to the results of model test, a comprehensive treatment scheme combining ARFMP with the floor regional control is proposed to ensure the safe mining of working face.

Resource utilization of stone waste and loess to prepare grouting materials

Loess, a terrestrial clastic sediment, is formed essentially by the accumulation of wind-blown dust, while stone waste (SW) is an industrial waste produced during stone machining. Utilising loess and SW to prepare environmentally-friendly supplementary cementitious materials can not only address environmental issues caused by solid waste landfills but also meet the demand of reinforcement of coal-seam floor aquifer for grouting materials. In this paper, the effects of the loess/SW mass ratio and calcination temperature on the transformation of calcined products are investigated and their pozzolanic activities are evaluated. The workability, environmental impact and cost of grouting materials based on cement and calcined products are also assessed. Experimental results reveal that higher temperatures favour the formation of free lime and periclase, which tend to be involved in solid-state reactions. Higher temperature and loess/SW mass ratio strengthens the diffraction peaks of dodecalcium hepta-aluminate (C12A7), dicalcium ferrite (C2F) and dicalcium silicate (C2S). The clay minerals in loess become completely dehydroxylated before 825 °C, generating amorphous SiO2 and Al2O3. Covalent Si–O bonds are interrupted and that disordered silicate networks are generated in the calcined products, which is confirmed by the increased strength of the Si29 resonance region at −60 ppm to −80 ppm. Although co-calcined loess and SW contain the most four-fold aluminium at 950 °C, recrystallisation depresses the pozzolanic activity. Hence, the loess/SW sample designated LS2-825 exhibits the better hydration activity. Additionally, grouting materials composed of cement and LS2-825 exhibit good setting times, fluidity, strength and a low carbon footprint in practical engineering applications, and they also provide the additional benefit of being cost effective.

Evaluation of Control Effect of Confined Water Hazard in Taiyuan Formation Coal Seam Mining in Huanghebei Coalfield

The shallow-layer resources in the Huanghebei Coalfield have been depleted, and the deep coal seam of the Taiyuan Formation (lower coal group) is the main continuous resource in mining at this stage. With the annual increase in mining depth, the exploitation of lower coal groups is being met with the influence of high ground stress, high water pressure, high temperature, and strong mining disturbances, as a result of which the threat of water inrush is particularly serious. Based on the grouting data of the coalface floor aquifer, this paper proposes an evaluation method for the control of water rushing into the coalface floor. By comparing the test data with mine electrical exploration data through ArcGIS, the results show that the water pressure threshold based on ArcGIS is twice the water pressure of the grouting reconstruction layer as the optimal solution. The research results can provide a reference for the prevention and control of water inrush in the lower coal group of Huanghebei Coalfield.

Simulation of Water Influx and Gasified Gas Transport during Underground Coal Gasification with Controlled Retracting Injection Point Technology

Underground coal gasification (UCG) may change the energy consumption structure from coal-dominated to gas-dominated in the years to come. Before that, three important problems need to be solved, including failure of gasification due to large amounts of water pouring into the gasifier, environmental pollution caused by gas migration to the surface, and low calorific value caused by poor control of the degree of gasification. In this study, a geological model is first established using the computer modeling group (CMG), a commercial software package for reservoir simulation. Then, the inflow of coal seam water into the gasifier during the controlled retracting injection point (CRIP) gasification process is simulated based on the geological model, and the maximum instantaneous water inflow is simulated too. Meanwhile, the migration of gasified gas is also simulated, and the migration discipline of different gases is shown. Finally, the pressure distributions in two stages are presented, pointing out the dynamic pressure characteristics during the UCG process. The results show that (a) the cavity width, production pressure, and gasifier pressure are negatively correlated with the maximum instantaneous water inflow, while the initial formation pressure, injection pressure, coal seam floor aquifer energy, and temperature are positively correlated; (b) CO2 is mainly concentrated near the production well and largely does not migrate upward, O2 migrates upward slowly, while CH4, CO and H2 migrate relatively quickly. When the injection–production pressure difference is 2 MPa, it takes 33.5 years, 40 years, and 44.6 years for CH4, CO, and H2 to migrate from a depth of 1000 m to 200 m, respectively. When the pressure difference increases to 4 MPa, the gas migration rate increases about two-fold. The aquifer (3 MPa) above a coal outcrop can slow down the upward migration rate of gas by 0.03 m/day; (c) the pressure near the production well changes more significantly than the pressure near the injection well. The overall gasifier pressure rises with gasifier width increases, and the pressure distribution always presents an asymmetric unimodal distribution during the receding process of the gas injection point. The simulation work can provide a theoretical basis for the operation parameters design and monitoring of the well deployment, ensuring the safety and reliability of on-site gasification.

Diffusion Mechanism of Slurry during Grouting in a Fractured Aquifer: A Case Study in Chensilou Coal Mine, China

Grouting is one of the main technical means to prevent water inrush hazards in coal seam floor aquifers. It is of great significance to elucidate the diffusion law of slurry in the process of grouting in fractured aquifers for safe mining in coal mines. In this paper, the mechanism of slurry diffusion in horizontal fractures of fractured aquifers was studied based on the Bingham slurry with time-varying characteristics; additionally, a one-dimensional seepage grouting theoretical model considering the temporal and spatial variation of slurry viscosity under constant grouting rate was established. In this model, the grouting pressure required by the predetermined slurry diffusion radius can be obtained by knowing the grouting hole pressure and injection flow. Slurry properties, fracture parameters, grouting parameters, and water pressure were the parameters affecting the slurry diffusion process. Looking at the problem of water disaster prevention of coal seam floor in the Working Face 2509 of the Chensilou Coal Mine, according to the aquifer parameters and model calculation results, a grouting scheme with a slurry diffusion radius of 20 m and grouting pressure of 12 MPa was proposed. Finally, with the comparative analysis of the transient electromagnetic method (TEM) and water inflow before and after grouting, it was verified that the design grouting pressure and the spacing of grouting holes were reasonable and the grouting effect was good.

Effect of Crack Propagation on Mining-Induced Delayer Water Inrush Hazard of Hidden Fault

Hidden faults in deep coal seam floor threaten the exploitation of coal resources. Under the influence of mining and water confined in the floor, the cemented filler in the hidden fault will be eroded by water flow, in order to investigate the fracture characteristics and water inrush risk of hidden faults in floors above confined aquifer. Using the 27305 working face as geological background, the influence of the seepage scouring filler on the mechanism of water inrush from hidden faults was assessed by developing a stress-seepage coupling model and employing the finite difference method to simulate the seepage process of hidden faults under the combined action of high ground stress and high confined water. The evolution of seepage, shear stress, and plastic zone was also assessed. The influence of the hydraulic pressure of the aquifer and the thickness of a waterproof rock floor on the formation of the water inrush pathway was analyzed. Results indicate that (1) under the influence of mining, the hidden fault experienced the change process of stress stability, stress concentration, and stress release. The shear stress increases first and then decreases. The compressive stress decreases gradually due to stress release. (2) Water inrush disaster will not occur immediately when the working face is above the hidden fault. The delayed water inrush occurs in the mined-out area when the working face advances to 160 m, the floor failure zone is connected with the hidden fault failure zone, and the delayed water inrush channel is formed. (3) With the mining advances, the water pressure of aquifer is the same. The larger-angle fault leads to the thinner thickness of floor aquifer. The greater the influence of hidden fault on coal seam mining, the higher the danger of water inrush.

Surrounding Rock Failure Characteristics and Water Inrush Mechanism of Roadway above the Aquifer in Nonuniform Stress Field

The expansion of the roadway surrounding rock failure zone may connect the floor aquifer and cause the roadway water inrush. In order to reveal the mechanism of the roadway water inrush above confined water, we established the force model of the roadway surrounding rock above confined water in nonuniform stress field and studied the shape characteristics and expansion law of the roadway surrounding rock plastic zone. The results show that the roadway surrounding rock will form three kinds of plastic zone under different lateral pressure coefficients: circular, elliptical, and butterfly; when the shape of plastic zone is circular or elliptic, the maximum radius increases linearly with the increase of regional stress; when the shape is butterfly, the maximum radius increases exponentially with the increase of stress. Under the condition of a larger bidirectional stress ratio, the surrounding rock of the roadway will show butterfly-shaped failure, and small stress change will cause malignant expansion of the plastic zone; when the plastic zone is connected with the underlying aquifer, confined water of floor will enter the rock mass from the water diversion point and eventually flood into the roadway, causing floor water inrush.

Management and Utilization of High-Pressure Floor-Confined Water in Deep Coal Mines

This study analyzed the failure characteristics of the coal seam floor induced by longwall mining to manage and eliminate possible hazards associated with high pressure (8.0 MPa) floor-confined water in the Xinjulong coal mine. The disturbed floor was divided into mining failure (MF), water-resistant (WR), and confined water intrusion (CWI) zones. The MF and CWI zones have lost their ability to resist the confined water, so the WR zone is the key for preventing water inrush from the floor aquifer. A thin-plate model for the WR zone indicated that the water pressure had to be decreased to 2.4 MPa or less. Multi-well pumping was used to decrease the floor aquifer water pressure from 8.0 to 2.1 MPa. Moreover, grouting was used to block water-conducting fractures in the floor strata. In addition, classification, purification, and recycling techniques enabled 84.8% utilization of the produced mine water.

Research on the required width of a fault waterproof coal pillar based on underground pressure control theory

Based on underground pressure control theory, this paper expounds the distribution characteristics of abutment stress and the resulting damage to a coal seam in front of a working face. In addition, the distribution and propagation features of vertical underground pressure in floor strata is characterized by simulation results obtained from 2D-σ finite element analysis software. Numerical simulation reveals that the peak stress line of the vertical underground pressure is inclined toward the coal mining direction of the working face. Based on the establishment of a mechanics model for water inrush from floor aquifer through a fault in a stope, a formula for calculating the distance between the bottom of the coal seam and the intersection point of the peak stress line with the fault is derived. The condition allowing water inrush from a floor aquifer through a fault is described as follows: the destroyed floor depth caused by underground pressure is not less than the distance between the bottom of the coal seam and the intersection point of the peak stress line with the fault. The water inrush pathway is composed of the fault and the floor strata destroyed by underground pressure. Based on gray multivariate nonlinear regression analysis, the formula for calculating the destroyed floor depth with four main factors is deduced: these factors are the mining depth, dip angle of the coal seam, inclination width of the working face, and Platts’ coefficient of the floor strata. According to the mechanics model for water inrush from a floor, the formula for a fault waterproof coal pillar in a stope is presented, and the rationality of the model and formula has been verified by cases of water inrush from floor aquifers in the Suncun Coal Mine and the Xiaoyun Coal Mine, China.

Natural occurrence of potentially harmful fluoride contamination in groundwater: an example from Nakuru County, the Kenyan Rift Valley

High fluoride concentrations have been known to have several health implications on humans and animal’s health. In this paper, we present a case study dealing with high occurrences and distribution of fluoride in different aquifers in Nakuru County, Kenyan Rift Valley. Water quality data from 32 boreholes and hydrogeological data were acquired from the Catholic Diocese of Nakuru Water program and from literature, respectively. Results show that more than 87% of the boreholes are characterized by fluoride levels higher than the limit recommended by the World Health Organization for safe drinking water. Fluoride levels range from 0.5 to 72 mg/l, with a mean of 11.08 mg/l, and show a strong positive correlation with the dominant physicochemical parameters and a negative correlation with calcium, borehole depth, and water hardness. The correlations suggest that minerals dissolution and evaporative enrichment might be the main processes of fluoride release and concentration in groundwater. Spatial analysis showed that fluoride concentrations were not confined to the type of aquifers, but rather to their locations. Low-fluoride aquifers were located close to the rift’s Bahati and Mau escarpments, while high fluoride aquifers were in the rift floor, where the fluoride hot spot was located. Dilution of groundwater by high rainfall and little residence time in the escarpments recharge zones lead to low fluoride concentrations. Accumulation of dissolved solutes in the rift floor aquifers as groundwater flows from the escarpments and evaporative enrichments from high temperature lead to high fluoride concentrations. These high concentrations of fluoride in water are known to cause serious health issues on animals and humans.

Deformation Monitoring and Stability Analysis of Shaft Lining in Weakly Cemented Stratum

Shaft linings in thick weakly cemented stratum have the disadvantages of large deformation and repeated damage after repair. Considering the typical geologic characteristics and the failure characteristics of shaft linings, we establish a multilayer automatic deformation monitoring system in this paper, and the monitoring system can realize the real‐time, continuous, and long‐term dynamic monitoring on shaft linings. Based on the concrete strength failure criterion under biaxial compression and the analytical solution for spatially axisymmetric problem of thick‐wall cylinders, the damage limit of the shaft lining in Xieqiao coal mine is obtained. Then, we choose three sections as the test area according to the typical damage forms of shaft linings to carry out the monitoring scheme on the auxiliary shaft in Xieqiao coal mine. The monitoring results show that the extreme value of the shaft lining deformation is 2.369 mm. And the shaft lining located in the border between the floor aquifer and the bedrock generates the most severe deformation, which is about 89.4% of the deformation limit. The shaft lining deformation increment fluctuates in certain range, which belongs to elastic deformation. Finally, we inverse the stress state according to the deformation value of the shaft lining, and the obtained additional stress is found to be lower than the ultimate compressive strength. Long‐term project practice confirms that the deformation monitoring results can reflect the real stress condition of the shaft lining and that the monitoring system can realize the real‐time dynamic evaluation for the status of the shaft lining.

Hydrological evolution and chemical structure of a hyper-acidic spring-lake system on Whakaari/White Island, NZ

White Island has a long and varied history of acid spring discharge and shallow ephemeral lake formation on its main crater floor. In the 12months prior to the onset of the 1976–2000 eruptive episode, mass discharge from the spring system increased ca. 10-fold, pointing to a strong coupling of the hydrothermal environment to the evolving magmatic system. Between 1976 and 1978, the formation of numerous eruption vents to 200m depth in the Western Sub-crater abruptly changed the hydraulic gradients in the volcano, resulting in the reversal of groundwater flow in the massif towards the newly-formed crater(s). This affected not only the style of volcanic activity (leading to phreatic-phreatomagmatic-magmatic eruption cycles), but also led to the demise of the spring system, with discharge from the main crater declining by a factor>100 by 1979. Eruptive activity ended shortly after a moderate Strombolian eruption in mid-2000, after which ephemeral lakes started to form in the eruption crater complex.Between 2003 and 2015 there were three complete lake filling and evaporative cycles, reflecting varying heat flow through the conduit system beneath the lake. Over these cycles, lake water concentrations of Cl and SO4 varied between ca. 35–150 and 5–45g/L respectively, with pH values temporally ranging from +1.5 to −1. Springs appeared on the Main Crater floor in 2004, and their discharges varied with lake level, pointing to the lake level being a primary control over the piezometric surface in the crater area. Springs closest to the crater complex show direct evidence of crater lake water infiltration into the crater floor aquifer, whereas distal spring discharges show compositional variations reflecting vertical displacement of the interface between shallow, dilute condensate and underlying acidic brine fluids.Source components for the spring fluids include magmatic vapour, dissolved andesitic host rocks, seawater and meteoric water. Lake waters, on the other hand, consist predominantly of magmatic vapour, meteoric water and solutes derived from host andesites and their altered derivatives. δ2H and δ18O signatures of the enclosing acid brine fluids, indicate they are predominantly seawater which have been affected by both vapour loss, but also mixing with arc-type vapour.An interesting finding of this study is that crater floor deformation correlates directly to both lake level and volatile emissions, in an apparent poroelastic response to the establishment of a hydrostatic water column in the eruption crater complex, and a net decrease in permeability owing to hydrothermal mineralization in the conduit (predominantly elemental sulfur and sulfate minerals). The hydrostatic pressurization of the vent environment also leads to increased gas pressures and flows through fumarolic channels, and consequent expansion of fumarolic areas on the main crater floor.A period of unrest, which commenced in August 2012 and lasted until October 2013, included the extrusion of a small dome into the eruption crater complex. This activity, and related high heat flow, led once again to evaporation of the lake, and ongoing phreatic eruption activity which has provided interesting insights into the role which elemental sulfur, associated hydrothermal alteration minerals and of course water play in regulating pressures in the magmatic-hydrothermal environment.

Study of the effects of hydraulic fracturing at different dip angles for the development of coalbed methane: a case study in the southeast part of Qinshui Basin, China

Considering the negative relationship between the production of gas and water, we should minimise water production. The dip angle of hydraulic fracturing can control not only the hydraulic connectivity between the roof and floor aquifers and the coal seam, but also the gas flow direction in the coal seam. Permeability test results show that, under the circumstances of natural fractures and cleats fewer impacts, the permeability of coal samples along the direction of the coal seam is obviously higher than that in the direction perpendicular to the coal seam (accounting for 79.2% of the samples). According to the dip angle of hydraulic fracturing, combined with the thickness of coal seam and the development status of natural fractures and cleats, hydraulic fracturing will be divided into three types: vertical dip angle of fracturing, horizontal dip angle of fracturing and inclined dip angle of fracturing. The optimal fracture dip angle: α = arcsin (H/Lmax). [Received: July 6, 2015; Accepted: June 4, 2016]

Critical Break Length of Floor Aquiclude During Longwall Mining Above Confined Aquifer

Floor water inrushes not only cause heavy casualties and property losses, but seriously restrict the production of coal mines. The aquiclude was simplified as the fixed - fixed beam under the uniform load, and critical break length formula of floor aquiclude in three failure modes including bending failure, shear failure and buckling of pressure bar were established. Then the critical lengths of aquiclude in three failure modes were calculated according to the geological and mining conditions of 15031 panel in Jiulishan coalmine. The results show that the critical break length in bending failure is the smallest, while the critical length in shear failure mode is the biggest. And failures of fixed – fixed beam in three failure modes all occur at two ends so that water inrush from floor may occur at open-off cut or working face. Additionally, three critical break lengths increase with aquiclude thickness but decrease with confined aquifer pressure. According to the calculated results, water inrush may occur at the working face or open-off cut when mining advance reaches 12.47 m, and thus three control measures of lowering aquifer pressure, increasing aquiclude thickness and shortening the distances of first and interval weighting of main roof were discussed. The conclusions can provide references to control water inrush from floor aquifer in coal mines.

Study of the effects of hydraulic fracturing at different dip angles for the development of coalbed methane: a case study in the southeast part of Qinshui Basin, China

Considering the negative relationship between the production of gas and water, we should minimise water production. The dip angle of hydraulic fracturing can control not only the hydraulic connectivity between the roof and floor aquifers and the coal seam, but also the gas flow direction in the coal seam. Permeability test results show that, under the circumstances of natural fractures and cleats fewer impacts, the permeability of coal samples along the direction of the coal seam is obviously higher than that in the direction perpendicular to the coal seam (accounting for 79.2% of the samples). According to the dip angle of hydraulic fracturing, combined with the thickness of coal seam and the development status of natural fractures and cleats, hydraulic fracturing will be divided into three types: vertical dip angle of fracturing, horizontal dip angle of fracturing and inclined dip angle of fracturing. The optimal fracture dip angle: α = arcsin (H/Lmax). [Received: July 6, 2015; Accepted: June 4, 2016]

Groundwater flow dynamics in the complex aquifer system of Gidabo River Basin (Ethiopian Rift): a multi-proxy approach

Hydrochemical and isotope data in conjunction with hydraulic head and spring discharge observations were used to characterize the regional groundwater flow dynamics and the role of the tectonic setting in the Gidabo River Basin, Ethiopian Rift. Both groundwater levels and hydrochemical and isotopic data indicate groundwater flow from the major recharge area in the highland and escarpment into deep rift floor aquifers, suggesting a deep regional flow system can be distinguished from the shallow local aquifers. The δ18O and δ2H values of deep thermal (≥30 °C) groundwater are depleted relative to the shallow (<60 m below ground level) groundwater in the rift floor. Based on the δ18O values, the thermal groundwater is found to be recharged in the highland around 2,600 m a.s.l. and on average mixed with a proportion of 30 % shallow groundwater. While most groundwater samples display diluted solutions, δ13C data of dissolved inorganic carbon reveal that locally the thermal groundwater near fault zones is loaded with mantle CO2, which enhances silicate weathering and leads to anomalously high total dissolved solids (2,000–2,320 mg/l) and fluoride concentrations (6–15 mg/l) exceeding the recommended guideline value. The faults are generally found to act as complex conduit leaky barrier systems favoring vertical mixing processes. Normal faults dipping to the west appear to facilitate movement of groundwater into deeper aquifers and towards the rift floor, whereas those dipping to the east tend to act as leaky barriers perpendicular to the fault but enable preferential flow parallel to the fault plane.

Multi-factor evaluation for the water yield properties of coal floor aquifers, based on GIS

Groundwater inrush in mines directly threatens safe coal production. Aquifer yield plays a crucial role during water inrush. Significant accidents occur in areas where aquifer yields are strong. The yield of an aquifer is influenced by many things and, in this study, five for which data are easily acquired were selected as evaluation indices for coal-seam floor aquifer yield. They comprise specific yield, permeability, drilling fluid consumption, aquifer thickness, and fault and fold distribution. Based on these indices, a comprehensive model for evaluating water yield was constructed. Using the Ordovician limestone aquifer below the coal floor of Danhou coal mine, Hebei province, China, as an example, index data relating to aquifer yield were collected and the index weight determined. A geographic information system was used to prepare a thematic map for each driving factor and, subsequently, to superimpose them. Comprehensive water yield evaluation indexes were obtained and, finally, a zonal map of different water yield properties was developed for the research area.

Seepage Field-Strain Field Coupling Analysis for Rock Masses of Coal Seam Floor during Mining Based on COMSOL

Based on the fluid-solid coupling theory, The fractured rock mass was described as a continuous porous medium, and consider the rock mass permeability coefficient has a relationship with the stress and strain. The dynamic change of permeability coefficient was achieved using the multi-physics coupling analysis software COMSOL, and found that the stress and strain of surrounding rocks changes a lot due to mining. The maximum stress and strain in the mudstone area reaches 2-3 times of the original one, which happened at the immediate roof of mined-out area. According to the analysis of seepage field, mining does not destroy water resistance of floor aquifer. Mining fissures do not connect limestone aquifers, and water in the limestone is less likely to flow into stop to cause damage.

Study on Coal Seam Mining Face Floor Failure under Imbalance High Water Pressure

According to the seam floor aquifer inhomogeneity,in order to analysis the coal deformation and failure law in pressurized water ,this paper established mechanical model of nonuniform pressure and fluidstructure interaction models,using numerical simulation software FLAC3D analysis fluidstructure interaction water inrush regularity .Analyzing the extent of damage from the coal seam floor, the stress and displacement when working face promote different distances, this paper carried out the basic rules of mining face water inrush. This study shows that by monitoring the position of bottom water inrush occurs easily, analyzing the influence of pore pressure by the mining ,it can be accurately analysis the risk of water inrush and play an important role in guiding prevention and control of water inrush.

Study on a New Coal Floor Water Inrush Evaluation Technique and its Application

To solve the problem of how to evaluate coal floor multi-aquifer which is under complex hydro-geological conditions, a new approach is presented, which is AHP vulnerability index method based on GIS. In the research the main controlling factors influencing coal floor water inrush and its weight factors are taken into account. The method truly reflects the nonlinear dynamical process of coal floor inrush under the influence of many factors. Applying this method to evaluate the water inrush vulnerability of No.13091 working face No. 21coal floor aquifer in L1-3limestone and Ordovician - Cambrian limestone in Pingyu 1st coal mine, the results have showed that this method is more effective comparing with other traditional water inrush coefficient methods. The problem of indicating coal floor water inrush under complex hydro-geological conditions has been solved by using this method.