Investigations of groundwater bursting into coal mine seam floors from fault zones

https://doi.org/10.1016/j.ijrmms.2003.01.004Get rights and content

Abstract

This paper presents a case study of investigations into retarded groundwater bursting along the fault zones in the seam floors in coal mines. In addition to in situ measurements of ground stresses, a number of rock samples from the fault zones at the site of the case study were taken, and laboratory tests were performed for conventional and special rheological rock mechanics properties. The effects of different parameters on deformation and failure of the fault zone materials were analyzed, and the mechanism of retarded groundwater bursting along fault zones was further revealed. In the case study, different scenarios accounting for the different development phases of the fault weakness zone and groundwater bursting as well as different groundwater pressures were considered, and each of them was simulated using the 3D visual elastic–plastic numerical mechanics model FLAC3D (Fast Lagrangian Analysis of Continua in 3 Dimensions). Through the case study, the applicability of rock mechanics modeling software for studying retarded groundwater bursting along a fault zone was examined. Several issues are discussed regarding the investigations of retarded groundwater bursting and a general structured approach to investigate and control this geological hazardous event is presented.

Introduction

Groundwater with high pressure can break through seam floors and burst into the tunnels and winning excavations constructed for coal mining. This phenomenon is called groundwater bursting and can be a severe geological hazardous event if an unexpected amount of groundwater were to appear suddenly from the underlying aquifers through the breached seam floor. This could result in significant loss of life and property.

Groundwater bursting can be instantaneous or retarded. Instantaneous groundwater bursting refers to situations when bursting immediately follows the construction of tunnel or coal mining excavations. Retarded groundwater bursting, however, occurs when the groundwater bursting happens with a lag time after the tunnel or winning excavations have been formed. The lag time could be many years. The latter type of groundwater bursting may result in significantly higher risk during mining since it may have been assumed that groundwater bursting will not occur and that the seam floor is strong enough to resist the water pressure.

Essentially, groundwater bursting occurs when the water pressure is greater than the strength of the seam floor beneath the tunnel or winning excavations. In general, fault zones have weaker strength than those areas unaffected by faults. Additionally, there is more complexity associated with investigations of retarded groundwater bursting than with contemporaneous bursting.

The study of groundwater bursting has been highlighted in past decades, with reports given on the progress of investigations [1], [2], [3], [4]. The most current research on this topic has been expanded from applying the individual disciplines of hydraulics or rock mechanics to applying and integrating a multi-disciplinary approach. In particular, the investigations of groundwater bursting have been conducted by integrating the geological foundation with the other related disciplines such as hydrogeology, engineering geology, rock mechanics, and mining engineering. A number of developments have been reported on the mechanisms of groundwater bursting, hydrogeologic and hydraulic models for its investigations, and the rock mechanics behavior. Wang, for example, presented a hydraulic model of groundwater bursting [5] which was used to explain several phenomena. In addition, the statistical hydrogeologic models for predicting the discharge involved in groundwater bursting were presented [6], [7]. Based on the geological and hydrogeological characterizations and modeling, a general procedure was introduced to predict the event and discharge of groundwater bursting and prevent the related hazards in mines [8].

Most of those investigations, however, do not directly account for the gradual change of strength of the seam floor, especially the fault zone, with time. It was observed that the strength of the fault zone in the seam floor beneath the tunnel and winning excavations constructed for mining decreases gradually with mining under groundwater pressure and penetration [9]. Records of groundwater bursting from many coal mines of northern China also indicate that groundwater bursting generally happens much later than the tunnel constructions and winning excavations for mining, which is why the term “retarded groundwater bursting” is used in the paper to describe this special feature of groundwater bursting.

This paper focuses on the investigation of retarded groundwater bursting along fault zones, provides a framework for understanding the fundamental process, and provides a mechanism of the retarded groundwater bursting along fault zones. Such a phenomenon occurred at Zhaogezhuang Coal Mine, a mining site in northern China, in winning face No. 9132, during winning around the A–B cross-sectional line (Fig. 1). Fault F3 cut through the layers beneath the water bursting point. The fault displacement varied from 15 to 30 m, and it was a gravity fault. A cross-section of the geological structure is shown in Fig. 2. In the first 5 months of winning excavations, the mining appeared normal and safe without any groundwater bursting. Unfortunately, groundwater burst along the fault zone around tram rail No. 9 and ventilating way No. 9 after 5 months of winning. It is interesting to note that groundwater bursting had not occurred since tram rail No. 9 and ventilating way No. 9 were constructed 12 years before. It was believed that the fault zone, which extended along and cut through the whole bedrock sequence beneath the excavation site, played an important role in inducing the groundwater bursting.

It was understood that the tunnel excavation caused an increase of rock tension and created more potential for groundwater penetration along the fault zone. Due to the winning excavations in the mine, rock tension around the excavation increased further. The fault zone was then active again, and the groundwater in the aquifers beneath the excavation site intruded around the fault zone. With the increase of excavation time and wetting of the fault zone, the strength of the rocks in the fault zone decreased gradually and the extension length of the fault weakness zone, in which fault zone materials had been wetted and then weakened, increased progressively. When the overall strength of the fault zone decreased to such an extent that its strength limitation was lower than the water pressure, and both the lower induced-uplift hydraulic connection zone related to the fault weakness zone and the upper mining-induced fracturing zone were connected to each other, groundwater bursting was manifested.

A further investigation of the mechanism and modeling of retarded groundwater bursting along fault zones was performed through a case study of Zhaogezhuang Coal Mine. Based on the observations, it is necessary to investigate the effect of different parameters on the strength of fault zones in order to establish the factors involved in groundwater bursting, especially water content, at laboratory scales. Therefore, a number of samples from the case study site were taken and laboratory rock mechanics tests were performed. The effects of different parameters on deformation and failure of fault zone materials were analyzed, and the mechanism of retarded groundwater bursting along fault zones further revealed.

In this paper, different scenarios were considered, and each of them was simulated using the simulation package FLAC3D for elastic–plastic numerical mechanics modeling. Through this study, the applicability of a rock mechanics modeling software package for studying retarded groundwater bursting was examined. Several issues regarding investigations of retarded groundwater bursting were discussed, and a general structured approach to investigate and control this geological hazardous event is presented.

Section snippets

Site conditions

Zhaogezhuang Coal Mine is located in Hebei Province, northern China. From the bottom to the top there exist two large rock formations: the Middle Ordovician System (O2), containing a large limestone aquifer; and the Permian System, containing coalbeds. Coalbed No. 12 was designed to be included in the next mining level, level 13. Between Coalbed No. 12 and the limestone aquifer, there exist a number of sub-layers with different rock mechanics properties. The faults cut the rock layers and

In situ measurement of original ground stress

The stress releasing approach was applied to the in situ measurement of the original ground stress. The ‘Voidal Enclosure’ method was adopted, and the measurement of ground stress was completed through the ‘Casing Core’ technique. Data from the measurements indicated that gravity dominated the original ground stress field of the surrounding rocks, with a slight addition from the tectonic stress field. The direction of the maximum principal stress is approximately vertical with a magnitude

Numerical simulations of retarded groundwater bursting

FLAC3D is a powerful software package for simulating the mechanics behavior of three-dimensional structures built of soil, rock, or other materials that undergo plastic flow. The explicit finite difference formulation is adopted in FLAC3D, and it makes FLAC3D ideal for modeling geomechanical problems that consist of several phases, such as sequential excavation, backfilling, and loading. The formulation can handle large displacements and strains and non-linear material behavior, and has the

Main factors on retardance or time lag effect of faults for groundwater bursting

The retardance effect of a fault is exhibited by gradually weakening of fault zone materials under the high water pressure of the bedrock aquifer and reducing resistance to groundwater bursting to break through the seam floor during the winning processes. A number of factors play a significant role in determining whether and when groundwater bursting happens. Based on the obtained investigation results from this study, along with comprehensive analyses on a number of records of groundwater

Summary and conclusions

Investigations of retarded groundwater bursting bear a critical significance for the safety of coal mining. Through a case study of Zhaogezhuang Coal Mine for forecasting retarded groundwater bursting, the mechanism of retarded groundwater bursting along fault zones has been further revealed. It has been demonstrated that the fault weakness zone plays a significant role in determining the occurrence and possibility of groundwater bursting. It was shown that gradual development of the fault

Acknowledgements

The authors are grateful to Kailuan Mining Bureau, Hebei Province, China for providing funding for this research. We also want to thank Dr. Chen Zhu for his suggestions on the manuscript.

References (14)

  • W. Xu et al.

    On the formation mechanism of collapse columns and water bursting in coal mines of Northern China

    Hydrogeol Eng Geol

    (1990)
  • J. Ding

    Characteristic of water bursting and the prediction of discharge rate in Shaft No. 1, Weijiazai, Lingdai Mine

    Carsologica Sinica.

    (1991)
  • Z. Ge et al.

    Preliminary research on the hydrogeology of water bursting in the upper mining shaft in an Ordovician limestone aquifer in a certain mine of Xuzhou, Jiangsu

    Jiangsu Geol

    (1994)
  • F. Sun et al.

    Mechanics analysis of bursting conditions in underground mines

    Hydrogeol Eng Geol

    (1998)
  • M. Wang

    Hydraulic conceptual model of the geological hazard-groundwater bursting in mining pit with multiple groundwater bearing beds structure

    Chin J Geol Hazard Control

    (1995)
  • M. Wang et al.

    Study on hydrogeological models of the geological hazard from water bursting in mining pits with multiple aquifer structure

    Chin J Geol Hazard Control

    (1991)
  • M. Wang et al.

    Applications of regression prediction model to ground water discharge in Yanmazhuang Mine, Jiaozuo Coal Field

    J Xiangtan Min Inst

    (1995)
There are more references available in the full text version of this article.

Cited by (191)

  • An overview on flooding induced uplift for abandoned coal mines

    2021, International Journal of Rock Mechanics and Mining Sciences
    Citation Excerpt :

    In that case, isotope survey of mine water and groundwater is an effective tool to determine the flooding source. Wu et al.54 documented that groundwater under high pressure can break through floor seams and burst into roadways. If an unexpected amount of groundwater appears suddenly from an aquifer by breaching the seam floor, work space or goaf will be flooded.

View all citing articles on Scopus
View full text