Mining-induced Stress Changes Research Articles

Large-scale destress blasting for seismicity control in hard rock mines: A case study

Destress blasting is a rockburst control technique where highly stressed rock is blasted to reduce the local stress and stiffness of the rock, thereby reducing its burst proneness. The technique is commonly practiced in deep hard rock mines in burst prone developments, as well as in sill or crown pillars which become burst-prone as the orebody is extracted. Large-scale destressing is a variant of destress blasting where panels are created parallel to the orebody strike with a longhole, fanning blast pattern from cross cut drifts situated in the host rock. The aim of panel destressing is to reduce the stress concentration in the ore blocks or pillars to be mined. This paper focuses on the large-scale destress blasting program conducted at Vale’s Copper Cliff Mine (CCM) in Ontario, Canada. The merits of panel destressing are examined through field measurements of mining induced stress changes in the pillar. The destressing mechanism is simulated with a rock fragmentation factor (α) and stress reduction/dissipation factor (β). A 3D model is built and validated with measured induced stress changes. It is shown that the best correlation between the numerical model and field measurements is obtained when the combination of α and β indicates that the blast causes high fragmentation (α = 0.05) and high stress release (β = 0.95) in the destress panel. It is demonstrated that the burst proneness of the ore blocks in the panel stress shadow is reduced in terms of the brittle shear ratio (BSR) and the burst potential index (BPI).

Microbiologically Influenced Corrosion (MIC) of Ground Support

Ground support is an integral part of strategies to ensure the stability of underground excavations in rock. In a mining environment the long term performance of ground support is influenced by several factors including ground conditions and mining induced stress changes. In corrosive underground environments ground support can be compromised leading to loss of performance and potentially falls of ground that may compromise the safety of workers and damage to equipment. An improved understanding of the role of atmospheric and aqueous corrosion on the long term performance can facilitate the selection of ground support. The contribution of microbacterial influenced corrosion (MIC) on the corrosion rate and degradation of ground support has received minimal attention in the past. This paper confirms the presence of MIC of ground support in underground hard rock mines which resulted in considerable higher corrosion rates in otherwise similar environments. This can have significant implications in establishing long term ground support strategies.

Computational Modelling of Groundwater Inflow During a Longwall Coal Mining Advance: A Case Study from the Shanxi Province, China

The paper investigates groundwater inflow into a longwall working face of the Xiegou Coal Mine located in the northwest of the Shanxi province, China. Three modelling approaches, including basic Darcy modelling, full poroelastic modelling using Biot’s classical theory, and the full Biot poroelastic modelling that takes into account permeability alterations of the coal seam and strata due to alterations in the mining-induced stress state, are used to estimate the fluid flow into the underground longwall coal mine as the excavation progresses. Based on plausible assumptions of hydrogeological structure, boundary conditions and mining conditions, a three-dimensional finite element model of the simplified geological setting is developed using the COMSOL software. The permeability changes in both the strata and the coal seam due to the mining are associated with the mining-induced stress changes and water pressure, and the correlation between them accords with the exponential relationship. The differences in the modelling results of three approaches were analyzed, and the simulation results are compared with the field measurements.

Mining-induced stress changes and rock burst control in a variable-thickness coal seam

In longwall mining, one of the factors most influencing safety in underground coal mining is the stress change, especially when such work is carried out within the thickness variation of a coal seam. This paper analyses the special case of a coal seam which suffers significant overstress as a result of variations in the geometry and dimension of the load itself, in particular, thinning. This is because such thickening and thinning locations of the thickness are generally areas where rock bursts occur most frequently. The results of the research were validated by measuring the stress state in the area under study. A numerical model was established using FLAC3D to understand the results of the field tests and map the zones in the model with a high risk of rock burst. The results of field tests, and the numerical modelling, show that thinning accentuates the stress concentration and energy accumulation. Furthermore, on the basis of a retrospective analysis of what was observed in a coal mine, some preventative measures were instigated as rock burst controls. Also, it was found that rock burst occurrence can be effectively reduced by the use of large-diameter drilling and break-tip blasting.

Prediction and follow-up of failure and fallouts in footwall drifts in the Kiirunavaara mine

A likely result of changes in rock stresses due to progressing mining is an increased number of compressive stress–induced failures. This paper presents the results from numerical analysis and observations of stress-induced fallouts in footwall drifts in the Kiirunavaara underground mine. A brittle-plastic “cohesion-softening friction-hardening” (CSFH) material model was used for simulating brittle fallouts. To account for mining-induced stress changes, the local model stress boundary conditions were extracted from a global model. The rock mass properties were based on field observations in the footwall drifts as well as on results from laboratory testing. A multi-stage analysis was carried out to gradually change the stresses to simulate mining progress. A parametric study was conducted in which strength properties, location, and shape of the footwall drift were varied. Yielded elements and maximum shear strain were used as damage and fallout indicators. The modelling results were sensitive to the shape of the drift. The location of the predicted fallouts was in good agreement with the location of observed fallouts for the case in which the drift roof was simulated flatter than the theoretical cross section. The results indicate that the true shape of the drift is different from the planned one.

Triggering of Seismicity Remote from Active Mining Excavations

Observations of seismicity and ground control problems in the Sudbury mining camp have shown that late-stage (young) sub-vertical strike-slip faults are sensitive to small mining-induced stress changes. The strength-limited nature of stress measurements made in the region indicates that these structures are in a state of marginal stability. Numerical continuum models are developed to analyze the behavior of such structures. In the models, shear strain localizations (faults) evolve such that there is close interaction between the fault system, stresses, and boundary deformation. Fault slip activity in these systems is naturally sporadic and reproduces the commonly observed Gutenberg-Richter magnitude frequency relation. It is shown that a relatively minor disturbance to such a system can trigger significant seismicity remote from the source of the disturbance, a behavior which cannot be explained by conventional numerical stress analysis methodologies. The initially uniform orientation of the stress field in these systems evolves with increasing disorder, which explains much of the scatter commonly observed in data sets of stress measurements. Based on these results, implications for stress measurement programs and numerical stability analysis of faults in mines are discussed.

Mining-induced stress change and consequences of stress path on excavation stability — a case study

Abstract For stability assessments as well as for support design, it is important to understand the factors leading to detrimental stress changes. Stress changes not only influence the demand on the rock support, they also change the support capacity of frictional support components such as plain cablebolts. Stress and stress changes are commonly predicted by numerical models but it is rarely possible to verify these predictions. This study presents a practical example illustrating the usefulness of stress change measurements in providing an accurate picture of the mining-induced stress changes and their value for numerical model calibration. Stress changes, associated with mining of the 565#6 stope at Winston Lake Mine, were measured by four CSIRO HI stress cells, two in the hangingwall and two in the back of a sill drift. In order to obtain the full stress history of the hangingwall, stresses at the installation time were calculated by use of a three-dimensional boundary element program — MAP3D. Two types of rockmass failure were observed at this mine: hangingwall delamination and collapse leading to ore dilution, and wedge-like failures in the backs of sill drifts. All stress cells were located such that they were affected by these failure mechanisms. Using measured and predicted stress paths, this study explains the nature of these two failure modes, as experienced at Winston Lake Mine. Excellent correspondence between measured and predicted stresses is achieved for both the hangingwall and the back of the sill drifts. Modelled stress rotations in the hangingwall follow closely the stress rotations determined in the field, except at the end of the monitoring campaign when the stress cells were affected by inelastic straining of the host rock. These measurements assist in understanding the failure processes and in defining the limitations of the adopted numerical models.

Observational design of underground cable bolt support systems utilizing instrumentation

The observational approach to cable bolt design uses visual observations as well as data collected from rock mass and cable bolt instruments to gain increased understanding of the response of the rock mass to the creation of mining openings and the interaction between cable bolts and the deforming or failing rock mass. Predictions of rock mass and cable bolt behaviour, made from models, can be used to determine the type of instruments required in the programme and then to compare the predicted to the measured response. Increased understanding of the interaction between the cable bolts and the rock mass can lead to changes in the type or pattern of cable bolts specified in design. A case history based on a cable bolting instrumentation programme carried out at Ansil Mine illustrates many of these principles in practice. As a result of the findings of this case history, the cable bolt design for Ansil Mine was modified to take account of the anticipated conditions of mining-induced stress change.

The value of long tendons in design of tunnel layout and support

A project was initiated from hypotheses that the use of longer and different types of tendon support could lead to improved control of tunnel deformation in changing stress fields. An extensive literature study was conducted. Subsequently a tunnel was supported with various lengths and types of cable tendons. The tunnel was subjected to mining-induced stress changes. Tunnel deformations were measured using instrumentation. Results from the literature study and tunnel deformation measurements indicated within context that: (1) long tendons or other support systems cannot substitute for good tunnel layout; (2) long tendons effect a retain-and-hold function while shorter tendons strengthen and reinforce highly fractured tunnel walls; (3) larger boreholes or weakergrout reduce support system stiffness; (4) cable tendons have high shear resistance and debonding tendency; (5) long tendons are beneficial in sidewalls but not the hangingwall, (6) long tendons induce a more stable sidewall deformation distribution; (7) the updip sidewall deformed less than downdip; (8) more deformation occurred with stress decrease than with stress increase.

Temporal imaging of mine-induced stress change using seismic tomography

In an effort aimed at monitoring mining-induced stress changes that may influence ground failure, the US Bureau of Mines conducted a series of active 3-D seismic tomographic surveys at the Homestake (Lead, South Dakota) and Lucky Friday (Mullan, Idaho) mines. Existing rock bolts were used to mount geophones and as strike points for introducing seismic energy using a 3.6 kg sledge hammer. Travel time measurements were recorded spanning up to 160 m between drifts at several levels. The tomographic uncertainty was minimized by computing iso-velocity surfaces of constant probability based on statistics from multiple reconstructions and a threshold criteria. Periodic imaging using tomographic velocity surfaces demonstrated that temporal and spatial changes in the local stress field occurred in response to ground failure and stope advancement at a gold and silver mine, respectively.

Sequential velocity imaging and microseismic monitoring of mining-induced stress change : Maxwell, S C; Young, R P Pure Appl GeophysV139, N3/4, 1992, P421–447

Sequential velocity imaging and microseismic monitoring of mining-induced stress change : Maxwell, S C; Young, R P Pure Appl GeophysV139, N3/4, 1992, P421–447

Cable bolt performance during mining induced stress change - three case examples : Kaiser, P K; Diederichs, M; Yazici, S Proc International Symposium on Rock Support, Sudbury, 16–19 June 1992 P377–384. Publ Rotterdam: A A Balkema, 1992

Cable bolt performance during mining induced stress change - three case examples : Kaiser, P K; Diederichs, M; Yazici, S Proc International Symposium on Rock Support, Sudbury, 16–19 June 1992 P377–384. Publ Rotterdam: A A Balkema, 1992

Effect of stress change on the bond strength of fully grouted cables

Abstract The bond strength of fully grouted cable bolts is primarily frictional and depends on the pressure at the cable-grout interface. Thus, any change in this interface pressure must affect the bond strength. The interface pressure increases with (load induced) dilation and may increase or decrease with the stress change due to nearby mining activities (excavation induced). In this paper, the effect of stress change on the bond strength is studied in the context of the bond strength model (BSM) previously established in a companion paper. A laboratory testing program was executed to evaluate the stress change effect on the frictional resistance of the steel-grout interface of a smooth bar and the laboratory results confirm the theory. It is concluded that mining induced stress change is one of the most important parameters controlling the cable bond strength. Theoretical results indicate that cables should be most affected by stress change in relatively soft rocks. Stress reductions of −40 MPa, often measured in deep mines, can cause the cables to lose most of their bond strength in low to medium stiff rocks. This study provides a new perspective on cable design and provides a methodology to increase the stability of openings.

Sequential velocity imaging and microseismic monitoring of mining-induced stress change

Sequential imaging of the temporal changes inP-wave velocity offers a practical tool to monitor a rock mass. Using established correlations between the location of seismic events and velocity structure, the temporal seismic potential characteristics of the rock may be monitored. Furthermore, the temporal velocity differences isolate the time dependent factors effecting velocity such as stress, while cancelling static factors such as lithology. Various sequential imaging techniques were compared with respect to accuracy. Differences between successive velocity images were found to have relatively high associated error estimates. However, images of velocity differences calculated from measured travel time delays between successive velocity surveys were found to have lower error estimates. In particular, travel time delays measured using cross-correlation techniques resulted in the most accurate sequential image. Two 3D sequential imaging studies were carried out at Strathcona Mine in Sudbury, Ontario, Canada. Results of the average static images indicated an association between the location of induced microseismicity and a zone of both high velocity and high gradient. Additional examples are described from the global seismology literature which also show a similar correlation between seismicity and velocity structure. We attribute this association to an interrelated stress and strength effect. The Strathcona Mine sequential images show zones of significantly decreased velocity in regions of concentrated microseismic activity, which are postulated to be indications of localized destressing and relaxation of the clamping forces resulting in the microseismicity. The zones of decreased velocity corresponded to an increase in the velocity gradient. One of the case studies also shows an increase in velocity in a zone of high static velocity, which is later the site of amN 2.5 mining-induced seismic tremor. The increase in velocity is believed to correspond to a region of stress concentration, resulting in the subsequent seismic tremor.

Observations of mine seismicity in the eastern Wasatch Plateau, Utah, U.S.A.: A possible case of implosional failure

In the summer of 1984, a three-dimensional, high-resolution microearthquake network was operated in the vicinity of two coal mines beneath Gentry Mountain in the eastern Wasatch Plateau, Utah. During a six-week period, approximately 3,000 seismic events were observed of which the majority were impulsive, higher frequency ( > 10 Hz), short duration ( < 2–3 sec) events probably associated with the caving of the roof from a longwall operation. In contrast, 234 of the largest located events appeared to occur predominantly beneath the mines to a depth of 2 to 3 km consistent with previous studies. The magnitudes of these events ranged from less than M c 0 to 1.6. In addition to the unusual depths of these latter events, an anomalous aspect displayed by the events was an apparent dilatational focal mechanism suggesting a non-double-couple, possibly implosional source. Implosional events have been observed in other studies of mine seismicity; however, the generally inadequate instrumental coverage of the focal sphere has cast some doubt on the validity of such mechanisms. Previously suggested source mechanisms for such implosional events have included tensional failure through strata collapse, and a shear-implosional displacement mechanism. Shear failure must be involved in the failure process of the Gentry Mountain implosional events as evidenced by well-defined shear waves in the observed seismograms. Simultaneous monitoring in the East Mountain coal mining area to the south by the University of Utah revealed typical shear failure events mixed with implosional events. The observed double-couple, reverse focal mechanisms at East Mountain were similar to mechanisms determined in previous studies and a composite focal mechanism determined in this study for a sequence outside the mining areas. This suggested that the shear events within the mining areas are being influenced by the regional tectonic stress field. Thus in addition to the seismic events associated with caving of the roof from the longwall operation, there appear to be at least two other types of mining-induced seismic events occurring in the eastern Wasatch Plateau, both submine in origin: (1) events characterized by apparent non-double-couple possibly implosional focal mechanisms and well-defined shear waves; and (2) shear events, which are indistinguishable from tectonic earthquakes and may be considered mining “triggered” earthquakes. The small mining-induced stress changes that occur beyond a few hundred meters from the mine workings suggest both types of seismic events are occurring on critically stressed, pre-existing zones of weakness. Topography, overburden, method of mining, and mine configuration also appear to be significant factors influencing the occurrence of the implosional submine events.