Stability Of Excavations Research Articles

Fall of Ground Management Through Underground Joint Mapping: Shallow Chrome Mining Case Study

The use of joint properties and joint mapping techniques are key for fall of ground management in underground mining. This paper outlines the use of probabilistic design approach in addressing potential falls of ground, based on identified keyblocks. A shallow chrome mine was used as a case study to identify potential causes of falls of ground with the aim of improving the existing fall of ground management system. The existing fall of ground management system comprises of visual observations, Ground Penetrating Radar (GPR) scanning and Borehole camera inspections. The mine is characterised by geological structures such as faults and joints, hence the system leans towards structural analysis. Joint mapping was carried out in the North and South sections of the mine using window mapping and scanline mappint techniques. The collected joint data from each section was used to evaluate rock fall probability. Rockfall probabilistic analysis carried out in the study indicates that about 80% of all key blocks formed are 1 m3 in size. Results show that larger blocks are more likely to fail through rotation whereas small key blocks are most likely to fall in-between support units. Further stability analysis was conducted through simulation of the effect of change in support spacing on excavation stability. Indeed support spacing plays a critical role in the overall stability of the excavation as opposed to the length and capacity of the support unit. This conclusion was drawn based on the improvement of the factor of safety during the simulation exercise. This research is based on an MSc Engineering study.

Study on Mutual Feedback Mechanism and Stability Control of Toppling Slope and Tunnels with High Excavation Rate

During the construction of large-scale hydropower projects in deep mountain gorge areas of the Western China, the difficulties, such as uncontrollable large deformation, great deformation caused by small disturbance, are often encountered in the process of implementation. In particular, due to the limitation of topography, geological conditions and the layout of structures, the slope and dense tunnel group with large section need to be excavated simultaneously in the toppling rock mass. The feedback action mechanism between slope and tunnel excavation is complicated and the deformation control of toppling rock is difficult, which leads to great challenges to engineering construction. In MW hydropower project in west of China, the intake slope, the spillway channel slope, four headrace tunnels, and the sand flushing tunnel, as well as the construction traffic tunnel are arranged in Huishi Ridge on the left bank, where two high slopes and six lager cross-section tunnels are simultaneously excavated and the excavation rate in the toppling deformation rock mass is over 50%, moreover, the lateral coverage thickness of tunnel body is less than the diameter of the tunnel, which leads to the great difficulty of stability control. The excavation stability and safe of the thin mountain ridge is the vital issue to the success of the whole project. Based on the study of the physical and mechanical parameters of toppling rock mass of MW HPP, the mutual feedback mechanism of toppling slope and tunnel group is revealed, an excavation method involving horizontal partitioning, vertical layering, reserving rock plug and reasonable skipping warehouse is proposed. Monitoring results shows that the slope and surrounding rock of the tunnels are stable during the operation period. The research of this paper based on the MW HPP can provide reference for other similar projects.

Analytic Study on the Force Transfer of Full Encapsulating Rock Bolts Subjected to Tensile Force

Full encapsulating rock bolts are widely used in mining and civil engineering to keep the stability of underground excavations. Nevertheless, decoupling at the interface between the bolt and grout (first interface) still occurs. To disclose the loading mechanism of the bolting system and preventing decoupling at the first interface, experimental and analytic studies were conducted in this paper. First, a nonlinear shearing-slipping law is used to describe the coupling and decoupling behavior of the first interface. Then, this shearing-slipping law is merged into the anchorage body. Following this, the whole loading process is divided into two components: elastic period and elastic-plastic period. Consequently, the analytic force-deformation relation of bolts is obtained. To confirm the rationality of this solution, experimental tensile tests on bolts were performed. It shows that there was a good match between analytic results and experimental tests. With the confirmed analytic solution, a parameter study is performed to investigate the impact of shearing-slipping parameters on the bolting performance. It indicates that increasing the shearing strength of the first interface and the shearing slipping have a positive impact in improving the bearing capacity of bolts. Inversely, the softening coefficient for the post-failure stage has a negative impact on determining the bearing capacity of bolts. Overall, the shearing strength of the first interface has a major impact in determining the loading performance of bolts.

Slope Stability Evaluation of Basement Excavation: A Case Study

A deep excavation should be designed and implemented together with the precautions to prevent any loss of stability and structural damage in the infrastructure and superstructure elements of the neighboring parcels and risks against construction worker's health and safety. This study represents an example of soil damage that may occur due to foundation excavation. The features of the soil affected by the excavation were analyzed through a geological survey and soil models were created for each analysis stages. The study investigates the slope stability analyses that take place during the foundation excavation with the commonly used limit equilibrium (LE) methods. In this article, slope problems that may occur around the excavation before the construction of the basement of a 3-storey school building in Istinye Neighborhood of Istanbul Sarıyer district are examined. First, geotechnical and structural parameters were determined by obtaining detailed drilling reports of the region. Then the study area was modeled with the Rocscience Slide 6.0 program based on the limit equilibrium method. Following the evaluation of the basement excavations, a beveled excavation was made for the safety of the basement construction after the excavation and the stability of the slope excavations was found to be sufficient.

An Improved Form of 2D SPH Method for Modeling the Excavation Damage of Tunnels Containing Random Fissures

The excavation damage of deep tunnels is one of the most important factors contributing to the failure of tunnel structures. In order to investigate the influence of tunnel shapes and fissure geometries, the kernel function in the traditional SPH method has been improved, which can realize the brittle fracture characteristics of particles and can be called the Improved Kernel of Smoothed Particle Hydrodynamics (IKSPH-2D). Meanwhile, the random fissure generation method in IKSPH has been put forward. Different tunnel shapes, fissure geometries, and locations are considered during the simulation of tunnel excavation, and results show that (1) the typical “V”-shaped shear damage zones appear after the tunnel excavation, which is consistent with engineering practice. Meanwhile, tunnel excavation also has an “activating” effect on the preexisting fissures. (2) The stability of circular-shaped tunnel is the best, while horseshoe shaped tunnel is worse, and the “U”-shaped tunnel is the worst. (3) Fissures with small and large dip angles have the greatest influence on the stability of tunnel excavation. With the increase of fissure numbers and lengths, the tunnel tends to be instable. (4) The IKSPH method gets free from traditional grids in FEM, which can dynamically reflect the fracture processes of tunnel excavation. Meanwhile, developing 3D IKSPH parallel program will be the future directions.

Preliminary analysis on excavation stability of Beishan Underground Research Laboratory

According to construction requirements of the first underground research laboratory (URL) for high-level radioactive waste (HLW) disposal in China, preliminary analysis on the excavation stability was carried out. Based on in-situ stress conditions and preliminary design of the URL, excavation induced stresses were revealed by numerical simulation. Then, through coordinate conversion, the excavation induced stresses were converted into local coordinate system perpendicular to the tunnel. Finally, an analytical method that compares rock strength with stress conditions is used to evaluate the excavation stability of surrounding rock in the local coordinate system. Results of this study show that: due to the high strength of Beishan granite and low level of the regional in-situ stress, the Beishan URL is highly likely to remain stable after excavation, the surrounding rock can hardly be destroyed by both tensile and compressive stress. Relatively unstable parts of the URL are the bottom of the shaft, rotation parts of the -560m-platform and some corners. These parts should be paid attention to in the future excavation process.

Stability analysis of an abandoned deep metal mine using numerical analysis tool: A case study

In the recent years due to rapid urbanisation, the key focus has been shifted to development of rural and remote areas. Abandoned mine sites are being evaluated for land reclamation with remediation measures as part of the development program. The abandoned mines come with inherent risk like unpredictable potential future subsidence and sinkhole formations on the surface. The identification of these potential risk areas stands as a major challenge for remediation. The inaccessibility to site makes it even more difficult to study the current geo-mechanical stress conditions for stability evaluation. Despite these limitations, it is essential that abandoned mine sites could be evaluated to atleast give an indicative understanding of current scenarios with regard to safety.Using numerical analysis tool, it is possible to simulate a model similar to that of the abandoned mine, to identify the potentially stressed zones and assess the stability of the underground excavation. The finite element method is used to develop a numerical model using GTS NX software. The software aided in evaluation of stress distributions and stability of the closed mine since the mine presents a hydro geological problem. The model analysis has proved to be very helpful in observing the mechanical behaviour of materials, modelling of rock systems and understanding the deformations in the rockmass.Three case conditions of varying the input parameter of peak ground acceleration (PGA) with a minimum PGA value of 0.06g to a maximum PGA value of 0.22g was used in the model studies of Champion Reef mine (3.2km deep gold mine). The vulnerable zones of seismicity were identified to be concentrated at a depth below 1500m from the surface and the maximum peak ground accelerations observed at the surface was found to be 0.1g to 0.22g. The model developed and approach used in this study is an attempt to present the overall view of the probable stability condition for a rough risk assessment study of the deepest Champion Reef metal mine in Kolar Gold Fields.

Numerical Investigation on Instability of Buildings Caused by Adjacent Deep Excavation

AbstractThe embedded ratio of retaining structures and improved depth of ground at an excavation base are two key factors for the stability of deep excavation. For a collapsed excavation in soft cl...

Inversion of the 3D in situ stress field in a complex valley area around the underground cavern group of the Yingliangbao Hydropower Station

The inversion of three-dimensional in situ stress fields in complex river valleys is of great significance for evaluating the excavation stability of deep underground caverns. Based on known in situ stress measurements, it is a major method for obtaining the geostress of the engineering area through the inversion geostress of the whole area with the application of geo-mathematical mechanics. However, the inversion results are often limited by the geological structure and heterogeneous alterated rock, as well as the measurement data reliability. A three-dimensional nonlinear neural network inversion was combined with a simulation of stratigraphic denudation by stages, tectonic compression, and alteration degree in the fault structure. The model was then applied to an underground cavern group of the Yingliangbao Hydropower Station. Comparative analysis showed that the inversion results were in reasonable accordance with the measured in situ stress values. Moreover, the reliability of the three-dimensional in situ stress field obtained using this method was further verified from the stress-induced failure during the cavern excavation. This technique is helpful for optimizing the design of subsequent excavation and reduces the potential instability of surrounding rock during a layered excavation.

Comparison study of Excavation Stability using Different Tunnel Excavation Methods

For the safety and efficiency of tunnel construction, optimal schemes for tunnel excavation with different surrounding rock grades should be proposed to provide references for engineering construction. Through 3D numerical simulation, the mechanical properties of the surrounding rock during tunnel construction were studied by 3D numerical simulation. For different surrounding rock grades, three different excavation methods were simulated. A comparative analysis of ground settlement, surrounding rock displacement, and maximum principal stress caused by different construction methods was also performed. The results show that different construction methods have a small impact on the displacement of surrounding rock. When the surrounding rock is of grade I, the maximum vertical displacement of the excavation surface is less than 2 mm. Bench cut method can be used for excavation with fast construction speed. When the surrounding rock is of grade IV, the stratum settlement and excavation surface displacement are large. The Center diagram method can ensure the construction progress and minimize the vertical displacement of surrounding rock. When the surrounding rock is of grade V, the displacement caused by tunnel excavation further increases. The double-side drift method can be adopted to minimize the stratum settlement and surrounding rock displacement.

Research on the Reasonable Strengthening Time and Stability of Excavation Unloading Surrounding Rock of High-Stress Rock Mass

This study is aimed at better understanding the deformation and failure mechanism of surrounding rock during excavation unloading of a high-stress rock mass and determining the reasonable reinforcement time for the surrounding rock. To fulfill this aim, true triaxial tests were carried out on different loading and unloading paths during the unilateral unloading of a high-stress rock mass. The variational condition for minimization of plastic complementary energy is obtained, the optimal reinforcement time is determined, and the range of the plastic zone in the surrounding rock reinforced by anchor mesh-cable-grouting is compared and analyzed. The results are as follows: (1) Based on the Mohr-Coulomb yield criterion and the deformation reinforcement theory of surrounding rock, the stable state with the minimum reinforcement force is obtained. (2) After the true triaxial tests on the unilateral unloading of the third principal stress were carried out under different confining pressures, loading continued to be performed. Compared with rock failure without confining pressure, in the conventional uniaxial compression test, the failure of samples is dominated by composite splitting-shear failure; the unilateral unloading stress-concentration failure is a progressive failure process of splitting into plates followed by cutting into blocks and then the ejection of blocks and pieces. (3) The relationship between the time steps of the surrounding rock stability and the excavation distance is obtained. The supporting time can be divided into four stages: presupport stage, bolt reinforcement stage, anchor cable reinforcement stage, and grouting reinforcement stage. (4) In the range of within 5 m behind the tunneling face, the plastic zone of the surrounding rock with support is reduced by 7 m as compared with that with no support. In the range of over 5 m behind the tunneling face, the plastic zone of the roadway floor with support is reduced by 2.6 m as compared with that without support, and the deformation is reduced by 90%. These results can serve as a reference for controlling the behavior of surrounding rock during excavation unloading of high-stress rock masses.

Influence of Pre-Stressing on Tieback Retaining Wall for Sandy Soils Excavations

Pre-stressed ground anchor systems or tieback systems are commonly used at wide and irregular-shaped excavations, with the advantage of lower cost and ease of construction compared to the braced excavations, but they come with the drawback on permits for excavations near buildings and tunnels. Research on tieback systems in sands was generally conducted. However, the studies on the correlation between the retaining wall deflection and pre-stress force are few. The objectives of this paper are to study the influence of pre-stress force, depth of excavation, wall embedment length, and soil shear strength that is represented by soil friction angle on the deflection and soil pressure acting on the retaining wall. The parametric study was conducted on an excavation in sand using the finite element method with the Hardening soil model. The results showed that a 50 kN/m increase in pre-stress force reduced the wall deflection on top of the wall by 0.005–0.083% of excavation depth. However, the pre-stressing influence in reducing wall deflection at excavations became less significant along with the sand density increase due to higher friction angle contribution to excavation stability. Moreover, the pre-stress force needed for stabilization of the wall with long embedment length is smaller than those on the wall with shorter embedment length, since the embedment length increase of 0.25 times of excavation depth reduces wall top deflection by 0.002–0.095% of excavation depth. Also, the increase of soil density reduces the need for wall embedment length, so at dense sand, the embedment length of 0.5 times of excavation depth is sufficient to support the excavation.

Experimental study on permeability evolution and nonlinear seepage characteristics of fractured rock in coupled thermo-hydraulic-mechanical environment:a case study of the sedimentary rock in Xishan area

An understanding of how the fracture permeability evolves in a coupled thermo-hydraulic-mechanical (THM) environment at real-time ultrahigh temperature has a great significance for many underground excavations. This paper designs a testing apparatus to take the effects of THM processes into account by coupling high temperatures and high triaxial stress. Correspondingly, the permeability evolution of fractured rock in the coupled THM environment under real-time ultrahigh temperature (20–650 °C) and triaxial stress (confining pressure of 5–25 MPa)/seepage pressure (0.5–6 MPa) conditions is studied. Then, using the X-ray micro-computed tomography technique, the evolution of the fracture aperture in the fractured rock subjected to various conditions is analyzed. The results indicate that for the coupled THM environment, the variations of the volumetric flow rate, nonlinear seepage parameters, and the fracture aperture of the fractured rock with the temperature and the triaxial stress are not straightforward, and the inflexion points of the curves happen at a confining pressure of 15 MPa and a temperature of 300 and 500 °C. Second, in the temperature range of 20–300 °C, in all the triaxial stress states, the volumetric flow rate and the fracture aperture decline slightly with an increase in the temperature due to the recoverable elastic compression on the propping asperities. However, in the temperature range of 300–650 °C (at a confining pressure equal to or lower than 15 MPa), the parameters rise slightly with an increase in the temperature due to the relatively large axial and lateral deformation of the semicylinders. Third, in temperature ranges of 500–650 °C (at a confining pressure equal to or lower than 15 MPa) and 300–650 °C (at a confining pressure higher than 15 MPa), the parameters rise sharply owing to the lateral fracturing of the semicylinders, so the variational trends are irregular. The findings of the current work can provide theoretical guidance on the assessment of the stability and compactness of high-temperature underground excavations.

Numerical analysis of deep twin excavations and boreholes for heavy oil production

One of the plans for the production of Turkey’s largest oil reserve, Bati Raman (1.85 billion barrels), is to use Mining-Assisted Heavy Oil Production (MAHOP). In this method, twin declines are excavated from the surface to the reservoir and a series of excavations (galleries) continue along the bottom of the reservoir. Fan-shaped steam injection and production holes are drilled in the reservoir from the crown of the galleries to use conventional Steam-Assisted Gravity Drainage (SAGD). The Bati Raman reservoir is situated at an average depth of 1450 m, producing 12° API heavy oil from a 60m thick calcareous reservoir rock. This research aims to numerically investigate the stability of excavations together with the initiation, formation and propagation of fracture networks between the fan-shaped boreholes and around typical twin excavations using FLAC3D. It is shown that the stability of twin excavations and pillars between them can be guaranteed with suitable support types. In addition, this innovative approach in heavy oil production in hard rock media successfully showed that the fracture networks created by MAHOP could be used effectively for oil recovery of reservoir rocks using SAGD.

Investigation into the influence of macro-scale geometry on the stability of underground excavations in layered rock masses

Underground excavations for the development of infrastructure, mining operations, waste management etc. require support measures to ensure stability both during their construction and operational phases, which highly depends on the rock mass strength and deformability. Discontinuities within the rock mass are intrinsic features which determine said strength and deformability and affect the excavation stability. Several defining parameters demonstrate that rock mass variability has significant control in the material response during an excavation, especially in sedimentary rocks. In this paper, the Thornhill Sandstone is examined by applying numerical analysis and its mechanical response is investigated under several scenarios with varying excavation geometries, stress conditions while incorporating the influence of discontinuities explicitly within the numerical model. A two-dimensional parametric analysis is performed by applying the Finite Element Method (FEM) and integrating Discrete Fracture Networks (DFNs). More specifically, four different excavation geometries and three different in situ stress regimes are investigated. Drawing on the results, this paper provides insights for tunnel design optimisation when layered rock masses are encountered during an excavation in various underground stress environments with different opening geometries.

Impact of Road Cuts in Slope Stability in Hilly Regions of Nepal

This paper reviews the geological and engineering aspects of rural road construction in the hilly areas of Nepal. The general background in geological, climatic and geographical setting is briefly presented in reference to the five-zone Himalayan model for the Nepal Himalayas. Then, alignment selection of rural roads is discussed in the context of the five zone mountain model. The impact of road cross section design and construction on mountain slopes has been studied. The cut width is a key geometric design parameter that has a significant impact on slope stability and volume of excavation. The choice of cut width in cross-section is reviewed and appropriate cut width in cross-section is recommended in terrain slopes to minimize slope failures and volume of excavation.

Analysis on excavation stability of tunnel portal with different design methods

The stability of tunnel portal has its particularity. Due to the influence of natural or human factors, it is easy to produce cracking, landslide, collapse and other diseases. Taking the No.5 highway tunnel of F3 bid in Georgia as the background, using the finite element analysis software MIDAS-GTS to establish a three-dimensional model to simulate the tunnel construction process. The tunnel portal adopts two design schemes of micro pile wall support and bolt mesh shotcrete support. Through the analysis and comparison of the transverse and vertical displacement and stress field characteristics of surrounding rock caused by two schemes of tunnel excavation, the influence of tunnel excavation on the stability of tunnel portal is studied. The numerical results show that the maximum transverse and longitudinal displacements of surrounding rock are 3.55mm and 8.73mm respectively, and the maximum tensile stress is 1.91mpa. Compared with the micro pile wall support design, the bolt mesh shotcrete support scheme is more feasible. The conclusion has a certain guiding role for the construction of highway tunnels in mountain areas.

Assessing TBM performance in heterogeneous rock masses

A major challenge that TBM performance is requested to deal with for a successful and effective progress is tunnelling through lithologically and geomechanically heterogeneous rock masses. Such heterogeneous environments are common and recent tunnel examples in the UK include the Hinckley Point C offshore cooling tunnels being driven through interbedded carbonaceous mudstone/shales and argillaceous limestone and the Anglo American’s Woodsmith Mine Mineral Transport System tunnel in Redcar Mudstone with beds of ironstone. This inherent geological heterogeneity leads to difficult tunnelling conditions that initially stem from predicting a sound and representative ground model that can be used to preliminary assess the TBM performance. In this work, an exhaustive review of existing TBM Penetration Rate (PR) methods identified that no models address the issue of parameter selection for heterogeneous rock masses comprising layers with different rock strengths. Consequently, new approaches are required for estimating rock mass behaviour and machine performance in such environments. In the presented work the Blue Lias Formation (BLI), which is characterised by its layered rock mass, comprising very strong limestone, interbedded with weak mudstone and shales, is investigated. BLI formation is considered herein being a representative example of lithological heterogeneity. Based on the fieldwork carried out in three localities in the Bristol Channel Basin (S. Wales and Somerset), geological models are produced based on which a geotechnical model is developed, and four ground types are determined. Implications of the current findings for TBM performance are assessed, including faulting, groundwater inflow and excavation stability with a particular focus on both PR and advance rate. A modified approach using the existing empirical models is proposed, developed and presented in this paper that can be used as a guide to determine TBM performance in heterogeneous rock masses reducing the risk of cost and time overruns.

Influence of the microstructure and roughness of weakness planes on the strength anisotropy of a foliated clay-rich fault gouge

Cataclastic rocks, as clay-rich fault gouges, are commonly present in brittle rock masses when fault zones appear during geological engineering projects. Highly deformed rocks that are of poor mechanical quality can lead to technical, safety, and economic problems in rock engineering. The aim of this study is to characterise the resistant behaviour of a highly deformed clay-rich gouge >40 m wide with a marked tectonic fabric that indicates strength anisotropy. We present the results of consolidated-undrained (CU) triaxial tests that were performed at low confining pressures (50, 150, and 300 kPa) on several sets of foliated gouge specimens with four different orientations in the tectonic fabric. Specimens were collected from the encapsulated rock cores of two research boreholes drilled through the Alhama de Murcia Fault (AMF), a main regional fault located in SE Spain. The strain–stress relationships and failure modes were established, indicating that the gouge behaves as hard soil or very soft rock. The test results were adjusted at each orientation using the non-linear Hoek and Brown criteria by considering the fault gouge as an intact material or as a tectonised rockmass. Here, we use the Geological Strength Index (GSI) as an indicator of the rockmass strength that depends on the direction of the tectonic fabric. However, the results from specimens with tectonic fabric that is oriented most favourably for failure were not the weakest in terms of rock strength. Such an anomalous result could be the result of asymmetry in the roughness of the weakness planes that is related to the original gouge microstructure characterised by the strong reorientation of clays in an S-C′ like tectonic fabric.Our results will be useful for practical applications that are related to the stability of slopes and/or shallow underground excavations in brittle fault zones, and provide an inexpensive and easy way to preliminarily evaluate the anisotropic behaviour of this type of brittle fault zones for future engineering projects.

Analysis on Deformation and Stability of Tunnel Anchor Excavation under Broken Rock Mass

Based on the concrete engineering support of the tunnel anchor of the Dadu River Bridge on the Sichuan-Tibet Railway, the geological background of broken rock mass, extremely developed joints and fissures, and poor self-stability of surrounding rock within the range of side anchors in Chengdu, a three-dimensional elastoplastic numerical calculation method is used. The current engineering geological conditions and the tunnel anchor design plan are to establish a three-dimensional fine numerical simulation model of the Chengdu side tunnel anchor that comprehensively considers rock mass unloading, stratum classification, and anchor tunnel excavation and support structure. The “excavation-support” process in each construction footage is simulated to realize the three-dimensional calculation of the dynamic construction and support of anchor tunnels, and to study the surrounding rock deformation, stress and plastic zone distribution changes during the excavation process. Comprehensive analysis found that the deformation of surrounding rock during excavation shows that it develops toward the empty surface, and the displacement distribution is obviously affected by the distribution of ground stress and rock formation. The greater the buried depth, the greater the deformation of the surrounding rock.