Excavation Disturbed Zone Research Articles

Modeling of spalling failure of in-situ rock mass for quantification of the source mechanics with flat-joint model and moment tensor

Spalling failure caused by excavation unloading in tunnels under high in situ stress is a common phenomenon. Here, we attempt to use a three-dimensional flat-joint model (FJM3D) combined with a moment tensor to simulate the process of spalling failure. FJM3D simulation showing the process of micro-crack initiation and propagation in the surrounding rock from a micro-perspective. The nature of the source mechanism is quantitatively assessed by moment tensor analysis. There are a large number of micro-cracks extending deep at the shoulder and foot of the tunnel. This model reproduces similar thin slabs and fracture orientations in Mine-by Experiment (MBE). The excavation disturbed zone (EDZ) characteristics around the MBE show that the tunnel crown and invert are dominated by tensile mechanisms, the tunnel sidewalls are dominated by implosive and shear mechanisms, and the tunnel shoulders are butterfly shaped with tensile and implosive sources. The nature of these sources are not purely tensile/shear cracks, but some isotropic/deviatoric components also play a role. The magnitude and orientation of the principal stress near the excavation face are linked to the advancement of the tunnel face and are related to the direction of micro-cracks initiation and propagation, which sheds new light on the failure mechanics of spalling failure.

Modelling of the Corrosion-Induced Gas Impact on Hydraulic and Radionuclide Transport Properties of Geological Repository Barriers

The geological disposal of high-level radioactive waste is the final step in the nuclear fuel cycle. It is realized via isolating the high-level radioactive waste in the geological environment with an appropriate system of engineered barriers. Radionuclides-containing materials must be isolated from the biosphere until the radioactivity contained in them has diminished to a safe level. In the case of high-level radioactive waste, it could take hundreds of thousands of years. Within such a long timescale, a number of physical and chemical processes will take part in the geological repository. For the assessment of radionuclide migration from a geological repository, it is necessary to predict the repository’s behavior once placed in the host rock as well as the host-rock response to disturbances due to construction. In this study, the analysis of repository barriers (backfill, concrete, inner excavation disturbed zone (EDZ), outer EDZ, host rock) thermo–hydraulic–mechanical (THM) evolution was performed, and the scope of gas-induced desaturation was analyzed with COMSOL Multiphysics. The analysis was based on modelling of a two-phase flow of miscible fluid (water and H2) considering important phenomena such as gas dissolution and diffusion, advective–diffusive transport in the gaseous phase, and mechanical deformations due to thermal expansion of water and porous media. The importance of proper consideration of temperature-dependent thermodynamic properties of water and THM couplings in the analysis of near-field processes was also discussed. The modelling demonstrated that such activities as 50 years’ ventilation of the waste disposal tunnel in initially saturated porous media, and such processes as gas generation due to corrosion of waste package or heat load from the waste, also led to desaturation of barriers. H2 gas generation led to the desaturation in engineered barriers and in a part of the EDZ close to the gas generation place vanishing soon after finish of gas generation, while the host rock remained saturated during the gas generation phase (50–100,000 years). Radionuclide transport properties in porous media such as effective diffusivity are highly dependent on the water content in the barriers determined by their porosity and saturation.

Stability of complex rock tunnel face under seepage flow conditions using a novel equivalent analytical model

The longitudinally inclined layered rock masses under seepage flow conditions have an important impact on tunnel face stability. To solve this issue, a novel equivalent analytical model is proposed in this paper. This model settles the puzzle that the traditional model of limit analysis cannot directly determine the hydraulic head by treating the distribution hydraulic head at the anterior aspect of the tunnel face with the equivalent method, that is, without the tedious calculation by the numerical software. The seepage force is directly considered as a body force in this analytical model. Then, combined with the mirror-image method, layered discretization technique, limit analysis method, and Hoek-Brown yield criterion, an analytical expression for stability determination can be derived. The main factors affecting the stability of tunnel face are analyzed, such as the inclination angle of the flow line, the inclination angle of the rock layer, hydraulic conductivity, rock quality, excavation disturbed zone (EDZ), and the groundwater table height. The calculated diagrams have a certain guiding effects for tunnel construction.

Experimental study on the physico-mechanical properties of Tamusu mudstone – A potential host rock for high-level radioactive waste in Inner Mongolia of China

Tamusu mudstone, located in Bayin Gobi Basin in Inner Mongolia of China, has been selected as a potential host rock for high-level radioactive waste (HLW) disposal in China. A series of tests has been carried out, including X-ray diffraction (XRD) tests, scanning electron microscopy (SEM) tests, disintegration tests, permeability tests and triaxial compression tests, to estimate the physico-mechanical properties of Tamusu mudstone in this work. The mineral composition of Tamusu mudstone was analyzed and it was considered as a stable rock due to its low disintegration rate, i.e. approximately 0.11% after several wet/dry cycles. Based on the results of permeability test, it was found that Tamusu mudstone has a low permeability, with the magnitude of about 10 –20 m 2 . The low permeability makes the mudstone well prevent nuclide migration and diffusion, and might be influenced by temperature. The triaxial tests show that Tamusu mudstone is a stiff mudstone with high compressive strength, which means that the excavation disturbed zone would be smaller compared to other types of mudstone due to construction and operation of HLW repositories. Finally, the properties of Tamusu mudstone were compared with those of Opalinus clay, Callovo-Oxfordian (COx) argillite, and Boom clay to further discuss the possibility of using Tamusu mudstone as a potential nuclear waste disposal medium.

Stress and displacement fields around an arbitrary shape tunnel surrounded by a multilayered elastic medium subjected to harmonic waves under plane strain conditions

This study focuses on the propagation of harmonic mechanical waves around both circular and non-circular tunnels. The excavation disturbed zone is explicitly modeled with a multilayered description of the surroundings of the tunnel. The purpose of this paper is thus to establish closed-form solutions for stress and displacement fields for circular and non-circular geometries taking into account the mechanical heterogeneity of the area adjacent to the tunnel. Particularly, the multilayered non-circular shape configuration is a great step not treated previously. The classical approach based on the wave function expansion method is used to determine the expressions of stresses and displacements around circular geometries. To handle non-circular tunnel shapes, the complex variable method based on the Muskhelishvili approach and conformal mapping functions is introduced. Results indicate that the excavation damaged zone strongly modifies the distribution of stress and displacement fields around the tunnel. Geometrical tunnel shapes also affect stress and displacement polar diagrams.

Evaluating changes in the degree of saturation in excavation disturbed zones using a stochastic differential equation

Deformation characteristics of sedimentary rocks significantly change with water content during the drying process. Therefore, the proper evaluation of such drying deformation phenomena is necessary for tunnel construction in which extremely small displacements are allowed, such as geological disposal. Furthermore, it is also essential to accurately assess water content changes in the rock masses. The excavation disturbed zone (EDZ) spreads around the tunnel owing to the excavation process. The EDZ has a higher hydraulic conductivity than an intact rock mass. Therefore, it is essential to predict water content changes in the EDZ within the scope of the drying deformation phenomena. In this study, we derived the exact solution to the Richards’ equation at the Neumann boundary, which can describe the drying phenomena in sedimentary rocks. Using Japanese tuff, we conducted permeability test and mercury intrusion porosimetry tests to obtain the water diffusion coefficient and verify whether its drying behavior can be described using the exact solution. Using the verified exact solution, we propose a new stochastic differential equation that could be used to explain the local decrease in water content and the increase in variations in the EDZ, and we applied this stochastic differential equation to the 2D tunnel problem.

Study on the excavation disturbed zone during tunneling in sandy cobble stratum considering the material meso-structure

The excavation disturbed zone plays an important role in the evaluation of tunnel stability. The identification of the excavation disturbed zone is perceived as a main concern. Taking the material meso-structure into account, this paper investigates the construction mechanical behavior in heterogeneous sand cobble stratum and aims to precisely predict the excavation disturbed zone. Firstly, the meso-scale numerical models were used to simulate tunnel excavation in sandy cobble strata, in which the large-sized rock blocks and fine-grained soil matrix were treated as separate constituents. Then, the stress redistribution and deformation performance in the ground during tunneling were explored in detail. Subsequently, combined with the deformation performance characteristics, a method for identifying the excavation disturbed zone was proposed. The boundary of the excavation disturbed zone can be identified by the displacement mutation point of the monitoring paths. Finally, a parametric study was performed to understand the influence of key factors on the excavation disturbed zone, including the rock meso-structure parameters, the soil matrix properties, and the tunnel geometry characteristics. The simulation results indicate that both the rock content and rock size have a significant impact on the excavation disturbed zone, the strength parameters and lateral pressure coefficient have a marked effect, and the tunnel radius and tunnel burial depth have a negligible influence on the excavation disturbed zone under the given conditions when normalized.

Deformation modulus of rock foundation in Deriner Arch Dam

Civil engineering structures such as gravity concrete dams, arch dams, pressure tunnels, foundations of high-rise buildings and bridges are generally founded on rock. Design and performance of these structures are highly dependent on the deformation modulus of the rock mass so that expected deformations and/or differential settlements stay within the tolerable limits of the structures. In situ deformation modulus measurements of rock masses are generally performed by using the borehole dilatometer and plate loading tests in Turkey. The diameter of plate loading test device is 30-35 cm. Tests are performed in small unlined tunnels excavated in different elevations of dam abutments. The deformations upon loading are measured on the loading plate in the plate loading tests. It is not possible to exclude the effect of excavation disturbed zone since deformations are not measured with these devices. Large diameter plate test system may have multiple level extensometers set in drill holes opened perpendicular to loading plates. Reading from these extensometers may be utilized to exclude the effect of disturbed zone close to surface together with stress distribution estimated by using elastic theory. There are several field test methods for determining deformation modulus and each method has its own shortcomings. The optimum methodology to correlate plate loading test results with corresponding in-situ deformation modulus values is to back-calculate deformation modulus by using settlements measurements during the construction of dam body. In this paper, Deriner Arch Dam settlements measured during the construction are used to back calculate deformation modulus. It is found that in situ deformation modulus is about two times higher than the average value determined by plate loading tests. This finding will have important effects on the depth of foundation excavations, concrete layers to fortify structure foundations and the amount of consolidation grouting

Comparative Investigation on the Compression–Shear and Tension–Shear Behaviour of Sandstone at Different Shearing Rates

Direct shear tests on rocks under compression–shear stress conditions have been widely conducted, whereas few have been performed under tension–shear stress conditions. However, rocks exhibit tension–shear failures in many scenarios, such as in the excavation disturbed zone in deep underground caverns and high slopes. A series of direct shear tests were performed with cuboid sandstone specimens under different normal tensile stresses (σn = − 3, − 2, and − 1 MPa) and compressive stresses (σn = 1, 3, and 5 MPa) at different shearing rates (v = 0.2, 1, 5, and 10 mm/min). The tension–shear tests were performed using an auxiliary device in combination with a compression–shear testing machine. The results showed that the fracture, shear stress–displacement curve, shear stiffness and shear strength were affected by both σn and v, and the differences in these mechanical behaviour between compression–shear and tension–shear cases were analysed in detail. The shear strength had a nonlinear relationship with both σn and v in the full region of tested normal stress (namely, the normal stress range from tension to low compression). The Hoek–Brown criterion $$(\tau = A(\sigma_{{\text{n}}} - \sigma_{{\text{t}}} )^{B} )$$ considering the shearing rate effect (the relationship between parameter A (B) and shearing rate v is represented by a natural logarithm function) was proposed as the optimal shearing rate-dependent strength criterion for sandstone in the tested normal stress range.

Gas permeability evolution of granite under confining pressure unloading tests

Tunnel excavations in rock usually cause initial stress release, which is different from the stress path in classical triaxial compression tests. To analyze the permeability variations in excavation disturbed zone, two different stress paths, e.g. classical triaxial compression and confining pressure unloading, are applied on granite, and the permeability at different damage levels is measured using gas as transport medium. The differences in deformation and failure mechanism and their impacts on gas permeability variation are discussed. The test results show that lateral deformation of samples in confining pressure unloading tests is obviously larger than that in classical triaxial compression tests. Unlike the permeability decrease-increase transition in classical triaxial compression tests, the permeability in confining pressure unloading tests shows an overall progressively increase trend. The permeability in both tests increases significantly near failure and achieves the same order of magnitude. The differences in permeability evolutions under the two stress paths are directly attributed to their different deformation mechanisms. The test results could provide a new understanding of permeability properties during the underground excavation.

Gas transport modelling at different spatial scales of a geological repository in clay host rock

The work presented in this article is a part of the international European Commission FORGE project which studied the behaviour of repository gases in the context of radioactive waste disposal. Experimental work is essential to understand the main transport mechanisms for gas and to determine the main geophysical parameters. However, while laboratory and rock experiments can help to investigate the short- and partly the middle-term behaviour of gas in a rock formation, long-term predictions have to be based on numerical simulations. Numerical simulation of long-term gas behaviour in a deep geological repository was one of the aims in the FORGE project. The objectives of the FORGE modelling were to describe the state-of-the-art consideration of gas in performance assessment, and to propose an updated treatment of gas issues in long-term safety assessments for high-level radioactive waste and spent nuclear fuel. Three benchmark exercises for a hypothetical geological repository in clay host rock ranging in scale from a single disposal cell to the whole repository were defined. To provide added value to this benchmark, a feature not yet well represented in typical gas simulations was introduced: the explicit representation of the interfaces between the excavation disturbed zone and the engineered elements within the excavation, such as waste canisters, bentonite plugs, and access drifts. In order to assess gas transport behaviour at the whole repository scale, models were developed with ‘mathematical’ or ‘numerical’ upscaling techniques for small-scale features. The assessment across different modelling scales revealed the main long-term gas migration pathways and led to the conclusion that the explicit representation of interfaces is not important.

Stability of large underground caverns excavated in layered rock masses with steep dip angles: a case study

The Wudongde Hydropower Station is constructed in layered strata that typically have steep dip angles. These features, together with the folds and faults that have developed in the strata, directly threaten the stability of the underground powerhouse. In this work, the behavior of the surrounding rocks under the original support measures is detailed in terms of rock mass deformation, anchor bolt stress, and extent of the excavation-disturbed zone, as obtained from safety monitoring and field testing. The characteristics of deformation and cracking of the surrounding rocks were also interpreted. To enhance rock mass stability, the support design and construction process were optimized and the positions liable to cracking treated. The efficiency of suggested scheme is verified by subsequent safety monitoring data and field testing. Then, a transversely-isotropic elastoplastic constitutive model for the layered rock masses was established. Numerical simulations are performed during the construction process to provide reference for decision making. The calculated deformations are in good agreement with the monitored values and they tended to be more stable. The plastic zones of the surrounding rocks fall within the range controllable using cables. Thus, the proposed measures are necessary and ensure sufficient stability. Finally, the mechanisms for the deformation and cracking of the surrounding rocks during the construction process are discussed. The work described aims to deepen our understanding of the mechanical properties of layered rock masses with steep dip angles and to improve the design philosophy.

A coupled hydro-mechanical creep damage model for clayey rock and its application to nuclear waste repository

In this paper we propose a new nonlinear elasto-viscoplastic damage model, based on a modified Mohr-Coulomb criterion, to study the creep and seepage in clayey rock during construction of a high-level radioactive waste repository through laboratory experiments and field tests. First, three types of damage evolution equations are constructed by using the relationship between the damage variable and the strain. Then, a self-healing model is investigated for the clayey rock by considering the damage, confining pressure, pore water, and duration of saturated state. By introducing the damage, permeability evolution and self-healing as the key factors, a fully coupled hydro-mechanical model for clayey rock is developed with the commercial software ABAQUS. The hydro-mechanical behaviour in the surrounding rock is simulated with the proposed model considering the actual construction of the repository. The numerical results show that the construction quality has a significant effect on the stability of the rock formation, and that the extent of the horizontal gallery disturbed by shield tunnelling is less than that of the test drift disturbed by using jackhammers method. The creep damage of the surrounding rock increases rapidly at the early stage and tends to stabilize gradually after 15 years, and the damage in the middle part of the surrounding rock is larger than that in the bottom and top parts. In addition, due to the self-healing effect of clayey rock, around three years later, the permeability of the excavation disturbed zone (EDZ) is close to that of the original clayey rock with an order of magnitude 10−19 m2. The present model can also be used to predict the long-term stability of tunnels.

Deep Geological Disposal of High-Level Radioactive Wastes and Coupled Thermal-Hydraulic-Mechanical-Chemical Analysis

In this paper, coupled thermal-hydraulic-mechanical-chemical(THMC) processes to be considered in deep geological disposal of high-level radioactive wastes and their characteristics are introduced. We also investigated and introduced the characteristics of an excavation disturbed or damaged zone(EDZ) since the coupled THMC processes may concentrate and play a significant role in EDZ, where irreversible mechanical deformation is induced, considering that a crystalline rock is dominant in our geologic conditions. Numerical approach can be more effective in dealing with various uncertainties in individual process and parameters as well as long time period so that the types and properties of coupled analysis to simulate coupled THMC processes were identified as well.

Stability of Large Parallel Tunnels Excavated in Weak Rocks: A Case Study

Diversion tunnels are important structures for hydropower projects but are always placed in locations with less favorable geological conditions than those in which other structures are placed. Because diversion tunnels are usually large and closely spaced, the rock pillar between adjacent tunnels in weak rocks is affected on both sides, and conventional support measures may not be adequate to achieve the required stability. Thus, appropriate reinforcement support measures are needed, and the design philosophy regarding large parallel tunnels in weak rocks should be updated. This paper reports a recent case in which two large parallel diversion tunnels are excavated. The rock masses are thin- to ultra-thin-layered strata coated with phyllitic films, which significantly decrease the soundness and strength of the strata and weaken the rocks. The behaviors of the surrounding rock masses under original (and conventional) support measures are detailed in terms of rock mass deformation, anchor bolt stress, and the extent of the excavation disturbed zone (EDZ), as obtained from safety monitoring and field testing. In situ observed phenomena and their interpretation are also included. The sidewall deformations exhibit significant time-dependent characteristics, and large magnitudes are recorded. The stresses in the anchor bolts are small, but the extents of the EDZs are large. The stability condition under the original support measures is evaluated as poor. To enhance rock mass stability, attempts are made to reinforce support design and improve safety monitoring programs. The main feature of these attempts is the use of prestressed cables that run through the rock pillar between the parallel tunnels. The efficacy of reinforcement support measures is verified by further safety monitoring data and field test results. Numerical analysis is constantly performed during the construction process to provide a useful reference for decision making. The calculated deformations are in good agreement with the measured data, and the calculated forces of newly added cables show that the designed reinforcement is necessary and ensures sufficient stability. Finally, the role of safety monitoring in the evaluation of rock mass stability and the consideration of tunnel group effect are discussed. The work described in this paper aims to deepen the understanding of rock mass behaviors of large parallel tunnels in weak rocks and to improve the design philosophy.

Determination of elastic modulus of claystone: Nano-/micro-indentation and meso-compression tests used to investigate impact of alkaline fluid propagation over 18 years

Micro-mechanical properties of a claystone were tested after undergoing alkaline perturbation on site (Tournemire, CD borehole) for 18 years. In a saturated context and outside the excavation disturbed zone (EDZ), the claystone exhibits a 11.6-mm black rim at the cement/paste interface, which shows a different mechanical behaviour from the rest of the claystone. Three sets of measurements of elastic modulus were performed using: (i) nano-indentation tests with a constant indentation depth of 2 μm, (ii) micro-indentation tests with a constant indentation depth of 20 μm, and (iii) meso-compression tests with a constant displacement of 200 μm. The increase of the modulus of deformability in the black rim is between 15 GPa and 20 GPa according to the scale. Moreover, an overall decrease of the modulus of deformability from the smallest to the largest scale is observed in each zone. In view of the mineralogy and petrographic observations, higher values of modulus of deformability in the black rim are related to carbonate content and its distribution. Precipitation of cementitious carbonates as inclusions and very thin partings leads to hardening of the claystone.

Direct Shear Tests of Sandstone Under Constant Normal Tensile Stress Condition Using a Simple Auxiliary Device

Tension–shear failure is a typical failure mode in the rock masses in unloading zones induced by excavation or river incision, etc., such as in excavation-disturbed zone of deep underground caverns and superficial rocks of high steep slopes. However, almost all the current shear failure criteria for rock are usually derived on the basis of compression–shear failure. This paper proposes a simple device for use with a servo-controlled compression–shear testing machine to conduct the tension–shear tests of cuboid rock specimens, to test the direct shear behavior of sandstone under different constant normal tensile stress conditions (σ = −1, −1.5, −2, −2.5 and −3 MPa) as well as the uniaxial tension behavior. Generally, the fracture surface roughness decreases and the proportion of comminution areas in fracture surface increases as the change of stress state from tension to tension–shear and to compression–shear. Stepped fracture is a primary fracture pattern in the tension–shear tests. The shear stiffness, shear deformation and normal deformation (except the normal deformation for σ = −1 MPa) decrease during shearing, while the total normal deformation containing the pre-shearing portion increases as the normal tensile stress level (|σ|) goes up. Shear strength is more sensitive to the normal tensile stress than to the normal compressive stress, and the power function failure criterion (or Mohr envelope form of Hoek–Brown criterion) is examined to be the optimal criterion for the tested sandstone in the full region of tested normal stress in this study.

Gas migration modelling in geological repository module in clay formation and sensitivity analysis

Corrosion of steel canisters disposed of in a geological repository for high-level waste leads to generation and accumulation of hydrogen gas which may significantly affect long-term safety of the repository. Numerical modelling can be used to predict the likely hydraulic evolution of such a disposal facility and to estimate the influence of generated gas on repository safety.This article presents a two-phase flow (not taking into account mechanical coupling) gas migration model developed in the original version of the computer code TOUGH2-MP (USA). An important and new aspect of this model is its capability to introduce a network of very thin interfaces in three-dimensional analysis of gas migration. This model was verified in the international benchmark exercises in the recent European Commission project FORGE (Fate Of Repository GasEs). The analysis was focused on the results of a second benchmark exercise, i.e. hydrogen gas behaviour in a part of a geological repository (module) situated in clay formation, and sensitivity analysis. The results showed that the interfaces, backfill and excavation disturbed zone are the main migration and accumulation media for gaseous hydrogen in the repository module. The peak pressure in the module did not exceed the geological environment pressure at a depth of 500m, and is not sufficient to disturb mechanical stability and functionality of the system of engineered and natural barriers.

Large scale three-dimensional seepage analysis model test and numerical simulation research on undersea tunnel

During the construction process of Qingdao Jiaozhou Bay Undersea Tunnel, the faults and other unfavorable geological discontinuities were often encountered. To study the water inrush mechanism in the faults, both physical model test and numerical analysis were carried out. The results of crown displacement and hydraulic pressure of the monitoring sections in the physical model and numerical model were analyzed in this paper. It was found that the displacement and hydraulic pressure in the process of tunnel construction are often interacted as both cause and effect, and the lower of hydraulic pressure is often accompanied with the growth of its displacement. The changing of the excavation disturbed zone during the excavation in the undersea tunnel was also studied. The results show that the excavation disturbed zone in fault is larger than that in surrounding rock mass, and the excavation disturbance effects in the filling type fault are both transient and persistent. When the displacement and hydraulic pressure in the undersea tunnel change sharply during excavation, there are relatively slow and continuous change trend of the displacement and hydraulic pressure. For practical purposes, to prevent water inrush in the undersea tunnel, more attentions should also be paid to the undersea tunnel after excavation.

Gas-pressure-induced integrity of salt formations

Abstract Besides other host rocks, rock salt formations are considered for the long-term storage of radioactive waste to exclude any threat to the biosphere. This means that the host-rock's integrity has to be guaranteed during construction, operation and in the post-closure phase of a repository. Gas-transport properties are key issues in the long-term assessment of the storage of high-level radioactive or toxic waste in salt formations. In this context, the impacts of disturbance induced by the excavation of the underground facilities and long-term effects during recompaction of the excavation disturbed zone (EDZ) are important items but in the past significant progress has been made. However, significant gas quantities may be generated in the long term (e.g. due to anaerobic corrosion, if humidity is present), resulting in a time-dependent pressure build-up that may affect the barrier integrity if the acting minimal stress is exceeded. The objective of this paper is to review the current understanding of the gas-transport properties in a salt environment at increased gas pressures associated with a radioactive waste repository. This knowledge has mainly been developed by laboratory work and fieldwork over the last decade.