Weak Rock Masses Research Articles

Assessment of damage zone thickness and wall convergence for tunnels excavated in strain-softening rock masses

There are two fundamental issues in all underground excavations, which are safety and economy. To ensure safety and expedite excavations, level of tunnel wall convergences and damage zone thickness should be predicted before the excavation starts, should be determined accurately, and monitored during the excavation by the tunnel designer. However, accurate prediction of these two parameters is difficult unless in-situ stress and deformation measurement tools are used. In this study, damage zone thickness and tunnel wall convergence relation was investigated for horseshoe-shaped highway tunnels by using empirical methods. For this aim two-dimensional plane strain finite element models were used. Totally 9 tunnel sections were selected for the empirical analysis from 5 different ongoing tunnel excavations, which are excavated in weak and fair quality rock masses and show strain softening post-failure behavior. With the help of data gathered through in-situ convergence measurements and predicted convergences and rock mass geotechnical properties an empirical analysis was conducted. Engineering characteristics of the tunnel routes were determined by means of geological mapping, drillings and laboratory studies. Tunnel wall convergences were measured by optical measurement devices in three-dimensional space. Then, numerical models have been created for each of the tunnel sections studied. For evaluation of damaged zone thickness; yielded elements, volumetric strain, and principal stress concentrations have been used. Damage zone thickness and convergences were estimated from the numerical models and the whole data were compared with the real convergence results. Findings have been compared with previous researchers’ convergence predictions and plastic zone calculation approaches. The results are in agreement both with the field measurements and previous empirical approaches. In this way, an empirical relation has been obtained for tunnel wall convergences and damage zone thicknesses. Besides, through an analysis of the relations of convergences at each query points in plane strain finite element model, a new empirical relation has also been put forth that gives “Convergence Constant” for modelled cross-section. This constant can be used as an auxiliary tool for prediction of next section convergences, on the condition that excavation has similar geological, geotechnical properties and topography.

Study on time-varying viscosity of cement and sodium silicate grout under seawater condition

For practical purposes, the grouting technique is an effective method to strengthen weak rock masses around subsea tunnels, while clarifying the effect of seawater condition on grout properties is the basis for engineering applications. This study mainly focuses on the time-varying viscosity of grout under seawater condition. The cement and sodium silicate (C-S) grout with different ratios that commonly used in engineering cases was selected in this study. Through the digital display rotating viscometer (NDJ-8S), the time-varying viscosity of C-S grout was obtained. Moreover, the comparative analysis of grout viscosity under seawater and freshwater conditions was made. The test results show that the time-varying viscosity of C-S grout was obviously influenced by seawater condition and volume ratio (C-S ratio). The variation process of C-S grout viscosity with time under seawater condition can be divided into three stages: (a) initial stage, (b) stable stage and (c) rising stage, while there are only two stages under freshwater condition. Furthermore, the viscosity of grout in seawater is much greater than that in freshwater at the initial stage. After that, the time-varying viscosity models of C-S grout with different ratios were obtained on basis of experimental data. It is hoped that the obtained data will provide aids to the design of grouting with C-S grout in the future.

Proportion optimization of grouting materials for roadways with soft surrounding mass

ABSTRACT Severe deformation and failure frequently occur in roadways with soft or weak surrounding rock and have greatly influenced safe and efficient mining of coal in many coal mines. Using portland cement, emery and fly ash as main raw materials, through laboratory tests, effect of water/binder ratio, cement/sand ratio, water/sodium silicate ratio, water-reducing agent, fly ash/cement ratio, and various performance indexes of grout of fluidity, viscosity, setting time, bleeding rate, compressive strength, concretion rate, and various performance indexes were systematically analyzed. An optimized mixture ratio of the main raw materials added in the grouting material proportion was determined through uniform design method, an optimal mixture ratio was determined by regression analysis. The results show that: 1) The flow performance is significantly affected by change of sodium silicate and water reducer, the compressive strength of grouting material increases significantly with increase in emery content, and decreases significantly with increase in water reducer. 2) An optimized mixture ratio among water-cement ratio, cement-sand ratio, water/sodium silicate ratio, water-reducing agent, fly ash/cement ratio in the grouting material is 0.75, 1.2, 0.08, 0.03, and 0.18, respectively. Field test demonstrated that the material has better performance in reinforcing weak and broken rock mass.

Creep Behaviours of Argillaceous Sandstone: An Experimental and Modelling Study

Understanding the creep behaviours of rocks is essential for the long-term stability of underground excavations in mining engineering. Creep behaviours are more important when the mining depth is greater, which leads to the emergence of weak rock masses and high in situ stresses. In this study, the creep behaviours of argillaceous sandstone (AS) were systematically investigated. For the experimental investigation, creep tests were conducted on AS with different confining pressures (3, 6, 9, 12, 15, and 18 MPa) using an MTS815.02 rock mechanics test system. The mechanical characteristics of AS were analysed. For the numerical study, a nonlinear creep model of AS under equal and different confining pressures was established based on rock creep theory and plastic theory. The results showed that confining pressure could effectively improve the creep failure strength of AS, accelerating its creep deformation rate and process and reducing the final expansion volume. The nonlinear creep model was embedded in the FLAC3D software, and the experimental and numerical results agreed well. The experimental investigation and proposed creep model can provide important guidance in underground mines for safe long-term stability of underground excavations.

Analysis of stability of the Baihetan arch dam based on the comprehensive method

In this paper, the stabilities of the Baihetan arch dam were analyzed by combining a geomechanical model test and the 3D nonlinear finite element method (FEM) based on the comprehensive method. The deformation characteristics and failure patterns of the dam, abutments, and foundations under different overload conditions were modeled and analyzed by the geomechanical model test. Then, the 3D nonlinear finite element calculation was conducted for complement and verification, by which the deformation characteristics and plastic failure zones of the dam and foundation under different overload stages were simulated. Based on the deformation behaviors, failure pattern, and the development of plastic zones of weak structure planes and rock mass in the dam’s abutments and foundation, the impacts of weak structure planes on the safety coefficient of dam abutment and foundation were analyzed. The geomechanical test results and FEM results demonstrated consistency in this study and provided an important basis for engineering safety assessment. The results indicated that the Baihetan arch dam meets the principal stability requirements; however, considering the large deformation and serious cracks in the middle-upper part of the left dam abutment, reinforcement treatment is proposed to improve the abutment stability of the Baihetan arch dam.

Time-dependent support pressure of underground gold mine in Indonesia

The mines expand slowly into deeper deposits through underground mining, and it is related to geotechnical challenges concerned with unexpected failure. The radius of the failure zone and the extent of rock mass deformation in the precinct of opening can be restrained by the implementation of internal support pressure. It is essential to primarily understand the time-dependent behavior of the rock mass. This study aimed to estimate the support pressure of underground gold mine in weak rock mass zone in Indonesia and evaluate the time-dependent support pressure. Rock mass quality Q-system is used as the basis for analysis in this study. To understand the time-dependent behaviour of the rock mass, the support pressure related to the short-term rock mass quality were evaluated. In this study of opening excavated in the weak rock mass, support pressure may progressively increase with time attributable to the creep behavior of the rock mass. The time-dependent behavior of rock mass is indispensable to be contemplated in the long-term design and continuance of underground excavation.

Upper bound solution for the bearing capacity of rock masses considering the embedment depth

The effect of the embedment depth is one of the most important issues in calculating the bearing capacity. The results of the previous studies show that in most cases, it was assumed that the footings are rested at the ground surface and the effect of the embedment depth was considered by applying an equivalent surcharge load to the ground surface. In this paper, the influence of the embedment of the footings on the bearing capacity of weak rock masses was investigated using the upper bound method of limit analysis. The modified Hoek–Brown failure criterion was used for the rock mass. The embedment depth of the footings was considered directly in the upper bound calculations. According to the obtained results, considering the embedment depth results in increasing the ultimate bearing capacity, which cannot be anticipated correctly in the methods which replace the embedment depth by an equivalent surcharge. The results of this paper were presented in the form of a design table that can be used easily in practical applications.

Face Stability Analysis for the Earth Pressure Balance Method in Nonhomogeneous Inclined Soil Layers: Case Study

Earth pressure balance tunnel boring machines (EPB TBMs) were successfully used for tunnel excavation in loose grounds. Determining the minimal support pressure in the tunnel face is one of the most critical steps in tunneling by the earth pressure balance method. This study aimed to investigate the tunnel face stability of the Dez–Ghomrood water transfer tunnel analytically and numerically. During the water transfer tunnel project from Dez to Ghomrood, a very weak rock mass similar to an alluvial zone was encountered. Considering the depth of the tunnel and also the layering of the ground, calculating the proper support pressure of the tunnel face was significantly complex. The analytical method was based on the slope of layers relative to the tunnel face and the limit equilibrium of forces. Numerical analyses were conducted based on three-dimensional finite-element simulations using the PLAXIS 3D tunnel. The results demonstrate that the formation of soil arching yields to the curvilinear distribution of the weight of upper levels, therefore, pressure reduction at the tunnel face. The geomechanics parameters of the ground, shield geometry, and ground convergence in the excavated section are the determining factors in calculating the support pressure of the tunnel face. The slope of ground layers should also be considered in the tunnel face failure mechanism and in support pressure. The operational support pressure is compared to the analysis results, and the good accuracy of the method is confirmed.

An Overview of Design and Construction practices of Himalayan Hydropower tunnels

Due to steep terrain and fast flowing rivers in the Himalayan region of Nepal, medium to mega size hydropower projects are constructing day-by-day. Tunnel is one of the best and short routes for water conveyance system for power production. Hundreds of kilometers of tunnels have been constructed and new tunnels are planned in this region. The availability of high head for hydropower generation, the tunnel cross sections are relatively small, up to 6 m diameter in size, and there is high rock cover above the tunnel alignment. This paper focuses on the design and construction practices of hydropower tunnel passes through weak rock mass with high rock cover of the Himalayan region of Nepal. Most of the hydropower tunnels undergo excessive deformation and support failure during the tunnel construction, which delayed the project as well as increases the cost of project. The paper first discusses the available construction practices for tunnel and underground structures. In the second part, the current practice employed during construction in the Himalayas is discussed along with the shortcomings of the methods and how it is addressed in the region.

New Method of Monitoring Tunnel Feet-Lock Pipe (TFP) Mechanics Using Fiber Bragg Grating (FBG)

Abstract Tunnel feet-lock pipe (TFP) is one of the most effective measures used to control tunnel settlement and is widely used in tunneling with soft ground or weak rock mass. Mechanical analysis is an important method to optimize TFP design and construction, and its results can be used to evaluate the structural stability of the tunnel. Although the traditional direct resistance method (DRM) can measure the mechanics of TFP, it is not suitable for harsh environments (humid environments, chemical etching, and electromagnetic field noise) or long-term monitoring. In this article, a method to measure the mechanics of TFP by using Fiber Bragg grating (FBG) is proposed. The strain and bending of TFP monitored by FBG is used to analyze the mechanics of TFP. In order to verify the feasibility and accuracy of the FBG method to measure the mechanics of TFP, an experimental scheme was designed to compare DRM and theoretical analysis. A supporting device and loading device were developed for the experimental scheme. Compared with the theoretical value, the error of strain and bending measured by FBG was stable within 5 με, the error of strain measured by DRM was approximately 15 με or even 20–30 με, and the error of bending measured by DRM was 10–15 με or even 25 με. Therefore, the precision of measuring the mechanics of TFP by using FBG is higher than by using DRM.

Dynamic and Probabilistic Multi-class Prediction of Tunnel Squeezing Intensity

Tunnel squeezing is a time-dependent process that typically occurs in weak or over-stressed rock masses, significantly influencing the budget and time of tunnel construction. This paper presents a new framework to probabilistically predict the potential squeezing intensity and to dynamically update the prediction during construction based on the sequentially revealed ground information. An extensively well-documented database, which contains quantitative data from 154 squeezing sections with 95 unpublished inventories is established. A Decision Tree method is employed to train a probabilistic multi-classification model to predict the tunnel squeezing intensity. The trained classifier is then integrated with a Markovian geologic model, which features embedded Bayesian updating procedures, to achieve a dynamic prediction on the state probabilities of the geologic parameter within the model and the resulting squeezing intensity during excavation. An under-construction tunnel case—Miyaluo #3 tunnel—is used to illustrate the proposed framework. Results show that the Decision Tree classifier, as opposed to other black-box models, is easy to be interpreted. It provides reliable predictive accuracy while leading to insights into the understanding of the squeezing problem. The strength-stress ratio (SSR) is suggested to be the most important factor. Moreover, the implementation of the updating procedures is efficient since only a simple field test (e.g. Point Load index or Schmidt rebound index) is required. Multiple rounds of predictions within the updating process allow different levels of prediction, for example long-range, short-term, or immediate, to be extracted as useful information towards the decision-making of construction operations. Therefore, this framework can serve as a pragmatic tool to assist the selection of optimal primary-support and other construction strategies based on the potential squeezing risk.

Nonlinear deformation behaviors and a new approach for the classification and prediction of large deformation in tunnel construction stage: a case study

Reasonable evaluation and prediction of squeezing condition to avoid large deformation disaster in tunnels have been an important research issue. In order to accurately and rapidly predict the large deformation under complex geological conditions, this paper proposes a classification and prediction method for quick identification of large deformation in tunnel construction stage in a case study of Muzhailing Tunnel. Previous prediction methods of large deformation are usually based on the stages of geological survey and engineering design, and there are great difficulties in obtaining some prediction indexes in soft and fractured strata, such as uniaxial compressive strength. In this paper, the nonlinear deformation behaviors of surrounding rock were analyzed. The geo-stress condition, strata occurrence, rock strength, rock intactness and groundwater condition were chosen as evaluation indexes (including eight sub-indexes). The classification of large deformation was carried out based on the deformation statistics in Muzhailing Tunnel. And the fuzzy prediction was conducted and compared with other methods in engineering. The comparison results show that the new method has higher accuracy and applicability in predicting the large deformation of weak rock mass. This study provides a new approach for rapid identification and prediction of large deformation in tunnel construction stage.

Two-stage analysis of interaction between strain-softening rock mass and liner for circular tunnels considering delayed installation of liner

A two-stage analysis of the support-rock interaction is carried out using a proposed numerical procedure. The procedure provides a simplified solution for readily obtaining the final support load and rock mass displacement that is easier to use than the conventional Convergence-Confinement Method (CCM). The procedure accounts for the strain-softening behaviour of the rock mass and the effect of delayed support installation. Based on the procedure, a method to determine the delayed distance and allowable displacement for the support installation is presented. A Modified Ground Reaction Curve (MGRC) that relates the rock mass displacement to the final support load is introduced. The effects of the rock mass quality, initial stress, support type and strain-softening, dilatancy behaviours on the delayed distance and allowable displacement are investigated. A comparison between the predictions of the MGRC and GRC is conducted. The results indicate that the influence of the rock mass quality and initial stress on the support-rock interaction is more significant than that of the strain-softening and dilatancy behaviours. An approximate support design using the GRC may be acceptable for a weak rock mass, whereas the GRC results in an overly conservative support design for an excellent rock mass.

Field Experimental Study on Mechanical Characteristics of Primary Supporting in Weak Surrounding Rock Tunnel

Since the poor self-stability of weak surrounding rock when applied the full-face excavation in a tunnel construction, a large surrounding rock deformation are tended to occur, which causes a big security risk. Under the engineering background of gaojiaping tunnel of zheng-wan high-speed railway, the stress of surrounding rock, steel frame, initial shotcrete stress, bolt axial force and extrusion deformation of working face of the tunnel are monitored and analyzed respectively. The results show that the surrounding rock stress, steel frame stress and initial shotcrete stress of the tunnel are significantly affected by surrounding rock lithology and construction disturbance. The plastic zone of surrounding rock can be preliminarily determined as 3m according to the peak value of bolt axial force. The extrusion deformation of the tunnel working face shows that the surrounding rock integrity of the test section is good. The experimental study is helpful to collect the deformation of weak rock mass and provide reference for establishing scientific and reasonable support system.

Optimum Sublevel Height and Drift Spacing in Sublevel Cave Mining Based on Random Medium Theory

Weak rock mass and poor flow of material can cause excessive ore loss and dilution in sublevel caving. This paper proposes a method to establish design parameters for an inclined deposit within a weak rock mass and evaluates its effectiveness through a case study. In order to solve the problem of low production capacity and poor safety conditions of sublevel caving mining at the Yingfangzi silver mine in China and to ensure a smooth production from the refractory ore body, it is necessary to select appropriate stope structure parameters. Based on random medium theory of gravity flow and the improved equation of ore discharge in random medium, an end wall drawing experiment is carried out to determine the optimum design and flow parameters. It was observed from the experiment that, due to the influence of the randomness of particle movement, there is a mismatch in the flow parameter measurement. The flow characteristics and distribution of the marker particles are different from that of the tested particles resulting in large drawbody shape distortion. The study finds that the flow parameters obtained by the new method are more practical and account for drawpoint width and recommends that the new method for determining the sublevel stope height is used when the drawbody is a non-standard ellipsoid. The outcomes have shown a significant reduction in the dilution and improvement in the recovery rates, and the safety and efficiency of mining have been greatly improved.

Estimation of rock mass properties in excavation damage zones of rock slopes based on the Hoek-Brown criterion and acoustic testing

When slopes are excavated in rock masses with the blasting method, blasting and stress relaxation will produce an excavation damage zone (EDZ) in the remaining rock mass. This study presents an estimation of the rock mass properties in the EDZ based on the generalized Hoek-Brown failure criterion and the varying disturbance factor. In this estimation, the disturbance factor is quantified by using the acoustic P-wave velocities tested in undamaged and damaged rock masses, instead of a single constant value chosen from the descriptive guidelines. Statistical analysis of the acoustic test data shows that with an increase in depth behind the slope face, the disturbance factor declines from 1.0 to 0 almost in a linear manner. For blasting excavation of slopes in weaker rock masses, the zone in the immediate vicinity of the slope face is fully damaged and thus over this range the disturbance factor remains the constant value of 1.0. As the disturbance factor linearly decreases within the EDZ, the deformation modulus, uniaxial compressive strength and equivalent cohesive strength of the damaged rock mass linearly increase to the undamaged values. By assigning the varying parameters to the rock mass in the EDZ, a numerical calculation based on the FLAC program is conducted to assess the slope stability. It is found that using the varying rock mass parameters within the EDZ will yield completely different failure surfaces and factors of safety from that uses the constant parameters. Therefore, it is significant to develop such an approach to quickly estimate the varying rock mass properties in the EDZ particularly when in-situ tests are not available.

Propagation Laws of Blasting Seismic Waves in Weak Rock Mass: A Case Study of Muzhailing Tunnel

In order to study the propagation laws of blasting vibration waves in weak rock tunnels, the longitudinal and circumferential blasting vibration tests in Muzhailing Tunnel were carried out, and the measured data were analyzed and studied using the methods of Sadov’s nonlinear regression, Fourier transform, and Hilbert–Huang transform (HHT) to provide a reference for the optimization of blasting design of Muzhailing Tunnel or similar weak rock tunnels. The results showed that the tangential main frequency decreases rapidly and the radial main frequency decreases slowly with the increase of proportionate charge quantity. Under a certain charge quantity, as the distance from the explosion source increases, the spectrum width of the blasting vibration frequency becomes narrower, the overall energy is more concentrated, and the vibration frequency tends to be closer to the low frequency. At a certain distance from the explosive source, the frequency of blasting vibration decreases gradually, and the amplitude of low‐frequency region increases with the increase of charge quantity. The vibration velocity on the left side of the tunnel is larger than that on the right side, and the vibration velocity at the vault and the arch foot of lower bench decreases rapidly, while the vibration velocity at the arch feet of upper bench and middle bench decreases slowly. The vibration frequencies of the left arch foot of the middle bench and the right arch foot of the upper bench are higher than those of other positions, while the frequencies of the left arch foot of the upper bench are the lowest. During tunnel blasting, the energy input to the strata media is mainly concentrated in the stage of the blasting of the cut hole. The blasting has more energy input to the left arch foot of the upper bench and the tunnel vault, which is consistent with the conclusion of frequency analysis.

Assessment of design parameters and the slope stability analysis of weak clay-bearing rock masses and associated spoil piles at Tunçbilek basin

Tuncbilek lignite basin situated in western part of Turkey has been increasingly excavated by open-pit and underground mining methods since 1940 to meet Turkey’s energy demand. The blasted and then the excavated stratified rock masses in this basin consist mainly of clay-bearing rocks such as claystone, mudstone, siltstone, and marl. It was found that the strength and deformation properties of these clay-bearing rocks decrease with the increase in water content. In addition, time-dependent attenuations were also recorded in the mechanical properties due to disintegration caused by physical weathering processes. Therefore, considerable time-dependent instabilities were observed in the slopes left after open-pit mining activities. In this study, it was aimed to determine the design parameters of weak rock masses and spoil piles at Tuncbilek basin and then to recommend safe slope geometry for open-pit mining activities. For this purpose, the geotechnical properties of rock masses and spoil piles were obtained based on field tests as well as characterizations and back analyses of the instabilities. This study reveals that shear strength parameters of undisturbed clay-bearing rock masses are 200 kPa and 30°. In addition, a cohesion of 150 kPa and an internal friction angle of 25° should be used for slope stability analyses of both weakly cemented conglomerate-sandstone and disturbed clay-bearing rock masses.

An analysis on the slaking and disintegration extent of weak rock mass of the water tunnels for hydropower project using modified slake durability test

Water tunnels built for hydropower passing through weak and heterogeneous rock mass pose challenges associated to slaking and disintegration, as they are first exposed to dry condition during excavation and are then filled with water to produce hydropower energy. Over the period of operational life, these tunnels are drained periodically for inspections and repair leading to drainage and filling cycles. The weakening of rock mass caused by cycles of drying, saturation and drainage may lead to the propagation of instabilities in the tunnels. Therefore, it is important to study the slaking and disintegration behavior of the weak rock mass consisting of clay and clay-like minerals. This paper assesses the mineralogical composition of flysch and serpentinite from the headrace tunnel of Moglicë Hydropower Project in Albania. Further, to determine the slaking and disintegration behavior of these rocks, extensive testing using both the ISRM, Int J Rock Mech Min Sci Geomech Abstr 16(2):143-151, (1979) suggested test method and a modified variant of this test are performed. Finally, comprehensive assessments, discussions and comparisons are made. It is found that the modified slake durability test better suits for the tunnels built as water conveying systems such as hydropower tunnels.

An analytical solution for analysis of toppling-slumping failure in rock slopes

When a slope consists of a horizontal weak rock or soil overlaid by some vertical strong slender rock columns, the slope is prone to a secondary toppling failure which is known as “toppling-slumping” failure. In the slope, rock columns impose pressure on the lower continuous weak rock or soil mass which leads to a differential settlement at the base of each rock column or a circular shear failure in the continuous mass. If the differential settlements reach a special threshold, rock column overturn and toppling-slumping will happen. However, the circular shear failure of the soil or weak rock mass which is a well-recognized failure mechanism will occur when shear stress in the slope exceeds its shear strength. In this paper, an attempt has been made to better understanding of the “toppling-slumping” failure mechanism through developing an analytical solution. For this purpose, first an Airy stress function is acquired for the lower continuous soil mass and its stress distribution and displacement are accordingly determined analytically due to the overlaid rock columns loads. Then, differential settlements at the base of all rock columns and shear stresses in the soil or weak rock mass are computed and compared with the toppling and shear failure criteria to assess slope failure. Finally, several design-charts are developed that determine the stability of a slope against toppling-slumping failure mechanism based on geometric characteristics of the slope and shear strength of its constituent material. Also, a Mathematica package code was developed to evaluate the stress distribution within the slope, and the safety factor. The results show that the vertical stress distribution plays a significant role in the settlement of the rock columns. Results also manifested that at a certain distance from the slope crest, the set of rock columns will act like a single column on a half-plane. The shear or toppling failure of the slope not only depends on the material's strength but also on the geometry and the normalized column distance.