Stability Of Underground Structures Research Articles

Experimental Study on the Horizontal Bearing Characteristic of a Strip-Walled Underground Diaphragm Wall

Researching and developing a new type of diaphragm wall foundation can solve the probl em that the traditional diaphragm wall structure may not meet the high standards of safety and stability of underground structures in some specific engineering environments. This paper focuses on the horizontal bearing characteristic of a new form of foundation, a strip-walled underground diaphragm wall, through a series of model tests. In the tests, nine plexiglass models with different section sizes, wall spacings and wall heights, as well as loading strategies (horizontal loads along and against the wall in the model), were conducted. The influence of the above factors on the horizontal bearing performance of the foundation and the soil resistance distribution around the wall was studied. The results show that when the horizontal load applied along the wall is greater than 50 N, the growth rate of total displacement at the top of the wall gradually decreases; when a horizontal load is applied against the wall, with a uniform change in wall height, the optimal wall spacing is 11 cm. When the same displacement occurs, the bearing performance of the model under the former loading strategy is generally 10% higher than that under the later loading strategy. In addition, the depth where the maximum bending moment along the wall occurred gradually moves downward with the increase in horizontal load, and the increase in wall spacing and wall height has a positive effect on the horizontal bearing characteristic. With the application of load, the maximum bending moment of the wall will gradually decrease along the depth. The increase in wall spacing and wall height can improve the overall flexural stiffness and horizontal bearing performance of the foundation. Lastly, the group wall effect coefficient, β, is put forward, and a simplified formula for calculating the horizontal bearing capacity of a strip wall foundation is proposed. In the formula, β is negatively correlated with the buried depth of the wall and positively correlated with the distance between the walls, and its coefficient is greater than 1.

Dynamic photoelastic experimental study on the influence of joint surface geometrical property on wave propagation and stress disturbance

Rock joints significantly affect the dynamic response of rock masses and the stability of underground structures . In this study, the dynamic photoelastic test was conducted to investigate the influence of joints on the stress field and wave propagation . The photoelastic equipment as well as the Split Hopkinson Pressure Bar (SHPB) were adopted for the test. The specimen sandwiched between the input and output bars was composed of two polycarbonate plates. One side of the plates was uneven with a few notches, while other sides were smooth and flat. The artificial joints were simulated by the contact of the uneven and flat sides of the two plates. The relation between the stress wave propagation and the stress field around the joint was studied when the joint has different contact area ratios, joint thicknesses, and joint surface distributions. The test results show that compared to the joint thickness, the joint contact area ratio and the joint surface distribution have a significant influence on the wave propagation and stress field near the joint. The transmitted wave is highly correlative with the stress field. This work provides a beneficial attempt to reveal the stress field disturbance and the effect of joint geometrical properties on wave propagation from the photoelastic test.

Generalized Relaxation Behavior of Rock Under Various Loading Conditions Using a Constant Linear Combination of Stress and Strain

Time-dependent behavior has been demonstrated to be an essential factor in determining the long-term stability of underground structures. Creep and relaxation experiments are commonly used to investigate time-dependent behavior by subjecting rock to constant stress and strain. However, both stress and strain of in-situ rock masses are likely to change with time, a phenomenon known as generalized relaxation that has not been thoroughly investigated. In this study, a newly proposed control method with a constant linear combination of stress and strain as a feedback signal is used in compression and tension tests to investigate generalized relaxation behaviors of rocks. The results showed that the stress and strain of generalized relaxation are dependent on values of α, which represented generalized relaxation direction. The isochronous curves are enclosed within stress–strain curves of different loading conditions. The variation of stress (∆σ) and strain (∆ε) increases with increasing stress level and decreases with increasing confining pressure. Also, ∆σ and ∆ε in region II are smaller than in regions I and III. Furthermore, by performing brittle rock tests, complete generalized relaxation curves are obtained; three stages are observed, which are similar to conventional creep and relaxation behavior. Finally, the time and generalized relaxation failure behavior of Class I and Class II rock are discussed. The study is a valuable resource for gaining a comprehensive understanding of the time-dependent behavior of rocks and improving the stability and safety of underground structures.

Role of Geologists During Tunnel Construction and Face Mapping to Decide the Required Tunnel Support

Abstract: The stability of underground structures is an important aspect during design, construction and execution Phase. Depending on the geotechnical conditions and influencing factors, different failure modes during execution mode can be expected, and depending on the potential failure modes, boundary conditions and specific construction measures to ensure stability have to be chosen. The most important is developing a realistic estimate of the expected ground conditions and their potential behaviour/failure modes as a result of the excavation. The variability of the geological conditions including local ground structure, ground parameters, stress and ground water conditions requires that a consistent and specific procedure is used. The other is to design an economic and safe excavation and support method for the determined ground behaviours. The discussion of role of geologists during design stage is beyond the scope of the present study. The main objective of this study is to present the role of geologists during the construction stage. Keywords: Geologist, Stress, Behaviour, RMR, Q Value, RQD, Rock Mass

Loading-Rate Dependency of Young’s Modulus for Class I and Class II Rocks

Understanding the time-dependent behavior of rocks is important for ensuring the long-term stability of underground structures. Aspects of such a time-dependent behavior include the loading-rate dependency of Young’s modulus, strength, creep, and relaxation. In particular, the loading-rate dependency of Young’s modulus of rocks has not been fully clarified. In this study, four different types of rocks were tested, and the results were used to analyze the loading-rate dependency of Young’s modulus and explain the underlying mechanism. For all four rocks, Young’s modulus increased linearly with a tenfold increase in the loading rate. The rocks showed the same loading-rate dependency of Young’s modulus. A variable-compliance constitutive equation was proposed for the loading-rate dependency of Young’s modulus, and the calculated results agreed well with measured values. Irrecoverable and recoverable strains were separated by loading-unloading-reloading tests at preset stress levels. The constitutive equations showed that the rate of increase in Young’s modulus increased with the irrecoverable strain and decreased with increasing stress. The increase in the irrecoverable strain was delayed at high loading rates, which was concluded to be the main reason for the increase in Young’s modulus with an increasing loading rate.

Loading Rate Dependence of Rock Strength Under Triaxial Compression

Knowledge regarding the time-dependent behavior of rocks is essential to estimate the long-term deformation and stability of underground structures. The rock strength increases with the increasing loading rate. However, the loading rate dependence in postfailure regions under confining pressure remains unclear. In this study, triaxial compression tests were conducted on four types of rocks to examine the loading rate dependence in both peak and postfailure regions. Results demonstrate that an increase in residual strength with a tenfold increase in the strain rate was approximately proportional to 4% of the residual strength. Furthermore, the increase in peak strength with a tenfold increase in the strain rate increased at a rate of approximately 4% of peak strength. The obtained results were applicable to all the sample rocks and can be easily employed for improving the constitutive equations. Finally, the effect mechanism of the confining pressure on the loading rate dependence of rock strength is discussed.

An empirical strength criterion for deep rock incorporating the effect of fracture intensity using distinct element method

Deep rock strength is one of the most important research areas in rock mechanics and engineering which is closely related to bearing capacity and stability of underground structures. Due to high ground stress, the mechanical behaviours of deep rock are different from those of shallow rock. In order to characterize deep rock strength precisely, a great number of rock strength criterions have been proposed over the past few decades to quantitatively describe the relationship between complex stress state and rock strength parameters. Some microscopic characteristics like fracture intensity(P32 ) have received some attention. They are thought to have significant influences on the strength of deep rock. Currently, most of the constitutive models and strength criterions focused on the shallow rock and macro mechanical parameters, and they can not directly explain the relationship between the microscopic fracture characteristics and the strength of deep rock. In this research, the effects of fracture characteristics and confining pressure on deep rock strength were investigated using distinct element method(DEM). Firstly, an improved bond contact model incorporating rolling and twisting resistances was implemented in the rock numerical specimens, and the Smooth-Joint contact model was used for simulating fractures of rocks. The fracture length distributions in specimens were characterised by an exponent parameter a. Then numerical simulation of triaxial compression tests were carried out on numerical rock specimens under different confining pressures and P32 . The results show an approximate linear relationship between the maximum principal stress and P32 . Rock strength decreases with the increase of P32 , and it decreases rapidly at a higher confining pressure. Based on the results, a new empirical strength criterion is proposed. The criterion considers the nonlinear relationship between maximum principal stress and confining pressure of deep rock, additionally reveals the correlation between microscopic fracture characteristics and rock strength. In principle stress space, the corresponding failure envelopes projected onto the π-Plane showed a good agreement with DEM simulation results, which indicates a good applicability for the prediction of deep rock strength.

THE EFFECT OF LIMESTONE POROSITY ON THE VELOCITY OF P- AND S-WAVES UNDER MECHANICAL AND THERMAL LOADING

The thermal and mechanical tests of different porosity limestone show that an increase in the axial load results in the higher velocities of elastic waves while elevation of temperature decreases them. Higher temperatures act to raise velocities of P- and S-waves with increasing mechanical load, which enhances acoustic strain-sensitivity of rock. The spectral analysis of the recorded signals shows that higher temperature shifts spectrum maxima to lower frequency region. It is found that size of pores has influence on attenuation frequency of ultrasonic signals. The authors describe new approaches to acoustic strain-sensitivity control in rocks and to stress measurement reliability enhancement toward stability of underground structures.

The Technology for Manufacturing HighStrength Pipe-Concrete Bearing Elements to Increase the Stability of Underground Structures

It is proposed to use high bearing small-size pipe-concrete units in the construction of underground geotechnical structures instead of large-sized reinforced concrete units (support blocks, pylons, columns, etc.). The analysis of the methods of manufacturing pipe-concrete units, their advantages and disadvantages is conducted. The process flowchart for manufacturing pipe-concrete units with barothermal pre-stressing of a concrete mixture in a steel pipe was developed to ensure the collaboration of a steel pipe and a concrete core and increase its strength. Experimental research in barothermal treatment of a concrete mixture in a steel pipe was performed using the laboratory model of a pipe-concrete unit, which showed the efficiency of the proposed technological scheme for the production of pipe-concrete units. The lines for further research in the development of a method for determining the optimal parameters of the technological scheme for manufacturing pipe-concrete units with barothermal pre-stressing of a concrete mix are planned.

Study on healing technique for weak interlayer and related mechanical properties based on microbially-induced calcium carbonate precipitation.

The weak interlayer refers to the filling material in shear belts or large-scale structural planes, which is usually composed of soil, fine sand and gravels. It is prone to argillization when encountering water and its mechanical strength and stiffness are generally low, which has adverse effects on the stability of underground structures. In recent years, research on reinforcement techniques for weak interlayers has been a hot topic in geotechnical field. As a new reinforcement method for structural planes, the microbial healing technique has attracted a lot of attention. In this paper, a study on the healing technique for weak interlayer based on microbially induced calcium carbonate precipitation (MICP) and related mechanical properties was conducted for the interlayer shear belt at Baihetan Hydropower Station in China. First, Sporosarcina pasteurii was activated in laboratory. Reinforcement of the weak interlayer was realized by utilizing calcium carbonate precipitation on the weak interlayer. Continuous monitoring of the precipitates on the weak interlayer by XRD and SEM indicated that the precipitates on the weak layer were microbially induced calcium carbonate. Its crystals were irregular fish scale-shaped cubes with size in the range of 5~20μm. With favorable crystal growth, the crystals and the particles of the weak interlayer were cemented together. Finally, the mechanical properties of the healed weak interlayer were tested and the variations of uniaxial compressive strength, shear strength and triaxial compressive strength with bacteria concentration were discussed. The test results indicated that the maximum uniaxial compressive strength, peak shear strength and triaxial compressive strength can be increased by 149%, 162% and 119%, respectively, which subsequently improve the overall strength of the shear zone or structural plane. This can provide a new idea for soft ground reinforcement in underground projects.

Understanding progressive rock failure and associated seismicity using ultrasonic tomography and numerical simulation

Monitoring the stability of underground rock excavation zones, such as tunnels and underground mines, is critical to their operational safety. The stability of these structures is related to the stress redistribution introduced by the excavation process and disturbance during the operation. Therefore, the characteristics of progressive rock failure behaviour at different stress conditions must be investigated. In this study, we address this problem using a laboratory experiment, coupled with ultrasonic tomography (UT) and numerical simulation. A time-lapse two-dimensional (2D) UT observation was conducted on a granite slab under uniaxial compression. This test was then reproduced numerically by the combined finite-discrete element method (FDEM). This innovative combination of technologies depicted the entire deformation and failure processes at macroscopic and microscopic scales. Quantitative assessments of the results suggested six precursory behaviours indicating the catastrophic failure of the rock: (1) decrease of the average wave velocity perpendicular to the loading direction, (2) increase of the heterogeneity and anisotropy of wave velocity, (3) exponential increase of seismic rate, (4) spatial localization of damage onto the failure plane, (5) increase of the dominance of shear failure, and (6) slight recovery of b-value, followed by a significant drop. An integrated monitoring and analysis of these indicators, accompanied by carefully calibrated numerical simulations, may provide vital information regarding the stability of underground structures.

Experimental Investigation on the Deformation, Strength, and Acoustic Emission Characteristics of Sandstone under True Triaxial Compression

The study of deformation, strength, and other mechanical characteristics of sandstone under true triaxial compression is significant for understanding failure mechanisms in rock and evaluating the stability of underground structures. Conventional and true triaxial compression tests for sandstone are conducted for different stress states in this study using the self‐developed true triaxial electrohydraulic servo test system combined with acoustic emission (AE) testing. This study presents an in‐depth and systematic investigation of deformation, strength, and AE characteristics. The results show significant differences in deformation, strength, and acoustic emission characteristics for the rock under conventional triaxial and true triaxial compression tests, respectively. The peak strength, axial strain, lateral strain, and incremental strain (in unstable crack growth stage) increase with increasing confining pressure under conventional triaxial compression, and the AE count gradually decreases while shear crack proportion gradually increases, indicating that increasing confining pressure gradually inhibits the shear slip effect along fractures, delays perforation of the rock shear fracture surface, and enhances the ability of the rock to withstand deformation and load. Under true triaxial compression, the peak strength increases and then decreases with increasing intermediate principal stress σ2 and the axial strain ε1 and lateral strain ε2 gradually decrease; besides, the lateral strain (expansion) of the rock is mainly in the minimum principal stress σ3 direction, and lateral expansion tends to decrease before increasing. AE events first weaken and then enhance with increasing σ2, and the proportion of shear cracks increases first and then decreases, indicating that the confining pressure gradually changes from the shear slip effect that controls crack offset to the damage effect that promotes crack tension with increasing σ2. In addition, the protective effect of confining pressure improves when σ3 increases.

Deterioration of weak rocks over time and its effect on designing tunnel support systems

The long-term behavior of rock mass plays a vital role in the overall stability of underground structures. One possible long-term scenario is the degradation of the rock mass, especially in unstable clay-bearing rocks susceptible to water ingress. Lack of efficient countermeasures in extreme cases could significantly affect the physical and mechanical properties of rocks and alter the engineering behavior to some extent. In this research, we describe the case of the Golab access tunnel where it is subject to failure in a specific section containing meta-shales. The situation was considered to have occurred as a consequence of long-term deteriorating phenomenon after a 5-year period. To characterize the rock mass, point load tests were conducted on the rock samples in failure zones and values were compared with the ones obtained through primary geotechnical studies. Slake durability index (SDI) was also used to assess the vulnerability of rocks to changes in moisture content and was correlated to point load values. Additionally, GSI was measured in the failed regions, and the decreasing trend was presented in accordance with time. By taking advantage of the engineering characteristics, ground reaction curves in short-term and long-term conditions were obtained. It was deduced that the temporary support system applied to the tunnel in this section cannot provide a safe margin against long-term strains imposed by the ground. Therefore, another support system was suggested and applied to the unstable sections of tunnel to prevent further hazards.

Application of a non-linear viscoelastic-plastic rheological model of soft coal on borehole stability

Time dependent deformation of coal has a significant effect on the stability of underground structures. In this study, taking the visco-elastic–plastic characteristics and the damage effect into account, a non-linear model was proposed to describe the creep behavior of soft coal. Meanwhile, the creep equations of the proposed non-linear model were derived and the three-dimensional creep equations were constructed by generalizing the one-dimensional creep equations. At the same time, the uniaxial creep tests on soft coal specimens under different axial stress conditions were carried out to validate the proposed model. It is found that the present model can not only reproduce the transient creep under low axial stress level, but also can accurately predict the steady and accelerating creep stages under high axial stress level. Finally, borehole stability in soft coal seam was investigated based on the proposed model. The results show that the vertical displacement and plastic zone range around borehole gradually increase while the increasing rate decreases and tends to be stable after 20 days. It can also be obtained that the creep duration tends to be stable when it is about 20 days after drilling excavation under the simulation conditions. The analysis results should be useful for the long-term stability of boreholes in soft coal seam for gas extraction.

Influence of specimen height-to-width ratio on the strainburst characteristics of Tianhu granite under true-triaxial unloading conditions

Strainbursts occur frequently near excavation boundaries, posing a significant threat to the safety of workers and stability of underground structures. A better understanding of the strainburst phenomenon is important for safe underground construction and risk management. This paper presents the results of an experimental study on strainburst behaviours of Tianhu granite with four specimen height-to-width (H/W) ratios. A series of tests under unloading conditions were conducted using a true-triaxial strainburst test system that was equipped with an acoustic emission (AE) monitoring system. A high-speed camera was used to capture the instantaneous strainburst process of the specimens on the unloading surface. The experimental results indicate that regardless of the H/W ratio, strainbursts occurred in all specimens but strainburst characteristics depend on the H/W ratio. When the H/W ratio changes from 2.5 to 1.0, the dynamic failure process of the unloading surface transforms from local rock ejection to full-face burst. The strainburst is more violent when the H/W ratio is small. Analysis of the cumulative AE energy revealed that the cumulative AE energy is not sensitive to the H/W ratio for ratios lower than 1.5. When the H/W ratio is greater than 1.5, the cumulative AE energy shows a decreasing trend during rock failure.

Index of Loading-Rate Dependency of Rock Strength

To estimate long-term deformation and stability of underground structures, knowledge of the time-dependent behavior of rock is essential. It is well known that rock strength increases with an increase in loading rate (loading-rate dependency), strain increases under constant stress (creep) and stress decreases under constant strain (relaxation). It has been reported that such time-dependent behaviors under different loading conditions are closely related to each other. The constant n, which represents slow crack growth due to stress corrosion, has been used to formulate the loading-rate dependency of strength (Sano et al. 1981) and the relation between creep stress and lifetime (Wilkins 1981). Lajtai et al. (1991) stated that rock strength is proportional to the 1/ (n ? 1)-th power of loading rate and that creep lifetime is inversely proportional to the n-th power of creep stress. Hashiba et al. (2006) presented the results of strength and creep tests which demonstrated the relation concerning n. Okubo et al. (2013) collected the values of n for some Japanese rocks and reported that the value for Sanjome andesite was consistent in uniaxial compression, uniaxial tension, Brazilian tension, three-point bending and direct shear tests in dry conditions. Quantitative comparison of the time-dependent behaviors of various rocks is essential but difficult to do using stress–strain relations or creep strain curves. The abovementioned expression for n represents the degree of time dependency and is suitable for the comparison of rocks or testing conditions. Furthermore, Hashiba and Fukui (2013) considered the value of n as an index of time dependency and proposed a rock mass classification incorporating this value. Okubo et al. (2013) reported the values of n for only nine Japanese rocks, one Japanese hardpan and one Chinese coal. In previous studies many other researchers have provided a large amount of information on the loading-rate dependency of rock strength. In this study, the values of n were calculated from the published data for many rocks and obtained from laboratory tests for three rocks. The degree of time dependency was compared between various rocks and the effects of the testing conditions were examined with the aid of the data. In the latter part of this paper, the relationship between the time dependency, variability and size effect on rock properties is discussed from a theoretical viewpoint considering the values of n and simple hypotheses.

Experimental Investigation on Creep Deformation Behavior of Medium-strength Marble Rock

The creep deformation behavior of rocks has significant effect on the stability of underground structures. This study presents the short-term and creep deformation behavior of medium-strength marble rock using a conventional uniaxial compression testing machine and a servo-controlled rheology testing machine. The uniaxial compressive strength is obtained by the uniaxial compression testing machine. During the creep behavior test, two types of rock specimens (dry and water-saturated) are specified to be used to perform the uniaxial creep tests. Two rheological failure modes and the relationship curves between axial/circumferential strain and stress levels of marble specimens are also obtained from the creep test results. Eventually, the creep deformation behaviors are compared with those of typical soft rocks. These creep curves combined with a given creep constitutive model would provide accurate parameters for long-term stability analyses of actual projects.

Results of experimental installation of roofbolting system in the mines

Application of AT roofbolting system in the mines around the world has proven to be a very effective way of rockbolting support, considering that the underground facilities supported by this type of support proved to be reliable in terms of their functionality. However, despite the large and well-documented experiences, as well as extensive scientific research dedicated to this type of support, there is no serious application in the mines in our region. This paper presents the results obtained during experimental research the possibilities of application AT roofbolting system in ensuring the stability of underground facilities. As the results, according to the tests carried out in the Bauxite Mines Niksic - mine 'Biočki Stan', Coal Mine Rembas - 'Strmosten' pit, as well as the Copper Mines Bor - 'Jama' the underground mine. Bellow the provided results confirm that in different geological formations can reach reliable data on the capacity of the bolt, and thus to achieve the necessary stability of underground structures using the AT roofbolting system.

An enhanced numerical procedure for modelling fully grouted bolts intersected by rock joint

The stability of underground structure made especially in jointed rock mass is of the utmost important to designer, engineers and operators. Rock bolting is generally being practised to reinforced excavation walls and roofs by minimizing the movement of rock joints. In this study, a numerical procedure has been developed in extended finite element framework (XFEM) to analyze the behaviour of grouted bolt intersected by a joint. A solid finite element intersected by a bolt and a joint along any arbitrary direction is termed as ‘doubly enriched’ element. Nodes of an doubly enriched element have additional degrees of freedom for determining displacements, stresses developed in the bolt rod as well as the displacements jump and traction along the joint. The paper also provides verifications of this procedure by solving two known examples (i) direct shear test performed on a bolted joint sample-experimental verification and (ii) reinforcement of a joint located in the vicinity of a circular tunnel-analytical verification.

岩石の時間依存性を表す指標とその利用に関する一考察

To estimate long-term deformation and stability of underground structures, it is indispensable to understand time-dependency of rock sample and rock mass. Previous experimental and theoretical studies showed that rock strength was proportional to the 1/(n+1)-th power of loading-rate and creep lifetime was inversely proportional to the n-th power of creep stress, where n was a constant depending on rock type and testing environment. The smaller value of n, the larger time-dependency of rock, which suggests that n can be a useful index of time-dependency. This paper introduced three loading and a creep testing methods to obtain the value of n, and showed their results in uniaxial compression, uniaxial tension and Brazilian tension tests for 38 rocks in the country and overseas. Many rocks showed smaller values of n under water-saturated than air-dried condition. The value of n was proportional to strength under water-saturated condition or low confining pressure, but not under high confining pressure under which failure was more ductile. Rock mass classification systems considering time-dependency of rock were examined as an application of testing data on design or construction of underground structures. An index n was implemented in four classifications, RMR, Q, GSI and RMi, and their proposed equations indicated that rating decreased with an increase in duration of use of structures or in degree of time-dependency of rock mass.