Safety Of Underground Structures Research Articles

Influencing Factors, Design Methods, and Buoyancy Reduction Measures for Basement Anti-Flotation Engineering

Basement anti-flotation design is crucial in modern urban construction. An increase in groundwater buoyancy can cause basement structures to uplift, leading to structural instability or even damage. To ensure the stability and safety of underground structures under various hydrogeological conditions, anti-flotation design must comprehensively consider factors such as construction, design, supervision, structure, and hydrology. During construction, improper dewatering measures or unreasonable construction progress may lead to an abnormal rise in groundwater levels, increasing the risk of anti-flotation. Design considerations must include sufficient safety margins, supervision must fully recognize the impact of groundwater, and the structural dead load must be adequate. Anti-flotation stability verification includes both overall and local anti-flotation, involving the calculation of groundwater buoyancy, structural self-weight, and overburden, and selecting appropriate anti-flotation stability safety factors. The assessment and selection of the anti-flotation design water level are also critical. Common anti-flotation measures include adding counterweights, tension-resistant piles, compression and tension-resistant piles, and hydro-pressure reduction methods, while reinforcement and repair methods include epoxy resin grouting and steel plating reinforcement. Through systematic analysis and comprehensive research, scientific basis and technical support are provided for anti-flotation design, enhancing design efficiency and reliability and ensuring the safety and stability of underground spaces. Future research will develop more accurate calculation methods, improve design standards, and explore new anti-flotation technologies and materials.

Обобщённая математическая модель основных задач теории упругости для анизотропных тел

Introduction. The assessment of the strength of underground structures is a complex task that requires a comprehensive approach. In many cases empirical methods based on experience are not sufficient. For effective application of analytical methods, it is necessary to use mathematical modelling. This allows to simplify the problem considerably, to consider different variants, to identify cases to obtain a solution in a closed form. Materials and methods. The model is based on a system of singular integral equations with a shift and complex conjugate terms. The solution of the system is the complex elastic potential. Systems of singular equations with a Carleman shift are considered on the space of random functions converging in the mean square to expand the possibility of practical use of the work results. Results. The results of the study are: a mathematical model formulation, the development of an algorithm for reducing a system of singular equations to a system of known Fredholm integral equations of the second kind, a method for determining the solvability conditions and the number of solutions. Discussion. As a result of the study: (1) a general algorithm for finding a solution to a mathematical model is obtained; (2) the methodology for determining the number of solutions and the type of solvability conditions is given; (3) The solution algorithm is extended for the stochastic case; (4) The possibility of wider application of the mathematical apparatus of functions of complex variable for research of strength properties of rocks is proved. Conclusion. The proposed comprehensive approach to assessing the strength characteristics of underground structures, combining a set of analytical methods in conjunction with mathematical modelling, expands the variability of calculation and allows to simplify the problem significantly and obtain a solution in closed form. Resume. The paper develops a mathematical model of the stress state of an elastic body, which allows solving the basic problems of elasticity theory for a variety of media. Suggestions for practical applications and directions for future research. The results of the study make it possible to improve the efficiency of analytical methods in the study of the strength of underground structures. The obtained results are applicable for predicting the strength and safety of underground structures.

A comprehensive study for the effect of sample geometry and lateral pressure on shear fractures using the short core in compression (SCC) method

Shear fractures are common in the rock mass, especially with increasing depth, due to the increase in lateral pressure. Hence, investigating these fractures is crucial for improving the stability and safety of underground structures at greater depths. The Short Core in Compression (SCC) method has recently been used by some researchers to generate shear fractures under mode II loading and initiation due to the ease of sample preparation and lateral pressure application. This study examines the impact of sample geometry and lateral pressure on the formation of shear fractures in rocks, by conducting numerical and experimental investigations on SCC samples. In this paper, the effect of sample shape, notch length, and lateral pressure was evaluated depending on the shear and the maximum principal stress distribution. To the best knowledge of the authors, the impact of these factors has not been sufficiently explored in the literature. Moreover, the mode II stress intensity factor, fracture toughness, and fracture path were investigated. The results provided the stress intensity factor for different geometries of the SCC sample that can be used for many applications in mechanical and geological engineering. Our findings showed that the resulting fracture must be evaluated due to the possibility of crack growth under shear σxy or maximum principal stress σmax, especially for Notch Length/Diameter ≥0.5. Additionally, lateral pressure mainly affects the distribution and magnitude of σmax, such that σmax is inversely proportional to this pressure. The results showed that the SCC sample with Height/Diameter = 2 is preferred because σmax is minimum for this sample shape, which promoted fracture growth under shear stress not tensile stress. Furthermore, as the lateral pressure increases, mode II fracture toughness significantly increases and the number of fragments that spalled off the specimen along the fracture path decreases.

Experimental analysis on the interaction between underground structures and sand layer under groundwater level change

Groundwater level change stands a momentous role in affecting the geotechnical construction stability and safety of underground structures. Global warming and active underground construction cause conspicuous changes in the groundwater level, which further leaves an impact on the underground structures’ serviceability. To reveal the interaction between underground structure and soil under groundwater level change in the sand layer, model tests of circular transportation tunnel and rectangular utility tunnel were carried out. With the self-designed experimental equipment and innovative experimental methods, the changes in tunnel stress, bending moment, buoyancy, and vertical displacement of the rise and drawdown of the groundwater level in the sand layer were studied. The results revealed the developments of concentrated structural forces during the rising and falling process of the groundwater water level, indicating critical locations that should be strengthened. Meanwhile, both tunnels showed the same movement trend: settling first, floating afterwards and settling at last. And it is concluded that no reduction is required when calculating buoyancy in sands using measured pore pressure. Conclusions can provide a notable reference for future related research and engineering designs.

A NEW CONSTITUTIVE MODEL FOR THE TIME-DEPENDENT BEHAVIOR OF ROCKS WITH CONSIDERATION OF DAMAGE PARAMETER

Deformation and time-dependent behavior of rocks are closely related to the stability and safety of underground structures and mines. In this paper, a numerical-analytical model is presented to investigate time-dependent damage and deformation of rocks under creep. The proposed model is obtained by combining the elastic-visco-plastic model based on the theory of over-stress and stress hardening law with the sub-critical crack growth model. The advantage of this model is that it is in incremental form and therefore can be implemented numerically. First, the governing equations of the model and its numerical computational algorithm are described. The proposed constitutive model is then implemented in the FLAC code using the FISH function. Determination of model parameters and calibration is done by various laboratory tests performed on a type of gypsum. The creep test was performed on gypsum under a stress of 13 MPa, which is equal to 70% of its compressive strength. After determining the parameters, by fitting the creep curve of the presented analyticalnumerical model, a good agreement is observed with the creep curve obtained from the laboratory data. It is also observed that during creep, the damage parameter and wing crack length increase.

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.

Оценка технического состояния подземных сооружений крупнейшей гидроэлектростанции Северного Кавказа Чиркейской ГЭС

Objective: Chirkeyskaya HPP is by far the most powerful hydroelectric power plant in the North Caucasus with the highest arched dam in Russia and the second highest dam in the country after the Sayano-Shushenskaya HPP. This explains why it is called the pearl of the Caucasus. Methods: For the operation and maintenance of this unique structure, a large-scale complex of underground structures for various purposes was built, the technical condition of which must be constantly monitored. To carry out work on the survey of underground structures, the management of the design and survey institute of JSC “Lengidroproekt” decided to attract specialists from the Department of Tunnels and Subways and the Test Center “Strength” of Emperor Alexander I Petersburg State Transport University. The work was successfully carried out at the end of 2015. Results: The safety of underground structures was objectively assessed. Recommendations for the repair and further comprehensive reconstruction of the Chirkeyskaya HPP have been developed. Practical importance: Carry out work on the survey of underground structures of Chirkeyskaya HPP is allowes elaborate of complex measures on safety from Chirkeyskaya HPP.

An Estimation Model for Hydraulic Conductivity of Low-Permeability and Unfilled Fractured Granite in Underground Water-Sealed Storage Caverns

The permeability of rock mass is closely related to the stability and safety of underground structure, especially in underground water-sealed storage caverns. With regard to the estimation approaches in predicting the hydraulic conductivity of fractured granite in water-sealed storage caverns, there are some limitations of parameter selection leading to poor applicability. Focusing on the contribution of the water conduction fractures (WCF) to the hydraulic conductivity, we attempted to propose a novel model, the CA model, for estimating its hydraulic conductivity based on the fracture orientation index and the normal stress index by analyzing the borehole wall imaging results and borehole water-pressure test results in the site of underground water-sealed storage caverns. The results indicated that the proposed model is suitable for low-permeability and unfilled fractured granite, exhibiting good effectiveness by clarifying the relation between geomechanical parameters and hydraulic behavior. Further, the parameters upon which the proposed model is based are representative and easy to obtain, which has certain guiding significance and reference value for analyzing the permeability characteristics of similar rock masses.

Seismic vulnerability assessment of multi-storey subway station structure

The safety of underground structure under seismic load is an important basis of the normal operation of underground rail transit system. Structural seismic vulnerability assessment based on incremental dynamic analysis method can evaluate the probability of the structure exceeding a certain limit state under a specific seismic intensity in terms of probability. In this paper, the seismic vulnerability of a multi-storey subway station structure is evaluated using this method, and a two-dimensional finite element model of both soil and structure is established by finite element software ABAQUS. The vulnerability curve is obtained through incremental dynamic analysis and mathematical statistics. Based on this curve, the probabilities of the structure exceeding four seismic limit states are obtained under the design seismic intensity and the rarely occurred seismic intensity at 7 degree. Results show that this station may attain slight damage under design seismic intensity of 7 degree, and may attain life safety at the rarely occurred seismic intensity at 7 degree.

Progressive failure behaviors and crack evolution of rocks under triaxial compression by 3D digital image correlation

In the study, a series of monotonic triaxial compression tests were carried out on Emochi andesite to evaluate the effects of confining pressure on progressive failure behaviors. The three-dimensional digital image correlation (3D-DIC) system consisting of six cameras was used to capture the images based on the transparent pressure cell. The effects of confining pressures on stress thresholds, energy evolution, field strain patterns and crack evolution were investigated qualitatively or quantitatively. The crack initiation (σci), crack damage (σcd) and peaks stress (σc) increased with confining pressure and the ratios σci/σc and σcd/σc respectively varied from 57.74% and 75.11% to 64.02% and 85.33%. Total energy U, elastic energy Ue, and dissipation energy Ud at different characteristic points increased with confining pressures. Field strain patterns indicated that localization developed rapidly in the post-peak region due to the acceleration of crack propagation and damage evolution. The failure mechanisms of Emochi andesite under the uniaxial and triaxial condition are respectively tension-shear destruction and shear destruction. Furthermore, the stress-strain response inside and outside the localization zone are characterized by strain accumulation or inelastic unloading. Finally, a method of confirming the stress level of localization initiation is proposed based on the DIC technique. The stress level decreases with confining pressure. The study provides a useful reference for understanding the failure process of rocks and improving the stability and safety of underground structures.

Applied Zone Model to Evaluate the Smoke Management in an Underground Structure

In Taiwan, since underground structures are more functional and complicated than usual buildings, for example, mass rapid transportation centers and underground shopping streets, performance-based designs are always adopted in allowing architects greater design freedom without any reduction in levels of fire safety of underground structures. Consequently, the function and efficiency of the smoke control systems in a public-use underground structure are becoming one of the important subjects for fire safety design engineers. This paper used a zone model as well as simplified mathematical models, which are commonly employed by fire safety engineering designers to investigate the smoke temperature and accompanying effect in an underground structure with a long corridor. Also, an evacuation assessment and escape time calculation were evaluated in this paper to address the strong relationship between evacuation and smoke control design. Further, experimental data collected from a full-scale underground corridor in Japan was used to verify the results from the model predictions.

Research on smoke control in underground structures

Abstract The purpose of the research described in this paper is to test smoke movement, to study the efficiency of smoke control by air control around the fire origin, and to determine control methods to protect evacuation zones from smoke. The four steps required to accomplish these aims are: (1) construction of mathematical models that describe the horizontal movement of smoke, the propagation velocity and heat loss of smoke; (2) a comparison between the mathematical models and full-size and scale models of an underground corridor; (3) a comparison between different ventilation systems and positions of fire origin in a scale model of an underground structure; and (4) simulation of the smoke movement of the underground structure using a one-layer zone model. A simulation model for evaluating the safety of underground structures is also proposed.