Geomechanical Model Test Research Articles

Model test and numerical simulation research on the mechanical response law of lager span and small interval tunnels constructed by CD method

In order to study the mechanics response law of lager span and small interval tunnels during the construction process, this paper took the Jinan Southeast Second Ring Engineering-Daling Tunnel as the research object, and developed a large-scale 3D visible and assemble geomechanical model test system. Based on that, we carried out a model test and numerical simulation of the construction progress of the Daling Tunnel, and we studied the mechanics response law by monitoring the deformation and stress changes of the surrounding rock in real time. Combined with the results of the model test and the numerical simulation, the mechanical characteristic response law of the construction process of the lager span and small interval tunnels was revealed, and we found that: 1) The deformation of the surrounding rock of the lager span and small interval tunnels showed a change law of “slow settlement-rapid settlement-tending to be gentle”. And the stress release of surrounding rock shown a change law of “load accumulation-load release-tending to be stable”; 2) The excavation of the section ③ caused the largest settlement of the vault surrounding rock, which accounted for about 18.5% of the settlement value. The order of the influence degree of the surrounding rock deformation of the vault was ③＞②＞④＞①, which were almost the same with the law of the stress release of the surrounding rock; 3) The excavation of the tunnel ② caused the largest convergent deformation of the surrounding rock of the side wall, accounting for about 26.4% of the total convergent deformation. The order of the degree of influence of excavation on the deformation of the surrounding rock of the side wall was: ②>④>③>①, which were almost the same with the law of the stress release of the surrounding rock. The technical methods and conclusions formed in this experiment have important guidance and reference significance for related projects.

Development and Application of Similar Materials for Foundation Pit Excavation Model Test of Metro Station

Geomechanical model tests provide an intuitive and convenient method for observing physical phenomenon due to their easy implementation compared to in situ tests and prototype tests. The success of model tests depends heavily on the appropriate selection of model materials and proportions. Therefore, a new similar material is developed by utilizing the orthogonal experimental design method to conduct a series of proportioning tests. The new material is mixed with barite powder, iron ore powder, quartz sand, liquid paraffin, rosin, gypsum powder, and water. The physical and mechanical properties are studied through uniaxial compressive tests, Brazilian splitting tests, and direct shear tests. The influences of various raw material factors on the parameters of the similar material are systematically studied through range analysis. The results demonstrate that the mechanical parameters of similar materials have wide variation ranges; the adjustment range is 42.0–279.0 MPa for the elastic modulus, 0.37–5.37 MPa for the uniaxial compressive strength and 2.23–2.65 g/cm3 for the density. The new similar material has illustrated advantages in terms of performance stability, low price, and convenient production, which can simulate the similar relationship with different geomechanical model tests. The similar material is applied to a 3D geomechanical model test of the foundation pit excavation of Shenzhen metro station, which proves that the similar material can realistically reflect the change of earth pressure in the excavation process. With the deepening of excavation, the earth pressure curve shows significant fluctuations, and as the retaining structure is displaced, the parts with large earth pressure changes should be strengthened. The research methods and results can provide reference for further geological engineering research.

Investigating the mechanism of splitting failure in deep high sidewall cavern based on complex function and strain gradient

Owing to excavation-induced unloading, high sidewalls of underground powerhouse are often subjected to splitting failure, which produces many adverse effects to the construction of the cavern. To reveal the formation mechanism of splitting failure, we proposed a novel elastic–plastic damage softening model based on the strain gradient theory and the elastic–plastic damage theory. Using the ODE45 program in MATLAB, the numerical solution of the displacement and stress of circular cavern was solved based on the newly proposed elastoplastic damage model. We then adopted the complex function method and used the Schwarz-Christoffel integral formula to obtain the mapping function from the outer domain of high sidewall cavern to the outer domain of the unit circle. Finally, the elastic–plastic region and the displacement distribution of high sidewall cavern were obtained by mapping the obtained elastic–plastic solution of the circular cavern under the axisymmetric condition. The numerical analysis results were consistent with the previous geomechanical model test results. The results presented that the stress of high sidewall of an underground powerhouse exhibits an oscillating variation with alternating peaks and troughs, which is the mechanical mechanism of splitting failure.

Geo-mechanical model test on the water inrush induced by zonal disintegration of deep tunnel under hydro-mechanical coupling

With the increase of burial depth, complex geological environments such as high in-situ stress and high seepage pressure will inevitably affect the generation of zonal disintegration and may induce water inrush disasters during the excavation of the deep tunnel. A true three-dimensional geo-mechanical model test is conducted to study the mechanism of water inrush induced by zonal disintegration under hydro-mechanical coupling. The distribution and variation of different physical information during excavation, including displacement, stress and seepage pressure, are comprehensively analyzed. Test results reveal that zonal disintegration may induce water inrush when the tunnel is excavated under the coupling action of high stress and high seepage pressure. The excavation disturbance causes the stress in the rock mass to be oscillating. As a result, rupture and non-rupture zones appear alternately around the tunnel. With the increase of seepage pressure, the rock mass in non-rupture zone, which acts as the water barrier, is damaged and connects with rupture zone to form a water inrush channel. Eventually, high-pressure water pours into the tunnel to form a water inrush disaster. The reproduction of this special phenomenon will provide an important test basis for further theoretical and numerical studies.

An Experimental Study on the Preparation of Soft Rock Similar Materials Using Redispersible Latex Powder as a Modifier.

The engineering geological problems of soft rock are common in large slope engineering and underground engineering surrounding rock. In order to study the change in mechanical properties of soft rock under the action of loading, excavation and rainfall, this paper carried out experimental research on similar materials of soft rock. The similar material of soft rock is prepared by using iron fine powder, barite powder and quartz sand as aggregate, gypsum as binder and redispersible latex powder as regulator. A single-factor influence test was designed with the content of redispersible latex powder as variation parameter. Analysis the influence of redispersible latex powder from the perspectives of physical and mechanical indexes, failure forms, stress-strain states and changes after water seepage. In addition, evaluate the feasibility of this similar material in geomechanical model test. Experimental results show that the density, compressive strength and Poisson's ratio of similar materials can be improved to a certain extent by the redispersible latex powder with low dosage. However, the above indexes show a significant downward trend with the increase in dosage when the dosage exceeds 2%. The deformation modulus always shows a downward trend, and this trend becomes more significant especially when the dosage exceeds 2%. With the increase in the redispersible latex powder, the stress-strain curves of similar materials show obvious elastic and plastic stages. The failure mode gradually changes to X-shaped conjugate failure, which is common in soft rock, and the material changes from brittle failure to plastic failure. In addition, this type of similar material with gypsum as cementing agent will cause serious damage and loss of bearing capacity after seepage. These methods produce similar materials with low strength, low deformation modulus and plastic failure form, which can be used to simulate the stability of soft rock engineering caused by loading or excavation. At the same time, it also sheds lights on preparing similar materials of hard rock.

Tunnel stability analysis of coral reef limestone stratum in ocean engineering

The islands and coasts in the South China Sea have developed coral reefs, which are the basis for the development of ocean resources and the construction of ocean engineering. However, the biogenetic coral reef limestone in the sea has the physical properties of much porosity, low strength and low density, which is obviously different from the traditional geological rock mass during the underground engineering construction. So in this paper, surrounding rock stability of coral reef limestone stratum in tunnel excavation is studied by physical model tests and numerical simulation. Firstly, according to the similarity theory, quartz sand, barite powder, high strength gypsum, calcareous sand and cement are selected to configure the similar materials of coral reef limestone by the orthogonal ratio test and physical mechanics test. Secondly, the triaxial geomechanical model tests of tunnel excavation in coral reef limestone stratum are carried out. The influence of tunnel shape, support method and rock strength on the tunnel stability is studied by analyzing the variation laws of displacement and stress of the surrounding rock. Finally, the numerical calculation of tunnel excavation in coral reef limestone stratum is conducted and the results of physical model tests are verified. The influence of tunnel depth on the surrounding rock stability is further studied. The results show that: (i) Compared with the horseshoe shaped tunnel, the displacement of the circular tunnel at the haunch and springer is significantly increased, and the stress concentration phenomenon occurs. (ii) The stress release rate and displacement of surrounding rock decrease significantly when the anchor support is added or the strength of reef limestone increases. (iii) The stress release rate and displacement of surrounding rock increase with the tunnel depth, so the support strength of surrounding rock should be increased.

Geomechanics model test and numerical simulation of 2G-NPR bolt support effect in an active fault tunnel

Geomechanics model test and numerical simulation of 2G-NPR bolt support effect in an active fault tunnel

A Case Study on Deformation Failure Characteristics of Overlying Strata and Critical Mining Upper Limit in Submarine Mining

Unlike land mining, the safety of seabed mining is seriously threatened by an overlying water body. In order to ensure the safety of subsea mining projects, it is of great importance to understand the failure characteristics and influencing factors of overlying strata deformation. Focusing on the Sanshandao Gold Mine, a typical submarine deposit in China, geomechanical model testing and numerical simulations were carried out. The results show that in the mining of a steeply dipping metal ore body, subsidence deformation mainly occurs on the hanging wall; the subsidence center is located on the surface of the hanging wall, and the uplift center is located on the upper surface of the ore body. The critical mining upper limit, which represents the minimum thickness of the reserved isolation pillar between the overlying seawater and the goaf, was determined to be 50 m in the Xinli mine; fault slip would occur if this critical value was exceeded. The dip angle and thickness of the ore body were negatively correlated with the vertical surface deformation. As the dip angle and thickness increased, the critical upper mining limit increased. When the fault was located in the footwall, the critical upper mining limit increased as the distance between the fault and the ore body increased, and the failure mode of the goaf was fault slip. When the fault was located in the hanging wall, the final failure mode of the goaf changed to a combined failure mode of overlying rock collapse as well as fault slip. These research results provide a theoretical basis for the selection of the reserved pillar height in the Xinli mining area, as well as a reference for safe mining practices under similar geological conditions.

True 3D geomechanical model test for research on rheological deformation and failure characteristics of deep soft rock roadways

Deep soft rock roadways generally present large deformation characteristics, and a vast number of workers attempt to repair them to no avail. A true 3D model test was performed based on the engineering background of the transportation roadway of the Huafeng coal mine in China. The selected analogous materials have a significant time similarity relationship with the surrounding rock, and a similar bolt-mesh-shotcrete support structure, according to the engineering background, was applied to the roadway. The test results showed that the evolution process of the rheological deformation of the roadway is two sidewalls → floor → vault → full-section convergence of the roadway. The large deformation zones of the surrounding rock are constantly transferred from the surface to the depth, and the bearing capacity of the bolts decreases rapidly after the deformation of the anchorage zones. The mechanical characteristics of the floor determine that the lower surrounding rock always repeats the failure process of the upper rock stratum, and the floor heave does not stop. The test results revealed the full-section convergence rheological deformation laws and migration laws of the internal key deformation locations of the surrounding rock and clarified the reasons for the floor heave. The model test results are verified through field monitoring, and opportunities and suggestions for secondary support are determined. These results could provide a reference for the failure process and control technology of the same type of deep soft rock roadway.

Evolution analysis of the Banbiyan dangerous rock mass in the three gorges reservoir area, China

ABSTRACT Due to the changing mechanical environment, it is difficult to determine the evolution process of the rocky bank. In this paper, the Banbiyan dangerous rock mass (BDRM) in the Three Gorges Reservoir area was taken as an example to analyze its failure characteristics. As for this specific evolutionary environment, a new judgment system was proposed, involving the comprehensive field survey method and indoor tests. The key information about the steep areas, interior areas, and underwater areas was obtained, which overcomes the limitations of the conventional survey methods. On this basis, combined with a simplified geo-mechanical model and dry-wet cycle tests, the evolution trend of BDRM under variable water levels was predicted. The results show that the stability of BDRM depends on many factors, including the self-weight of overlying rock mass (the location of the broken area), the strength of the broken area, the water pressure of the trailing edge crack (rainfall) and the reservoir water level. According to the calculation results, the BDRM is likely to slip along the broken area after 46 dry-wet cycles. The application of related methods can provide an important reference for judging the evolution trend of unstable rocky banks worldwide.

Failure and stability analysis of deep soft rock roadways based on true triaxial geomechanical model tests

The deformation and failure of deep soft rock roadways become more prominent. In this paper, two true triaxial model tests were carried out to investigate the deformation and failure law of deep soft rock roadway excavation and the stability characteristics of the surrounding rock after the support. The surrounding rock stress and roadway deformation characteristics can be divided into three stages: advance influence, rapid change and stability. The deformation and failure sequence during the excavation process of the model roadway is “vault → sidewalls → arch springing → roof collapse”. Roof stability is the top priority. After the supporting structure is applied, the surrounding rock stress increases obviously and the roadway deformation decreases. The model test results are verified through field monitoring, and the key deformation parts of the deep soft rock roadway and the characteristics of the supporting structure force are clarified, which can provide reference for later research.

Evolution Patterns of Frost-Heaving Pressure with Partial Ponding in Cold Region Tunnels

The calculation of the frost-heaving pressure is one of the key issues in the frost-resistant design of tunnels in cold regions. To reveal the frost heave mechanism of partial ponding behind the tunnel linings, a three-dimensional geo-mechanical model test was developed that only considered the frost heave load. A self-designed water bladder equipment was set up to simulate the partial ponding. The frost heave response characteristics of the surrounding rock with partial ponding were simulated to analyze the change law of temperature field and frost-heaving pressure under freezing by using air-conditioning to reduce the ambient temperature of the model test. The changes in the internal temperature of the surrounding rock and the evolution of the frost-heaving pressure over time under different thicknesses of partial ponding and different levels of surrounding rock were compared. The theoretical calculation value of frost-heaving pressure and the test values of the model test were compared and analyzed using the numerical simulation method. The results showed that the temperature of the surrounding rock presented three-stage changes, which showed a lagging characteristic relative to the environmental temperature. The empirical equation for the relationship between the frost-heaving pressure and time was obtained by nonlinear fitting of the experimental results. Taking advantage of the regular tetrahedron calculation model, the paper established a semiempirical frost-heaving pressure model considering time and space effects, which was identical with the frost-heaving phenomenon from the experiments. Theoretical analysis, experiments, and numerical simulation show that the frost-heaving pressure increased along with the depth of partial ponding and the elastic resistance coefficient of surrounding rock, which could provide references for revealing the frost-heaving pressure of partial ponding behind tunnels of cold regions and has a significant meaning in the engineering application.

Geomechanical model test for mechanical properties and cracking features of Large-section tunnel lining under periodic temperature

The stress state of tunnel linings is complex and variable because of the repeated climate changes and deterioration of surrounding rock. In the long run, the cracking of the lining and the weakening of the bearing capacity will be aggravated. Thus, the bearing capacity and cracking mechanism of tunnel linings under thermal stress should be explored. Here, a tunnel geomechanical model test system is independently developed to simulate the thermal stress and the pressure of surrounding rock on a large-section lining of a tunnel during operation . Air cooling circulation and active loading are applied. Then, the deformation, internal force variation and crack characteristics of the lining under temperature–load coupling are investigated. Results show that, the cracks generated in the lining structure are mainly in the form of bending tension cracks and compression shear cracks. Bending tension cracks occur at the vault and inverted arch, whereas compression shear cracks occur at both side walls of the tunnel. The deformation of the tunnel lining entails vertical extrusion and lateral expansion. When compression shear cracks occur on the side walls, the inverted arch clearly rises. The crack width increases under repeated temperature changes, but no new crack occurs. The axial force distribution of the lining structure is symmetrical, and the bending moment distribution is similar to that of a horn. When the second compression shear crack occurs on both side walls, the lining structure is destabilised. The bearing capacity of lining decreases, and the ultimate bearing capacity decreases by 4%.

Experimental investigation of mechanical characteristics for linings of twins tunnels with asymmetric cross-section

A large-scale 3D combined geomechanical model test is conducted in this study to investigate the mechanical characteristics for linings of asymmetrically closely-spaced twin-tunnels constructed in sandy ground. A series of experimental results (i.e., excavation and overloading) effectively reveal the mechanical features of the linings and the stress release and displacement characteristics of the sandy ground for asymmetrical twin-tunnels during the excavation and overloading under the condition of great buried depth (vertical stress) and high stress coefficient (the ratio of lateral to vertical stress). Afterward, numerical analyses are conducted by a finite difference program FLAC3D to verify experimental results. The influences of construction sequence, stress coefficient, clear distance and lining stiffness on mechanical behaviors of linings and stratum are discussed in some details. The experimental results show that during the excavation of the asymmetrically closely-spaced twin-tunnels, the linings are in a state of compression. The lining of the smaller diameter tunnel which excavated later is susceptible to bias pressure. The bending moment located on the lateral side of the larger diameter tunnel close to the smaller diameter tunnel decrease when the smaller diameter tunnel is excavated. The change of stress release direction in the middle soil pillar is the main reason that results in the change of stress state in linings. The overloading process only causes the bending moments increasing continuously, but not affect the basic distribution patterns in linings. Numerical results indicate that if the smaller diameter tunnel is excavated first, the continuous tunneling of the larger diameter tunnel will cause sharp changes in lining moment of smaller diameter tunnel, but it will not cause serious adverse effects on the existing tunnel lining (larger diameter tunnel) if the larger diameter tunnel is excavated first.

Study on fractured rock-like materials for geomechanical model test based on 3D printing technology

The geomechanical model test has been verified to be an important way to solve some puzzling geotechnical issues. So far, there are mainly three methods used to construct geological models: pouring, tamping, and small block masonry. However, several disadvantages arise when using these artificial methods, including consuming too much time and cost, difficulty in guaranteeing uniformity of the density of different layers, most importantly, being unable to simulate detailed tectonic discontinuities, like faults or fissures. In view of the fact that three-dimensional printing (3DP) technology can easily produce complex fractured structures, 3DP technology is introduced to the research of the geomechanical model test. However, what kind of material is suitable to simulate rock mass and how to fabricate fractured rock-like material specimens are still two complicated problems. In this paper, a cement-based material is used to simulate fractured rock-like material specimens. This kind of material is proved to reach low strength and low elastic modulus by adjusting the mix ratio. What’s more, a method to fabricate small block masonry used in geomechanical model test with 3D printing fracture network is proposed. A series of tests are conducted for these 3DP fractured models, and the corresponding results show that their mechanical properties and failure characteristics are consistent with experimental results of other rock specimens with certain similarity, and geomechanical model test. These fundamental experimental investigations demonstrate the potential feasibility and prospects of 3DP technology in geomechanical model test.

Analysis of the influence of asymmetric geological conditions on stability of high arch dam

Abstract Geological conditions play a decisive role in the stability of arch dam engineering, and the asymmetric geological conditions of the abutment have a very negative impact on the safety of the arch dam. This article takes Lizhou arch dam as the research object, and determines that the arch dam is preliminarily affected by the geological asymmetric characteristics. Through the geomechanical model test method, the overload failure test of the Lizhou arch dam was carried out, and the resistance body, the instability deformation of the structural plane of the two dam abutments, and the influence of each structural plane on the dam body are obtained, and the safety factor is determined. According to the test results under the condition of asymmetric foundation of arch dam, for the structural plane which affects the geological asymmetry of the arch dam, the corresponding reinforcement measures are carried out. The feasibility of the reinforcement scheme is verified by the finite element method, and the safety factor after reinforcement is obtained. According to the results, it is suggested that some engineering measures can be taken to reduce the geological asymmetry between the two banks and ensure the safe and stable operation of the arch dam in the future.

Mechanical properties and reasonable proportioning of similar materials in physical model test of tunnel lining cracking

Similar materials of surrounding rock and lining suitable for crack of tunnel lining are developed through numerous matching tests according to the numerous theory of geomechanical model test to study the mechanism of crack damage of long-span tunnel lining and the variation rules, such as load and deformation, of physical fields. Similar material of surrounding rock is composed of quartz sand as coarse aggregate, barite powder as fine aggregate, and oil, vaseline, and paraffin as binder. Similar material of lining is based on gypsum and quartz sand, diatomite, fly ash, barite powder, and cement as additives. Physical and mechanical parameters such as uniaxial compressive strength, elastic modulus, and cohesion of materials are controlled through uniaxial compressive, direct shear, and splitting tests. The influence laws of mechanical parameters under various ratios are systematically studied. Test results show that the mechanical parameters of similar material of surrounding rock made of quartz sand, barite powder, and vaseline are stable. The range of variation is moderate, and the fabrication is convenient. The adjustment range of physical and mechanical parameters of similar materials in surrounding rocks is 30.1–52.2 MPa of elastic modulus, 0.23–0.33 MPa of uniaxial compressive strength and 2147–2396 kg/m3 of gravity. In the range of 1.2–2 water paste ratio, gypsum mixture is linearly weakened, and various admixtures strongly influence the mechanical properties of similar materials. Quartz sand as admixture can better replace aggregate in concrete. When the sand content is less than 20%, adding quartz sand into gypsum material can significantly improve the elastic modulus of the material, and the compressive strength changes minimally. When the sand content is more than 20%, the elastic modulus is basically unchanged and the compressive strength gradually decreases. When this similar material is applied to the model test of Fucunchuan tunnel lining crack in the Huangyan Road Capacity Expansion Project, the mechanical and physical properties meet the test requirements. The material can effectively simulate the development and evolution of cracks, provide material basis for obtaining good test results, and serve as a reference for the selection of similar materials for a similar model test.

Failure Mechanism and Numerical Simulation of Splitting Failure for Deep High Sidewall Cavern Under High Stress

With the increase of the depth of the underground engineering, the phenomenon of splitting failure of the deep rock will appear, which is very different from the shallow cavern. In order to reveal the formation mechanism of splitting failure, mechanical model test and numerical simulations of splitting failure were carried out respectively. Using the Pubugou Hydropower Station as the engineering background, a three-dimensional (3D) geomechanical model test was conducted relying on a high stress three-dimensional load test system. The splitting failure phenomenon of high sidewall cavern was observed, and the oscillation variations of displacement and stress were measured. Based on strain gradient theory and continuum damage mechanics, an elastic–plastic damage softening model for splitting damage was established. The relationship between rock damage and energy dissipation was analyzed. Based on the strain energy density theory, the splitting failure criterion based on the strain gradient is established. A numerical analysis method for splitting damage was proposed, and a regional disintegration calculation program was developed based on a commercial finite element code. The numerical simulation results are in basic agreement with the 3D geomechanical model test.

Experimental research and application of automatically formed roadway without advance tunneling

Longwall mining is a widely used coal mining method in underground engineering, in which a considerable roadway tunnelling is required. This makes it difficult for the roadway surrounding rock control. In order to realize the safety of coal mining and also economic impact, a new mining method called automatically formed roadway (AFR) without advance tunnelling is proposed. The S1201-II working face of the Ningtiaota Coal Mine is used as an engineering background to explore this method. We conduct numerical comparisons between the AFR and the traditional mining methods. The mining pressure distribution and evolution of the two mining methods under the influence factors of different pillar width, in-situ stress level, roof cutting height, and angle are systematically analyzed. Furthermore, a geo-mechanical model test system for the new mining method is independently developed. In order to clarify the control mechanism of the AFR surrounding rock, the model test is performed. Corresponding engineering suggestions are then proposed according to the results of the numerical simulations and geo-mechanics model test. The in-situ application results show that the new mining method satisfies the surrounding rock control requirement.

Comparative study of model tests on automatically formed roadway and gob-side entry driving in deep coal mines

Automatically formed roadway (AFR) by roof cutting with bolt grouting (RCBG) is a new deep coal mining technology. By using this technology, the broken roadway roof is strengthened, and roof cutting is applied to cut off stress transfer between the roadway and gob to ensure the collapse of the overlying strata. The roadway is automatically formed owing to the broken expansion characteristics of the collapsed strata and mining pressure. Taking the Suncun Coal Mine as the engineering background, the control effect of this new technology on roadways was studied. To compare the law of stress evolution and the surrounding rock control mechanisms between AFR and traditional gob-side entry driving, a comparative study of geomechanical model tests on the above methods was carried out. The results showed that the new technology of AFR by RCBG effectively reduced the stress concentration of the roadway compared with gob-side entry driving. The side abutment pressure peak of the solid coal side was reduced by 24.3%, which showed an obvious pressure-releasing effect. Moreover, the position of the side abutment pressure peak was far from the solid coal side, making it more beneficial for roadway stability. The deformation of AFR surrounding rock was also smaller than the deformation of the gob-side entry driving by the overload test. The former was more beneficial for roadway stability than the latter under higher stress conditions. Field application tests showed that the new technology can effectively control roadway deformation. Moreover, the technology reduced roadway excavation and avoided resource waste caused by reserved coal pillars.