Engineering geological physical simulation

1 summary of engineering geological simulation test

When analyzing and evaluating some complex geological phenomena related to major projects, it is often necessary to use simulation research methods for in-depth demonstration and evaluation. Simulation research can be divided into physical simulation and numerical simulation. The former includes photoelastic simulation, electrical simulation and geomechanical simulation test of similar materials. The latter adopts numerical calculation methods such as finite element, boundary element and discrete element.

The basic task of simulation research is to demonstrate some or all topics by reappearing the formation and evolution process of complex geological phenomena: ① to verify whether the mechanism model or conceptual model established by geological analysis is in line with reality, and to make a deeper (quantitative) analysis of its evolution mechanism; (2) Quantitatively evaluate the correlation between the main controlling factors and the dominant internal and external forces in the evolution process of geological phenomena, and demonstrate whether the established evaluation model is reasonable; (3) quantitatively evaluate the evolution and development trend of geological phenomena or processes under environmental conditions, and demonstrate whether the established prediction model is credible; ④ Quantitatively evaluate the effect of engineering design or treatment measures, and demonstrate whether the proposed countermeasures and schemes are effective and optimized.

In recent ten years, great progress has been made in the research of engineering geological simulation test in China. On the basis of introducing foreign advanced technology extensively, the research approach suitable for China's national conditions is explored, which is characterized by attaching importance to prototype modeling analysis and whole process evolution simulation. In the simulation study of geological disasters and complex rock mass stability, a research system with China characteristics has been gradually formed. This paper mainly introduces the basic principle, method and application of geomechanical model test of similar materials.

Principle and method of geomechanics simulation test

2. 1 model design

According to similarity theory, in addition to geometric and mechanical similarity, prototype and model materials are required to have similar deformation and fracture process characteristics, and their stress (σ) and strain (ξ) curves should satisfy the following relations:

Surface crust and ergonomics

Where: Cξ is the strain similarity coefficient (the ratio of prototype Cp to model Cm, the same below); Cξ is the similarity coefficient of residual strain. According to the dimensional analysis, the following relations can be obtained:

Surface crust and ergonomics

Where Cσ, CE, CL, Cδ and Cρ are the similarity coefficients of stress, elastic modulus (variable modulus), geometric size, displacement and material density, respectively.

In the model design, according to the geometric similarity coefficient drawn up in the design, other items can be calculated according to Formula (2).

Coefficient, according to the coefficient to determine the material selection, model making and loading system design.

2.2 Model materials

Barite powder, zinc oxide powder, diatomite, magnetite powder, iron powder, lead particles, polystyrene particles, quartz sand, etc. Usually used as aggregate. The binder can be gypsum, paraffin oil, glycerin, engine oil and epoxy resin.

The mechanical properties of the module with gypsum as the main cementing material are determined by the aggregate ratio and water-gypsum ratio of the cementing material, which has been systematically studied by the Rock Foundation Office of the Yangtze Academy of Sciences (Table 1). When paraffin oil is used as cementing material, it needs to be pressed into blocks by some special equipment for making modules, or rammed and shaped. The mechanical properties of the module are not only related to the material selection and proportion of aggregate, but also largely depend on the pressure or module density ρ (Table 2).

Table 1 Test Results of Mechanical Properties of Gypsum Cementing Module

Table 2 Test results of mechanical properties of powder compression module

2.3 Structural plane simulation

Shear strength is usually used as the control condition. Friction coefficient similarity coefficient Cf= 1, cohesion CC=Cσ. Hard structural surfaces (such as joints and cracks) are simulated by module contact surfaces. The shape of modulus should be determined according to the combination form of cracks. When the connectivity of cracks needs to be considered, the model can be made into a mosaic module.

For weak structural planes (such as faults with good continuity, weak interlayer, bedding plane, geological contact surface, etc.). ), aluminum foil and polyethylene film are used as sandwich materials for simulation, and talcum powder can also be sprayed to obtain low friction coefficient. The simulated f value can vary from 0.08 to 0.75. When it is necessary to simulate the high compression performance of weak structural planes such as faults, toilet paper or cork with appropriate thickness can be selected as cushion.

2.4 Loading system

The external load is loaded by different types of jacks or pressure pillows (bags). In order to simulate pore (void) water pressure, sand cushion aeration (water) method can be used in the module, and water can also be directly injected into the model in different ways.

Dynamic loads can be applied by installing vibrators on the active surface to simulate vibration effects as needed. A more perfect method is to put the whole model on a vibration table and simulate the dynamic environment with a three-dimensional vibration table.

Dead load is a volume force. In order to make the model fully reflect the role of rock (soil) self-weight in the evolution process, it is best to make the density of the model close to the prototype. In the large-scale model test, the tension rod pressure compensation system can be used to pressurize the model in layers and compensate the necessary dead load. The pull rod is connected with the base to which the tension is applied through rubber bands, and the number of rubber bands determines the tensile stress borne by the pull rod, thus simulating the gradient of gravity field. In the small-scale model test, the model can be placed in the rotating bucket of centrifuge, and the dead weight stress can be increased by high-speed rotation. The Yangtze River Academy of Sciences and the Institute of Water Resources and Hydropower Research have installed large-scale high-speed centrifuges with a diameter of 6m.

2.5 measuring system

The conventional method for measuring displacement is to directly measure with a dial indicator or to measure with a multi-point strain gauge of a strain gauge or displacement sensor. This kind of test is very necessary in the test, but it has great limitations. Because the obtained data can only reflect the information of fixed measuring points, it is difficult to describe the signs of bending deformation and fracture of the whole section. This method is not suitable for some models with very soft materials. According to the characteristics and special requirements of model test, the following test techniques have been developed and adopted in China: ① Tracking photography or rapid photography; (2) Copy the carbon powder grid, and observe the location and characteristics of the rupture of the measurement model after large deformation; ③ White light speckle method is used to measure the micro-displacement trace of the whole section of key test points; (4) Projection grid method, which is used to measure the in-plane displacement of large section points of soft material model; ⑤ Image moire method is used to measure the out-of-plane displacement of the whole section or a certain area of soft material model.

3 Application of stability evaluation of engineering rock mass

3. 1 Rock mass stability simulation of dam foundation

The proposed Three Gorges Dam on the Yangtze River is 65,438 0.75 meters high, and the dam foundation is pre-Sinian amphibolite plagiogranite, with hard and complete rocks. However, there are relatively developed gently inclined cracks in the dam foundation of the left powerhouse, and faults intersect with them, forming a possible wedge-shaped sliding body (Figure 1a). The following main problems should be considered in the overall stability of the powerhouse pit section: ① the conditions of possible sliding bodies and their influence on dam stability; (2) Whether the elevation difference of 79m from the shore to the riverbed foundation surface will cause harmful uneven compression deformation; ③ The maximum depth of foundation pit excavation in the backwater direction is 120m. What is slope stability? In order to demonstrate the above problems, it is necessary to understand the displacement field of each dam section under design load, the safety degree under overload and the possible failure mechanism, and put forward suggestions for foundation treatment. For this reason, the Yangtze Academy of Sciences completed a large-scale geomechanical model test (Figure 1b) with technical consultation and measuring equipment provided by the Italian Institute for Model Structure Test (ISMES). CL= 150, Cρ= 1 and CE=Cσ=CL of the model. Barite powder, engine oil (instead of paraffin oil), lithopone powder (instead of zinc oxide powder) and other compression modules are used for molding. The connectivity rates of gently inclined faults and faults are 10% ~ 50% and 50% respectively. Load with a series of jacks to change the rock mass density (ρ) to represent uplift pressure.

Figure 1 geomechanical simulation of the stability of the left powerhouse dam section of the Three Gorges Dam

The main conclusions are as follows:

(1) Under the design load, the safety reserve of each dam section can meet the requirements, and the safety reserve of the foundation surface is higher than that of the gently inclined fracture surface. When the dam is overloaded (3.5N), some dam sections will slide along the first gentle crack surface or be accompanied by heel cracking.

(2) Under the design load, the relative displacement between the dam site and the powerhouse is very small. The maximum horizontal displacement of adjacent dam sections is 1 1. 1mm, and the maximum relative displacement of settlement is only 4. 1mm, which is beneficial to the design of pressure pipes and water-stopping facilities between dams.

(3) Under the design load, the upper and lower slopes are stable. When the load is 3.5 ~ 3.8 N, the lower slopes of individual dam sections slide obviously along the gently inclined fracture surface.

The test results show that corresponding reinforcement measures should be taken for gently inclined cracks and faults in the south section.

3.2 Simulation of Bolting and Shotcreting Support Mechanism for Surrounding Rock of Underground Cavern

The stability of surrounding rock of underground cavern in medium strength homogeneous rock mass under different measures is studied experimentally. Four 50 cm×50 cm×20 cm models (Figure 2a) are made of gypsum, sand and other materials, which respectively simulate four situations: no support (I in the figure), anchor support (II), shotcrete anchor support (III) and anchor reinforcement (IV). Bolt reinforcement is to insert the bolt when the hole wall deformation has basically reached a stable state after the hole is formed. The model is pressurized in a triaxial loading test device to simulate the in-situ stress field. The compressive strength of the model material RC=2MPa, tensile strength St=0.2MPa, elastic modulus E= 1.25× 104MPa, Poisson's ratio μ=0. 17, φ = 4 1, c=0.45MPa, and spraying. ..

The main conclusions are as follows: ① In the homogeneous material cavern, the bolt-shotcrete support also has obvious reinforcement effect; ② Different measures have no obvious influence on the deformation and failure form of surrounding rock of the cavern (Figure 2b); ③ Bolt support and bolt reinforcement have obvious effects on improving the bearing capacity and deformation stiffness of the cavern.

Fig. 2 Simulation test of action mechanism of cavern supporting measures

Simulation Study on Formation Mechanism of 4 Xi 'an Ground Fissures

There are many views on the causes of Xi 'an ground fissures. On the basis of reappearing the formation and evolution process of ground fissures, this study demonstrates whether the genetic viewpoint of "tectonic gravity expansion" can explain this special geological phenomenon. Ground fissures developed in Cenozoic caprock, with a thickness of about 5km, surrounded by four tensile faults. Lintong-Chang 'an fault in the south is the boundary fault between the basin and the Qinling fold belt, and it is an active normal fault. The mantle uplift axis is near the urban area of Xi and passes in the direction of NEE. The model adopts a mixture of barite powder, diatomite and paraffin oil, which is paved and compacted in layers and confined in a frame representing the surrounding faults, with C L= 10000. Plexiglass plates are used as cross-section observation windows on both sides. The pedestal is placed on the arched steel beam, which can be lifted and lowered to simulate mantle uplift. The south baffle can be pulled up and tilted to simulate the extension of Lintong-Chang 'an fault. Image moire method is used to measure the small out-of-plane displacement on the top surface (Figure 3).

Simulation study on the relationship between Jibazi landslide and rainstorm in the Yangtze River

On July 24th, 1982, Jibazi landslide occurred near Yunyang County of the Yangtze River, which was a partial resurrection of the old landslide. In order to prove the quantitative relationship between landslide revival and groundwater hydraulic gradient, a simulation study was carried out. The model is made of different proportions of fine gravel, sand and soil according to the prototype structure. The sliding surface k is covered with polyethylene film. Pull out rubber pipes with different depths along the longitudinal section, and measure the pressure head of the side pressure pipe. Artificial water spraying simulates rainfall and rainstorm.

The "rainfall" in the model fell at 3: 30 in the morning after nearly 20 hours. The appearance after sliding can be compared with the actual situation. At start-up, the hydraulic gradient of groundwater in the sliding body is 65438 0.9%, which is close to the actual calculated value. This value can be used as a reference value to evaluate the stability of landslide under rainstorm conditions.

Fig. 3 Geomechanical Simulation of Xi 'an Ground Fissures