Insufficient theory of gravity formation
According to the theory of gravity, Huanglashi area is an anti-slope, and it is also a soft and hard rock layer, in which flexure and interlayer dislocation are developed, and the slope has a material basis that is easy to deform; In addition to the small faults in the east-west direction, there are also SN- trending faults in this area, which makes it have deformable boundary conditions. The Yangtze River is cut to form a high slope with lateral unloading. Under the long-term action of lateral rebound relaxation and gravity, the rock mass is stretched and bent, and the original cracks and interlayer dislocation further expand the stretching. 1 tunnel, the horizontal depth of rock mass relaxation deformation is 2 15m, and the vertical depth is 100m ... Through the numerical analysis of the Second Design Institute of the Ministry of Railways and the Design Institute of the Yangtze River Water Conservancy Commission, it is found that at the vertical depth of the slope 100m, the direction of the maximum principal compressive stress is almost parallel to the slope slope, and the compressive stress at the foot of the slope is/kl. The stress increment at the rock bridge of gently inclined fault is 2.5MPa, which will not destroy the rock mass, nor will it uplift and expand the rock mass on the upper wall of gently inclined fault. There should be superimposed forces that are transformed after they are generated.
The exploration of late structural transformation in 4.4.2.2 has not been strongly demonstrated.
For the formation mechanism of buried gently dipping extension normal fault, the influence of late structural transformation must be considered. Therefore, the Three Gorges Exploration Corps of the Yangtze River Water Resources Commission entrusted China Geo University to study the microstructure of gently dipping faults in this area to confirm this point.
(1) Study on microstructure of gently inclined faults:,,,, etc. On the basis of macroscopic deformation study, the gently inclined faults are systematically sampled, and their microstructure characteristics are studied by various means. The research results were published in the proceedings of the fourth academic conference of the Rock Mechanics and Engineering Society of Hubei Province and Wuhan City in May, 1992. Excerpts are as follows:
Fault breccia breccia is mainly protolith and calcite fragments with sharp edges and different sizes. Only the breccia is slightly cemented, showing semi-diagenetic state and loose cementation. Cements are calcareous and clayey, and breccia and cements are the materials that the original rock was crushed in situ in the process of fault transformation, reflecting that the fault displacement is small and the active age is relatively new.
Fault gouge The fault rubs in motion, resulting in fault gouge. New active faults are easy to form and retain fault gouge, and old faults are often dehydrated and transformed into broken silty rocks. Microscopic analysis shows that about 20% of the broken particles are broken powder rock containing broken particles, and the broken particles and broken powder are composed of parent rock and calcite microcrystals. Differential thermal analysis shows that clay minerals in fault gouge have no obvious mineral change and element migration. It shows that fault reconstruction occurred in the low temperature crust surface environment.
Microcrack. Micro-cracks are very developed in gently dipping fault zone, including east-west, north-south and NE50, among which the east-west direction is the most developed, and the maximum density under microscope is 1mm, with 5 cracks, which are tensile and filled with calcite. The growth direction of calcite fiber in tectonic tensile vein is east-west, and its maximum principal stress direction is east-west. In the east-west fractures, E- twins are developed in calcite veins in the middle and early stages of the fractures, and the calcite veins are basically not deformed in the later stages, showing the characteristics of multiple activities. East-west microcracks obviously cut SN- direction microcracks, and the horizontal offset is 65438±0mm, which indicates that the formation period of east-west microcracks is the latest.
Microstructure of deformed minerals. Calcite in microcracks breaks and slips in the crystal during deformation, forming calcite E twin. According to the statistics of many rock samples under optical thin-section microscope, the occurrence rate of one group is less than 30%, and the occurrence rate of the second group is less than 20%. According to the empirical formula established by Jamison et al. (1976), the differential stress of normal fracture activity of the landslide is estimated by using different groups of E twins.
Dislocation structure of deformed minerals. Ultra-micro dislocation analysis of micro-fracture calcite veins was carried out by transmission electron microscope, and compared with the micro dislocation analysis of Shiyingpo in the east-west regional fault F6 in Baotahe coal mine area on the north side, in order to help identify its formation mechanism.
The distribution of free dislocations in deformed mineral crystals is uneven. No steady-state creep characteristics. The calcite pulse density is 106 ~ 108 /cm2. The deformation time is 107 ~ 109 /cm2. Used to calculate the differential stress of the strongest fault activity, the gently dipping fault is 50MPa, which is basically consistent with e-twin's estimation. F6 of the regional steep fault is 133.7MPa, and dislocations slip to the same slip surface during deformation, forming a dislocation wall with high density arrangement. A domain surrounded by dislocation walls in different directions is called subgrain. Dislocation wall and subgrain are dislocation structure types of plastic deformation at medium and high temperatures. In this study, there are only a few dislocation walls in the deformation time of F6 fault. It shows that the deformation mechanism of F6 is different from that of gently dipping fault in landslide area.
In the low temperature deformation environment, displacement lines in different directions often cross each other, forming a dislocation network. With the increase of deformation, dislocation lines in different directions are intertwined, which becomes dislocation entanglement and forms local high dislocation density. Brittle fracture is easy to occur and release action energy.
It is preliminarily understood that the gently inclined tensional normal fault in the landslide area is the transformation and deformation of brittle fracture in the modern shallow surface environment with low temperature and low pressure, and its maximum differential stress is 50MPa, which is obviously different from the formation mechanism of regional structural faults. However, taking the growth direction of calcite fiber in the extensional vein under microscope as EW direction, the principal stress field is determined as EW direction, which is consistent with the direction of modern neotectonic stress field and seems to be a derivative fracture of neotectonic movement.
(2) Temporal and spatial analysis of structural reconstruction: original low-order compression-shear small faults in landslide area. After the brittle fracture near the surface at low temperature and low pressure in the later period, it is transformed into a tensile normal fault, and its change should follow the basic principles and laws, and be judged from time and space.
Tectonic movement and its characteristics in this area. After examination by the State Seismological Bureau, the Seismological Bureau of Hubei Province put forward a special study report on earthquakes in this project area in June 1990, in which the structural part pointed out that Yanshan movement was an extremely important tectonic movement in the geological history of this area, and under the strong action of the north-south horizontal compression force field, the caprock and ancient block were strongly folded and fractured. Since the late Cretaceous, the principal stress field in this area has changed, and the movement has changed from compression to extension, resulting in large-scale uplift and subsidence block movement. Form some long graben depressions. The neotectonic movement basically inherited the characteristics of the previous movement, accompanied by the inherited activities of the old faults. SN-trending activity is obvious, followed by NE and NW-trending activity, and the near EW-trending activity is relatively poor, and the new fault activity is extremely weak. The neotectonic movement in the Three Gorges area is a large-scale uplift, which has three stages: five-level leveling, multi-layer karst landforms and multi-level river terraces.
Relationship between terrace change and topography in canyon area. There are six terraces along the Yangtze River in the Three Gorges area, from Chongqing to Yichang. Yichang is well developed, and its I-VI terrace is determined by 14C, and its age is 1, 2.5, 9, 42, 73, 165438+ million years. There are five terraces in Chongqing, and the age of terraces I-VI is equivalent to that of terraces II-VI in Yichang. There are I-IV terraces, and Wushan, 56 kilometers upstream, has II-IV terraces. According to the natural slope, the corresponding terraces in the upstream and downstream are connected, and the corresponding ground elevations of the landslide area and terraces at all levels are 95 m, 1 14, 147,187,207,238 m respectively. According to Mr. Xu's Research and Application of Neotectonics and Mr. Li Xingtang's Theory and Method of Regional Crustal Stability Research, the discussion is as follows: 165438+ million years of Yuanmou movement, the uplift of central Yunnan and the subsidence of Panxi Rift Valley stopped the southward flow of Jinya water system, formed the Panxi Lake environment, and deposited the lower Xigeda layer. In 500,000 ~ 703,000 years and 200,000 ~ 500,000 years, it rose and fell again, and deposited the upper Xigeda layer and Xigeda-like layer. Han Wenfeng and others (1993) think that the tectonic uplift before 14× 104a made the Qinghai-Tibet Plateau rise to 1000 meters. The activity of (3 ~ 5) ×104a caused great structural changes in Qaidam basin. Before 1× 104a, the Yellow River captured the Zoige Basin. The above-mentioned intermittent uplift geological events in the west are reflected in the corresponding six-level terraces in this area.
Reconstruction and inference of gently inclined faults. F48 fault with gentle dip angle in No.2 cave was reconstructed at (64.5 ~ 71.1)×104a, then at 15× 104a, and then at 4.65× 104a.
(3) Problem: The compression-shear gentle dip fault was transformed into an extensional normal fault, which was formed in shallow low temperature and low pressure environment. The protruding part of the river after scouring and cutting is a discontinuous carrier on the plane in the east-west direction, and there is no force transmission feature in the east-west direction. The deformation caused by tectonism is obvious in the deep, and the low-order deformation should be related to the deep. The enhanced reflection is only formed in the carrier part of the surface layer, which does not conform to the natural law. Earthquake theory can't explain its situation characteristics and development. The results of fine exploration and scientific research cannot be confirmed by tectonism.
The hidden thermal stress in 4.4.2.3 is now another way to explore the reconstruction of gently dipping faults.
Because its influence was not paid attention to in the past, relevant information and parameters were not paid attention to in exploration and scientific research monitoring. Now we can only use empirical data from books to explore.
(1) Calculation formula and parameters: The temperature difference stress mentioned in this book refers to the differential stress formed by the temperature difference between different depths of rock mass due to temperature drop. Because the cold front propagates inward from the surface, the tensile stress formed by cold shrinkage is upward from the surface, which is a problem of reaction and strain. The mechanical parameters of rock mass are deteriorating in strain, which must be determined according to the actual situation and master its specific anisotropic characteristics. Based on this, it is solved by the equation 1. 17. Its formula is:
Application of reaction strain rock mechanics in engineering
Where σ'x, σ'y and σ'z are strain equations of physical equations, which are different from thermal stress equations σx, σy and σ z, and бtr represents tensile stress that only affects the time dimension in the z direction. ν is Poisson coefficient, the volume of the landslide ν = 0.38 ~ 0.5, and the loose deformed rock mass ν = 0.3 ~ 0.47. бtrl represents the tensile stress in Z direction, which is influenced by the time dimension and the scale dimension of the vertical slope surface. The stress equation here has no required parameters, so the formula 1. 12, namely σ TTL = β eδ T, is used to calculate its tensile stress.
Table 4.2 Deformation parameters and thermal stress coefficient of rock mass in this area
As there is no experimental research result of relevant thermodynamic parameters, only the empirical values in Table 1.3 are used, and Table 4.2 is established according to Table 1.3 and local conditions. According to Table 4.2, the equivalent deformation parameters and thermodynamic stress coefficient values of rock mass in this area are estimated. See Table 4.3.
Table 4.3 Equivalent Deformation Parameters and Thermal Stress Coefficient of Rock Mass
(2) Gravity and possible maximum temperature difference stress: Calculation background: On the upper wall of the gently inclined extension normal fault, there are slumping accumulations and rock masses disturbed by relaxation deformation, and their bulk densities are 2.0t/m3 and 2.5t/m3 respectively. The annual average temperature in this area is 18℃, and the buried depth should be below the loose disturbed rock mass. Therefore, the study areas are all in the temperate zone. Every depth difference of 65438 0℃ in the rock mass, the slump accumulation body is about 3.0 ~ 3. 1m, and the relaxation disturbed rock mass is about 5.5 ~ 6.5m According to Tono. 1, No.2 and No.3 caves, and find out the thickness of various rock masses. When the horizontal length of the accumulation body is 49 ~ 50m and the average slope is 30, the vertical slope thickness is 24.5~25m, and the maximum temperature difference is 8℃. The horizontal length of relaxed rock mass is 50 ~ 52m, the thickness of vertical slope is 25 ~ 25m, and the maximum temperature difference is 4℃. However, the horizontal length of 1 cave is 97m, the vertical slope thickness is 4 1m, and the temperature difference is 6.3℃. The slope in this area is 25 ~ 37, which has great influence on the vertical thickness of rock mass everywhere, so the change range must be determined according to the actual situation. The deadweight of footwall rock is calculated according to the formula H 1γ 1γ 2, and the temperature difference stress is calculated according to β1E1δ T1β 2E2δ T2. The results are shown in Table 4.4 below.
Table 4.4 Gravity and Possible Maximum Temperature Difference Stress
(3) Derivation of the force on the fault plane and the concentrated stress at its end: Due to the change of terrain slope, the cold contraction tension and gravity on the fault plane form outward tensile resultant forces in different directions, and the tensile resultant forces in different directions form a unified resultant force, which becomes the tension-shear leveling potential field force. Because the fault plane has no tensile characteristics, the tensile force is concentrated at the end of the fault, and the value of this concentrated stress is as follows:
Application of reaction strain rock mechanics in engineering
Where σLtw is the tensile tensile shear force formed by the resultant force of depth dimension and W gravity. L is the fault length, which is about 100 m according to borehole measurement. See Table 4.5 for the solution results of the formula.
Table 4.5 Research results of gently inclined fault strain
The differential stress formed by the concentrated stress and the dead weight at the fault end is consistent with the differential stress of 30 ~ 50 MPa in microstructure research. It is proved that the temperature change from the surface to the interior obviously lags behind in the deep with the seasonal change, so it generally reaches the maximum possible temperature difference stress and lasts for a short time. Only special climatic conditions, such as ice age, will have obvious influence, which also shows that its activities are intermittent and active. SN-oriented tensile shear stress produces EW-oriented tensile joints, in which calcite fiber also increases EW, which can be explained as a misunderstanding of the role of EW-oriented principal stress.
When the temperature in the accumulation body changes rapidly, and the temperature difference between the upper and lower reaches 20℃, the cold shrinkage Zhang Liwei is 0.54MPa, the elevation angle is 65 and the gravity is 0.48MPa, so the resultant force is 0.23MPa and the elevation angle is 0.5, which is a relatively standard leveling potential field force. This force is greater than the tensile strength of loose rock mass, which will lead to cracks parallel to the slope direction, rainwater infiltration, hydrostatic pressure superposition and temperature difference stress increase, which will adversely affect stability. This important factor should be paid attention to in slope stability analysis.
(4) Analysis and summary of hidden force: The neglected hidden force of temperature difference stress is the leading role in the formation of gently dipping normal faults. According to the book data, combined with the engineering practice, the calculation parameters are determined, and the faults are calculated one by one in combination with the fault background conditions, and the results are consistent with the hidden microstructure analysis, and the ideal verification effect is obtained.