In the process of deep-buried tunnel construction, due to the long tunnel line, large buried depth and complex geological conditions, problems such as high ground temperature, rock burst and high-pressure water gushing will occur if the treatment measures are not appropriate in the construction process. In view of this, taking the actual project as an example, this paper analyzes and discusses the main geological problems existing in the deep-buried tunnel project, so as to ensure the smooth construction and provide reference for similar projects.
Keywords: deep-buried tunnel project; Disaster geology; High pressure water gushing
1 project overview
Taihang Mountain expressway Handan Dongpo Tunnel is located in the south of Lingdi Village in Wu 'an City, the east of Qishuiling Village and the northeast of Dongpo Village in Shexian County. The tunnel is a separated extra-long tunnel with a total length of 3 134m. The left picture is ZK38+624~ZK4 1+740, and the length is 3116; The right picture is K38+642~K4 1+776. The maximum buried depth is 176m. Taking this project as an example, this paper analyzes and discusses the main geological problems of deep-buried tunnel engineering.
2. The problem of high ground temperature in deep-buried tunnel
In the deep underground tunnel project, geological problems are the key areas to be explored and studied. First of all, we should predict the natural ground temperature. Once the ground temperature exceeds 30℃, it is generally called high ground temperature. High ground temperature will not only worsen the working environment of deep-buried tunnels, but also seriously reduce the labor productivity of workers and even cause great harm to the lives of on-site construction workers. In addition, it is more difficult to select construction materials for deep-buried tunnels [1]. The geothermal value changes with the buried depth of underground engineering, but the relationship between the maximum buried depth of underground engineering and the increase of geothermal value is not linear, because the high geothermal problem of this deep-buried tunnel is mainly caused by the high content of radioactive heating elements in groundwater activities and recent magmatic activities.
3 Deep-buried Tunnel and Rock Burst High Ground Stress Problem
Rock burst is one of the outstanding geological problems in deep underground tunnel engineering. The deeper the underground tunnel project is buried, the higher the ground stress is. The difference between deep-buried tunnel engineering and near-surface engineering lies not only in high horizontal tectonic stress, but also in high ground stress in surrounding rock. It not only causes high compressive stress on the side wall of the adit, but also causes high tensile stress on the top of the adit, which will lead to the instability of the surrounding rock of the chamber and bury hidden dangers. Due to the existence of high geostress, some surrounding rocks with high cohesive soil content and low hard rock content may undergo plastic extrusion. With the continuous release of high ground stress, the underground tunnel will be deformed and the tunnel will suddenly become smaller in a short time. For example, when the tunnel face is 30m away from the main tunnel, the deformation length of the tunnel body is 40m, and the damage of the supporting structure will be very serious at first. According to the measurement and calculation, the tunnel vault subsidence is between 10~20cm, the tunnel arch foot and side wall are squeezed and displaced to varying degrees, and even cracks appear in the concrete [2]. At this time, it is necessary to design a set of scientific and effective construction scheme, combining rigidity with softness and comprehensive management. In order to suppress the high ground stress, about 10000 super-long anchor rods are considered, and the total length is required to exceed 1 1× 104m, and the cross section in the underground tunnel is changed into a ring shape to form an arch shape, so as to meet the design requirements of flexibility first, rigidity second and resistance. Rockburst is affected by earthquake blasting, vibration of adjacent rockburst or mechanical external force, but the most basic reason affecting rockburst is the structural characteristics of rock. Through a large number of data analysis, it is found that the arrangement of rock particles is directional or random, the rock is cemented or crystallized, and the calcareous or siliceous cementation is ultimately related to the strength of rockburst. For example, (1) the rockburst strength of randomly arranged rocks such as granite and diorite is stronger than that of surrounding rock particles such as directionally arranged gneiss, granite gneiss and mylonite; (2) The rockburst strength of deep magmatic rocks connected by crystallization is greater than that of sedimentary rocks connected by cementation; (3) The blasting strength of siliceous cemented rock in the diversion tunnel of Tianshengqiao Ⅱ Hydropower Station is greater than that of calcareous cemented rock in Guancunba Tunnel.
4 high-pressure water gushing in deep-buried tunnel
In the process of deep underground tunnel construction, besides high ground temperature, water inrush has become another difficult problem to be solved urgently in tunnel operation. Due to the complex geological conditions, many geological units with large water inflow will be excavated in the section where the tunnel passes, and generally there will be large water inflow or high head pressure. When the groundwater pressure in deep rock mass is extremely high, it will lead to hydraulic fracturing of rock mass. This shows that under the action of high water head pressure, intermittent cracks and cracks in rock mass show a certain direction near the water inrush point, and the extended cracks and cracks are finally opened after fusion due to the influence of network-like intertwined structural cracks. With the increase of water inflow from deep rock mass in tunnel, the groundwater pressure is getting higher and higher, which will lead to hydraulic fracturing of surrounding rock in deep-buried tunnel engineering. Once hydraulic fracturing happens, it will quickly connect the cracks, the tension and degree of cracks in the cracks will become larger and larger, and the permeability of water gushing will become stronger and stronger. Coupled with the influence of hydrodynamic pressure, the crack will expand again, causing shear deformation and displacement of the filler on the crack surface. Whether it is a deep-buried tunnel project or a shallow-buried tunnel, geological disasters are mainly manifested in the collapse and earthquake caused by fault fracture zone, unconformity contact surface of rock mass and unfavorable combination section of structure, as well as gas explosion, harmful gas, karst collapse and debris flow [3]. Among them, the gas explosion mainly refers to the explosion of methane CH4 in the relatively closed coal measures structural strata, which is extremely disastrous due to the shock wave and violent oxidation.
5 bedrock fissure water
5. The meaning of1bedrock fissure water
Only the insoluble groundwater stored in hard rock cracks can be classified as the traditional bedrock fissure water. According to the basic characteristics of water-bearing media, groundwater can be divided into three types: pores, fractures and karst, but there is no corresponding relationship among groundwater, rocks and pores in rocks. Water storage pore system has dual porous media. In groundwater exploration, people have made new explorations on the types of water storage pores. Bedrock fissure water mainly exists in the fissure-dominated water storage space controlled by hard or semi-hard rocks that meet the geological structure conditions, and it is groundwater with movement and enrichment laws. Part of dissolved fissure water in dissolved rock and semi-hard rock in pore fissure water belong to bedrock fissure water, which is fundamentally different from other types of groundwater in that it is not strictly controlled by geological structural factors. Water-bearing fractures in rocks are mainly divided into diagenetic fractures, structural fractures and weathering fractures according to their causes. If it must be compared with weathered fissure water and diagenetic fissure water, it must be a structural fissure with concentrated water source and large water quantity.
5.2 Characteristics of bedrock fissure water
Because of the different main control factors, the basic law of distribution and enrichment of bedrock fissure water in different water storage structures and the factors that determine the main control are basically the same, with unique distribution and movement laws. The basic characteristic theory of bedrock fissure water enrichment in China is water storage structural system, and its main characteristics are as follows. (1) bedrock fissure water has complex and diverse burial and distribution laws. The space and channel for storing and transporting bedrock fissure water is called rock fissure. The size and shape of bedrock fractures, as well as the occurrence of fracture development zones that control burial and distribution, are all affected by geological structure, stratigraphic lithology and geomorphological conditions. Buried and unevenly distributed bedrock fissure water has the characteristics of irregular aquifer, diverse forms and banded distribution [4]. Such as brittle and plastic stratum, will produce strong water retention. If cracks develop in the fold structure, such as fold axis, turning point, anticline dip angle, etc., it will be easier to form water-rich sections, while the water-bearing capacity of the crush zone is relatively poor. (2) In complex bedrock fissure water, due to the inhomogeneity of storage space medium, the groundwater movement state of the same aquifer with different burial depths is also different. Geological structure is the most basic factor for the formation and distribution of pores in rocks, which mainly shows that the development of rock fractures and the storage of fissure water are influenced by geological structure and formation lithology, and the movement law of bedrock fissure water is also restricted by geological structure. Because of the different groundwater levels, even in the fractured water with the same bedrock, there are sometimes diving and sometimes confined water [5]. Laminar flow, pipeline flow, turbulent flow and open channel flow are different states of water movement under the special shapes of rock cracks and caves. Therefore, the heterogeneity of bedrock fissure water and strong sense of direction are the fundamental reasons for the complex and irregular seepage of fractured rock mass.
6 conclusion
In the deep underground tunnel engineering, several prominent geological problems are high geostress and rockburst, high pressure water inrush and high ground temperature. In addition, there are earthquake damage, gas explosion, water gushing and mud bursting, surrounding rock collapse, karst collapse, mud flow and so on. Therefore, in this complex and systematic deep-buried tunnel project, the study of disaster geology is a key step for the smooth development of tunnel project. Before the construction of tunnel engineering, effective and targeted defense measures should be taken according to the specific conditions of tunnel engineering.
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