1 overview
In order to meet the needs of rapid and sustainable urban economic and social development, Guangzhou has planned the rail transit project according to the urban development pattern of "East, West, South and North". By 20 10, Guangzhou will have 9 urban rail transit lines with a total length of 255km and a total of 164 stations. Large-scale subway construction has brought large-scale geotechnical engineering investigation. According to the characteristics of subway engineering and the specific geological conditions in Guangzhou, the key and difficult problems in geotechnical engineering investigation of Guangzhou subway are discussed.
Subway engineering can be divided into underground lines, ground lines and overhead lines according to the way of line laying; According to functions, it can be divided into stations, sections, rolling stock depots, parking lots, substations and control centers. According to the construction methods, it can be divided into shield method, mining method, cut and cover method, cover excavation method and immersed tube method. The diversity of subway line laying methods and construction methods leads to the diversity of subway engineering foundation types and structural forms (such as piles, underground diaphragm walls, supports, anchors, composite foundations, etc.). Therefore, the geotechnical investigation of subway has the characteristics of railway tunnel, urban high-rise building, deep foundation pit and hydrogeological investigation. Different route laying methods, construction methods and structural forms put forward different requirements and emphases for geotechnical engineering investigation.
2 Engineering geological and hydrogeological conditions in Guangzhou area
Guangzhou geotectonics is located in the structural unit of the Central Guangdong Depression of the South China Fold System. Under the action of Caledonian, Indosinian, Yanshan and Himalayan tectonic cycles, folds and faults of different scales developed in the range, and sedimentary rocks, magmatic rocks and metamorphic rocks all developed. Among them, Guangcong fault in the northeast and Shougouling fault in the east-west direction divide Guangzhou into three structural areas, and control the distribution of Quaternary sediments and sedimentary centers in each area.
2. 1 The structural area is widely distributed to the west of the fault.
Located in the northeast and southwest of Guanghua sag, the main structure is northeast, which is composed of Upper Paleozoic and its folds and associated faults, as well as Permian and Tertiary syncline basins. The west of Baiyun District is located in this structural area. Quaternary strata in this area are mainly composed of artificial fill, alluvial sand layer, alluvial soil layer and Pliocene and Holocene eluvial layers. The underlying bedrock is mainly Carboniferous, Permian, Triassic and Tertiary clastic rocks and carbonate rocks, with limestone, marl, carbonaceous limestone, carbonaceous mudstone or shale, sandstone, mudstone, conglomerate and gray conglomerate. The adverse geological processes in this structural area mainly include karst, coal mine goaf, fracture and weathered deep trough.
2.2 The structural area east of Guangcong fault and north of Shougouling fault.
It is located in the west of Zengcheng uplift, and the east of Baiyun District and Luogang District is mainly located in this structural range. The Quaternary in this area is mainly composed of artificial fill, alluvial soil layer, alluvial sand layer, river and lake sedimentary soil layer and Pliocene and Holocene eluvial layer, and the underlying bedrock is mainly Yanshanian intrusive rock. The bad geology and special soil in this structural area are mainly granite weathering residual soil, spherical weathering, weathered deep groove, sand liquefaction and so on.
2.3 The structural area east of Guangcong fault and south of Shougouling fault.
Located in the east of Sanshui fault basin, it is an inherited structure composed of Mesozoic Cretaceous wide east-west folds and a series of NW-trending faults formed in the late Yanshanian and Himalayan periods. Yuexiu, Liwan, Tianhe, Fangcun, Haizhu, Huangpu and Panyu are mostly located in this structural area. The Quaternary system is mainly composed of Holocene and Upper Pleistocene artificial fill, marine and continental deposits, alluvial and diluvial deposits and residual deposits, and the underlying bedrock is Cretaceous sedimentary rocks and Sinian metamorphic rocks. The typical adverse geological effects of subway engineering in this structural area mainly include sand liquefaction, soft soil, Cretaceous eluvial layer, weathering zone softening when meeting water, etc.
Guangzhou is located in the Pearl River Delta, with criss-crossing rivers, rich groundwater and shallow buried depth. Groundwater can be divided into quaternary pore water, bedrock weathering fissure water and carbonate karst fissure water according to its occurrence mode. The groundwater level is obviously influenced by seasons and tides. Quaternary pore water is locally confined, and karst fissure water in concealed karst is mostly confined. In addition, there are many folds and faults in Guangzhou, which are affected by structural cracks and fracture zones, and the hydrogeological conditions are complex.
3 subway engineering requirements for geotechnical engineering investigation
Geotechnical engineering investigation of subway can be divided into engineering feasibility investigation, preliminary investigation, detailed investigation and construction investigation by stages. In addition, special hydrological investigation, special fracture investigation, special soft soil investigation and geophysical exploration can be supplemented at a certain stage when necessary. Different investigation stages have different investigation accuracy and requirements. Taking the detailed survey as an example, the main objectives and requirements of geotechnical engineering survey in the detailed survey stage of Guangzhou Metro can be summarized as follows:
(1) On the basis of preliminary investigation, find out the engineering geological and hydrogeological conditions of the site in detail, and conduct key investigation for complex sections or sections with special requirements.
(2) Find out the unfavorable geological conditions and the distribution characteristics of special rock and soil in detail.
4 Key and difficult points of geotechnical engineering investigation of Guangzhou Metro
4. 1 main points
4. 1. 1 Measurement should meet the needs of structural design and construction methods.
For open-cut and cover-cut methods, exploration holes are generally arranged about 2m outside the structural sideline, and open-cut and air duct holes can be arranged along the centerline. Boreholes should be arranged outside the structure 1 times the excavation depth. Geotechnical engineering investigation needs to find out the stratification and thickness of rock and soil; Find out the occurrence, fluctuation and slope of bedrock; Identify bad geology; Find out the groundwater type, water level, water quantity, recharge source, permeability and corrosiveness to concrete and steel structure; Judging the possibility of piping and floating loss; Judging the liquefaction characteristics of sand layer; Judging the possibility of dewatering in foundation pit; Grading the diggability of soil and rock; Evaluate the environmental bearing capacity of foundation pit excavation; Provide geotechnical parameters (such as gravity, shear strength, Poisson's ratio, unconfined compressive strength, static lateral pressure coefficient, foundation coefficient, characteristic value of pile foundation bearing capacity, bond strength between rock and anchor body, etc.). ) enclosure structure (pile, wall, soil nail, anchor rod) and permanent pile foundation design; Provide engineering geological vertical and horizontal sections, etc.
During shield construction, the exploration holes are staggered 3 ~ 5m outside the tunnels on both sides along the line. Geotechnical engineering investigation needs to find out the stratum structure, sequence and the distribution of karst caves, lenses and obstacles in the stratum. For soft soil, loose sand layer, boulder-bearing stratum, stratum with high silty clay content, stratum with uneven soft and hard surfaces, and stratum with great influence on the selection and construction of shield machine, we should focus on investigation. Find out the joint development of hard rock and the basic quality classification of rock mass; Find out the groundwater level, permeability coefficient and corrosivity, and estimate the water inflow of tunnel face (as a reference index to measure the damage consequences of tunnel instability); Provide geotechnical physical and mechanical parameters required for mechanical calculation and selection of shield machine and cutter; Classify the diggability of rock and soil, and provide vertical and horizontal profiles of engineering geology.
For mining methods, exploration holes should be arranged 3 ~ 5m away from the excavation scope as far as possible. The primary task of geotechnical engineering investigation is to accurately classify the surrounding rocks of tunnels. In addition, the investigation should also focus on finding out the hydrogeological conditions and estimating the water inflow per unit length of the tunnel (1m or10m); Find out the structural fracture zone, water-bearing loose surrounding rock, expansive surrounding rock, karst, softening and disintegrating surrounding rock when meeting water and surrounding rock that may cause rockburst; Classify the diggability of rock and soil, and provide vertical and horizontal profiles of engineering geology. The freezing construction also needs to provide stratum water content, groundwater flow rate, rock and soil layer temperature and thermophysical indexes within the excavation range.
4. 1.2 The survey should focus on identifying unfavorable geology and special rock and soil.
The geological conditions in Guangzhou area are complex, and there are many adverse geological effects affecting subway engineering, such as fracture, karst (including Cretaceous red bed dissolution and cavity), goaf, granite eluvial layer and weathered zone, spherical weathering, weathered deep groove, soft soil and hard rock.
When the rock mass in the fault zone is broken, avoid the pile foundation as far as possible. If it is impossible to avoid it, cross the fault zone. Because of the complex groundwater activity in fault zone, it is a great threat to underground tunnel construction. In the investigation stage, we should find out the scope, occurrence, structural fracture and water abundance of the fault zone. The activity of faults may have a certain impact on subway engineering. If necessary, the fault should be investigated specially to provide data for the design consideration of whether the structure needs special treatment.
Guangzhou Metro Line 2, Line 3, Line 5 and Line 6 all pass through karst development areas, and karst caves are also exposed in Cretaceous red beds in geotechnical engineering investigation from Zhujiang New Town to Chigangta, Ruyifang and Shuiyin Road. Karst and red bed caves have great adverse effects on underground lines and overhead lines. For underground lines, the shield machine may fall, and the karst confined water may break through the water-resisting floor or expose the karst aquifer during open excavation or mine construction, resulting in karst water inrush; For overhead lines, karst and karst caves lead to difficulties in selecting bearing stratum, sticking in pile foundation construction, slurry leakage and ground collapse, which affect the safety of ground buildings and underground pipelines; If there is hidden karst under the pile foundation and tunnel, the structure may sink due to the influence of load; The development of karst (especially soil caves) also brings adverse effects to subway operation. For example, the development of soil holes may cause structural settlement and affect subway engineering and environmental safety. Geotechnical engineering investigation should try to find out the distribution and hydrogeological conditions of karst caves and soil caves. Due to the limitation of drilling and the local irregularity of karst development, drilling should be combined with geophysical exploration when necessary.
The adverse effects of mined-out area on subway engineering are mainly manifested in the ground collapse and cracking caused by the collapse of mined-out area, as well as the destruction and deformation of the upper rock mass of mined-out area, which leads to the sinking and fracture of subway tunnel. The Permian strata in the northern suburb of Guangzhou (Sanyuanli to Jiahe) contain coal. Since the beginning of mining in Qing Dynasty, a large area of mined-out areas and complicated coal mining roadways have been formed. Because the plane position and space conditions of mined-out area have great influence on the selection of route laying scheme and construction method of the North Line, we should find out the scope and depth of mined-out area through investigation and study, and analyze the influence degree of coal mined-out area on subway line, so as to provide basis for the design and construction of the North Line.
Hard rock has great influence on the selection and construction method of shield machine. Generally, when the natural uniaxial compressive strength of rock is higher than 80MPa, a single-edged disc hob is selected, and the hob spacing is designed according to the rock strength and the development of joints and cracks. When shield tunneling in hard rock stratum, the cutter wear is serious, and sometimes even the cutter is damaged and the cutter head is worn, so the tunneling efficiency is extremely low. At this time, mining method (drilling and blasting method) should be selected for construction. Therefore, it is necessary to find out the distribution, strength, timely content (abrasiveness) and crack development of hard rocks. There are hard rocks such as granite, limestone, quartzite and diabase in Guangzhou, which should be identified in geotechnical engineering investigation.
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