Research And Application Of Dewatering Methods In Beijing Subway Construction

Traffic pressure in Beijing has been growing since mid-1980s and it has become a striking problem affecting normal functions of the capital city and restricting its economic and social development. The way out for traffic in Beijing is to establish rail rapid transit road network in the city as soon as possible and form the city complex public traffic system in which rail rapid transit backbone and ground traffic connect and supplement each other. It is necessary for solution of traffic problem in Beijing to speed up construction of its subway road network and fully display its key role in public passenger transport system. There are usually two difficult problems in the process of subway construction: First, most part of subway line is below the level of underground water and characteristic of multiple points, big length, and long time of construction, and it is difficult to handle underground water in the process of construction. Second, subway line is mostly located in downtown area where buildings and underground pipelines are dense, and this is another difficult problem in the process of construction as to how to handle the relations with traffic and occupation of land, prevent ground subsidence, and ensure the safety of surrounding ground and high buildings. Of above two problems, how to undertake underground precipitation to ensure normal subway construction and how to contain ground settlement due to underground precipitation can be summarized as the problem how to undertake scientific precipitation during construction. According to the rock properties and water content of the 287.4 km2 of shallow groundwater aquifer within the fourth roundabout of Beijing, the distribution of shallow groundwater (above 30m) in Beijing may be divided appropriately into following areas: The urban area is divided into 5 areas according to single water yield of shallow wells (30m) and introduction is given hereunder: 1. Rich-water area I (single well yield of shallow groundwater over 30 m3/h) The area covers mid-west of Beijing, the aquifer is mainly composed of sand cobbles and coarse medium sand, average aquifer thickness is 13m, and infiltration coefficient of aquifer is 60~150 m/d. 2. Rich-water area II (single well yield of shallow groundwater at 20~30 m3/h) The area covers the center of the city, the aquifer is mainly composed of gravel cobbles, round gravel, and medium fine sand, and underground water has embedded depth at 15~17m. 3. Medium-rich water area III (single well yield of shallow groundwater at 10~20 m3/h) The area covers Xibahe, Dongzhimen, Jianguomen, east of Lvjiaying, and west of east fourth roundabout in the east, reaches Xizhimen in northwest, Xibahe, Shuguangli in northeast, Beixinqiao and Houhai in the south, taking the form of "U". The aquifer is mainly composed of sand-gravel and medium finesand. 4. Weak-rich water area IV (single well yield of shallow groundwater at 5~10 m3/h) It mainly covers Anzhenqiao of the north third roundabout and north of Madianqiao in U form in the north, reaches Dongsheng Road in northwest, and Taiyanggong in northeast. The shallow aquifer of the area is poor, the formation is mainly composed of sandy clay and silty sand, etc., water yield is not stable, and infiltration coefficient is 20~40 m/d. 5. Poor-water area V (single well yield of shallow groundwater less than 5 m3/h) It is located on the mid-axis in the north of the city and over Gongzhufen. The shallow formations of this area contain basically no aquifer, and huge thickness of clay is mixed with some amount of silty fine sand. If the thin layer of silty fine sand is made proper use, water yield will be less than 5 m3/h. Underground water is one of the emphases for consideration in subway construction. It decides on the selection of construction method and seriously affects project cost and complex costs of construction. According to the study of the geohydrologic condition in North China, the geohydrologic condition in the area of construction should be studied amply for tunnel construction under quaternary geological conditions of North China. Then the most proper methods of controlling water and digging wells shall be chosen according to the level of underground water level, water content of underground aquifer, and rock properties of the aquifer, etc. Such preparation will save great money and enhance construction efficiency and safety, achieving twice the result with half the effort. At present, there are mainly following methods of controlling undergroundwater for subway construction in Beijing: (1) Block method: Water block by continuous wall, rotary blow-out piles, and mud injected into partial aquifer; water stop by fixed injection; deep-mixing cement wall blocking soil and water; water block through freezing method. (2) Well-point precipitation: Mainly including 1. Light well point; 2. Injection well point; 3. Deep well point (pipe well point); 4. infiltration well point (self-infiltration well); 5. Electric percolation well; 6. Radiation well point; 7. Covered drain (pipe) and open drainage, etc. Following materials shall be collected and known for design of precipitation engineering at certain place: l. Underground water distribution and burial conditions; 2. Dynamic water pressure and distribution of flowing network; 3. Formation permeability (infiltration coefficient); 4. Supply source and supply boundary materials. Hydraulic parameters of aquifer concerned in precipitation design consist of following two kinds: The first kind indicates the characteristics of the aquifer. The parameters indicating the permeability of aquifer include infiltration coefficient K and transmissibility coefficient T=KM, and M implies aquifer thickness. The parameter indicating water storage of aquifer is water storage coefficient S for artesian and feed-water degree for water-table aquifer; the parameter indicating the transmission speed of water head or water level in aquifer is pressure transmission coefficient for artesian, expressed in a. The second kind are parameters indicating interaction between aquifers after precipitation, including percolation coefficient B, indicating the scope of influence of underground water level in aquifer after precipitation, e.g., influence radius R.Indoor test and site test may be used to fix the hydraulic parameters of aquifer. With site test, pumping test will be conducted on site to determine the hydraulic parameters of aquifer. Due to economic limitation, it is impossible to conduct pumping test at each station or between all stations during subway precipitation construction so as to determine geohydrologic parameters (infiltration coefficient K, influence radius R, etc.). So pumping test is conducted over some chosen representative areas to deduce the parameters of aquifers for subway construction in Beijing. The results of pumping test over different formations in different parts of Beijing show: 1. Characteristics of underground water in southwestern part of Beijing In the west, south, and area before mountains of Beijing, the formations mainly consist of sand, cobbles with shallow embedment, underground water there is phreatic water with embedment usually about 10~20m, and infiltration coefficient is usually over 100m / d. 2. Characteristics of underground water in the center of Beijing In the center of Beijing, the formations mainly consist of sand and cobble-gravel, and the aquifer is in sand, cobble formation. Phreatic water exists in 10~25m sand, cobble formations, underground water level is 10~20m, aquifer is 3~5m deep, and infiltration coefficient is usually 50~75m / d. Artesian water exists in 25~35m sand, cobble formations, underground water level has embedment at 20~22m, aquifer thickness is 5~10m, and infiltration coefficient is usually over 75m / d. 3. Characteristics of underground water in northeastern part of Beijing In the east and north of Beijing, underground water has shallowXIIembedment and possesses several aquifers. If distribution of precipitation wells over ground system is not available during precipitation construction due to limitations of ground conditions (such as bridge area, river section, railway, traffic hub, ground buildings, etc.), underground conditions (such as underground pipeline, underground structures, etc.), or air conditions, radiation well precipitation may be adopted. Beijing No. 5 Subway Line first adopts radiation well technology for precipitation. In view of the construction and precipitation effect, radiation well precipitation is applicable to wide range of formations, aquifers such as cobble-gravel formation, sand formation, silt formation, etc., adopt horizontal wells for precipitation, and anticipated precipitation effect is available if correct construction process is adopted. Due to the special structure of radiation well, the hydraulic conditions are different from pipe well, big-opening well during pumping. The exact method of theoretical calculation is by far not available to fix the water yield of radiation well that usually refers to the materials of pumping test. According to analysis and summary of test data, calculation of the precipitation water yield of radiation wells for subway construction in Beijing usually adopts the formula away from water: lRqKHh?=???lg0.751. 36(2 2) During subway precipitation construction in Beijing, observation materials of radiation wells are displayed in the pumping process, and the flow of underground water is somewhat related to the time t. The water yield of radiation wells will decrease with time. With the time coefficient, the functional relationship between flow quantity Q and time T is as follows:XIIIBy calculating the flow change curves of all radiation wells, we may get 0.0345-the average value of time coefficient. Through analysis of above factors affecting the flow of radiation wells, the calculation of the flow of radiation wells for subway construction in Beijing shall take into consideration following factors: With the time function t, the formula for calculation of the flow of radiation wells is: Q = αqne?at In which: a=0.0345 Thus, the changes in the flow of radiation wells can be known at any time during actual construction. This formula is obtained through the author's analysis and summary of test data and is proved at some stations to render basic uniformity between the result of calculation and actual data. Radiation well construction consists of vertical well construction and horizontal well construction. Different methods shall be adopted according to the conditions of site and formations. The method of precipitation is restricted by various factors in the process of subway construction, such as geographical position of subway station, geohydrologic condition, relative position of construction structure and aquifer, etc., so that the method of precipitation shall be chosen according to actual circumstances in principle so as to achieve the best result. Most part of the subway line passes the region where buildings and underground pipelines are dense. It is difficult as to how to contain ground settlement and ensure the safety of surrounding ground and high buildings in the process of construction.Settlement calculation will mainly depend on the soil nature of formations. Proper parameters shall be chosen to check calculation in numerous methods. Usually different parameters shall be chosen in different areas to conduct calculation and forecast the settlement. Beijing ground settlement monitor station established in Bawangfen in eastern suburb of Beijing in 1991, which mainly contains bedrock mark as deep as 251.16m and level marks at different depth, conducts profound study of the law of occurrence and evolution of ground settlement in Beijing and generalize a set of soil mechanical method applicable to the forecast of ground settlement in Beijing according to detailed monitor materials, e.g., integral leveling method that calculates the settlement of each formation according to its soil mechanical parameters respectively and then add them together. The formulae for calculation of the final settlement of water-release and consolidation of clayey formations and sandy formations are as follows: lg()1 000PS 粘= H+×εCc P+ΔP EsS 砂= H0×ΔP This formula is applicable to the area where underground water is mined for long and there is large area of ground settlement. Usually subway engineering precipitation is precipitation during the construction period; since there is small precipitation during the construction period (precipitation at 2~4rn, certain single-formation with maximum precipitation about 6m) and tunnel construction is in sections; when one section is completed, the precipitation for that section may be stopped, so that the time of precipitation is not long for certain part. Thus if no void forms, ground settlement due to precipitation is quite limitedaccording to the conditions of subway construction in Beijing. Following works shall be done properly to prevent ground settlement during digging tunnels: 1. Prevent move of solid grains. 2. Adopt effective construction method to shorten the time of precipitation and dig rapidly. 3. Recharge underground water. With precipitation recharge, set up specially designed shallow pumping wells and deep recharge wells at both sides of the structure along the subway line. Pump out underground water with shallow pumping wells in proportion and then use deep recharge wells to recharge the water, which passes quality test, into deep aquifer below 40m, which can guarantee digging wells without water, ensure the safety and stability of ground buildings, and protect water resources. Subway construction in Beijing may be divided into following kinds according to the site conditions of different sections and concrete characteristics: First kind: Xidan Station to east section of Tian'anmen West Station. The target formation of precipitation is single artesian, single artesian pump may be used for precipitation, and water may be pumped out at shallow formations and recharged into deep formations. Second kind: East of Jianguomen Station-Dabeiyao Station and Redianchang Station-open and shield digging interface section. The target formation of precipitation is single lower groundwater level, single phreatic water pump may be used for precipitation, permeated water may be introduced and pumped for precipitation at some places, and water may be pumped out at shallow formations and recharged into the shallow formations mainly at the water-table aquifer.Third kind: Dabeiyao Station to Redianchang Station. The target formation of precipitation shall drain the water-table aquifer, lower artesian water head at some place and distribute pumps to introduce and pump water for precipitation with sand percolation wells intermittently. The underground water in that section has been seriously polluted and can not be recharged. Four kind: Tian'anmendong Station to east section of Jianguomen Station. The target formation of precipitation shall drain phreatic water and lower some artesian water head, distribute mixed precipitation wells with sand percolation wells and recharge wells intermittently, and pump out water at shallow formations and recharge it into deep formations. Concrete method: According to underground water reserve of all sections, make overflow self-infiltration sand well of different apertures from ground or inside pilot tunnel, drill pump wells in the middle and deep-level recharge wells at proper places. Drill self-percolation sand wells and pump wells into first artesian aquifer, drill recharge wells into the second or third artesian aquifer. So phreatic water may freely percolate into the first artesian aquifer through self-percolating sand wells, then pump wells may be used to pump water from the first artesian aquifer and recharge into the second and third artesian aquifers, which may drain the phreatic water. Through joint percolation of sand wells and pump wells, perched ground water is basically drained to guarantee digging tunnels without water. This method combining precipitation and recharge saves underground water resources considerably whole increasing water balance, strengthens local bearing force, and enhances the stability of the foundations of surrounding buildings.