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Large flow pump caisson construction method in Tianjin flow plastic silt
Tianjin Economic and Technological Development Zone sewage treatment plant is to improve the investment environment and the establishment of environmental protection infrastructure projects, the sewage pump room was built in the former known as the salt Wang Zi sea sedimentation tank, about 2m below the surface of the earth for the filling of soil, and then All of them are sediment-shaped flow-like silt for many years. The underground pumping station of the pumping station has a depth of 13m and a convex shape. Due to the arrangement of substations, dehumidifier rooms and mud cake storage areas nearby, it is not possible to adopt the open-cut foundation pit or deep foundation maintenance structure and design it as a polygonal caisson structure. It is very difficult to control the speed and direction of caisson sinking, so it has not yet been solved so far Construction problems. In accordance with the design intent, the three stations of the 18th Bureau of China Railway Construction contracting for this project are to prefabricate the DJM piles at the foot of the sink shaft to form a continuous diaphragm pylon underground continuous wall. For the shaft wall of the sunk shaft, Supporting the wall body, the process of sunk well sinking in the silt becomes a controllable process and successfully solves this construction problem. 1 Construction Preparation 1.1 Excavation and filling to reduce the initial elevation According to the geological conditions of caisson parts, in order to ensure the sinking of the sinking stage of the equilibrium subsidence, the artificial fill layer is excavated, the caisson precast and the initial elevation set Is 0.48m, thus creating two favorable conditions: one is due to the initial settlement of the silt, its water content and bearing capacity uniform, easy to calm; second, the total settlement of caisson down 2.5m, the upper third Section 0.5m thick caisson can not be used as caisson construction, but after pouring caisson back cover, so that not only reduces the caisson weight (still able to meet the sinking weight requirements), but also shorten the sinking depth, serve two purposes. 1.2 DJM pile construction powder spray pile, reinforcing the soft soil under the sinkhole edge so that the sinkhole in the continuous powder cement pile formed by the continuous diaphragm wall sinking. DJM construction should pay attention to the following points: ①The position should be accurate, the outer edge of the pile along the outer edge of the line tangent shall not be exposed, so as not to sink caisson cement soil squeeze to the outside of the shaft, the loss of support. ② pile bottom should be deep caisson edge design elevation 16m. ③ outer ring pile at the end of the foot and the pile top 1.0m range of cement 10% (according to the weight of the pile), the remaining pile 7%. ④ The inner circle pile cement volume is 10%. ⑤ inner and outer two rows of piles spacing 10cm, in order to ensure excavation of the inner side of the pile does not hurt outside the pile. 2 caisson prefabrication 2.1 prefabricated site layout As the caisson shaft wall is built together pump wall pool, so choose the pump pool position for caisson prefabricated position. In order to facilitate the construction and consider the need of concrete pouring in the caisson support wall, the surface of the soft soil in the site is compacted with 15cm and 8% lime soil, and the bottom of the support wall is made another 10 limestone platform. 2.2 Prefabrication The original design of the open caisson is three sections. After the topsoil is dug, the elevation of the ground is exactly the top elevation of the second section of the caisson. Therefore, the third section of caisson is no longer regarded as the caisson structure, while the two sections of caisson Shen then place the back cover then hit. In order to reduce the weight of the first pouring concrete, to avoid excessive sinking and uneven settlement caused by concrete cracking, the first section of concrete pouring completed in two, the first casting edge (pouring 1.0m) until the concrete strength to design strength Over 70%, pour the remaining part and construct the second section in turn. Two sinkholes concrete all completed, a sink in place. 2.3 Bladed bottom die and bottom die support wall According to the preliminary design, two rows of powder spray pile reinforce the soft soil layer. The original ground is muddy clay, the allowable bearing capacity is 80kPa, the cement content is 10%, the unconfined compressive strength is up to 600kPa for 7d, and the compressive strength is up to 800 ~ 1000kPa for 28d. The total amount of caisson concrete is 444m3, according to the capacity of 2.5t / m3, the total weight is 3610t. The sinkhole bottom area is 44.88m2. Therefore, when the sole surface of the blade edge is used as the support surface, the bearing load is 804 kPa. Based on the above calculation, and taking into account the error of DJM construction, the sinking edge is placed on the top of DJM pile and the bearing capacity can not fully meet the requirements. However, during the precasting process of sinking well, Support bottom wall to increase support bottom mold to share the weight of part of caisson. 3 Sunk Well Sinking 3.1 Preparation The sunk well must begin to sink after the concrete strength has reached the design strength. Make the following preparations before sinking: ①Marking marks on the wall of the sunk well, set the standard observation points at the four corners of the sunk well, The amount and balance of the situation; set a vertical line at the central axis to observe the caisson displacement and balance. ② remove the module. ③ Excavation of the surface of the bottom of the lime-soil support wall demolition, caisson slightly subsidence, but then the edge of the blade edge bearing capacity, caisson stop. 3.2 Subsidence coefficient calculation The subsidence factor formula: K = Q / (f · h · L)> 1 (1) where Q? - caisson gravity gravity? F friction coefficient, soft soil 9.8 ~ 11.76 kN / m2 h? - The maximum sinking depth L? - The perimeter friction coefficient of the caisson outer wall takes the maximum value of the soft soil. The subsidence factor of the general structure caisson can still reach 3.0 since the gravitation, Stagnation problem. The problem is that when the sinking depth reaches the requirement, it will still sink more than that, so control measures must be taken. 3.3 DJM piles continuous wall control mechanism of sinking ①guide and prevent sudden sinking, flood According to the preliminary design concept, in the borehole density range, under the edge of the blade, pre-playing two rows of DJM pile strata to prevent caisson Sudden sinking, settling too fast and a comprehensive measure of infiltration, the role of the principle is as follows: First, the powder spray pile to form a continuous wall of cement land, for the caisson is a closed clip in the silt in the load-bearing wall The sinking process of the whole sinkhole is also the excavation process of this bearing wall, so that the sinking speed and the smoothness of the sinkhole can be completely controlled by manually removing the powder jetting pile. Second, the implementation of open caisson in silty soft soil, in the process of digging soil subsidence prone to flooding, that is, flow-shaped plastic silo outside the well due to large differences in soil surface when the instability and slippery , Sliding arc edge into the hole at the foot of the foothole, so that the hole is only unearthed but the soil elevation does not drop, and the soil elevation subsidence, easily lead to uneven soil pressure, resulting caisson inclination. Pre-beating the blade at the foot of the first two lance spray pile, forming a 1.2m thick cement soil wall to prevent the formation of soil outside the wellbore sliding wall sliding inrush. (2) Controlling the design depth of caisson sinking When the caisson sinks to the design elevation and the soil below the bottom of blade edge is still silt, it can be known from the subsidence coefficient that the carrying capacity of the silt is not enough and will sink more than the designed depth At the foot of the next prefabricated powder spray pile underground continuous wall, can make the bottom surface sinkhole to the design elevation, fell on the DJM pile diaphragm wall underground surface to solve the problem of control caisson sinking depth. When sinking need sinking, only need to chisel pile head, sink well to overcome the friction and support strength of the soil with its own weight and sink, once the blade edge to the pile head to stop sinking. According to the above inference, when the caisson is about to reach the design elevation (with a difference of 50 ~ 70cm), the DJM can be pre-cut at the design elevation to control the caisson accurately and stop sinking. 3.4 Excavation method The surface layer of hard soil crushed by artificial clay, the remaining mud using high-pressure water gun mud, a layer of 50cm, layer stripping stripping down. In order to prevent the support wall and the ground beam to bear the bending moment and catch the mud, the soil should be washed into the pot bottom type (middle low, four weeks high). At the bottom of the sinkhole, each exposed layer of powder jetting pile is exposed. The inner layer of powder jetting pile is dug and cut along the same height first, and the outer layer of powder jetting pile is dredged intermittently so that the remaining powder jetting pile is crushed by a caisson and manually removed . 3.5 Caisson Correction Because of the role of DJM, the caisson generally does not have a large deflection. To ensure that the caisson in-position error is within the allowable range of the standard, the following control measures are taken: a. With the caisson sinking, the horizontal and Central axis monitoring, at any time to adjust the position and height of dredging pile. b. When the sinkhole sinks close to the place, stop the chiseling, dig the soil for 24h and observe the settlement. If there is no obvious settlement, it can sink once and stop taking other measures to stop the sinking. If there is obvious settlement, the reason should be found and increased Stop the measures. c. When the caisson is close to its place, if the axial displacement or inclination exceeds the allowable range, unilateral compaction and filling, unilateral excavation and deloading, counterweight and other means can be used to correct it. 4 sinkhole bottom caisson sinking is completed, the deviation should meet the specifications: ①axial displacement of not more than 1% well depth; ② elevation: +40 mm, -60mm; ③ slope ≯ 0.7% well depth. Caisson in place 2 ~ 3d, the edge has been a stable fall in the top of the powder spray pile, you can carry out the caisson backfill. In order to avoid the buildup of groundwater to form a large buoyancy, the top-sealing concrete can be divided into 2 to 3 holes on the bottom plate, and the seepage pipe buried in the well. After the pump tank structure is completed, the wellhead is blocked. 5 Conclusion In the flow-shaped silt stratum implementation of caisson, due to poor bearing capacity of the formation, the friction coefficient and other small features, easily sinking in the process of sinkhole sinking, influx, Shen speed too fast and excessive settlement and tilt Too large and so on, it is difficult to control. The design and construction of the caisson, take full advantage of the characteristics of cement soil, precast caisson at the foot of the pre-two rows of powder spray pile in the soft soil layer to form a suitable strength for continuous bearing wall body, in the caisson sinking process In the form of a reliable guide. Through the sub-section, sub-site chiseling powder pile head to adjust the supporting force, accurate control of caisson attitude and sinking speed, depth. Through the above construction process can be seen in similar soil caisson design and construction, by changing the edge area and the length of the powder injection pile, diameter, strength (by adjusting the amount of dust to achieve) and other means to adjust the bearing capacity, Various methods, simple process, low cost, is a successful construction process.