1 project overview: Xiamen west railway station consists of station building and platform canopy project, rigid frame bridge and reserved subway civil works, with a total construction area of 162 409m2, including station building 109 028m2, platform canopy 53 38 1m2, with a total building height of 66.78m The plane projection of the main station building is work. The lateral earth pressure is supported by the combination of L-shaped retaining wall and pile-sheet wall on both sides of the exit staircase. Among them, the pile-sheet wall needs 46 rectangular anchor piles to provide anchoring force, and its section size is mainly 2.0m-2.5m, and its depth is about 20m, and the field water level is low and the local water inflow is large. There is no special hole-forming bit for rectangular piles, and manual hole-digging construction method is adopted (see figure 1 2). The construction points of rectangular manual digging pile include hole wall support, concrete pouring in the hole and safety protection. It is a high-risk operation to use hole wall support in the construction of large-size and super-long manual digging piles, and the design of hole wall support is related to the safety of workers at the bottom of the hole and the quality of the pile body. The main reason for the failure of manual digging pile retaining wall is that the earth pressure around the upper part of retaining wall is transferred to the side of retaining wall to produce lateral horizontal force, and the strength of retaining wall material is insufficient, resulting in hole collapse. There are two forms of support for rectangular hand-dug piles in Xiamen West Railway Station: ① pouring reinforced concrete on site in areas with small water output and good soil quality; (2) If you encounter a thick quicksand layer and silt layer, especially a large amount of groundwater, with the collapse of a large amount of mud and sand around the retaining wall, the concrete retaining wall becomes suspended, loses its support, and then produces cracks until it breaks off. In this stratum, the construction technology of precast pipe open caisson excavation is adopted. 2. 1 construction technology of cast-in-place reinforced concrete retaining wall The key of cast-in-place reinforced concrete retaining wall is the setting of locking retaining wall. The deformation and collapse of soil at the orifice are easy to occur due to the disturbance of stacking materials and earth-moving vehicles at the orifice of manual digging piles. If the lock is not firmly set, it will easily lead to the collapse of the top orifice, which will cause potential safety hazards. Therefore, the lock of manual digging pile must be firm and stable. The specific design parameters of the lock and retaining wall are shown in Figure 3. When the operator enters the hole, two solid blocks will be supported horizontally and vertically at the lock. The retaining wall is designed as a cone with a narrow top and a wide bottom. If manual digging pile construction is carried out in areas with water, the height of retaining wall should be higher than the surface water level, and drainage and dewatering measures should be taken. During excavation, when the excavation depth is less than 1m, wall protection shall be carried out (see Figure 3b). When encountering quicksand and silt layer, the excavation depth of each step should be appropriately reduced to 50cm, and the reinforcement wall protection should be strengthened. In the stratum with large water inflow, drainage holes should be reserved at the retaining wall in advance. In the process of construction, in order to ensure the accuracy of the hole location, it is necessary to check whether the position of the dug pile is skewed or displaced before digging every day. Especially, check the formwork before pouring the retaining wall, and then check the retaining wall after demoulding. Some mud and sand around the wall collapse, and the retaining wall is prone to displacement and skew after pouring concrete. Attention should be paid to inspection and timely correction. 2.2 Precast caisson excavation construction technology Precast concrete well pipe is used for caisson excavation wall protection, and the inner diameter of well pipe is the same as the design size of pile foundation. The pipe thickness of each prefabricated well is 10cm, and the length is ≤1m. Strengthen according to the design, add 2 ~ 4 lifting lugs and reserve drainage and grouting channels with a diameter of about 2 ~ 4 cm, and butt joint at the top and bottom of each retaining wall. During the construction process, the well pipe shall be lowered slowly into the well with a chain hoist or an electric hoist, and the verticality shall be adjusted after each lowering. In the stratum prone to collapse and with large water inflow, grouting materials are filled into the soil pores in the reserved channel of the retaining wall by filling, infiltration and compaction, so as to reinforce the soil around the retaining wall and protect the mud and sand around the wall from collapsing, thus increasing the friction around the pile. The pressure grouting material can be selected from fly ash, early strength cement concrete, lime-clay mixture, etc. Its construction technology is as follows. 1) After the paving site is leveled, ordinary sleepers will be placed vertically, densely laid along the periphery of open caisson, and the gaps between the skids will be filled with sand (see Figure 4). The number of planking shall be determined according to the quality of 1 section open caisson and the bearing capacity of foundation (sand cushion). The number n of slideways per meter along the toe of the blade can be calculated according to the following formula: in the formula, g is the gravity per unit length of the caisson in section 1 (kn/m); A is the bottom area (m2) of each stow-wood in contact with the sand cushion; F is the design value of bearing capacity of foundation or sand cushion (kN/m2). 2) After the steel bar binding of precast open caisson is completed, the internal and external templates and supports of open caisson are erected on the skids, and the seams of the templates outside the shaft wall are along the vertical direction to ensure the flatness. In order to avoid the influence of external formwork diagonal bracing on traffic, all supports are set in the well, and the combined steel formwork and steel tie rods supporting the wood diagonal roof are horizontally pulled. A double-row steel scaffold construction platform is set on the outside of the shaft wall. Concrete shall be poured in layers, vibrated by plug-in vibrator, transported by bucket, and chute shall be set when the pouring height of concrete exceeds 2m to prevent segregation. 3) The cushion concrete can be pumped only after reaching the design strength. When drawing mats, they should be carried out in different zones, sequentially, symmetrically and synchronously, and a special person should be appointed for unified command. When several groups of gaps are drawn, they can be backfilled, and then backfilled once every group is drawn. Backfill materials shall be sand-gravel blocks and fully compacted by layered sprinkling. The backfill height should be determined so that the mass finally distributed on the positioning skids will not damage the skids, and the bearing stress under the skids will not exceed the ultimate bearing stress of the original ground. In the process of cushion pumping, 1 measuring points should be set at the upper, lower, left and right sides of the open caisson to observe the subsidence. In the process of cushion extraction, if there are any abnormal phenomena such as inclination, backfilling with sand, crushing of cushion and sharp increase of settlement, it should be handled in time. 4) Dig the upper part of the sinking well, hollow out the blade feet, backfill the sand with stones after each section is hollowed out, and after the last section is hollowed out and backfilled to the positioning bearing pad, gradually dig back the filler in layers, and the open caisson sinks. The soil at the bottom is very soft. First, dig 4 050cm in the middle, and gradually expand the excavation symmetrically, hierarchically and synchronously. Before sinking, apply engine oil on the outer wall of open caisson to reduce the lateral friction with soil during sinking. When the upper part is manually excavated to the rock stratum, it is excavated by air shovel and hoisted by crane. 3. After excavation to the design foundation elevation, when the local foundation soil is sandy or cohesive soil, gravel or gravel cushion should be laid, and the back cover should be sealed immediately after passing the inspection. For dug piles with large water output, the drainage hole should be blocked to prevent water from flowing into the hole. When cleaning the bottom, if the rising speed of water seepage is less than or equal to 6 mm/min, the common concrete pouring method can be used to seal the bottom; If the water seepage is large, underwater concrete pouring should be adopted. In order to reduce the accumulation of groundwater, the water in the adjacent hole should be pumped out at the same time when any digging pile is sealed, so as to reduce the influence of the water in the adjacent hole on the working hole. When the hole depth exceeds 6m, attention should be paid to prevent concrete segregation. Generally, the mixed concrete is packed into a solid canvas bag or barrel with a volume of 1 ~ 2m3, and the concrete is tied with ropes. When the concrete is sent to the bottom of the hole, pull the hook to send the concrete to the bottom of the hole. Continuous operation can quickly block the bottom of the hole and block most or all of the groundwater at the bottom of the hole. If there is more groundwater and the dug pile is deeper, lift the pump and the bottom overflows nearly or more than 20cm. At this time, when the overflow in the hole is basically calm, use the conduit to reach the bottom of the hole and transport the mixed early strength concrete. When the amount of concrete exceeds 30cm above the bottom section retaining wall, then slowly dismantle the conduit. Because of the water pressure, the back cover concrete is basically dense, and the concrete is pumped after the final setting. Because the back cover concrete has exceeded the retaining wall at the bottom section, there is no underground water gushing. Remove the loose part of the surface concrete (the cement slurry in this part of the concrete will escape into the water) and transport it to the outside of the hole before continuing the next process. 4 Safety precautions for manual digging pile construction 1) There shall be no more than two people working under the hole, and wear safety helmet, raincoat, etc. When working, there is a contact signal with the personnel on the hole. Do not place hammers, hoes, stones and other falling objects around the ground. Every 1h, underground personnel and ground personnel are exchanged once. 2) Underground personnel should pay attention to observe the change of hole wall, and take timely support measures in case of collapse or cracks in wall protection. In case of danger, contact signals should be sent out in time to evacuate quickly, and effective measures should be taken to eliminate the danger as soon as possible. 3) Ground personnel should pay attention to the contact signal under the hole and react quickly. Always check whether the brackets, pulleys and ropes are secure. When hoisting, it shall be hoisted stably, and the hoisted earthwork shall be emptied and discharged to a distance of 2m from the orifice. 4) During construction, electrical equipment such as pumping, lighting and ventilation. A brake and a leakage protector shall be provided. Three-core rubber wires should be used for power supply lines, and the wires should be overhead and not dragged on the ground. Always check whether the wires and leakage protectors are in good condition, and set a ladder in the hole. 5) When pumping water from the hole, the drainage outlet should be more than 5m away from the orifice, and ensure smooth drainage on the construction site. Dig the pit and pour the retaining wall on the same day. When leaving the construction site, cover the orifice and set up obvious warning signs. 6) Construction personnel and inspectors shall open the hole cover 10 minutes before entering the hole. If there is any abnormal smell, it should be reported in time, and the operation can be carried out only after the harmful gas is eliminated. Dig a hole with a depth of 6 ~ 10m, and ventilate it at least once a day; if it exceeds 1 0m, ventilate it at least twice a day. If underground workers feel poor breathing, they should be ventilated in time. Conclusion Manual digging pile is widely used in construction because of its flexible operation, low site requirements and no limitation of aperture and shape, but it is also a high-risk operation, in which wall protection, drainage and ventilation are important links. During construction, the site hydrogeological conditions should be carefully analyzed, the support form should be selected, and the construction scheme should be determined.
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