1 general situation of the project: ground subsidence and local cavities appear within the range of nearly 100 meters along the sidewalk of the main road on the south side of a residential area, which has caused great safety hazards to residents' traffic and adjacent buildings. The reason is that the main sewer with an underground diameter of 600㎜ and a depth of -5.6 meters is broken, which washes the underground soil layer and takes away the surrounding soil with the underground water flow, resulting in the ground settlement and uneven settlement inclination of the 38#, 32# and 29# residential buildings. Pipelines must be excavated and repaired. The details are as follows: 1) The surrounding situation is complicated. The pipeline along the construction section is only 6.5 meters away from the nearest Building No.38, and the gas pipeline, heating ditch and six-hole communication cable are buried above the pipeline. Buried depth1.0 ~ 2.0m. 2) Geological conditions ① Fill: plastic, uneven in nature and about1m thick; (2) The interbedded layer of Q4 yellowish brown loam and light loam is about 2m thick, [r] =1.2 ㎏/cm ㎏ es = 60 ㎏/cm; ③ In Q4, gray light loam and thin loam layers appear alternately, with a thickness of about 5m, [r] =1.0 ㎏/cm ㎏ es = 50 ㎏/cm; ④Q4 silty clay, gray, containing decomposed and undecomposed organic matter, with a thickness of about 4m, [r] = 0.8 ㎏/cmes = 40 ㎏/cm; 3) Hydrological conditions: the buried depth of groundwater is 0.65 ~ 1.9 m, which belongs to diving. 4) Find out the damage point of the pipeline? Pipeline detector (Germany IPEK- camera detection system) is used to detect underground pipelines. See figure 1. The accurate location and degree of pipeline rupture are detected, which provides the most direct basis for the correct treatment scheme. The technical features of the pipeline detector are ① 1.80-degree wide observation angle ② simple and quick operation, and all leakage points such as misconnection and rupture in the pipeline can be detected once; ③ accurate location of pipeline defects and leakage points (accuracy 1㎝). ? Figure 1.5) According to the measurement, there are uneven settlements in buildings 38#, 32# and 29#, with 38# slightly larger, but all within the allowable inclination range [δ/η≤11000], which must be effectively controlled immediately. See Figure 2 for the general plan before construction. 2 construction difficulties 1) The buried communication lines around the foundation pit and the special gas pipeline in the heating trench must be strongly supported. 2) The groundwater level is high and located above the basement; The foundation pit is deep and close to buildings, and the surrounding water should be prevented from entering the pit when the underground structure is constructed. To ensure that the supporting water stop body does not leak, beware of the collapse of the foundation pit soil wall and ensure the safety of the foundation pit. At the same time, to ensure the safety of buildings. 3) Due to the long construction period, it is necessary to ensure smooth drainage during the construction period, which will not affect the normal life of three nearby residential quarters. 4) It is forbidden to construct at night in residential areas, and adjacent piles constructed at intervals cannot meet the requirements of normal lap joint, so joints should be properly handled to ensure no water leakage after excavation. 5) Take effective measures to ensure the quality of pile body. In particular, how to do a good job in the upper part of the pile to ensure the quality of the pile top below the ground1m. 3 Design of foundation pit support scheme According to the excavation depth of foundation pit, the geological conditions in the site, the actual situation around the site and the specification requirements, considering the feasibility, economy and construction convenience of the construction period, it is decided to adopt the deep mixing pile support and water stop scheme. 1) The deep mixing piles are engaged with each other, with two horizontal piles ≥ 15㎜ and vertical piles ≥20㎜, forming a reliable waterproof curtain. 2) Insert 3M-long bamboo ∮ 100 as steel dowel in the deep mixing pile, and control the top height below the natural ground-1m.. Inserting steel bars is beneficial to coordinate deformation with piles and ensure the stability of foundation pit slope. Pile up and insert bamboo. The implementation efficiency is high and the effect is reliable. 4 Design of deep mixing pile 1) Apply the design principle of gravity retaining wall, consider the most unfavorable points, and choose three rows of grid layout, with reinforcement width of 2.9M, pile length of 9M (below -9m for impermeable clay layer) and effective pile length of 8m; After calculation, the anti-overturning KP value is ≥ 1.5, which meets the support requirements. Its main function is to stop water and slope protection. 2) 43. 1M south of building 38, 40.9M south of buildings 29 and 32? 700 double-headed piles with three rows of lattice layout. Between 38 # and 32 #, three rows of piles are arranged within the expansion bend 10M of the heating pipeline. Refer to figs. 3 and 4. 3) ordinary portland cement (P0.32.5) is selected as the cement, and the dosage is 15%. 5. Overall construction sequence of main construction technology: surveying and setting-out deep mixing pile construction (inserting bamboo bars at the same time)? Maintenance of the upper roof structure? Earthwork excavation and pipeline support? Well point precipitation? Sewage diversion discharge? Construction survey monitoring? Maintenance of sewage pipes? Pavement repair 1) Deep mixing pile construction ① The construction technology adopts the four-mixing and two-spraying pile-forming technology, with water cement ratio of 0.6 ~ 0.7 and aw= 15%. The technological process is as follows: measuring, positioning, aligning, and making curing agent slurry? Delay drilling for 9 minutes? Pre-mixed sinking to design depth delay13min? Stir for 9 minutes? Repeated sinking? Delay 13 minutes? Displacement of repeated pile drivers ② Main technical measures A According to the characteristics of soil layers within the range of reinforcement depth shown in the engineering geological report, two ZJ37 double-shaft deep mixing pile drivers with power of 37KW are selected to meet the requirements of this project. B, the key to ensure the pile length is to control the penetration depth ≥9.0M and the shotcrete depth within the range of-1.0m ~-9.0m. c, control the verticality of the pile driver to ensure that the verticality of the pile is controlled at ≤ 0.5%; After the pile body is formed, the lap length (horizontal ≥ 15㎜, vertical ≥20㎜) shall be ensured to ensure the tightness of the water-stop curtain. D, strictly implement the specified time and speed of pile sinking and lifting, and the speed should be uniform to ensure the uniform mixing of piles. The sinking speed of this project is ≤1m/min; The lifting speed is controlled at 0.5 ~ 0.6m/min. E. Before spudding, the technical director of the project shall make technical disclosure to the operators on drilling speed, frequency of re-mixing, speed and frequency of shotcreting and slurry stopping, especially on cement dosage and cement slurry water-cement ratio. D, the cement slurry in the hopper should be prepared according to the cement dosage and water-cement ratio requirements of each pile, to avoid long storage time, and it is forbidden to park for more than 2 hours. C, ensure the quality of underground 1M pile head. When the upper part of the pile is delivered, draw a dividing line with an elevation of 1m ~ 2M on the pile frame. When the pile drill is lifted to the 2m scale line, the speed will slow down and the mixing will increase. When lifting to 0.5M above the ground, stop mixing for several seconds to ensure that the protective layer at the top of the pile is 0.5m, and finally lift the ground. D stop slurry surface control in flush with the natural ground. Do a good job of cleaning the floating slurry on site to ensure the walking and displacement of the pile driver. E, bamboo stick construction: use steel pipe casing with a diameter slightly larger than ∮ 100, take out the pile-forming drilling tool, press the bamboo stick into the pile, and mark the elevation scale line with red paint on the upper part of the casing to ensure the height control of the bamboo stick. The upper part of steel dowel is reserved with 150㎜ to meet the requirements of connection and anchorage in the capping beam. (200㎜ thick coping, C20 concrete ∮ 10@200 two-way reinforcement). Joint treatment between piles. During the construction process, due to the segmented operation of the two machines and the overlapping construction stopped at night, part of the cement-soil pile has solidified and hardened. In order to ensure the water-stop effect, the water-stop surface is reinforced by outsourcing piles, pressure grouting between outsourcing piles and engineering barrier piles, and small-diameter piles are added internally to ensure the water-stop effect. See figure 5. ? (3) Strengthen the monitoring of the construction process, so as to facilitate the observation and comparison of building settlement and inclination, keep the permanent observation protection point away from the operation area, and do a good job in observing the original data. 2) Earthwork excavation, pipeline support, measurement, monitoring and maintenance shall be carried out 28 days after the completion of pile construction. In order to prevent quality and safety accidents caused by mechanical excavation of supporting piles, earthwork excavation is carried out in layers. Firstly, the earthwork is dug below the ground-1M, and the pile top is reinforced with C20 concrete with a thickness of 200㎜ and ∮ 10@200, and the early strength agent is mixed in the concrete. At the same time, the gas pipeline, heating trench and six-hole communication cable with a buried depth of1.0 ~ 2.0m above the pipeline shall be excavated, and steel pipe piles and steel trusses shall be used for effective support in the earthwork excavation area. Finally, dig into the maintenance basement in layers. The amount of earthwork excavation shall be subject to the requirements of basement maintenance working face. Pay attention to the safety slope of the upper part of the foundation pit except the vertical plane of the pile foundation. The pile foundation can be used as a fulcrum, and wooden boards and horizontal supports can be set as temporary supports at the foundation position of1.5m. In the process of excavation, closely monitor the survey, handle the abnormal situation in time, and ensure the safety of pipelines, buildings and personnel. See Figure 6 for the excavation profile. ? 3) Well-point precipitation should not be too deep to meet the requirement of 500㎜ below the bottom of the sewage pipe because the water-stop supporting pile blocks the flow of groundwater from north to south. Dewatering adopts a single row of well points, which are buried in the south side of the maintenance pipeline. During construction, continuous precipitation shall be carried out to ensure that the groundwater level is 500㎜ lower than the earthwork excavation surface. 4) Divergent discharge of sewage In order to ensure the smooth discharge of sewage during the construction period and the normal life of three nearby residential quarters, the sewage is intercepted upstream, pumped out by the sewage pump, and diverted to the downstream sewage well through the temporary drainage pipe. At the same time, the downstream sewage well is blocked at the mouth side of this pipeline until the sewage pipeline is repaired and restored. 5) The sewage pipeline repair and pavement repair shall be excavated to the maintenance basement, and the pipe bottom foundation shall be rebuilt, and the waterproof interface shall be made according to the specification requirements. After the pipeline is repaired, the well point shall be removed, and the earth shall be compacted and backfilled by layers. Return the supports of communication lines, heating ditches and special gas pipelines to the ground one by one. After the pipe joint is repaired, remove the blockage at both ends and restore the sewage circulation. Finally, restore the road surface. Conclusion 1) Pipeline detector (IPEK- camera detection system in Germany) is used to detect underground pipelines that people can't reach, accurately detect the location and degree of pipeline rupture, and provide the most direct basis for the correct treatment scheme. It is a good method to accurately find out the location and degree of urban underground pipeline rupture. 2) The project lasted more than three months from piling to maintenance. From the excavation situation, the water stop effect is good. Through measurement and monitoring, no abnormality is found, which proves that the support quality is good. Achieved the expected purpose. 3) According to the characteristics of this project, three rows of lattice layout are selected, and 3M-long∮100 bamboo is inserted into the deep mixing pile as steel dowel, and the pile top capping structure forms the cooperative work of the piles, which effectively ensures the stability of the foundation pit slope and realizes effective water stopping. The implementation efficiency is high, the effect is good and the engineering cost is saved. It is an attempt and innovation for small and medium-sized supporting and water-stopping projects. I hope my answer can help you.