First, the karst caves of pile foundation are mainly hidden in the stratum, which is generally close to the irregular state of horizontal development, which brings many difficulties to pile foundation construction. In pile foundation engineering, the emergence of karst caves will have a serious impact on drilling and concrete pouring in pile foundation construction. Treatment steps of karst caves in pile foundation construction 1. Understand the geological conditions and pile foundation types. Karst cave treatment in pile foundation construction is to know the geological conditions of the engineering site in detail and determine the pile foundation type of engineering design. According to the geological, pile type and hydrological data of pile foundation, we can master the location of karst cave, roughly infer its scope, filling situation and nature, and formulate corresponding construction scheme and construction technical guarantee measures. This work needs to be surveyed and checked by professionals, and the geological histogram of each hole position needs to be sent to professionals to ensure the authenticity of the location, size and filling of the cave, and to allow designers and managers to adopt corresponding construction schemes according to specific geological conditions. 2. Choosing a karst cave treatment scheme When choosing a karst cave treatment scheme, it is necessary to refer to the construction effect and economic analysis of completed related projects, comprehensively evaluate the quality and safety, construction difficulty, construction progress and construction cost of each scheme, and choose the most popular construction scheme among various schemes. For some areas where karst caves are widely distributed and complicated, the construction difficulty will increase and the selection range of karst cave treatment schemes will be narrow. This is because the requirements for karst cave treatment are higher in complex areas, and the safest treatment scheme must be selected through effective analysis. 3. Formulation of quality inspection standards The formulation of quality standards mainly includes comprehensive evaluation of karst cave treatment effect. One is to ensure the firmness and reliability of the retaining wall, but to prevent the hole wall from leaking and collapsing. When formulating quality standards, the most important thing to consider is the possible loopholes and weak points after karst cave treatment, because weak links are the places that are most likely to cause quality problems after karst cave treatment, so relevant quality management departments need to refer to the measurement results and suggestions of karst cave determination technicians. 4. Determine environmental protection measures Environmental protection measures mainly include water and soil, vegetation protection measures, water environmental protection measures and atmospheric environmental protection measures. At the same time, attention should be paid to the management of production and domestic waste during the construction process, and the construction waste should be properly and reasonably treated after the construction. Three. Principles of karst cave treatment The principles of karst cave treatment in pile foundation construction mainly include the following five aspects: (1) Ensure the detailed investigation of each pile and provide the basis for selecting the corresponding construction scheme; (2) Geotechnical tests are carried out on the cave filler to detect physical parameters such as bulk density, porosity and water content; (3) For each pile, the corresponding karst cave treatment scheme and pore-forming method should be designed; (4) For each treatment scheme, careful calculation should be made to improve the construction scheme and guide the construction; (5) When encountering a large cave, the supervision engineer and design unit must be invited to check and report to the supervision for approval before implementation. Four. Main measures for karst cave treatment in pile foundation construction 1. Treatment measures of grouting cement mixture A common measure to treat karst caves in pile foundation construction is grouting cement mixture to effectively enhance the bearing capacity of karst caves. When adopting this treatment method, we should pay attention to that if the drilling hole passes through the karst cave and the slurry leaks too fast to achieve the purpose of reinforcement, we should take centralized throwing and filling measures. Centralized throwing and filling refers to the centralized filling of clay, flaky, bagged cement or concrete. The premise of this method is the filling time. For the cave with a height of 2-5m and no filler in the cave, when the hole is drilled to the rock at the top of the cave, the mud suddenly drops and the supplementary mud cannot rise, so the natural cone is formed by filling the gravel, mixed sand and bagged cement first. 2. Treatment measures of injecting cement mortar The measures of injecting cement mortar to treat karst caves in pile foundation construction are used in complex karst areas. Generally, there are many layers in karst caves, so as to ensure the accurate survey of karst caves. There are thick silt layer, sand layer and gravel layer in the soil layer. Grouting uses the slurry formed by cement, first filling the cracks in the cave, and then backfilling the cavity. Drilling grouting holes is also a further exploration of geological conditions. The fillings in the cave are in a loose or soft plastic state, so cement slurry can be directly injected, and cement mortar can be injected into the parts with large gaps. According to the pile diameter, the minimum diameter of enclosure and the formation law of accumulation body, the sand filling amount is calculated. In order to prevent high pressure in the hole from stopping sand filling, other holes are used as decompression holes, which can be stopped after the pressure is stable. 3. Rubble clay wall construction treatment measures Rubble clay wall construction treatment measures are used for pile foundation construction and karst cave treatment. Specifically, when drilling is about 1m away from the top of the karst cave, enough rubble and clay are prepared. After the drill bit passes through the roof of the karst cave, the drill bit is taken out, the clay and rubble are put into the hole, and the hole is filled in layers in proportion, and the clay and rubble are squeezed into the hole wall to realize artificial wall building. Pay attention to the change of water level in the hole during impact. If the mud in the hole drops rapidly, it should be replenished in time. When the punching depth is less than 50cm from the top of the hole, put forward a punching hammer, continue to throw flaky and clay into the hole, then punch the hole, and so on, until the stroke gradually increases to the normal punching speed after passing through the hole. V. Drilling and treatment measures concerned 1. When drilling meets a karst cave (1), firstly, analyze whether the hole wall is stable or not, and whether there will be sticking and hole burying. If it is definitely not, then take a closer look at the leakage of washing liquid. If the leakage of washing liquid is not serious, a small hole can be slowly ground at the bottom of the hole by depressurizing drilling. When the depth of slim hole reaches about 0.5m, normal drilling can be resumed. (2) If the hole wall is stable and the lotion leaks seriously, clay or mud balls can be used to block the water. When the hole depth is greater than 15m, clay or clay balls are not easy to reach the bottom, so they can be pressed down with drilling tools. (3) If the hole wall is unstable, but the height of the cave is not large, serious collapse will not occur, and clay or mud balls can also be used to protect the wall. (4) If the hole wall is unstable and the height of the cave is high, serious collapse or even scrapping may occur. First, the depth position of the cave should be studied. If the depth is not large, such as well depth 15m, wall protection and variable diameter drilling can be used. The original aperture is 1 10. You can run 108 casing, and then drill with 89 drilling tools. I would like to add here: experienced managers often leave room for one or even two floors when determining the tunnel diameter in order to prevent accidents in the tunnel when constructing in complex areas. (5) If the hole wall is unstable and the cave depth is large, such as casing wall protection and variable diameter drilling method. When the cost is too high because the casing cannot be recovered, the method of re-drilling after cement slurry consolidation can be adopted. In order to save time, an accelerator can be added to the cementing slurry, and salt can be used when there is no accelerator temporarily. 2. Concrete Pouring Method When pouring into the cave (1) after hole formation, install a conduit to pour C20 concrete, and stop pouring until the top of the cave is 2 meters, and then make a second hole formation after 6 hours, so that a new hole wall with certain strength and stability can be formed in the cave to ensure the smooth pouring of pile concrete. (2) Follow-up treatment measures for steel liners Another common measure for karst cave treatment in pile foundation construction is the follow-up treatment of steel liners, in which steel liners are generally made directly in the factory, with a wall thickness of 12 ~ 15 mm, and the diameter of the liners is selected according to the pile diameter and the number of karst caves. When multi-layer liners follow up, the diameter of impact hammer should be adjusted according to the diameter of liners. Due to the long length of steel lining, the precision and accuracy of manufacturing, processing and splicing of steel lining are required, especially the quality control of splicing in construction site. Therefore, before splicing the linings, the diameter and length of each lining must be checked section by section, marked well, and guide steel plates set to ensure its straightness. The lining is spliced segment by segment, cast until it cannot fall off by itself, and then struck to a predetermined depth with a vibrating hammer.
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