【Excerpts】Discussion on the treatment technology of anchor pile repair

        With the development of engineering construction, both cast-in-place piles and prefabricated pipe piles are improving in theory and construction technology, and have been widely used, and the frequency of class III defective piles is gradually decreasing. Due to the engineering geology and hydrogeological conditions, construction technology and construction organization factors and the influence of underground obstacles, the engineering construction, especially class III pile pile has obvious defects, affect the bearing capacity of pile structure, can not meet the requirements of structural safety and durability, although class III pile often use grouting reinforcement, pipe pile pile, increase cap, but encounter deep broken pile or lack of pile, still need to take other methods, so it is worth discussing.

1、Project overview

（1） Engineering design overview

        The total construction area of a large group project in Wuhan is about 510,000 square meters, including 11 towers and 5 floors of podium, 26~33 floors above ground and 2 floors of basement. The tower of this project adopts φ 800 bored cast-in-place pile foundation, the designed effective pile length is 33.6~36.1m, Ra = 4300 KN, the strongly weathered mud siltstone and silty mudstone are selected as the bearing layer, and the pile end is required to enter the bearing layer is not less than 0.8m.

        There are a large number of old reinforced concrete foundations of the project site, and most of the soil layer in the excavation depth is silt soft soil, which brings great difficulties to the construction of the pile.

（2）Lack of pile situation

        After the earthwork excavation of the tower, one of the cast-in-place piles (AG 3-121) of the 2-2 axis / 2-F axis ZCT 7 cap was found at and nearby, and no pile top and pile body reinforcement were seen, which was judged to be missing.

        The geological situation near the pile is: below the ground to 4m for miscellaneous filling, plastic silt and clay, 6~17m thick soft plastic or silt silty clay below 4m, down for silty clay, silt, fine sand, coarse sand mixed gravel and strongly weathered muddy siltstone, silty mudstone, etc.

2、Cause analysis of missing pile

（1）The influence of underground obstacles

        The site near the pile is originally a workshop and residential building of 3 to 5 floors, with prefabricated square piles and cast-in-place pile foundation. In the process of construction, the design pile position must be offset and redrilled or irrigated. This may cause the design deviation, and then the pile is missing due to negligent rig transfer.

（2）Bad weather impact

        In the process of concrete pouring of pile foundation, the rainstorm was encountered. Due to the undulating slope and the slippery road, the concrete transportation was difficult, and the continuity of cutting could not be guaranteed, which led to the deep broken pile.

3、Pile shortage treatment scheme

（1）Primary election treatment plan

        Based on the previous engineering experience and combined with the surrounding environment near the missing pile, the preliminary selected treatment plans include: re-pile, increase cap, anchor pile, etc., and the final treatment plan is determined through feasibility analysis and economic and technical comparison.

        Because the anchor pile is mainly carried out after the construction of the bottom plate, especially after the main body reaches a certain stage, the workers have a good working environment and will not affect the construction period. Finally, the owner, the design, supervision and the construction unit jointly decided to adopt the pile repair scheme of the anchor pile.

4、Plan implementation

        In this project, 10 anchor static pile are reinforcing, and the vertical bearing capacity of the anchor static pile Ra = 450 KN (section 300300). The pile pressing force is 900 KN. At the pile end, the (4-2) silty fine sand layer is used as the bearing layer, and the depth of the pile holding layer is> 1.5m. There are 14 holes reserved for the anchor static pressure pile. In order to prevent the need of repiling due to ground obstacles, 4 of them are used as backup holes. In the construction of anchor pile, oil pressure value and pile length are controlled in both directions, and oil pressure value is mainly used.

        The ast anchor static pressing pile, the pile body adopts C30 concrete, welded steel joint, the pile section length is 2.0m. The anchor rod adopts 8.8 grade high strength bolt, and the length anchored into the cap is not less than 40d. During the construction of the foundation floor, the construction unit of the anchor pile has 4 piles, and Φ 30 / 32 bolt is used. The length of the anchor rod into the cap is 1280mm (the anchor rod is fixed with the middle steel mesh). The length of the top surface of the anchor rod exposed cap shall not be less than 120mm.

        The construction conditions of this project are: the construction of anchor static pile only after the joint cap is completed and the concrete reaches the design strength.

        After the static pressing pile of the anchor rod shall be cleaned and washed before the pile sealing after the side surface of the pile top, the anchor rod shall be welded, and the debris, water and floating slurry should be removed in the pile pressing hole. The C45 micro-expansion early strength concrete should be poured and compacted in the pile pressing hole (including the unused spare hole).100 high pile cap concrete is poured into the roof formwork of pile hole.

5、epilogue

        The selection of class III foundation pile or missing pile should take into account many factors such as structural safety reliability, construction schedule requirements, cost control, pile defects, engineering hydrogeological conditions, construction conditions and construction difficulty. Considering the above factors, the anchor static pile pile method is selected to ensure the progress of the project and control the construction quality, which is recognized by the owner and the supervision parties.

——Source: Baidu online library

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