Pipe pile is widely used in the field of construction, but due to improper operation and other reasons often lead to pipe pile deviation. This article mainly through a case of pipe pile deviation, explain the cause of pipe pile deviation and its solution.
Case of pipe pile deviation
Causes of deviation of pipe pile and its solution
A residential area × building residential building foundation, the design adopts C60, 400 thin-wall prestressed commercial concrete pipe piles 293, the pile length is 24m, the full section of the pile into the bearing layer (clay layer) is greater than 3m, using 10+10+4m welding pipe piles, single pile design bearing capacity standard value of 550 kN. After the piling is completed, the pile top is about 2.5m below the natural ground.Soil stratification (from top to bottom) in the excavation range of the building is as follows: (1) miscellaneous fill; Silty clay, mostly soft plastic, can not be used;(4) -1 silty clay is a highly compressible soil with poor mechanical properties.The site of the foundation was originally a pond, and the bottom plate was located in the mixed fill soil and silty clay layer, with a depth of about 2.8m. The longitudinal spacing of thin-wall prestressed concrete pipe piles is 1.1 ~ 1.6m.First, mechanical excavation was used to reach 0.6-0.8 m above the elevation of pile top, and then manual excavation was used. During the mechanical excavation, a single-bucket backhoe was used to pull back from east to west and dig to the excavation depth at one time. The earth was temporarily stacked on the south side of the foundation pit with a height of about 1.5m. The construction went smoothly. However, when the foundation groove was repaired and dug manually, it was found that the deep black silt in the foundation pit in the southwest region arched the silty clay on the ground, and some of the piles deviated the next day.After a review of pile positions, it was found that 88 piles with offsets of 11 ~ 50cm, 14 piles with offsets of 51 ~ 80cm, and 8 piles with offsets of > 100cm were found. The pipe piles in the area north of the shaft are basically offset. The distribution of offsets has an obvious rule, that is, it decreases from south to north and increases from east to west.
(1) Reason analysis of pipe pile deviation: this area used to be the edge of the pond. The soil quality of the south and north sides is quite different. The silty clay layer on the north side is better (= 19.1kn /m3, c = 13kPa, = 22.6°), while the silty clay layer on the south side is worse (= 16.9kn /m3, c = 6.7kpa, = 13.4°). The pile pressure on the south side causes the slippage of silty clay to the southwest area, which causes the deflection of prestressed high-strength concrete pipe piles.
(2) Solution idea pipe pile deviation : in order to determine the damage degree and integrity of the squeezed pipe piles, the low-strain dynamic test was carried out first. It was found that the pipe piles with an offset less than 50cm were not broken, most of the pipe piles bodies were intact, there were no obvious defects, there were some local cracks, and the damaged parts were all 5 ~ 10m away from the top of the pipe piles.The pile with an offset of more than 50cm has obvious defects and local cracking is serious.If the original pile type is used to make up the pipe piles, the construction period is long, the cost is high, and the claim for breach of contract will be caused. Therefore, the following two solutions are considered at the same time: (1) push up method (that is, pile top applies horizontal thrust) to reset the pile. According to the formula in the technical specification for construction pipe piles foundation (JGJ 94-94), the horizontal deformation coefficient of pipe piles = 0.6495m-1 is calculated, and then the allowable horizontal thrust value is obtained from formula Rh = 3EI oa/Vx (where oa is the allowable displacement of pile top, and 40mm of soft soil is taken; Vx is the horizontal displacement coefficient of pile top, when ×h(pile length)≥4, 2.441 is taken. EI is the flexural stiffness of pipe piles), that is, Rh = 124.91kN.It is safe to use horizontal thrust less than Rh for prestressed high strength commercial concrete pipe pile. During construction, the soil in front of the pile should be cleared first to reduce the required horizontal thrust to the greatest extent, and then the off-alignment pile can be reset with horizontal thrust less than Rh to ensure the safety of the pipe piles. According to the above treatment ideas of construction, the time limit is short, the treatment cost of about 3,000 yuan per root.
(3) Bolting static pipe piles. With the help of anchor pipe piles to make up for the partial bearing capacity lost by pile deviation, and according to the actual deviation of engineering pile, it can be handled flexibly. The anchor bolt pipe piles hole is reserved when pouring the pile cap, and the rest is constructed according to the original design, which will not affect the construction period and project quality. But the average cost of each pile is about 7,000 yuan. According to the above economic and reliability analysis, it is decided to adopt two methods for comprehensive treatment in each case: namely, the jacking method is used for the treatment of pipe piles with deviation less than 50cm, and the bolting pile is used for the treatment of pipe piles with deviation more than 50cm.
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