Experimental Study On Load Transfer Characteristics Of DX Pile In Soft-hard Interbeded Soil

DX pile is a new kind of varied cross-section pile with high bearing capacity, small settlement and simple and fast construction that is recently developed. The existence of bearing bell makes the DX pile become friction and end-bearing pile which is born by bearing bell, pile shaft and root of pile, its bearing mechanism is very complex. This paper aims to explore the variation rule of pile tip resistance, bearing bell resistance and pile side resistance in different soil layers, systematically analyze the distribution law of soil stress surrounding bearing bell in sand and silty clay, on this basis, this paper analyzes the reasonable design space of bearing bells in different soil layers, put forwards correction coefficient of bearing bell resistance in different soil layers, and modifies the formula for the bearing capacity of DX pile in the original specification. The study mainly obtains the following conclusions:(1) Compared with the soil layer that silty clay is under the sand layer, in the sand layer under silty clay, which exists weak layer in lower soil layer, the bearing capacity of DX pile will decrease about20%; at the later stage of the loading, the settlement curve flattens out, the lower plate is set in sand layer that can improves the later bearing capacity of DX pile.(2) In the sand layer under the silty layer, because the lower layer is soft soil, load mainly is borne by the upper part of the pile; the load of DX pile is mainly borne by the lower portion of the pile and it can mobilize more tip resistance in the silty clay under sand layer.(3) The theoretical analysis and experimental results show that, when the bearing plate is located in the sand layer, its stress influence is in the range of2times disk diameter below bearing plate and2times disk diameter away from the center line of the pile. When the bearing plate located in the silty clay layer, its stress influence is in the range of1times disk diameter below bearing plate and1times disk diameter away from the center line of the pile.(4) Combination of DX pile load transfer regularity in different soil layers and multiple calculation and analysis of engineering example show that, the use of influence coefficient of bearing plate in different soil layers for the correction of calculated value of bearing plate resistance which is specified in the calculation formula of JGJ171-2009conforms to actual situation. (5) In the loading process on top of the pile, free surface appears at the top of bearing plate, through a complex stress adjustment, soil arching effect forms in range of about1times disc diameter on the top of the bearing plate. The stability of soil arch is related with the nature of soil and the depth of bearing plate:the stability of soil arch on the top of bearing plate in sand layer and on the top of the lower bearing plate is better.(6) When the distance between plates is too large in the sand layer which is under silty clay layer, not only because of the larger settlement and smaller stress of soil surrounding upper plate, the bearing performance of upper plate will be reduced, at the same time the bearing capacity of lower plate is also affected. Suitable distance between bearing bells can make the load from the upper plate passed to the lower plate in soil layer with better properties timely, thus improve the bearing capacity of DX pile. This paper suggests that the distance between plates in the sand layer which is under silty clay layer is taken as two times disc diameter.(7) In the silty clay layer which is under the sand layer, when the distance between upper and lower plates is four times disc diameter, the bearing capacity of soil surrounding upper plate can make full play. When the distance between plates is too small or too large, the bearing capacity of plates will be influenced. The soil stress surrounding lower plate decreases with the distance of plates increasing. This paper suggests that the distance between plates in the silty clay layer which is under the sand layer is taken as four times disc diameter.