Application Of Prefabricated High-strength Tubular Stiff Tube In Soft Foundation Treatment Of Expressway

Based on the field test on Haiqi Expressway,theoretical comparison and numercal simulation method were used to study on application of prefabricated high-strength tubular stiff tube for soft foundation treatment of Expressway.The prefabricated high-strength tubular stiff tube discussed in this paper is a kind of rigid pile.This prefabricated high-strength tubular stiff tube constructed by centrifugation and pre-tensioned.The main contents and achievements of this paper are as follows:(1)The technical characteristics and ecomomic benefits of prefabricated high-strength tubular stiff tube,prestressed concrete pipe pile and bidirectional soil cement mixing pile are compared and analyzed.prefabricated high-strength tubular stiff tube,in similar condition,reduce the construction cost about 10% than prestressed concrete pipe pile;the construction cost between prefabricated high-strength tubular stiff tube and bidirectional soil cement mixing pile is close,but the former is due to the use of prefabricated technology,it easy to control the quality of construction.The construction data of the prefabricated high-strength tubular stiff tube are analyzed.The applicable scope and construction technology of the prefabricated high-strength tubular stiff tube are summarized and given.(2)The field test is based on the Haiqi Expressway project.The field test is to study the properties of composite foundation,pile-soil stress ratio,pore water pressure,geogrid strain and tension force,settlement characteristics and lateral displacement of the embankment.The monitoring results show that the stress on the pile is larger than soil,and the difference of them is getting larger;the soil pressure at the center of the embankment is greater than the soil pressure at the shoulder.The load transferred by soil arching effect is about 4 times that of the membrane effect.The pile-soil stress ration is 5.3~8.6,it increases with the heigher embankment filling height.The pore pressure decreases with the heigher embankment height,and the velocity of the shallow pore pressure is greater than that in the deep.The settlement of the embankment in center is greater than the settlement in shoulder,and the differential settlement of the pile and soil is less than 0.9mm.(3)The existing design method of piled embankment is studied,and compare the difference of various method through the several engineering case.Through comparison,it is found that the calculated values of pile and soil load calculated by Hewlett and Randolph method,Japanese rule and Van Eekelen method are more close to the measured values,besides the application scope of them is wide.When the triangular pile plan is used,the results of each method have deviations with measure value.When calculating the membrane force,the German EBGEO method is the closest to the Haiqi monitoring value.(4)Base on Haiqi field test,using ABAQUS to simulate different engineering conditions by changing the pile length,pile spacing,cushion thickness,pile cap size and shape,the composite foundation settlement,pile-soil stress distribution,pile-soil stress ration.By comparing the simulation results,summaried the key design parameters of prefabricated high-strength tubular stiff tube composite foundation.The numerical simulation results show that the pile-soil ratio will be 12~16 and the settlement of embankment will be 179 mm after 180 days of preloding time.With increasing the size of pile cap,the selttlement are reduced,pile-soil stress ratio increase,when the ratio of pile cap and pile spacing over 0.5,the effect will no longer obvious.Increasing of the cushion thickness and stiffness of geogrid can reduce total settlement and differential settlement of pile and soil,the pile-soil stress ratio increases,but the rate of that is limited,the cushion thickness is suggested to be 0.4~0.6m.(5)Starting from practical engineering application,discussed the design contents,process and factors to be considered in the design of prefabricated high-strength tubular stiff tube applying to the expressway building in soft foundation.A set of systematic design method for reinforced embankment is given.The contents include pile selection and pile design,precast pile cap design,cushion design,pile bearing capacity checking and settlement calculation.