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1、Value of Load Transmission and Carrying Capacity of Pile,Tongji University Gao DazhaoMarch 2010, Shanghai,Preface,Many decades of research history on the load transmission characteristics of single pile during vertical load. Research focus on two aspects: assumed load transmission function and analy
2、tical calculation; verification of the tests and values of the carrying capacity What should be paid more attention to is: how to coordinate the rules of the load transmission and the formal simplified method determining the carrying capacity of single pile?,The mechanism of load transmission sugges
3、ts that the addition rules of the pile end resistance and pile lateral friction are different during the increasing load on the pile head, which are not synchronous. Pile lateral friction relates with the relative displacement between pile and soil, due to the max compression deformation of the pile
4、 at pile head, the relative displacement between pile and soil decreases along with the depth, thus the lateral friction decreases along with the depth and the pile lateral friction of shallow soil is firstly represented. But current formal simplified estimate method assumes that the addition of the
5、 pile end resistance and the lateral friction is synchronous.,Content,Pile test verification of the load transmission Defects of the formal simplified method of the predicted single pile carrying capacity Two safety factors to differentiate pile end resistance and lateral friction Project applicatio
6、n cases concerning load transmission,Pile Test Verification of Load Transmission,In the 1970s Shanghai measured the change of the pile axial force along with the depth during static load test of pile to verify the law of load transmission. In recent 20 years the verification tests of load transmissi
7、on have been carried out in many areas and been used for projects. The development of modern measurement techniques has provided more convenient and reliable ways for measuring the axial force.,Measured Data of the Distribution of Axial Force along the Depth,When the vertical load is applied to pile
8、 head downward displacement of the upper part of the pile against the soil due to compression occurs, thus upward friction of the surrounding soil occurs on the lateral sides, the downward transmission of the load along the pile shaft is the continuous process of overcoming the friction and spreadin
9、g towards soil through it. Suppose the axial force is Q, the axial force is the function of the load at head, N and depth, Z. Qf（N, Z）,Changes of the Axial Force along with Depth under Different Load,The ratio of the length and diameter L/d is one main factor which affects the load transmission, the
10、 effect of the surrounding soil on the carrying capacity decreases with the increasing ratio L/d and when the ratio reaches 100 the effect of the surrounding soil is nearly zero. The important meaning of this finding is that it corrects the one-sided understanding of “the longer the pile, the higher
11、 the carrying capacity”. It is not economical to gain high end resistance through the addition of the pile length and placement of the pile tip on deep hard soil. The cost is added much rather than the carrying capacity.,The Longer the Pile, the Smaller Percentage of the End Resistance,According to
12、statistics from Shanghai,According to the data from Fujian Province full scale test have been carried out for ten cast-in-place piles with the length from 27m to 46m to test the load transmission. The result suggests that the relative displacement needed for the lateral utmost friction is very small
13、, for the cohesive soil it is 1-3mm while for the non-cohesive soil it is 5-7mm. Except two piles which support the rock end, carrying capacity of other piles (force bearing layer of the pile end is pebble, gravel, rough sand or residual silty clay) under design load is smaller than 10% of the head
14、load.,Defects of the Formal Simplified Method of Predicted Single Pile Carrying Capacity,Current simplified method of predicted single pile carrying capacity is practical and applicable to general pile foundation. With the long piles and super long piles are widely used, the defects of this method b
15、ecome more and more apparent in contrast with the increasing length of pile and the misleading to the engineers is also apparent, for example, they misunderstand that the longer the pile the higher the carrying capacity and in order to gain high carrying capacity they try to search the hard soil or
16、rock layer dozens of meters below the earth to use as the force bearing layer for the pile ends.,Supporting Action of the Foundation Soil towards Pile,If the pile end resistance and the lateral friction are added synchronously, the following expression is always true for any load period whether unde
17、r limiting condition or working condition:,Values of the Vertical Load of Foundation Pile,“Technical Code for Building Pile Foundations” stipulates: vertical carrying capacity should be determined by the following expression: Quk - standard value of single pile ultimate carrying capacity，which can be determined by pile test or estimated by formal experience formula； K- safety factor，K2 Whether the safety factor can be placed into the ? ?,
