Based on tests and numerical simulation data of overlength cast-in-place piles

the composition coefficient of pile shaft compression is obtained through back analysis; and a study on curve characteristics between composition coefficient and other parameters

the influence factors of composition coefficient

and the forecast of composition coefficient is carried out. The following conclusions can be drawn. Firstly

under ultimate bearing capacity

the change range of composition coefficient of overlength cast-in-place bored piles is 0.278 5－0.988. So

it is not rational to adopt 0.5 or 1.0 for composition coefficient. Secondly

the composition coefficient increases with the increase of load on the pile top

pile shaft compression

total pile shaft displacement and pile shaft rigid body displacement. The shape of composition coefficient versus load on the pile top curve is various. Thirdly

the correlation between composition coefficient and length-diameter ratio is not obvious. But in generally

the composition coefficient decreases with the increase of length-diameter ratio. For overlength rock-socketed piles in different sits

there are good correlation between composition coefficient and length-diameter ratio

while for overlength non-rock-socketed piles in different sits

the correlation is not obvious except for five piles whose correlation coefficient is 0.9901 in a site. Fourthly

the composition coefficient versus load curves whose Edt/ Ect are smaller are larger than the curves whose Edt/ Ect are bigger. Fifthly

the average correlation coefficient between composition coefficient and load on the pile top for many piles in six sites is 0.9776. So

these regression curves can be taken as important reference for the forecast of composition coefficient for these sites and similar sites.