Study on negative friction of bored piles in backfilled clay

Generally,no negative friction is found along pile shafts in static tests.However,negative friction really exists near the surface,as newly backfilled clay is involved.A method was introduced in detail to estimate the average friction along the shaft of bored pile with the assistance of instrumentation of KL re-bar stress meter bonded on the steel cages.On the basis of the calculated average friction,the following conclusions could be drawn.Firstly,negative friction occurs in the range of 0.2⁰.4 pile length from the pile head,and it is mainly induced by consolidation settlement of backfilled clay around the pile head.Secondly,the magnitude of negative friction gradually reduces as getting deeper.Finally,the location of the neutral point,i.e.the location of the zero friction point,keeps changing until deformation of foundation soil becomes stable.