Due to the low rate of consolidation of clay

the time effect of negative skin friction on pile is obvious. However

the correlative research work is not deep enough and needs to be continued. The model tests of negative skin friction on pile and double pile are designed and conducted

which can apply pile load and large surcharge load on soil surface. And the soil in the test adoptes sand and clay interlayer. During the consolidation of clay

pile stress

pile settlement and settlement at each soil layer are measured. The results indicate that settlement and negative skin friction have significant time effect. The pile settlement increases with soil settlement under the surcharge load

which increases rapidly in early stage and slowly in later stage. The negative skin friction in clay is smaller than that in sand on the initial stage of loading

and the change rule is similar to the settlement. Since the settlement stability time of sand layer surrounding pile base is short

the neutral point rises slightly with the increase of pile settlement. Furthermore

the settlement of the double pile with small pile spacing is small under the action of the same load because the dragload is relatively small. The group effect coefficient of the double pile under negative skin friction is from 0.71 to 0.77 at 3D pile spacing and there is no group pile effect when the pile spacing reaches 6D under the present model test conditions.