Model Test Study On Vertical Bearing Behavior Of Composite Foundation With Defective Pile Under Unilateral Row Of Piles

The sinking pipe pile or bored pile is usually used to treat soft soil foundation in composite foundation.During the construction process due to many links and complicated construction technology,the quality of pile is susceptible to be affected by various factors,necking,fracture,expansion and other defects are often appeared.The occurrence of defective piles is detrimental to the settlement control of composite foundation.In order to meet the requirements of existing specifications and not cause waste of resources,it is possible to consider some measures to strengthen the composite foundation with defective piles.The existing research and engineering practice have shown that the settlement of composite foundation can be effectively controlled by using laterally restrained piles on both sides of the composite foundation of the defective piles.Based on this,model test is conducted to consider the single side of the composite foundation in the defect pile to set up a single row of piles on the adjacent composite foundation with defective reinforcemen,the displacement,soil pressure and strain data of the defective pile under different loads were measured and applied to the corresponding treatment by applying four cyclic loads.The displacement,soil pressure and internal force of defective piles with different defective types and different defective locations during the first loading and unloading are compared with each other,and the displacement,soil pressure and internal force of the defective pile under four cyclic loads are analyzed by self contrast analysis.Conclusions are drawn as follows:(1)With the same defect location and loading,the expanded pile has the smallest settlement,the maximum thrust amount and the strongest effect of the pile,the necking pile takes second place,the broken pile has the maximum settlement,the smallest thrust amount and the weakest effect of the pile.The upward movement of the defect position causes the settlement to tend to increase,the thrust amount to tend to decrease,the pile body to tend to weaken.(2)With the load increased,the stress on the top of defective piles and the inter-pile soil are gradually increased.With the same defect location and low pressure stage,the top of the expanded pile has the maximum stress and pile-soil stress ratio,the necking pile takes second place,the broken pile has the minimum stress and pile-soil stress ratio.The reinforcement effect of lateral constraint piles is going to play gradually at high pressure stage.The curves of pile-soil stress ratio and loading of the necking pile,the broken pile and the middle expanded pile are convex,have obvious peak value.The curves of pile-soil stress ratio and loading of the top and bottom expanded pile are concave.For the low part of the expanded pile,the upward movement of the defect position causes the stress and pile-soil stress ratio tend to increase.Compared with the top or bottom expanded,the reinforcement effect of the middle expanded pile is optimal.(3)The axial force at the defect site is obviously changed.The axial force of the necking part is significantly reduced.The necking of upper part for defective pile causes the position of the axial force peak and the neutral point of lower part to move down,the necking of lower part for defective pile causes the position of the axial force peak and the neutral point of upper part to move upward,the necking of middle part for defective pile causes two peaks and two neutral points of the upper part and the lower part respectively.The axial force of break location decreases because of the axial force loss.The break of upper part for defective pile causes the position of the axial force peak to move down.The axial force of the top of the expanded pile decreases with depth,and the axial force of the expanded part is significantly improved,showing the shape of "convex belly".All defect location is demarcation point of positive and negative frictional resistance.There are 2-3 neutral points in the necking pile.The top of the lower part for the broken pile is of negative friction,the break of upper part for defective pile causes the position of the neutral point of lower part to move down.Both of the top and the expanded section of the expanded pile are of positive friction.(4)As the same defective types piles with different defective positions,the variation curve of axial force and friction resistance along the depth is obviously different,but the variation curve of axial force and friction resistance near the defective position is similar.As the same defective positions piles with different defective types,the opposite distribution shapes of axial force and friction resistance along the depth is showed.(5)The curve of settlement,thrust amount and pile-soil stress ratio of composite foundation with defective piles during the four reloading and repetitive unloading showing memory effect.When the maximum loading of the(i+1)-th cycle larger than the maximum loading of the i-th cycle,the(i+l)-th loading curve will be back to extension line of the first loading curve.