Experimental Research On The Vertical Bearing Characteristics Of The Long Spiral Pressure-grown Expanded Bottom Pile

Long spiral pressure grouting belled pile is a new type of pile developed and widely used in China.Its unique geometric structure and construction technology of pile body change the relationship between pile and soil,so it has higher bearing capacity.Although it is widely used in engineering,people still lack of understanding of its bearing characteristics and failure mechanism.Based on the summary of the research on the bearing characteristics of pile foundation at home and abroad,relying on the Nanchang Hongxin school project,in order to have a more in-depth understanding of the bearing characteristics of long spiral pressure grouting belled pile,this paper carries out field static load test,indoor model static load test,numerical simulation and theoretical analysis in turn:(1)The load settlement relationship and the distribution of lateral resistance of axial force of long spiral grouting pile and long spiral grouting pile are analyzed through field test.The test results show that the ultimate bearing capacity of long spiral grouting pile with the same geometric size is 1.21-1.44 times of that of long spiral grouting pile.Q-s curve is a slow change type,without significant inflection point,the axial force attenuation is faster and the lateral resistance is more fully exerted in the depth range of6-12 m under the pile body.(2)Through the analysis of the static load test results of three kinds of model piles in the room,it is concluded that the bearing capacity of the long spiral grouting pile is better than that of the straight pile and the traditional pile.Compared with the straight pile,the bearing capacity of the pile is 57%-84%,and that of the pile with enlarged bottom is increased by 20%-38%.In the limit state,the side friction of the long spiral grouting pile is 1.2-1.5 times of the pile,which is 1.7-2.4 times of the straight pile;in the whole test process,with the increase of the force applied on the pile top,the ratio of the end resistance in the load is first less than the proportion of the side friction in the load,and then increases gradually to exceed the ratio of the side resistance.(3)Based on the model test,the numerical simulation is carried out,and the validity of the model is verified.The influence of pile length,pile diameter,bottom expanding diameter,modulus of pile body and ratio of thread distance to diameter on bearing capacity of long spiral grouting pile is further studied.It is concluded that except for the small influence of pile elastic model on bearing capacity,other factors have a great influence,and the optimal distance diameter ratio is about 2.5.(4)Combined with the test and simulation results,the bearing characteristics of the pile are analyzed theoretically,and the failure mode of the thread part of the pile body of the long spiral pressure grouting belled pile is analyzed,as well as the influence of three factors on the bearing capacity of the long spiral pressure grouting belled pile,such as the size of the thread,the compaction effect of the soil around the pile,and the enlarged end of the pile.The calculation formula of ultimate bearing capacity is derived.