Field Test Study On Bearing Behavior Of Rock-socketed Pile Based On Pile Stress Test

With the continuous expansion of the city scale and the construction of various infrastructures,the rock-socketed piles are widely used in projects where the settlement requirements are relatively strict and the upper load is relatively large.Although the rock-socketed pile has the advantages of simple construction,small settlement,high bearing capacity,weak group pile effect and good seismic performance,there are still many shortcomings in the research on the bearing capacity of rock-socketed piles.Before the 1990 s,people mixed the rock-socketed piles with the end-bearing piles.They thought that the end-supported piles were rock-socketed piles and the rock-socketed piles were designed as end-bearing piles.However,in the 1990 s,people continued to analyze and summarize in the project,and found that the lateral friction of the rock-socketed piles can not be ignored.When the load is small,the lateral frictional resistance plays a major role in balancing external load;When the load is large,the lateral frictional resistance and the end resistance work together.Based on a project in Indonesia,the field test and research on the bearing capacity and deformation characteristics of rock-socketed piles were carried out by single pile vertical compression static load test,horizontal static load test and high strain.The 6 test piles are divided into two groups of 800 mm and 600 mm according to the diameter.Each group has 3 test piles and 6 test piles have different pile lengths.The main work and research results of this paper are as follows:1.The vertical bearing capacity characteristics of rock-socketed piles are studied in detail through high and low strain test and vertical compression static load test.The measurement and analysis of the pile body integrity,Q-s curve,pile axial force,pile end resistance,and pile side frictional resistance during the test were carried out.At the same time,the ultimate bearing capacity of the test pile was tested by high strain test.(1)The Q-s curves of the test piles are all slowly changing.The settlement gradually increases with the increase of the load,the settlement is less than 40 mm,the rebound rate is large,the residual settlement is small,and the maximum load value is the design ultimate bearing capacity.The s-lgt curve is approximately straight,and there is no obvious downward bend,indicating that the bearing capacity of the test pile has great potential;the ultimate bearing capacity of the test pile S-01~S-03 is greater than 7200 kN,the ultimate bearing capacity of the test pile S-04~S-06 is greater than 4800 kN.(2)Under the same load,the axial force of the pile gradually decreases with the increase of the buried depth,and the decreasing rate increases gradually.At the same depth,the axial force of the pile increases with the increase of the pile top load,and the load reduction rate is the largest at the bedrock,indicating that the side friction resistance is fully utilized.(3)The lateral frictional resistance of the pile is asynchronous.As the load increases,the lateral frictional resistance of the pile gradually develops.The maximum lateral friction of the six test piles is between 136.2 and 166.4 kPa.The lateral frictional resistance of each layer of soil piles increases with the increase of applied load,and the change is relatively flat,and finally stabilizes.(4)The pile end resistance increases with the pile top load.When the load is small,the pile end resistance tends to 0.The reason is that the load is small,the side friction resistance plays a full role;when S-01~S-03 is loaded to 3600 kN and the test pile S-04~S-06 is loaded to 2400 kN,the end resistance and the loading force are basically linear.When applied to the maximum load,the pile end resistance reaches the maximum value,and the maximum end resistance ratio of the test pile S-01~S-03 exceeds 50%,showing good characteristics of the friction end pile;the maximum end resistance ratio of test pile S-04~S-06 is less than 35%,showing the characteristics of the end bearing friction pile.2.The horizontal bearing capacity of rock-socketed piles is studied by horizontal static load test of single pile.The test adopts one-way multi-cycle loading method to obtain the horizontal ultimate bearing capacity of rock-socketed piles,and then determine the horizontal bearing capacity limit value and the proportional coefficient m value of the horizontal resistance coefficient of the foundation soil includes horizontal force-time-displacement curve analysis,horizontal force-displacement gradient curve analysis,horizontal force-m value curve analysis and horizontal displacement-m value curve analysis.(1)The horizontal force-time-displacement curve and the horizontal force-displacement gradient curve show that the test pile with a diameter of 800 mm has an ultimate bearing capacity of 210 kN or 270 kN,and the ultimate bearing capacity of a pile with a diameter of 600 mm is 140 kN or 180 kN.(2)Horizontal force and displacement are not linear with m value.With the increase of horizontal force and displacement m value,when horizontal force is less than critical load,m value decreases faster with horizontal force increase;when loaded to critical load,the m value of the S-01~S-06 pile is between 4.22 and 15.63 MN/m4;when it is greater than the critical load,the m value remains basically unchanged with the increase of the horizontal force.In summary,the vertical bearing capacity of the rock-socketed pile is significantly greater than the horizontal bearing capacity,and the vertical bearing capacity is 26.7~34.3 times of the horizontal bearing capacity,which also reflects the strong compressive performance of the pile foundation but the characteristics of bending and shear strength are weak.It is suggested that the vertical bearing potential of rock-socketed piles should be fully utilized in the project,which has a strong practical significance for the optimization design and cost saving of rock-socketed piles.