Model Test And Three-Dimensional Discrete Element Study On Bearing Characteristics Of Screw Piles

Screw pile is a kind of compound cast-in-place pile with abruptly changed cross-section,which consists of cylinder-shaped cross-section and screw-shaped cross-section.Due to its peculiar screw configuration,it effectively improves the extreme bearing strength of screw pile.At the same time,it also makes the mechanism of pile-soil interaction more complex,while the research on the bearing property of screw pile is not deep enough.Therefore,studying the bearing effect of screw piles can better apply screw piles to engineering practice.In this article,the influence of upright bearing features and design parameters on the bearing feature of helical piles under different overburden loads is investigated through indoor tests and numerical simulations by discrete element method.In summation,the following are the primary research details and details.(1)The Q-S curve,pile body axial force,pile side and pile end resistance and load-sharing ratio of pile and soil under different overburden loads are compared and analyzed by laboratory model tests.The summing-ups indicate that the extreme bearing strength of screw piles is approximately 50.8%～62.4% better than that of direct piles under the effect of different overlying loads.The axial force distribution curves of upright piles and helical piles show the trend of " top of pile is large,bottom of pile is small",the velocity of reduction of axial force of pile is much more than that of upright pile,the extreme end resistance of upright pile accounts for about 40.2%～49.4% of the whole load,while the end resistance of the screw pile accounts for about 12.9%～15.1% of the whole load.In the extreme scenario,the structural capacity of the helical pile composite foundation is 30.8 % superior to that of the straight pile.Moreover,the screw pile can effectively weaken the influence of negative friction resistance,and the range of negative lateral friction of the screw pile is much narrower when compared to that of the straight pile.(2)Based on the results of indoor trial,the mesoscopic parameters of the established particle flow model were calibrated and the validity of the model was verified through back analysis.The effects of the pile shaft design parameters on its load carrying features were studied.The following summary was obtained: As the material modulus of helical pile shows an accumulation of pile-soil stress ratio,and their pile top settlement gradually decreased,but at the same time,the utilization rate of pile material gradually decreased with the change,Try to select screw piles with smaller pile length and diameter,provided that the bearing capacity meets the programme requirements;When the length of the screw section of the screw pile is(1/3～1/2)L,the load borne by the soil around the pile accounts for a high proportion of the absolute load,the stress focus appearance at the mutative cross-section of the straight rod and screw portion boundaries is not significant;When the screw pitch is(0.8～1.2)D,better load-bearing performance of screw pile,stronger degree of pile lateral stiffness,better stabilization of soil settlement around the pile;When the thread width is(0.16～0.24)D,the bearing characteristics of the threaded pile are improved.(3)Using the established discrete element model,the effects of pile modulus and single pile replacement The load carrying property of the helical pile of compound base was examined and the following summary was proved: With the increase of the elastic modulus of helical pile material,the ratio of the stress at the top of the pile to the surface of the soil between the piles gradually increases,and the scale of load borne by the pile itself in the total load increases.When the elastic modulus of the pile becomes a critical value,continuing to augment has less effect;as the replacement rate of the single pile increases,the percentage of the pile top pressure on the surface stress of the soil between the piles gradually decreases and the percentage of the load borne by the soil volume perimeter of the pile within the combined load increases.After the ratio of pile cross-sectional area to affected area increases to 10%,The clamping effect will be minimized.When the single pile turnover rate is 10% to 14%,the utilization rate of soil around the screw pile composite foundation is bigger,And the effect of improving bearing capacity is obvious.