Finite Element Analysis And Test Study On Deformation Mechanics Of Soils Under Percussion-Compact

The vibrate force produced under the high frequency impact of DTH hammer destroy the soil original structure, make the soil liquefy, and forming drilling hole at last. Compact with other methods, this drilling technique not only have the merits of compact drilling, but also have the merits of vibration drilling and air drilling. So it is widespread used in many fields such as geotechnical anchoring engineering and trenchless engineering because that the drilling efficiency is higher and the hole wall is smoothly and stability.Because the action object of the bit is soil straty, it is necessary to establish the mechanics model and finite element model between soil and drilling tool. By analysizing the interaction mechanics and understatnding the stress and strain distribution state in soil, we can design the optimum structurecompact bit, which is significant to raise the drilling efficience, improve the DTH percussion compact drilling technology, and further to advance the technology level of our nation's geotechnic drilling engineering.The research work in this dissertation is based on a scientific project supported by geology investigates bureau, China. On the basis of tutor's right guidance, the author conducts an investigation into the compact mechanics of DTH percussion compact drilling by FEM method and the following work has been completed in this paper.The deformation mechanics of soil in process of DTH percussion compact drilling is very complicated, which is not noly involve the problem of soil engineering properties and constitutive relation, but also involve theproblem of finite formation and contact friction. So it is the high nonlinear problem including material nonlinear, geometrical nonlinear and contact nonlinear, etc. each nonlinear problem is very important and difficult to resolve with conventional methods in geotechnic engineering. Aim at the contacting face to face, the FEA model of impacting drilling of DTH is established by adopting Drucker-Pager rule and regarding soil as DP materials in this paper. And the finite element analysis code of ANSYS is adopted. And through drilling tool-soil contact and impact loading on compact bit, the drilling process is simulatd.The results modeled by ANSYS show that the penetrate rate of step bit is the fatsted one, next to is spade bit, and following one is taper bit. The last one is six pyramidal bits. On the contrary, the friction stress of six pyramidal bits is largest. Following is bullet bit, then is taper bit, spade bit and the smallest one is step bits.The results simulated by ANSYS indicated that compact soil mechanics are different between step bit and taper bit, which are used popularly in engineering field. Under the behavior of impact loading, the soil failed nearby the top of taper bit firstly, then slowly split and flow to the lower and bilateral side of bit driven by the bit conical face. While the behavior of step bit acting on soil can be considered as mini-shear failure. Stress concentration appeared at each step variable region made the soil failure firstly almost simultaneously. Thus the whole failure area is larger and the penetrate rate is faster than the taper bits. From the contour of displacement distribution, we can see that the axial displacement of soil which distribute as parabolic shape along the conical face is largest at the top of the taper bit and at the joint between bit and drill pipe, while the smallest displacement appears at middle of conical face. Axial displacement of soil at each step contact region is almost equivalent for step bit. Andvalues of displacement are all larger than taper bit's. The principle of radial displacement distribution is similar between step bit and taper bit. The compact effect of step bit is explicit than taper bits because axial upward displacement of step bit is larger, namely the swelling phenomenon is obviously for step bit.The effects of soil mechanics properties on penetration rate are analysized by ANSYS. And results show that the smaller of cohesion and internal friction angle, the faster of penetrate rate, because that the soil is easily to yield. When the value of cohesion and internal friction increase to a certain value, the penetrate rate of taper is almost equivalent to the step bit's. In addition, the penetration rate decreases with increasing of elastic modulus and Poisson's ratio. Because the penetration rate is affectd obviously by friction for taper bit, it is reduced sharply with increasing of friction coefficient. While the step bits reduced very slowly.Five different type bits are designed and tested including step bit, taper bit, bullet bit, spade bit and six pyramidal bit. The experimental results are approximate with the modeled results. In addition, some step bits with different numbers of step are test in field either. And results indicate that 9 is the best number for step bit.The innovations of the paper are as following:Firstly, the drilling process of percussion compact drilling technology is modeled by finite element analysis code in this paper. And the interact mechanics between drilling tool and soil is simulate by this FEM model.Secondly, At the same time, the influences on drilling efficiency by different soil mechanical properties are researched in this article.