Stability Analysis Of Hole Wall In Rotary Excavation Of Soft Soil And Study On Measures To Prevent Collapse Hole

With the rapid development of China’s economy and the improvement of urbanization level,the development and utilization of urban underground space resources are greatly promoted.In the construction process of large-scale underground shopping mall,urban rail transit and underground parking lot,a large number of pile foundation projects are bound to face,and their complexity and construction difficulty are constantly improving.Especially in the eastern coastal area,deep soft soil layer conditions are often encountered in the construction process,most of which are muddy soft soil.In the process of rotary excavation construction in this stratum,the hole wall shrinkage and hole collapse are easy to occur.In order to effectively solve the problems of hole collapse and necking of rotary drilling rig in high sensitivity soft soil layer,high-pressure jet grouting pile is often used to pre strengthen the soft soil around the pile hole,and then the rotary drilling rig is used to form the hole with mud wall protection.At present,the theoretical research of this engineering measure lags far behind the engineering practice.In this paper,a series of researches are carried out by means of theoretical analysis,indoor test and numerical analysis:(1)The mechanical analysis model of hole wall in cement soil reinforcement area is established,and the theoretical solution of hole wall stress and displacement is solved.The reinforced area of high-pressure jet grouting pile is equivalent to the circular hole wall lining.The stress analysis of the model under different conditions is carried out by using the basic principles of elastic-plastic mechanics.The analytical solutions of stress and displacement in the cement soil reinforcement area and the fracture radius r of the water soil reinforcement area are obtained through the Mohr Coulomb yield criterion and the plastic flow rule＿p.It provides theoretical guidance for determining the thickness of cement soil reinforcement under different soil layers.(2)Through unconfined compressive strength and triaxial shear tests,the influence of curing environment,cement content and age on the strength of cement soil is studied.The test results show that the unconfined compressive strength of cement soil under the condition of sludge box curing is about 0.86 times of the strength value under the standard curing condition;the strength of cement soil increases with the increase of cement content,and increases with the increase of age;according to the test results,the strength parameter values of cement soil under different cement content and age are counted,which can be used for practical engineering design and numerical simulation For reference of calculation parameters.(3)Through FLAC3 D software,a three-dimensional mechanical analysis model of rotary excavation hole wall is established.The horizontal displacement and plastic zone characteristics of hole wall under different working conditions are analyzed.The optimal reinforcement thickness of soil under different pile diameter and pile length conditions is further determined.The correction coefficient K is obtained by comparing the numerical simulation results with the theoretical calculation formula,so that the theoretical analysis can better guide the engineering practice.