考虑冻胀效应的冻结壁弹塑性分析

Elasto-plastic analysis of frozen wall considering frost heave

在冻结法施工中,地层冻结引起介质体积膨胀而产生的冻胀力使地应力场产生重分布,基于这一特点建立了井筒未开挖和开挖后的冻结壁与其周围土体相互作用的计算模型。首先,利用修正的冻结壁本构方程,计算出井筒未开挖时受冻胀影响的地层应力弹性解;然后,把冻结完成时的状态作为起始状态,基于Drucker-Prager屈服准则推导出开挖后的冻结壁弹塑性解。通过工程实例分析可以获得开挖后的冻结壁及其未冻围岩区的应力场和位移场,结果显示考虑冻胀后环向应力曲线明显不连续,以及在考虑冻胀影响之后,井筒开挖后的塑性半径和冻结管处所受的应力变小,而冻结壁外边界处的应力相对增大;参数分析表明开挖前后的冻结壁外所受应力随着冻胀线应变的增大而增大,而冻结管处的应力和塑形半径随着冻胀线应变的增大而减小,适当选取冻结管的安放位置可避免冻结管断裂问题。此外,冻结管的安放位置对塑性半径影响不大,中间主应力系数对冻结壁应力布和塑性半径均有重要影响且具有区间效应性。该计算模型较好的符合工程实际,能为冻结壁的设计提供一定的借鉴意义。

The frost force is produced by the ground freezing to cause volume expansion which results in redistribution of crustal stress in the freezing method construction.In accordance with this special characteristics,the calculation model during digging and after digging was established based on interaction between frozen wall and surrounding rock.Firstly,the analytical elastic solution for stratum stress influenced by frost heave could be obtained through modifying the constitutive equation of frozen wall before the excavation.Secondly,the state of the freezing completion was regarded as the starting state,the analytical elasto-plastic solution for frozen wall could be derived based on Drucker-Prager yield criterion after the excavation.The stress and displacement field of the frozen wall and surrounding rock after digging was solved by the case study.The results show that circumstantial stress exhibits a distinct discontinuity of circumstantial stress curve considering frost heave.After shaft excavation,the plastic radius and the stress on freezing pipe decrease and the stress at the outside boundary of the frozen wall increase after considering the influence of frost heave.Before and after shaft excavation,the parameter analysis shows that stress on frozen wall increase with increasing the linear strain of frost heave and stress on freezing pipe and plastic radius decrease with increasing the linear strain of frost heave.The proper selection of the position of freezing pipe could avoid the fracture of the freezing pipe.In addition,the location of freezing pipe has little effect on plastic radius.The intermediate principal stress coefficient has important influence on the stress distribution of frozen wall and plastic radius and has interval effect.The calculation model can well satisfy the actual engineering.It may provide a certain reference for the design of frozen wall.