软弱围岩隧道台阶法施工中拱脚稳定性及其控制技术

以铁路隧道为背景,采用数值分析手段,并结合部分现场监测资料,对软弱围岩隧道台阶法施工过程中隧道拱脚变形特征、上台阶基底围岩失稳形态及拱脚稳定性控制技术进行分析。结果表明:(1)软弱围岩隧道施工过程拱脚沉降和水平收敛均表现突出,在实际工程中,拱脚变形控制是软弱围岩大变形控制的关键之一。(2)拱脚部位围岩屈服程度相对较高是该处施工变形显著的根本原因。在台阶法施工过程中,基底围岩的破坏形式随台阶高度的增加由拱脚局部失稳逐渐向基底整体剪切失稳过渡。(3)从对隧道拱脚及洞周变形控制效果出发,IV级围岩可采用长台阶法施工,V级围岩宜采用短台阶法施工,而VI级围岩应采用微台阶法施工。(4)从不同台阶高度条件下极限位移的计算结果可以看出,对于软弱围岩隧道,施工中在确保掌子面稳定的前提条件下,适当增加台阶高度有利于围岩的稳定。(5)扩大拱脚和临时仰拱2种工法对控制拱脚及洞周变形均有明显的效果。扩大拱脚技术适用于拱部沉降显著的工程,而临时仰拱技术则更适用于水平收敛显著的区段。

ARCH SPRINGING STABILITY AND ITS CONTROL TECHNIQUES DURING CONSTRUCTION OF TUNNELS WITH WEAK SURROUNDING ROCKS BY BENCH CUT METHOD

Taking a railway tunnel as background,combining with numerical analysis method and in-situ tests,the deformation characteristics of arch springing and destabilization mode of surrounding rocks during construction of the tunnel by bench cut method were analyzed.In addition,corresponding control measures for arch springing stability were discussed.The following results are achieved.(1) During the construction of tunnel,the settlement and horizontal convergence of arch springing are notable.Therefore,the control of arch springing deformation is one of the key measures to prevent the occurrence of large deformation of weak surrounding rocks.(2) The high yield degree of surrounding rocks at arch springing is one of the prime reasons for large deformation of arch springing.During the construction of tunnel by the bench cut method,damage forms of bedrock at foundations change from local destabilization of arch springing to integral shear failure with the increase of bench height.(3) Based on the control effects of arch springing deformation,the section of tunnel with the surrounding rock of class IV can be excavated by long bench cut method,the section with the surrounding rock of class V should be excavated by short bench cut method,and for the section with the surrounding rock of class VI,the micro-bench cut method is recommended.(4) For tunnels with weak surrounding rocks,proper bench height increment is beneficial to the stability of surrounding rocks under the condition that the working face stability is ensured.(5) The techniques of widening arch springing and setting temporary inverted arch are of obvious effects for controlling the deformation of arch springing.But the technique of widening arch springing is suitable for projects where arch settlement is remarkable,and the temporary inverted arch technique is more applicable to the sections with remarkable horizontal convergences.