关于土体隧洞围岩稳定性分析方法的探索

 长期以来，地下隧洞围岩稳定性分析没有科学合理的方法，一直停留在以洞周某点位移或塑性区大小的经验值作为判断稳定性的依据。隧洞洞周位移或收敛位移受围岩弹性模量、洞室形状大小等因素影响，洞周不同部位的位移值也不相同，很难找到统一的位移判据标准。以塑性区大小作为围岩稳定性的判据要优于位移标准，但围岩塑性区受泊松比、洞室形状大小等因素影响，不同软件计算出的塑性区大小也有差异，这种方法同样也不可靠。由此可见，传统的经验分析法不够合理。为此，提出将基于有限元强度折减法求出的安全系数作为稳定性分析判据，该判据有严格的力学依据，有统一的标准，而且不受其他因素的影响。以黄土洞室为例，提出洞室的剪切与拉裂安全系数的概念，通过不断折减土体的抗剪强度参数c，j 值或c，j 与抗拉强度，使黄土隧洞围岩塑性区不断扩展，直至塑性应变或位移发生突变时，即表明隧洞发生剪切破坏，此时的折减系数即为剪切安全系数。通过不断折减土体的抗拉强度参数，使黄土隧洞内临空面处(不包括底部临空面)围岩出现第一个单元拉裂破坏时，即表明隧洞发生拉裂破坏，此时的折减系数即为拉裂安全系数。该研究仅是对围岩稳定性分析方法的尝试性探索，供同行分析、讨论。

EXPLORATION OF STABILITY ANALYSIS METHODS FOR SURROUNDING ROCKS OF SOIL TUNNEL

For a long time，stability analysis methods of surrounding rocks in tunnel are unscientific and unreasonable. It still judges the stability of tunnel by the empirical criterion of displacements of tunnel perimeter or sizes of plastic zones of surrounding rocks. Displacements of tunnel perimeter are affected by elastic modulus of surrounding rocks or shape and size of tunnel，and displacements of tunnel perimeter are different at different positions. So it is difficult to get a unified displacement criterion standard. Judging the stability of surrounding of rocks by sizes of plastic zones is superior to displacement criterion. But plastic zones are affected by Poisson¢s ratio or shape and size of tunnel and size of plastic zones calculated by different softwares are different. The method is also unreliable. So the traditional empirical methods are unreasonable. The paper puts forward safety factors based on the strength reduction finite element method as the stability criterion. This criterion is based on the strict mechanical foundation，which has a universal standard and can not be affected by other factors. Taking a loess tunnel for example，the concepts of shear safety factor and tensile safety factor of a tunnel are proposed. With the reduction of c，j or c，j and tensile，plastic zones of rock mass keeping expanding，when the value of the nodal displacement or plastic strain has a big jump compared with that before failure，this means that loess tunnel reaches shear failure，and the reduction factor is just the shear safety factor. With the tensile reduction，loess tunnel reaches tensile failure when the first element of rock mass around loess tunnel free face fails in tension except for the bottom free face，and the reduction factor is just the tensile safety factor.