中文摘要：

对于极不规则的隧道围岩超欠挖轮廓，分形几何无疑是一个新思路。在测量数据的基础上，应用分形几何来定量描述围岩超欠挖轮廓线，表明隧道各类围岩的超欠挖曲线具有自相似性。在此基础上，进一步分析了各类围岩超欠挖曲线的分形维数与围岩类别之间的关系。研究表明，隧道围岩超欠挖曲线分形维数能较为直观地反映围岩类别，但其并不随着围岩类别的变化而呈线性变化；对于洞径一定的隧道，围岩超欠挖曲线的分形维数更能反映岩体结构和洞轴线之间的关系，这为准确预测掌子面前方围岩超欠挖情况奠定了理论基础。

英文摘要：

Overbreak-underbreak of tunnel is a common phenomenon. To overbreak-underbreak of tunnel, fractal geometry is a new idea. On the basis of applying fractal geometry, it indicates that the overbreak-underbreak curve of similarity. According to analyzing the relation between rock mass classification and the very irregular the measured data, tunnel has a self- fractal dimension of overbreak-underbrak curve of tunnel, for a certain diameter tunnel, the fractal dimension reflects better the relation between structure of rock mass and tunnel axes. This is a basis of the prediction of the overbreak-underbreak state of the tunnel.