黄土隧道锚杆受力与作用机制

 为探讨黄土隧道锚杆作用效果及机制，对陕西省吴堡-子洲高速公路上3座黄土隧道中的48根锚杆应力进行现场测试和统计分析，结果发现：黄土隧道在钢架支护条件下，拱部系统锚杆受压且应力值较小；拱脚处锁脚锚杆以受拉为主，锁脚锚杆应力普遍大于拱部锚杆应力。从土体的变形和锚杆与土体的锚固效果2方面分析黄土隧道拱部系统锚杆的力学状态，分析认为隧道开挖后，浅埋黄土隧道拱部发生整体沉降，锚杆并不存在锚固段；深埋黄土隧道开挖后土体产生较大塑性区，目前以“短而密”原则设计的系统锚杆也不存在锚固段；锚杆与土体采用水泥砂浆或药卷式锚固剂黏结效果差，因而黄土隧道锚杆锚固力不大；锚杆锚固于初期支护上，并伸入土体中，从内部约束土体变形，在初期支护施作后，相对于土体的后续变形，拱部系统锚杆受到土体向下的摩阻力，相当于桩承受负摩阻力，因而拱部系统锚杆受压。综合以上分析表明，在黄土隧道中，钢架支护条件下的系统锚杆支护效果不明显，可以取消。工程实践证明，钢架支护条件下黄土隧道取消系统锚杆，可减少施工环节，更有利于隧道施工安全和结构稳定，可缩短工期和降低工程造价，有着特别显著的社会经济效益。

STRESS AND ACTION MECHANISM OF ROCK BOLT IN LOESS TUNNEL

In order to study the action effect and mechanism of rock bolt in loess tunnel，the stresses of 48 rock bolts in 3 loess tunnels in Wubao—Zizhou Expressway in Shaanxi Province are measured by in-situ tests and statistically analyzed. It is shown that under the condition of steel arch support，the arch systematic rock bolts in loess tunnel are compressed and the stress is very small；the feet-lock bolts in arch foot are mainly subjected to tension and the stresses of feet-lock rock bolts are larger than those of arch systematic rock bolts. The mechanical state of arch systematic rock bolts in loess tunnel is analyzed from the deformation of soil and anchoring effect of rock bolt and soil. It is revealed as follows：(1) After tunnel excavation，the arch of shallow loess tunnel settles entirely；and there is no anchoring section in systematic rock bolts. (2) After tunnel excavation，there is a large part of plastic area in deep loess tunnel；and there is no anchoring section in systematic rock bolts. (3) The anchoring effect of rock bolt and soil by cement mortar and anchor agent is not very well；so the anchoring force of rock bolt in loess tunnel is small. (4) Rock bolt is anchored in the first lining and inserted into the soil，which can constrain the soil inside. After the construction of the first lining，the arch systematic rock bolts are subjected to downward frictional resistance from soil，corresponding to negative friction in pile；so the arch systematic rock bolts in loess tunnel are compressed. From the above analysis，it is shown that under the condition of steel arch support，the support effect of systematic rock bolts in loess tunnel is not obvious，which can be ignored. Engineering practice shows that ignoring systematic bolts under the condition of steel arch support can reduce construction procedure，making construction safe and improving the safety of support structure；it also can reduce the engineering cost and shorten the construction period. It will achieve obvious economic and social benefits.