五车道大跨分岔隧道小净距段中夹岩柱支护效果研究

Study on Supporting Effect of Middle Rock Pillars in Small Clear Distance Section of Long-span Forked Tunnel with Five lanes

厦门芦澳路与海沧疏港通道交叉处拟建一座地下互通式立交隧道,隧道分岔处由单洞5车道大跨段直接过渡至主洞3车道匝道2车道的小净距段,不设置连拱段,最大开挖跨度30. 46 m,最大开挖面积450. 41 m2。分岔隧道由大跨段直接过渡到小净距段,交界处中夹岩柱最小厚度仅为1. 2 m,中夹岩柱的稳定性是小净距隧道设计与施工的重点与难点。基于以上背景,首先通过三轴压缩试验确定岩石力学指标,并基于Hoek-Brown估算方法得到岩体参数,进一步运用数值计算方法对小净距隧道中夹岩柱的支护效果进行研究。结果表明:①花岗岩试样属于弹脆性岩石,其弹性模量与围压呈正相关;②由于围岩强度较高,预应力对拉锚杆相比较于普通锚杆可以缓解中夹岩柱的应力集中,施工中建议采用低预应力对拉锚杆;③主洞隧道初期支护的左、右拱脚的安全系数最小,说明左右拱脚应力集中较为明显,施工时应加强左右拱脚处支护;④岩石的强度和稳定性与围压关系较大,施工时建议采取主动支护的手段,及时地对隧道以及中夹岩柱进行封闭支护,使得围岩尽快的处于3向受力状态。

An underground interchange tunnel is planned to be built at the intersection of Luao Road and Haicang Evacuate Passage in Xiamen.The bifurcation of the tunnel transitions directly from the single-hole long-span section with 5 lanes to the small clear distance section which consists of main tunnel with 3 lanes and rump with 2 lanes.There is no double-arch section,the maximum excavation span is 30.46 m,and the maximum excavation area is 450.41 m2.The forked tunnel directly transitions from the long-span section to the small clear distance section.The minimum thickness of the middle rock pillar at the junction is only 1.2 m.The stability of the middle rock pillar is the key and difficult point in the design and construction of small clear distance tunnel.Based on the above background,the rock mechanical indicators are determined by triaxial compression test at first,and the rock mass parameters are obtained based on the Hoek-Brown estimation method.The supporting effect of the middle rock pillar in the small clear distance tunnel is studied by numerical calculation.The result shows that(1)The granite sample is elastic and brittle rock,its elastic modulus is positively correlated with confining pressure.(2)Due to the higher strength of the surrounding rock,the pre-stressed anti-tension bolts can alleviate the stress concentration at the middle rock pillar compared with ordinary bolts,so it is recommended to use low pre-stress anti-tension bolts in construction.(3)The safety factor of the left and right arch feet of the initial support of the main tunnel is the smallest,indicating that the stress concentration of the left and right arch feet is more obvious,so the supports at the left and right arch feet should be strengthened during construction.(4)The strength and stability of the rock have great relationship with confining pressure.It is recommended to adopt active support during construction to promptly seal and support the tunnel and the middle rock pillar,so that the surrounding rock will be in a 3-way stress state as soon as possible.