小净距隧道下穿薄层采空区引起的地层移动分析

Analysis of Ground Settlement Induced by Small-Space Twin-Tube Tunnel Excavation Underneath a Mined-Out Area

小净距隧道下穿采空区地层施工时，相邻隧道的施工扰动会出现叠加，严重影响采空区地层的稳定性。文章开展了小净距隧道下穿采空区地层施工的室内相似模型试验，对施工过程中围岩及采空区的沉降进行了监测。试验考虑了采空区倾角为15°和25°两种情况，通过控制岩块抗压强度和孔隙度两个指标，实现了采空区地层离散性的模拟以及模型与原型的相似。结果表明：采空区地层在左、右洞及中岩墙上方有3处沉降极值点，其中先行洞上方沉降最大；倾角越大，采空区地层沉降越大、沉降范围越广；小净距隧道开挖引起的最大沉降较单洞高35％-74％，且先行洞开挖对后行洞上方采空区地层沉降的影响小于后行洞开挖对先行洞上方采空区地层沉降的影响；采空区地层沉降过程可分为6个阶段，其中左、右洞开挖面通过时沉降发展较快。因此，在实际施工过程中，为了避免后行洞开挖时扰动范围的扩大，后行洞开挖宜在先行洞开挖引起的采空区地层沉降稳定或二次衬砌施作后进行。

During the construction of a small-space twin-tube tunnel underneath a mined-out area, the distur- bance from an adjacent tunnel excavation will be superimposed and the stability of the mined-out area is seriously affected. An indoor similar model test of a small-space twin-tube tunnel underneath the mined-out area was car- ried out and the settlement of the rock mass and around the mined-out area during excavation was monitored. Con- sidering the two cases of formation dip angles of 15 ° and 25° in the mined-out area, the simulation of formation dis- creteness in the mined-out area and the similarity of the model to prototype was achieved by controlling the rock compressive strength and porosity in the mined-out area. Test results show that： 1） there are three extreme ground settlement points located in the left tunnel, right tunnel and middle rock wall of the mined-out area, respectively, with the maximum ground settlement oceuring above the pilot tunnel; 2） the greater the dip angle is, the larger the settlement is and the wider the range of settlement is; 3） the maximum settlement induced by the small-space twin- tube tunnel excavation is about 35% to 74% higher than that of single-tube tunnel excavation, and the impact of pi- lot tunnel excavation on the settlement above the subsequent tunnel is less than that of the following tunnel excava- tion on the settlement above the pilot tunnel in the mined-out area; and 4） the settlement process of the mined-out area can be divided into six stages, and the settlement develops quickly during excavation of the left and right tun- nel underneath the mined-out area. In order to avoid the extension of the disturbance range during the following tun- nel excavation, excavation of the following tunnel should be performed after stabilization of the mined-out area set- tlement induced by the pilot tunnel excavation or installation of the secondary lining of the pilot tunnel.