大地电磁测深法在深埋隧洞勘察中的应用

Application of magnetotelluric sounding method to exploration of deeply buried tunnel

针对大深埋超长越岭隧洞埋深大、地质与水文地质条件复杂、地面自然条件复杂严苛等问题,为了在隧洞建设中现场勘察隧洞沿线的工程地质条件和水文地质特征,选择对地形适应性较强、探测深度大、具有一定探测精度的大地电磁测深法,结合部分隧洞工程勘察实例分析了不同岩性、断裂构造特征、岩体富水性和围岩类别的电磁特性,并利用施工开挖和钻探等方法对勘察精度和可靠性进行比较。据此,总结提出了判别标准,同时也分析了该方法的适用条件和局限。结果表明:电阻率很低的软岩岩体富水性一般较强,硬岩地下水活动强烈的岩体电阻率主要受地下水性质控制,岩石强度和完整性的影响微弱。随着埋深加大地应力会逐渐提高,岩体压密导致电阻率逐渐提高,不宜采用浅部的电阻率指标判断深部岩体的富水性。软岩Ⅲ类、Ⅳ类、Ⅴ类围岩按电阻率分类标准分别为>200Ω·m、50~200Ω·m、<50Ω·m;硬岩优于Ⅱ类、Ⅲ类、Ⅳ类、Ⅴ类围岩按电阻率分类标准分别为>5 000Ω·m、1 000~5 000Ω·m、100~1 000Ω·m、<100Ω·m。大地电磁测深法是深埋地下工程较为有效的勘察手段,可供类似隧洞工程参考借鉴。

Aiming at the problems of deeply buried depth, complicated geological and hydrological conditions, complicated and severe land-surface natural conditions, etc. of extra-long deeply buried mountain-crossing tunnel, the magnetotelluric sounding method with stronger topographic adaptability, large exploring depth and certain exploring precision is selected to analyzed the electromagnetic characteristics of various lithologies, fractural structure features, water abundances of rock mass and surrounding rock types in combination with a part of actual cases of the explorations for tunneling project, so as to make the in situ explora- tions of the engineering geological conditions and the hydrogeological features along tunnel, and then the precisions and reliabili- ties of the explorations are compared through the methods of construction excavation, drilling, etc. On the basis of this, a judgement criterion is summarized and proposed, meanwhile, the adaptable condition and limitation of the method are analyzed as well. The result shows that the rock mass water abundance of the soft rock with low resistivity is normally stronger and the rock mass resistivity of the hard rock with intense ground water activity is mainly dominated by the property of ground water, for which the effects from the strength and integrality of the rock are weak. Along with the increase of the buried depth, the ground stress is to be gradually increased, while the compaction of rock mass is to cause the gradual increase of the resistivity, thus the water abundance of the deep rock mass is inappropriate to be judged with the shallow-depth resistivity index. The resistivity-based sur- rounding rock classification standards for the soft rocks of Class III, Class IV, Class V are 〉 200Ω·m, 50 N 200Ω·m and 〈 50Ω·m respectively, while the resistivity-based surrounding rock classification standards for the hard rocks those are better than the rocks of Class Ⅱ, Class Ⅲ, Class Ⅳ and Class V are 〉 5 000 Ω·m, 1 000 - 5 000 Ω· m, 100 N 1 000 Ω· m and 〈 100Ω·m respectively. On the whole, the magnetotelluric sounding method is a more effective exploration method for the deeply buried underground project, thus can be taken as a reference for the similar tunneling project.