由于对第三系地层的工程性质认识不足，国内经验表明在第三系地层中修建隧道具有很高的风险。结合蒙华铁路中条山隧道勘察设计实践，利用勘探、室内试验和原位测试等手段获取原始数据，总结研究认为中条山南麓第三系地层岩性可划分为中等胶结砾岩、弱胶结砾岩、弱成岩砂岩、半成岩泥岩和半成岩砂质泥岩，地层成岩程度低，暴露易风化，遇水易软化，泥岩和砂质泥岩具膨胀性，水文地质特征因岩性而异，隧道施工中容易出现塌方、大变形和突水涌砂，基底需慎重处理，否则在铁路运营期容易出现翻浆冒泥。

It is believed that, due to the ignorance of the engineering geological characteristics of Tertiary strata, there are great risks in the construction of tunnels located in Tertiary strata. In the paper, the property of Tertiary strata are investigated by means of exploration, laboratory test and insitu test, on basis of the investigation and design of Zhongtiaoshan tunnel on Menghua railway. Conclusions drawn are as follows: The Tertiary strata at the southern foot of Zhongtiaoshan mountain can be classified into moderatelycemented conglomerate, weaklycemented conglomerate, weakdiagenetic sandstone, semidiagenetic mudstone and semidiagenetic sandy mudstone; The strata has such characteristics as low diagenesis degree, being easy to be weathered after exposure and being easy to be softened when water is encountered; the mudstone and sandy mudstone have swelling property; the hydrogeological characteristics of Tertiary strata depends on the lithology; collapse, serious deformation, water inrush and sand inrush are common risks during tunnel construction; the base ground of the tunnel should be carefully treated, otherwise floor mudding may occur during the operation of the tunnel.