Engineering geological properties of altered rocks and implications for railway construction in the Sanjiang orogenic belt, Southwest China

The Yunnan-Tibet Railway, currently under construction, crosses five altered rock zones in the Sanjiang orogenic belt, southwestern China. They are mainly montmorillonitized rock zones formed by hydrothermal alteration. Based on field investigation and laboratory tests, this paper presents the geological characteristics and engineering properties of the altered rocks encountered during the construction of the railway. Being characterized by a high montmorillonite content, high degree of alteration, high physico-chemical activity and high swelling potential, these rocks always lead to engineering geological problems. The concept of “alteration coefficient” is put forward; the greater the alteration coefficient, the higher the alteration degree and swelling potential. Countermeasures are proposed to tackle the engineering problems during tunnel construction caused by the altered rock.