Abstract: The engineering geological characteristics of the waterrich moraine Milin tunnel on LhasaNyingchi railway, causes of collapse, and corresponding countermeasures are studied in detail. The research results show the following. (1) The waterrich moraine stratum in the plateau is characterized by good water permeability and is prone to frost heaving at low temperatures. The shear strength increases with the increasing rock content and density of surrounding rocks, and decreases with the increasing fine particle content and water content of surrounding rocks. The surface water and groundwater have strong effects on the immersion, softening, migration, and destruction of surrounding rocks. (2) The interaction between the high content of fine particles in the moraine stratum and the development of groundwater would cause an instability in the surrounding rock, and further cause the tunnel to collapse. (3) Grouting reinforcement should be strengthened in the area of backfilling of the ballast body and surrounding rock overhead, a safety arch protection and cavein drainage system should be set up, and rational excavation and support methods should be adopted.

Key words: waterrich moraine tunnel, collapse cause, countermeasure, geological and mechanical model