Abstract: The geological conditions in the Lanzhou area are defined by a complex combination of large-particle, water-rich pebble strata and Tertiary hypabyssal red sandstone. A systematic evaluation was carried out to understand how the unfavorable characteristics of these formations-particularly the water-softening susceptibility of red sandstone and the high permeability and coarse particle size of cobble layers-affect support-system selection, groundwater control, and environmental protection. Corresponding engineering strategies were then developed to address these key technical challenges. Based on in-situ testing, a diaphragm wall was used for the first time in this region’s complex pebble strata. To prevent the traditional anchorage system from softening or disturbing the intact red sandstone outside the deep excavation, two 116 meter super-diameter ring bracing systems were implemented. A comprehensive groundwater-control scheme-centered on preventive, layered dewatering-effectively resolved issues related to the difficult dewatering of red sandstone and the protection of natural foundations. The successful completion of this project has significantly expanded the technical experience available for deep excavation engineering in Lanzhou and provides meaningful guidance for future projects in similar geological environments.

Key words: Pebble Soil Layer, Red Sandstone, Deep Excavation, Diaphragm Wall, Ring Support, Systematic Dewatering