Analysis of historical evolution characteristics and collapse process on July 18, 2023, of a dangerous rock mass at Guanyinyan in Baoxing County, Sichuan Province, China

On July 18, 2023, on the K54 + 670 K54 + 700 section of G351 National Highway in Baoxing County, Ya’an City, Sichuan Province, the rock mass (30° 25′ 4.8″ N, 102° 50′ 21.84″ E) located at the entrance of Leying in the Guanyinyan Open-cut Tunnel and affected by Lushan earthquakes and heavy rainfall factors suffered a rock mass collapse accident (hereinafter referred to as the Guanyinyan collapse, or collapse). It is a typical delayed rock mass collapse event that occurs under the influences of historical earthquake sequences and rainy season rainfall. The collapsed area has the maximum thickness close to 2 m, and a volume of about 1500–2000 m3. Some collapsed rocks have rolled into the interior of the tunnel and the riverbed, and boulders with a diameter of nearly 5 m have been found in the river channel. Most of the collapsed material accumulates on the road, approximately 1260 m3, burying some 30 m of the highway, severely affecting the normal use thereof. This article used a detailed field investigation, unmanned aerial vehicle (UAV) photogrammetry, historical image comparison, and numerical simulation methods to gain a deeper understanding of the engineering geology and historical earthquake situation in the study area. A three-dimensional site model was also established to identify the rock mass structural joints and accumulation conditions in the collapsed area, and the deterioration and fracture factors and historical evolution process of the dangerous rock mass were briefly analyzed, Finally, a comparative analysis was conducted between the simulated results using the three-dimensional finite discrete element method (FDEM) and the dynamic process as evinced by of real-time video footage. The collapse mainly experienced a local instability stage, stress adjustment stage, and overall collapse-accumulation stage. The further understanding of its motion characteristics and energy evolution can provide a basis for subsequent protection design and also a valuable experience for similar research in the region.