22 / 2025-07-09 22:25:51

Preliminary exploration of the formation mechanism of the accumulation layer landslide-debris flow disaster chain induced by extreme rainfall——Taking the collapse of the Houzi River Grand Bridge in Sandu County, Guizhou Province as an example

disaster chain,extreme rainfall

In recent years, extreme rainfall has occurred frequently, causing an increasing number of geological disasters. At around 7:40 am on June 24, 2025, a catastrophic collapse occurred in the approach section (K1264) of the G76 Xiamen-Chengdu Expressway Houzi River Extra Large Bridge in Sandu County, Guizhou Province. The day before the disaster, Sandu County was hit by heavy rainfall, of which Dujiang Town experienced a extremely heavy rainstorm of 212.2 mm, breaking the historical record. According to preliminary investigations, the Houzi River Bridge is located in a V-shaped valley in southern Guizhou, with a relative height difference of 200-300 meters and steep terrain (slope of 30 ° -40 °), providing terrain and hydrological conditions for the formation of disaster chains. The landslide body is a tectonic erosion of the middle and low mountain accumulation layer, with a surface layer covered by about 14 meters thick gravel soil, and the underlying bedrock is shallow metamorphic sandstone. Gravel soil has high permeability, while bedrock has low permeability, forming an unstable interface. Continuous rainfall leads to saturation of the rock and soil mass, significantly reducing the shear strength, and forming a wedge-shaped sliding surface on the hillside behind the bridge abutment. The volume of the landslide is about 5.6×10⁴m ³, and the main sliding direction is perpendicular to the bridge axis, forming a direct impact on piers 9 # to 11 #.

After the landslide starts, the rock and soil mass flows down the valley in the form of high-speed debris flow. The 9#pier on the left first tilted due to lateral thrust, causing the main beam between piers 9and 10 to fall after the pile foundation was cut off; Under the coupling effect of left collapse load and residual landslide thrust, the right piers 9 # to 10 # became unstable, resulting in the collapse of the three span continuous beam between piers 9 # to 11 #. The bridge structure adopts a 50 meter span first simply supported and then continuous T-beam, with a hollow thin-walled pier (maximum pier height of 70m) at the bottom. Its resistance to lateral displacement is insufficient to resist landslide thrust, and it eventually collapses in a "V" shape. Over 200m of bridge deck components mixed with landslide bodies to form debris sources, which were carried and discharged by the floods of the Houzi River. A debris flow accumulation with a length of about 150m and a thickness of 3-5 m was formed downstream in the valley, including concrete blocks, vehicles, and crushed stone soil.

The landslide debris flow bridge collapse disaster chain of Houzi River Grand Bridge reveals the hierarchical mechanism of extreme rainfall accumulation layer landslide structural damage debris flow transformation. Under the influence of heavy rainfall runoff groundwater conversion on the slope of the accumulation layer in Guizhou, the instability of the guide edge interface is exacerbated, and the inadequate resistance of bridge pile foundation design to lateral loads in mountainous areas amplifies the consequences of the disaster. It is suggested that in the future, bridge projects crossing valleys in mountainous areas should adopt measures such as optimizing the depth of pile foundation crossing cover layers and setting up slope groundwater dynamic monitoring networks to enhance the resilience of infrastructure.