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The objective of this work was to study the minimum support force of the shield excavation face of sand-gravel stratum. Based on the geological conditions obtained from the exploration of the 18th line of the Chengdu Rail Transit and the shield excavation parameters obtained from excavation, particle flow DEM PFC3D and FLAC3D were used as tools; the digital triaxial compression test was performed to calibrate the mesoscopic parameters of the soil; numerical excavation models were built for deep-buried and shallow-buried sand and gravel stratum shield tunnels; surface settlement and minimum support force under different depths and different supporting forces were obtained in the end. The stability of the excavation face under the condition of 10 kPa∼60 kPa support force was analyzed; the excavation surface gradually tended to stabilize, and the surface subsidence decreased with the increase of support force. When the supporting force was greater than or equal to 3 kPa, the excavation surface was stable and the surface settlement speed gradually decreased to zero with time. While analyzing the stability of the excavation face of 5 m, 10 m, and 15 m buried deep gravel tunnels and the influence of the support force on the surface settlement, the minimum support force that did not consider the surface settlement was reduced and the minimum support force required to control the surface settlement was increased as the tunnel depth decreased. Using this method, the depth of the excavation sand-gravel tunnel and the support of the excavation face could be obtained to meet the requirements of surface subsidence control and the economic benefits of the project could be improved.

On August 17, 2020, debris flows successively occurred in four ravines along the main stream of Yazhezaozu Village, Pingwu County, Sichuan Province. This event resulted in multiple collapses and interruptions along the G247 national highway, and extensive damage to the Jiumian expressway project site and laborer residences, among other areas. A significant volume of driftwood carried by the debris flow converged into the main stream, leading to the blockage of downstream road bridges and culverts, causing backflow and village flooding, thereby exacerbating the disaster risk. To prevent similar disasters in the future, post-disaster investigations using field surveys and remote sensing interpretations explored the characteristics of this debris flow disaster's risk. A primary focus was placed on analyzing the risk amplification effect caused by blockages in main stream road bridges and culverts due to debris flow. The results indicated: (1) The '8.17' debris flow in Yazhezaozu Village was a low-frequency, large-scale, rare, and extensive group occurrence, with an eruption frequency of approximately once every 50 years. The debris flows in Azu Gully and Zapai Gully were extremely large-scale, while Mashizha No. 3 Gully and Duobu River No. 5 Gully were large-scale, with an affected debris fan area of about 16.66 x 104 square meters. (2) Improper layout of main stream road bridges and culverts resulted in the blockages of driftwood carried by the debris flow, forming dammed bodies, increasing the affected area by 16.78 x 104 square meters, and enlarging the risk range by about 1-fold. (3) In well-vegetated, moist mountainous areas, when constructing main stream road bridges and culverts, it is advisable to appropriately increase the axial spacing between bridge piers, allowing for a certain width and operational space in the river channel. This will prevent the amplification of debris flow disaster risks caused by driftwood blockages. This study aims to provide guidance for the reasonable planning of main stream road bridges and culverts in similar mountainous areas and relevant prevention and early warning of debris flows. [ABSTRACT FROM AUTHOR]