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Sandy cobble strata with large granular particles are typically traversed by shield tunnels, leading to high tool wear and excessive surface settlement. Soil conditioning in the laboratory removes large particles owing to test device limitations and may not reflect field tunneling involving large cobbles and boulders. This study proposes a hybrid experiment/theory method for soil conditioning in sandy cobble strata having large particles. Viscosity, stirring, and slump tests are conducted to obtain suitable parameter ranges of different soil conditioning agents for soil samples (< 30 mm). To consider large particles, the average liquid bridge force per unit length F / d and the liquid flow velocity u in the foam's Plateau border are chosen to represent the static and motive characteristics of the samples. The calculation method for the liquid flow velocity in the Plateau border is determined, and a relationship between the particle clearance and the soil sample porosity ratio is proposed. Finally, the suitable parameter range for soil conditioning is revised by the theoretical part considering particles larger than 30 mm. The hybrid method was applied to two shield construction cases in Chengdu and Lanzhou, China. The results indicate that the bentonite injection ratio I b and the foam injection ratio I f should be increased by 42.7% and 60.6% on average for the two cases, respectively. The proposed soil conditioning hybrid approach provides a beneficial effect in the form of stable construction parameters while improving cutting tool life and can be used to guide soil conditioning in sandy cobble strata. [ABSTRACT FROM AUTHOR]

This study investigated several ground collapses in sandy cobble strata induced by shield tunneling in two intervals of Lanzhou Metro Line 1 and proposed corresponding control measures. Ground collapses were classified into three types (A, B, and C). The distribution of low-compactness zones was the primary cause of the collapses that occurred after steady excavation (A-type), indicating that advanced grouting should be conducted beneath these zones. The grouting area should be a semi-circular ring in the transverse section, with a grouting ring height and thickness of 1.05 m and 0.51 m, respectively. A poor tunneling state caused by fluidity reduction in the cutter chamber was the main reason for collapses caused by restarting the shield after long-term shutdown (B-type). We suggest rotating the cutterhead every 12 h during long-term shutdown and supplementing the bentonite until the cutterhead torque falls below 3500 kN·m. Collapses caused by cutterhead blockage (C-type) were mainly due to encountering large boulders during tunneling. We suggest pulling the front shield back with hinge equipment when the cutterhead is blocked, with a backward distance greater than the penetration distance (35 mm) and a face pressure above the limit support pressure calculated using the method presented in this paper. [ABSTRACT FROM AUTHOR]

This paper presents a landslide that occurred in loess hills resulting in structural damage to the Zhangjiazhuang tunnel of the Lanzhou–Xinjiang high-speed railway line. The characteristics of tunnel deformation and structural damage are provided in detail. To ensure future safety of the tunnel, investigations consisting of field reconnaissance, borehole investigations, seismic activity and rainfall records analysis, and numerical simulations were conducted to identify the regional geology and landslide characteristics. Using these investigations, the probable formation mechanism of this sliding failure is determined to be (i) an ancient groove that provides a natural slip bed leading to stress concentrations and dislocation failure at the back of the slope; (ii) seepage channels that are well-developed on this loess hillslope, through which an extreme rainfall event 6 months prior to the landslide saturated the strata materials; and (iii) finally, the limit state of the materials is reached in an earthquake under undrained condition, and the landslide is triggered. The findings presented in this paper can provide a reference for the control and treatment of landslides in loess areas. [ABSTRACT FROM AUTHOR]