Jinshang Wang

Intensive agricultural areas across China suffer severe nitrogen surplus, yet its spatiotemporal drivers remain unclear. Using the nutrient balance method, this study quantified cropland nitrogen surplus and its intensity in 47 prefecture-level cities in Beijing, Tianjin, Hebei Province, Shandong Province, and Henan Province from 2010 to 2022. The Geographically and Temporally Weighted Regression (GTWR) model was applied to analyze six driving factors. Nitrogen surplus decreased from 6.71 million t to 4.11 million t, representing a reduction of 38.7%, with a marked decline occurring after 2016. Surplus intensity dropped by 30.7% from 29.13 t/km² to 20.20 t/km², and nitrogen use efficiency rose from 46% to 60%. The GTWR model proved to perform well (R² = 0.749). Both the multiple cropping index and the agricultural industrial structure shifted from positive to negative effects. Specifically, the effect of the multiple cropping index declined from 0.325 to −0.183, showing the largest absolute average annual change. Urbanization rate, agricultural mechanization level, and per capita cropland area exerted negative effects, whereas the agricultural planting structure varied among regions. The multiple cropping index was the most critical factor. Therefore, optimizing cropping systems is an effective pathway to reduce nitrogen surplus. Future research should focus on incorporating climate factors and exploring the interactive effects among driving factors at finer spatial scales (e.g., county level), to achieve more precise nitrogen management.

Shield tunneling of water diversion tunnels in water-rich sandy pebble strata is susceptible to stratum cavity development and ground subsidence disasters. Existing research lacks a quantitative theoretical system for the dynamic evolution of excavation-induced stratum structural zones and dedicated physical simulation platforms, thereby limiting in-depth understanding of relevant disaster mechanisms. To fill this knowledge gap, this study established a three-zone mechanical model for shield tunnel excavation based on ellipsoid instability theory, classifying the overlying strata from the top to the bottom into a subsidence zone, critical stable zone, and collapse zone. The formula for the minimum stable layer thickness of surrounding rock was derived and a stratum collapse criterion (H ₁ + H ₃ ≥ H) proposed. In conjunction with practical engineering, similarity model tests were conducted to reveal the spatiotemporal evolution law of the three structural zones. Results showed that unbalanced shield support force triggers the transformation of stable soil to collapsed and subsided soil, and that the stable layer thickness is the core factor dominating ground subsidence. As the shield advances, stratum instability experiences four sequential stages: vault crack initiation, crack propagation, collapse arch formation, and stratum restabilization. Stratum cavity volume and construction disturbance intensity govern soil collapse and subsidence development. The theoretical collapse zone height of 9.06 cm and ground subsidence of 1.26 cm agreed well with measured values of 9.87 and 1.1 cm, respectively. Monitoring data indicated that soil within 30 cm above the tunnel is severely disturbed as the main collapse-affected zone, while soil at 60 cm depth reveals only slight consolidation subsidence and maintains stability. The proposed quantitative theory and test method provide a solid theoretical and experimental basis for disaster prevention, subsidence early warning, and optimized support design of shield tunnels in water-rich sandy pebble strata.

Ensuring face stability is a pressing concern in slurry shield tunneling under high water pressure. Although slurry infiltration and filter cake formation are known to affect stability, the governing role of seepage forces in the failure mechanisms remains insufficiently understood, and existing models often oversimplify the regulating effect of filter cake permeability. To address this gap, a combined numerical–theoretical approach is developed that explicitly incorporates seepage effects into the failure analysis. A three-dimensional seepage model was developed to simulate transient pore water pressure distribution ahead of the tunnel face, considering filter cake properties, stratum permeability, and applied slurry pressure. Based on the computed seepage field and a wedge-prism composite failure mechanism, an upper-bound limit analysis model was formulated that accounts for the work done by seepage forces. Results reveal a filter cake permeability threshold of 1.0 × 10⁻⁷ m/s, below which further reduction yields negligible stability improvement. Parametric studies quantify the influences of internal friction angle, cohesion, depth-to-diameter ratio, and permeability contrast between soil and filter cake. Validation against field data from the Maliuzhou Tunnel demonstrated that the calculated limit pressures consistently lie below the field-measured slurry pressures, confirming the model’s reliability and its practical utility for guiding slurry pressure selection in complex ground conditions.