Zhanping Song

To investigate the fatigue properties of bedded salt rock and the influences of mudstone interlayer on them, a series of fatigue tests were conducted on bedded salt rock under different cyclic loading conditions. The experimental results showed that the characteristics of the stress–strain curve under cyclic loading of bedded salt rock were similar to those of pure salt rock. The mudstone interlayer had an inhibition effect on the residual strain in the salt rock layers, which became more pronounced as the upper-limit stress increased. The macroscopical fatigue failure modes differed significantly between pure and bedded salt rock specimens. Pure salt rock underwent X-shaped conjugate shear failure, whereas bedded salt rock specimens demonstrated interlayer splitting failure, characterized by progressive fracture propagation initiating at the mudstone interlayer and extending into salt rock layers. Moreover, a damage variable was defined by the ratio of accumulative plastic strain to ultimate plastic strain. A novel characterization method for the plastic strain increment was proposed through the integration of the Manson–Coffin equation and Ramberg–Osgood model, with the plastic strain increment utilized to represent the accumulative plastic strain rate. The fatigue life incorporating stress increment and loading frequency was introduced into the fatigue damage rate equation, enabling the derivation of an accumulative damage model describing the entire fatigue deformation process of rocks. The unified function expressions were established between the model parameters and load factors through fitting analysis. Comparative analysis using this model revealed distinct fatigue damage evolution mechanisms between bedded salt rock and pure salt rock.

To address the practical engineering challenges of compaction difficulty and low efficiency associated with traditional backfilling methods for narrow and confined urban foundation trenches, this study proposes an eco-friendly backfill material: sodium silicate-activated fly ash-cement composite fluidized stabilized soil, using in-situ loess from the Xiong'an New Area as raw material. Material components were optimized through curing agent ratio determination and single-factor tests. The development laws of mechanical properties, the micro-scale solidification mechanism, and the flow-filling characteristics within narrow trenches were systematically elucidated by integrating scanning electron microscopy and computational fluid dynamics (CFD) simulations. The results indicate that a fly ash-to-cement mass ratio of 1:1 serves as the critical benchmark for optimizing the composite curing agent proportion. This ratio effectively inhibits excessively rapid early water evaporation and provides a stable hydration environment for the later pozzolanic reaction of fly ash. The optimal mix proportion achieves a balance between high fluidity and high strength. A water content of 51.25% ensures sufficient cementitious reactions. A sodium silicate content of 2.0% strikes a balance between activating fly ash and avoiding excessive slurry viscosity. A curing agent content of 35% facilitates the formation of a continuous and dense cementitious network. Under the optimal proportion, the material exhibits a spread flow of 175 mm and a 14-day compressive strength of 4.53 MPa, meeting the requirements for pumping construction and strength. The compressive strength of fluidized stabilized soil results from the synergistic interaction of fluidity, molding compactness, and water loss behavior. This is manifested microscopically by the generation of cementitious products, pore filling, and particle cementation, and macroscopically reflected in the coupled effects of physical water migration and chemical water consumption. Adopting a single-pour length covering three utility tunnel Sect. (9 m) and a pouring speed of 5 m/s combined with a terminal speed reduction process can significantly enhance the compactness and construction quality of trench backfilling. This approach facilitates the formation of a stable flow field and reduces air bubble retention in corners. Field application demonstrates that this process can meet the dual requirements of construction efficiency and quality. The research findings provide a theoretical foundation and key technical guidance for the construction of underground comprehensive utility tunnels in the Xiong'an New Area.

The disturbance of the surrounding soil by tunnel excavation will inevitably lead to surface settlement. When calculating the surface settlement caused by tunnel excavation, various prediction methods have assumed that the tunnel convergence mode is bilaterally symmetrical. This assumption ignores the influence of asymmetric convergence of the tunnel, and the resulting surface settlement is also symmetrically distributed. To address this limitation, this study proposes an asymmetric tunnel convergence mode and develops a corresponding surface settlement prediction model. The proposed convergence mode decomposes tunnel deformation into three components: uniform radial convergence induced by ground loss, biased ovalization deformation under asymmetric pressure, and rigid-body translation along the bias direction. Three bias-related parameters are introduced to quantitatively characterize the asymmetric convergence behavior. Based on the stochastic medium theory (SMT), a prediction model for surface settlement caused by the bias tunnel was obtained using coordinate transformation and double integral numerical processing. Through actual engineering cases, the applicability of this method was verified, and the influence of relevant parameters on surface settlement was analyzed. The proposed model extends conventional SMT from symmetric to asymmetric tunnel convergence and provides a practical analytical tool for predicting surface settlement in biased-pressure tunnelling conditions.

To reveal the mechanical response mechanism of bolted connections during the jacking construction of prefabricated utility tunnels, this study combines field monitoring and numerical simulation based on the utility tunnel project in Xiong’an New Area. The evolution characteristics and spatial distribution patterns of bolt stress are systematically investigated. The results indicate that during the jacking and unloading stages, bolt connection stress exhibits a V-shaped spatial distribution with opposite evolutionary trends. This reflects both the weakening effect of transient construction loads on the connection system and the self-recovery characteristics after unloading. Segments at the launching stage experience the most complex loading conditions, with bolt stress displaying a nonlinear W-shaped gradient distribution. The mechanical mechanism transitions from being jacking-load dominated to stratum-load dominated as construction progresses. The distribution of bolt stress shows pronounced non-uniformity, with higher values at the upper edge and lower values at the lower edge. Compared to the lower edge, bolt stress at the upper edge of segments 1–5 attenuates by an average of 25.52–37.63%. This non-uniformity results from the coupled effects of ground friction, which dissipates jacking stress at the bottom while inducing forward tilting of the upper structure, thereby causing greater compression and rebound of the waterproof rubber strip at the upper edge. The non-uniformity coefficient decreases linearly with construction progress, reflecting a transition from an unstable to a uniformly stressed state. The cumulative effect of jacking load exhibits stagewise attenuation along the utility tunnel. In segments 1–2, significant strain concentration occurs, with peak compressive and tensile strains. In segments 3–4, direct jacking influence weakens, but delayed strain deterioration is observed, with maximum compressive strain reaching only 69.72% of that in the initial stage. By segment 5, the jacking effect attenuates to background levels. The relationship between bolt stress and jacking spacing follows an exponential decay function. Bolt stress during the loading stage is approximately 76.99–89.07% of that during the unloading stage. Deformation at the roof is significantly greater than that at the floor and sidewalls, consistent with bolt stress distribution patterns.

The construction of new undercrossing tunnel inevitably disturbs the surrounding rock and soil, posing significant risks to the structural integrity of existing overlying tunnel. To accurately and efficiently predict this interaction, a refined theoretical framework is proposed. Based on Hamilton's principle, a flexural differential equation for an equivalent beam representing the existing tunnel is established, explicitly accounting for the axial force and material homogeneity. The Rayleigh-Ritz method is then employed to derive an analytical solution using two Rayleigh-Ritz functions that satisfy the specific boundary conditions of a fixed-end beam. By incorporating Peck formula, the stratum settlement at the existing tunnel's depth is characterized by the maximum settlement Smax and the settlement influence radius r. A simplified deformation coordination mechanism is introduced, where the existing tunnel is modeled as a beam with a span of 2r, constrained by the peak settlement Smax. Validation against multiple field cases demonstrates that the proposed method provides a reliable and slightly conservative prediction of the settlement profile, ensuring a safety margin for engineering practice. Parametric analyses further reveal that the lining thickness T, tunnel clearance d, and volume loss rate are critical factors governing the mechanical response. This study provides a practical and mathematically rigorous tool for assessing the impact of undercrossing excavation in urban geotechnical engineering.