Study on the mechanical characteristics of pipe curtain group construction in the backfill soil stratum based on model tests

Under backfill layer conditions, pipe-roof construction using multiple closely spaced jacked pipes has been widely adopted in shallow-buried undercrossing projects to satisfy the increasing demand for underground space utilization and structural presupport. However, during sequential pipe jacking, the superposition of soil disturbance induced by multiple pipes considerably alters the mechanical properties of the surrounding ground. As a result, abnormal pipe loading responses and uncontrolled surface settlement are likely to occur, posing potential engineering risks. Existing studies have mainly focused on single-pipe jacking, whereas systematic investigations on the coupled effects of pipe spacing on soil pressure redistribution around pipes, pipe deformation, jacking force evolution, and surface settlement during multipipe jacking remain insufficient. This deficiency is particularly evident in backfilled ground, which is characterized by a loose structure and high stress sensitivity. Therefore, the objective of this study is to systematically investigate the influence of pipe spacing on the mechanical response and deformation behavior of the ground–pipe system during multi-pipe jacking in backfilled ground. It also seeks to clarify the interaction mechanisms between adjacent pipes and the surrounding soil under pipe-roof construction conditions.

Based on the Chengdu–Chongqing Expressway undercrossing project, a series of physical model tests were designed and conducted according to similarity theory. A model box was constructed to simulate backfilled ground conditions, in which three pipes were sequentially jacked under different pipe spacing configurations. During the tests, vertical and horizontal soil pressures around the pipes, radial strain in the pipe segments, jacking force during advancement, and surface settlement were continuously monitored. The effects of geometric parameters on the interaction between the pipes and surrounding soil, as well as the superposition of disturbances during multi-pipe jacking, were analyzed across different spacing conditions. These physical model tests enabled continuous observation of the entire jacking process and provided a basis for comparing the mechanical responses of the leading pipe and the subsequent pipes under identical jacking conditions.

Experimental results indicate notable differences exist in the mechanical responses between the leading pipe and subsequent pipes. When subsequent pipes advanced to the monitoring section, the vertical soil pressures above and below the pipes, as well as the horizontal soil pressures in the inter-pipe zone, were considerably lower than those observed during the first pipe jacking stage. This phenomenon suggests that the presence of the leading pipe alters the original stress field and induces evident stress redistribution and partial unloading in the surrounding soil. Moreover, with increasing pipe spacing, vertical and horizontal soil pressures around the pipes exhibited an overall decreasing trend, indicating that larger pipe spacing can effectively reduce the extent of disturbance zones and alleviate stress concentration in the soil. In terms of structural response, the maximum radial strain was observed at the pipe crown after completion of jacking, and the magnitude of radial strain decreased progressively with increasing pipe spacing, demonstrating that larger spacing is beneficial for reducing pipe deformation. Meanwhile, under the same jacking length, the jacking forces required for the second and third pipes were greater than those for the first pipe, which can be attributed to cumulative soil disturbance. However, the peak jacking force decreased as the pipe spacing increased, indicating that appropriate spacing can effectively reduce construction resistance and improve jacking efficiency. In addition, surface settlement measurements show that during subsequent jacking, the settlement trough shifted toward the advancing pipe, accompanied by a continuous increase in settlement magnitude and affected area.

This study systematically reveals the controlling role of pipe spacing in soil pressure redistribution, pipe deformation, jacking force demand, and surface settlement during pipe-roof construction in backfilled ground through physical model tests. The results demonstrate that the presence of a leading pipe induces notable stress field modification, while increasing pipe spacing effectively weakens the superposition of disturbance, thereby reducing soil stress levels, structural deformation, and jacking force requirements. The findings provide valuable experimental evidence and theoretical support for pipe spacing optimization and construction safety control in pipe-roof construction conducted in backfilled ground.