Settlement Analysis of Box Jacking Projects

Abstract

Box jacking (BJ) is a trenchless method to install reinforced concrete boxes underground. Over excavation (overcut) is required during box installation to reduce friction force between box segments and surrounding soils. Surrounding soils may collapse during project execution or after project completion and cause surface disturbances and potentially damage road embankment. The main objective of this paper is to develop a statistical model to estimate maximum surface settlement associated with box jacking using eight factors including 1) soil modulus of elasticity (E), 2) soil friction angle (φ), 3) soil unit weight (γ), 4) soil cohesion (c), 5) box culvert height (h), 6) box culvert width (w), 7) overcut size (s), and 8) depth of box culvert from the surface (H). PLAXIS 2D, a finite element modeling software, was used to generate box jacking models under different site and project conditions. In total, 300 box jacking models were generated and corresponding maximum surface settlements were recorded. Then, multiple linear regression (MLR) analysis was conducted to investigate the relation between considered factors and maximum surface settlement. The minimum depth of box from the surface (H) was assumed to be at least 2h. Therefore, the suggested MLR model is not applicable for box culverts installed in depth less than 2h. It was concluded that the soil cohesion (c) was the most significant factor in determining maximum surface settlement. Results showed that soil friction angle (φ) did not have any correlation with maximum surface settlement.