Analysis of Vertical Earth Pressure Acting on Box Culverts through Centrifuge Model Test

Due to the lack of surface space, most structures are heading underground. The box culvert is underground infrastructure and serves to protect the buried structure from the underground environments, but it has a different characteristic from other structures in that the inner space is empty. Therefore, in this study, the vertical earth pressure which is the most significant effective stress acting on a box culvert was measured by conducting a geotechnical centrifuge model test. A box culvert was installed following the embankment installation method, and the vertical earth pressure acting on it was measured considering the cover depth, gravitational acceleration, and loading and unloading conditions. The soil pressure measured was greater than the existing theoretical value under high cover depth and the unloading condition, which is considered as the variability of many soils or the residual stress acting under the loading condition. Finally, a goodness-of-fit test was conducted as a part of variability analysis. The measured earth pressure was found to be considerably larger than the existing theoretical value, and the variability was large as well. This means the existing theoretical equation is under-designed, which should be reflected in future designs.

Characteristics of Cantilever Retaining Wall Composed of Upper Wall and Lower Wall by Physical Testing

Based on the finite element method, the earth pressure and deformation of the cantilever retaining wall composed of upper wall and lower wall are compared and analyzed. The results show that the maximum lateral displacement of the fill occurs near the top of the upper wall, the lower wall has the tendency of overturning to the free surface, while the upper wall has a certain deviation from the free surface. The vertical earth pressure acting on the heel plate of the upper wall and the lower wall presents non-linear characteristics. When the cantilever retaining wall composed of upper wall and lower wall is unstable, there are two slip surfaces in the filling. The first slip surface runs through the filling based on the heel plate root of the lower wall, and the second slip surface runs through the upper filling based on the heel plate root of the upper wall.

Model Test on the Influence of Surcharge, Unloading and Excavation of Soft Clay Soils on Shield Tunnels

In view of the influence of symmetrical surcharge, unloading and excavation of soft clay soils on shield tunnels, the surface settlement, tunnel settlement, vertical additional earth pressure and tunnel additional confining pressure are measured by an indoor model test. The changes of confining pressure, tunnel settlement, surface settlement and vertical earth pressure caused by surcharge, unloading and excavation are studied and analyzed. The results show that, in soft clay, surcharge will cause tunnel settlement, and unloading and excavation will generally cause tunnel uplift. There is hysteresis in surface settlement and tunnel settlement above the shield tunnel caused by surcharge and unloading; the change of additional confining pressure of the shield tunnel caused by surcharge is mainly concentrated in the three directions of 3, 5 and 7 of the radar chart. These points belong to weak points. Unloading and excavation can effectively reduce the additional confining pressure of the tunnel in these three directions; excavation will cause the increase of vertical cumulative additional earth pressure and tunnel confining pressure in local directions. The influence range of surcharge in soft clay is wider than that in sand, but the depth is relatively shallow.

Modification of Vertical Earth Pressure Formulas for High Fill Cut-and-Cover Tunnels Using Experimental and Numerical Methods

The high-filled cut-and-cover tunnel (HFCCT) is a solution to reclaim more useable lands due to the unique landforms of Loess Plateau in northwestern China. Because of the ultrahigh backfill, the estimation of vertical earth pressure will significantly affect the design and safety of the cut-and-cover tunnel (CCT). The current methods for estimating the vertical earth pressure are either to overestimate or underestimate the vertical earth pressure on the top of HFCCT. To more precisely estimate the vertical earth pressure distribution, the vertical earth pressure based on the soil column pressure, γh (h: the height of backfill above the CCT), needs to be properly modified. Considering different influential factors, four corresponding coefficients are proposed: k0: cross-sectional shape of CCT effect, k1: stiffness of backfill effect, k2: width of CCT effect, and k3: coupling effect of slope angle, θ, and the ratio of B/D. It is found that k0 has little influence; the k3 and k1 reduce and k2 amplifies the γh. The corresponding general forms for these coefficients are determined based on finite element analysis results. A general equation for estimating vertical earth pressure for the HFCCT including these four coefficients is proposed. Meanwhile, this general form is verified by the numerical analysis results and experimental results for different cases. Therefore, this proposed equation is applicable to estimate the vertical earth pressure for existing or newly designed HFCCT. Furthermore, this proposed method can significantly reduce the computational work in engineering analysis.