The degree of mechanical reinforcement provided by plants depends upon its roots distribution in the soil and mechanical properties of the roots. The mechanical properties and distribution of root traits (root diameter and number) in the soil of the standing forest depends on the tree stem diameter. This variation of root traits with tree stem diameter is rarely investigated. Therefore, this research presents the effect of tree stem diameter on the distribution of roots within the standing forest of Cunninghamia in the Longchi forest area, Sichuan province, China. In this area, shallow landslides take place frequently. We investigated the root traits distribution for trees with different stem diameters, i.e., 220 mm, 320 mm, 450 mm, and 468 mm, to show the variation of roots distribution in the soil with stem diameter. The root architecture of the selected trees was studied by step excavation method of the root zone accompanied by measurement of roots physical parameters (roots number and roots diameter) and indices (roots area ratio (RAR), roots biomass (RB), and roots distribution (RD)). We measured the root’s maximum tensile strength by performing root tensile tests in the laboratory. The field and laboratory-measured data were used to estimate the root cohesion by both the commonly used model Wu and Waldron Model (WWM) and Fiber Bundle Model (FBM). The results indicate that the tree stem diameter correlates with both the root distribution and the tensile strength. The roots indices and root cohesion increase with an increase in the diameter of the tree. Further, RAR decreases with depth and lateral distance from the tree stem, while the maximum values were observed in 10 cm depth. The relationship between roots diameter and roots tensile strength is established through power function. The average root cohesion estimated for a tree with stem diameter 220 mm is 23 kPa, 29 kPa for 320 mm, 54 kPa for 450 mm, and 63 kPa for 460 mm. This effect of stem diameter on the increase of soil shear resistance should be considered while evaluating the stability of slopes in standing forests. The comparison between WWM and FBM for investigated species suggests that WWM estimates the cohesion values greater than FBM by 65%.

Equivalent linear time history analyses are conducted to calculate the seismic response of various types of cut‐and‐cover single box tunnels. A finite‐element numerical model is calibrated against the results of centrifuge tests. The calculated tunnel responses compare favourably with the measurements. A validated model is then used to quantify the seismic response of box tunnels. The flexibility ratio (F) is illustrated to have a governing influence on the tunnel response. It is shown that the previously developed relationship between F and the racking ratio (R) is applicable for a wide range of F up to 20. It is also shown that an increase in F accompanies corresponding increase in R, the spectral acceleration in the tunnel lining, and the shear stress along the tunnel lining‐soil interface. The thrust in the tunnel lining is also revealed to increase with F, although the calculated value is significantly lower than the pressure on yielding walls. Additionally, the surface settlement is shown to increase with an increase in F.

Hyperbolic type nonlinear models used in 1D site response analysis are reported to predict favorably the nonlinear soil response at small strains, but may provide poor fits at large strains. It has been shown to result in unrealistic site response for deep and soft sites particularly in regions of high seismicity, where implied strength correction is demonstrated to be important. In this study, the importance of the implied strength correction for shallow sites at a region of moderate to low seismicity is investigated. This paper uses a quadratic/hyperbolic model that can capture the stress-strain response at large strains in addition to the small strain stiffness dependence. A total of fifteen measured site profiles are used and a suite of 1D site response analyses are performed using a range of input motions. The strength correction is shown to be relevant even for shallow sites and moderate intensity ground motions. The difference between non-corrected and corrected analyses is conditional on the site period. The overall difference is demonstrated to be more prominent in soft sites. A quantitative estimate of the difference between two types of analyses is presented.