No abstract

Plant roots are known to provide mechanical reinforcement to soils upon shearing and seismic loading. However, the effects of different stress paths on root reinforce-ment are unclear. Moreover, whether, and how, roots provide resistance to soil lique-faction upon cyclic loading have rarely been studied. The objective of this study is to conduct a series of undrained triaxial tests to investigate the monotonic and cyclic behaviour of rooted sand. Roots of vetiver grass (Chrysopogon zizanioides L), which has been advocated for use in shallow slope stabilisation purposes, were used for testing. The root diameters ranged between 0.3 to 1.5 mm, while the root volume ra-tios were 0.23%, 0.45% and 0.67%. It was discovered that the root reinforcement ef-fect was anisotropic and path-dependent. Along the extension path when the major principal stress was perpendicular to the predominant root orientation, the root-induced increase in soil friction angle was approximately 10o. This increase was much greater than the case along the compression path where the change was min-imal. The presence of roots prevented the limited flow failure (which occurred in the unreinforced sand), and the failure mode of root-reinforced soil switched to cyclic mobility. The liquefaction resistance was improved with an increase in root volume, and this improvement was more remarkable at higher cyclic stress ratios.

No abstract

Piled raft foundations are among the most commonly used support structures for offshore projects. When a raft foundation alone does not satisfy the design requirements, piles may be added to improve the ultimate load capacity and the settlement performance of the raft. In this study, design criteria were developed for the undrained behavior of a piled raft system based on an examination of average and differential settlements, raft bending moment, and pile butt load ratio. Raft settlements were evaluated by a series of three-dimensional finite element analyses. The average settlement values for the piled raft were highly influenced by the number of piles and the raft thickness. Optimal design configurations of piles for cohesive soils are discussed. Increasing the pile spacing decreased the pile butt load ratio by allowing for a more uniform load distribution between the piles.