Effect of Grain Size and Shape on Undrained Behaviour of Sands

The stress–strain and stress path characteristics of sands are influenced by their grain size, shape, and packing. Morphological characteristics and size of particles play important role on the undrained shear strength of sands. Often, effects of these parameters are complex and cannot be easily distinguished. This study advances the knowledge of the role of particle size and shape on the undrained shear strength of sands. To eliminate the consequence of morphological characteristics, two sands with different particle sizes but similar angularity, and another sand with different roundness were selected for the study. These morphological characteristics for all three sands were determined from the analysis of scanning electron microscope images. F131 sand with higher median grain size and lower shape factors (rᵣ and rₛ) had highest undrained peak shear strength and phase transformation value. Undrained strength (qₚₜ) and effective principal stress (P′ₚₜ) in phase transformation point had direct relationship with grain median grain size (D₅₀) and inversely effect of shape factor (rᵣ and rₛ). F131 and F161 sands represented highest peak and ultimate steady-state strengths, respectively. Flow potential appeared to be directly proportional with (rᵣ and rₛ) and inversely with D₅₀. The peak index decreased with increasing shape factors (rᵣ and rₛ)

Possible frost heave mechanisms in an unsaturated high-speed railway formation

The Harbin-Dalian high-speed railway in north-eastern China has a significant portion of track foundation built on seasonally frozen ground. Wide-spread frost heave has been observed during the first winter of its operation and the heave occurred mainly in coarse fills that were considered not susceptible to frost heave. This paper analyses alternative mechanisms that may have caused the observed frost heave. The three most likely mechanisms are: poor quality control of fine content in the coarse fill, the top-down water supply mechanism and the bottom-up water supply mechanism. The likelihoods of these mechanisms are analysed against observed field data, using a one-dimensional frost heave model. The results indicate that the most likely explanation for the unexpected frost heave is a combined action of different mechanisms