This paper presents experimental work aimed at proving the feasibility of using Distributed Fibre Optic Sensing (DFOS) as an early warning system for sinkhole detection. 1g experiments were conducted using a plane strain trapdoor and scaled to provide insight into the formation of a sinkhole in sand, in which DFOS cables are laid at selected depths. The DFOS data are compared with the geomechanics of the soil displacement, recorded using Particle Image Velocimetry (PIV). It was demonstrated that the DFOS exhibits a signature strain profile at the location of the sinkhole, allowing a sinkhole to be located using the DFOS data. Differences in the PIV and DFOS data are however apparent, notably the strain magnitudes. Nonetheless, it is possible to estimate the size and location of the sinkhole at depth using the DFOS data. Using a preliminary study of the development of the zone of subsidence, for a range of relative densities, it is then possible to predict the extent of the damage zone at ground surface. Such results show the potential for the incorporation of DFOS in the construction of critical infrastructure to enable early detection of sinkhole formation and thus opportunity for remedial action to prevent catastrophic failures.

In this Discrete Element Modelling research, triaxial compression tests of particle assemblies were simulated to study the mechanical behaviour of methane hydrate sediments with two different hydrate formation patterns: pore-filling and cementation. The soil particles were modelled using spherical or elongated particles (two aspect ratios 1.5 and 2.0). Hydrates were modelled as smaller particles and were placed either inside the pores in a random manner (the pore-filling case) or around the soil particle contacts (the cementation case). Compared to the pure soil samples, the hydrates essentially influenced the mechanical behaviour of the hydrate-bearing soil samples, and the behaviours varied due to the different hydrate growth patterns. The behaviour with elongated soil particles is much closer to that of the natural hydrate-bearing sandy sediments retrieved from the Nankai Trough than the behaviour with spherical particles. The observed macroscopic strength behaviour is also explained by the microscopic contact-type related contributions (soil-soil contact, soil-hydrate contact and hydrate-hydrate contact) to the deviatoric stresses.

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The shear wave velocity method has become an increasingly popular means to evaluate the liquefaction potential of granular soils. Understanding the fundamental mechanism underlying existing empirical or semi-empirical relationships is important for better assessing their reliability. This paper presents a particle-scale study of the correlation between cyclic resistance ratio (CRR) and the shear wave velocity corrected for overburden stress (V s1). The discrete-element method was used to simulate a series of undrained stress-controlled cyclic triaxial tests together with shear wave velocity (V s) measurements. Discrete-element method modelling with various relative densities, confining pressures and micro-parameters was performed under various cyclic stress ratios (CSRs), and the onset of liquefaction was illustrated through both macroscopic and microscopic responses, for example, inferred excess pore-water pressure, mechanical coordination number and redundancy index. The inter-particle friction was identified as the key micro-parameter that governs the liquefaction resistance of granular soils. A micro-scale CRR-V s1 correlation considering two independent micro-parameters, inter-particle friction and particle shear modulus, was then obtained and further validated with the outcomes from three dynamic centrifuge model tests performed on silica sand no. 8. This study demonstrates that the CRR-V s1 correlation is particle specific, thus soil specific, and the particle mechanical properties should be included in the V s-based method for future liquefaction evaluation of granular soils.