Andrew Trafford's research while affiliated with University College Dublin and other places

This paper assesses the use of Distributed Acoustic Sensing (DAS) for shallow marine seismic investigations, in particular the collection of seismic surface wave data, in an intertidal setting. The paper considers appropriate selection and directional sensitivity of fiber optic cables and validates the measured data with respect to conventional seismic data acquisition approaches ,using geophones and hydrophones, along with independent borehole and Seismic Cone Penetration Test (SCPT) data. In terms of cable selection, a reduction of amplitude and frequency response of an armored cable is observed, when compared to an unarmored cable. For seismic surface wave surveys in an offshore environment where the cable would need to withstand significant stresses, the use of the armored variant with limited loss in frequency response may be acceptable, from a practical perspective. The DAS approach has also shown good consistency with conventional means of surface wave data acquisition, and the inverted Vs is also very consistent with downhole SCPT data. Observed differences in phase velocity between high tide (Scholte wave propagation) and low tide (Rayleigh wave propagation) are not thought to be related to the particular type of interface wave due to shallow water depth. These differences are more likely to be related to the development of capillary forces in the partially saturated granular medium at low tide. Overall, this study demonstrates that the proposed novel approach of DAS using seabed fiber-optic cables in the intertidal environment is capable of rapidly providing near-surface shear wave velocity data across considerable spatial scales (multi-km) at high resolution, beneficial for the design of subsea cables routes and landfall locations. The associated reduction in deployment and survey duration, when compared to conventional approaches, is particularly important when working in the marine environment due to potentially short weather windows and expensive downtime.

Despite some 9% of Norway's area being underlain by peat the geotechnical characteristics of the material have not been well documented. As peatlands form an excellent carbon sink there is much pressure on planning authorities to avoid the excavation of peat for infrastructure development. In engineering projects in peat often the resulting settlements both in the short and long term are of greatest concern. Because of the high variability of peat, several, largely empirically based, methods exist for predicting settlement on peat. These were often developed based on specific cases. These techniques have shown to be inadequate to generically predict long term creep settlements and the development of settlement with time. Here the engineering characteristics of peat from several sites in the Trondheim area of mid-Norway are studied using a series of field and laboratory tests including three types of oedometer test. The laboratory and field data, together with empirical correlations found in the literature, were used to provide input into the commercially available constitutive model Soft Soil Creep (SSC) in the computer code PLAXIS. The model was initially calibrated using the laboratory test results and then applied to the back-analysis of two full scale field trials in the Trondheim area, where the peat properties were significantly different. The modelling showed that SSC captured well the vertical settlement versus time behaviour of the peat. Guidance is provided for selecting the critical input parameters for SSC such as stiffness, yield stress and permeability. This work contributes towards efforts being made to bridge the gap between the numerical modelling community and practicing engineers.

The long-term sustainability of the offshore wind industry requires the development of appropriate investigative methods to enable less conservative and more cost-effective geotechnical engineering design. Here we describe the novel use of distributed acoustic sensing (DAS) as part of an integrated approach for the geophysical and geotechnical assessment of the shallow subsurface for offshore construction. DAS was used to acquire active Scholte-wave seismic data at several locations in the vicinity of a planned windfarm development near Dundalk Bay, Irish Sea. Complimentary additional datasets include high-resolution sparker seismic reflection, cone penetration test (CPT) data and gravity coring. In terms of fibre optic cable selection, a CST armoured cable provided a reasonable compromise between performance and reliability in the offshore environment. Also, when used as a seismic source, a gravity corer enabled the fundamental mode Scholte-wave to be better resolved than an airgun, and may be more suitable in environmentally sensitive areas. Overall, the DAS approach was found to be effective at rapidly determining shear wave velocity profiles in areas of differing geological context, with metre scale spatial sampling, over multi-kilometre scale distances. The application of this approach has the potential to considerably reduce design uncertainty and ultimately reduce levelised costs of offshore wind power generation.

A significant challenge for engineers working with peat is the estimation of the undrained shear strength (su). This is particularly the case for shallow, slightly overconsolidated peats found on natural hillsides and at the toe of embankments. Existing laboratory and in situ techniques may not be suitable due to issues such as access to remote areas, the resolution and accuracy of measuring equipment, sampling disturbance effects, and the influence of fibers on the measured results. It is logical to attempt to correlate su with shear-wave velocity (Vs) because both parameters depend on the same set of fundamental factors. A lightweight portable probe has been developed that can be used to resolve the low Vs values in peat reliably and repeatably. It was not possible to determine su directly from the Vs measurements. However, a strong correlation was found between normalized Vs (by water content, w) and su (derived from direct simple shear tests). A single operator can collect a continuous profile of Vs and w simultaneously and therefore derive a one-dimensional (1D) su profile for the full peat thickness. Some limitations in the use of the derived empirical equations are outlined.