Abstract Over the past four decades, ongoing deformation of an 18-m-thick peat deposit within the flat-lying Mercer Slough has resulted in damaging deflections, and near-collapse in three cases, of pile-supported Interstate 90 bridges and a major water line on the east side of the slough. The peat is partially underlain by a dense sand unit, which includes a highly pressurized aquifer that produces artesian flow 1–2.5 m above the ground surface. Inclinometers on the east side of the slough show the peat flowing toward the structures and then apparently directed west along the interstate centerline. Large displacements recorded in several inclinometers near the center of the slough suggest a length of deforming peat that approaches 600 m, which is likely initiating retrogressively. Potential causal mechanisms include poor engineering characteristics of the peat, presence of high hydrostatic pressure transmitted within and beneath the peat, seasonal water-level variations of Lake Washington and induced hydraulic gradients within the peat, dredging of the Mercer Slough channel, puncturing of the underlying aquifer by numerous pile foundations, and fill placement along the eastern margin of the slough. The peat is flowing around the pile/shaft foundations; however, excessive lateral loads are still being applied to the foundations in a poorly understood and unpredictable manner. The most severe deflections have occurred in the outermost structures where the peat is primarily flowing transverse to the structures.

Abstract In the UK national derived geological datasets are increasingly being produced, many of which are based on NEXTMap digital terrain model (DTM) or digital surface model (DSM) data. These include groundwater level and land stability datasets. Any DTM is a model of the land surface and under different conditions may have differing degrees of accuracy. This paper compares the NEXTMap data, derived from airborne Interferometric Synthetic Aperture Radar (IfSAR) data, with other frequently used datasets derived from contours and point data; in particular, the Integrated Hydrological Digital Terrain Model (IHDTM), a terrain model that was originally derived from Ordnance Survey (OS) 1:50 000 scale contours, and a DTM interpolated from Land-Form PROFILE data. This initial comparison of the DTMs has highlighted some issues with the NEXTMap data: first, that of elevation inaccuracy in woodland areas; and, secondly, the shadowing effect caused by the side-looking scanner. It also highlights the problems of using DTMs created from contour data in areas of low relief. The development of an uncertainty layer would enable a user to decide whether the DTM was appropriate in certain areas, and could also be incorporated into uncertainty models for the derived national datasets.