Reply to Discussion on ‘The lines of evidence approach to challenges faced in engineering geological practice’, The Nineteenth Glossop Lecture: Quarterly Journal of Engineering Geology and Hydrogeology, 52, https://doi.org/10.1144/qjegh2018-131

The case history summarizes a commercial site investigation report undertaken by a team of specialists from various disciplines. The aim was to better understand the mode of slope failure at the site location known as Upper Osprey on the northeastern coast of the Isle of Portland in order to support the safety case for the proposed development, starting with a desk study. I thank Professor Brunsden for commenting. For clarity and ease of reading in the following text the convention adopted with reference to figures is B1996 for those published by Brunsden et al (1996) and P2019 for those published by Privett (2019). It is important to note that the West Weare to Penn’s Weare crosssection in B1996, figure 18 does not pass through the site. Penn’s Weare is c. 2 km to the south of the site, on the southeastern coast of the Isle, with a different form of slope instability referred to by the number 5 in a circle on B1996, figure 11. However, the mass movement profile in B1996, figure 16c at King’s Pier does pass through the centre of the site and so at the desk study stage was the one adopted to represent the preliminary ground conditions beneath the site. The cross-section was extended westwards to the centre of the Isle to incorporate the hypothetical clay extrusion model and to show the Portland Formations and the full thickness of the Kimmeridge Clay Formation (KCF). This was achieved by appending the central portion of B1996, figure 18 to a mirror image of B1996, figure 16c, thereby producing the composite crosssection of P2019, figure 22. Whether the failures are ‘deep-seated’ is arguably more significant to the proposed development than whether they are ‘rotational’. This is because it is the lowest credible depth of landsliding that has a significant influence on the ability, or otherwise, to provide stable foundations and on the depth of the subsurface safety valves to minimize future risk of blow-out. The thickness of the Portland Formations and the thickness of the KCF each account for approximately half of the vertical distance between the cliff top and the sea; and the western boundary of the site is horizontally central between the cliff and the sea. Based on this, the cross-section of B1996, figure 16c suggests that beneath the site most of the thickness of the KCF down to sea-level has failed. Similarly, in B1996, figure 18, although sea-level is not marked per se, the base of the coastal failures is close to sea-level and so the full thickness of the KCF down to sea-level is shown as potentially subject to clay extrusion and failure. I am grateful to Professor Brunsden for acknowledging the ambiguity in the description of themassmovement profiles in B1996. This became evident during the desk study when it was attempted to transpose the sketched form of the failures incorporated in P2019, figure 24a on to the actual topography obtained from the LiDARwith true horizontal and vertical scales in P2019, figure 24b. Note that P2019, figures 25 and 32 are drawn also with true vertical and horizontal scales. The remainder of the case study goes on to demonstrate how the site investigation programme was geared to refining the conceptual site model of the failures, which do indeed include non-circular landslides. In the final analysis, the findings of the high-resolution stratigraphy, seismic modelling and numerical stability modelling informed the revised conceptual site model presented in P2019, figure 32. This indicated that all the past and likely future failures are restricted to the superficial layer given the shorthand term ‘colluvium’, plus small patches of KCF Zone 2 (coloured yellow and red, respectively on the figure). This is not considered ‘deepseated’ in relation to the overall scale of the slope, but at c. 20 m is ample to have accommodated the large and rapid 1792 slip. No evidence was found at this locality for clay extrusion or lateral spreading in the KCF (coloured light blue on the figure) as a driving mechanism for the landsliding under the climatic conditions projected for the life of the proposed facility.