Geological and engineering characteristics of the pyritic Triassic Westbury Formation and assessment of weathering implications for construction and material reuse

During the early phase of ground preparation work for the Hinkley Point C site located on the North Somerset Coast, a much greater presence of Westbury Formation was exposed at shallow depths than had been anticipated. Fortunately, these occurrences were located away from the critical area of the development. However, the previous feasibilities studies carried out by others in support of the tender requirements had classified the very weak dark grey pyritous mudstone of the Westbury Formation as unsuitable for reuse as earthwork material. Consequently, the unexpected situation also presented further uncertainties for earthwork slopes, water-gallery foundations, and stockpiling. Under the circumstances decision was taken to re-examine the suitability of the Westbury lithologies for re-use as engineered fill due to the large volumes of excavated material and develop effective management of the exposed slopes. As the engineering behaviour of the Westbury lithology is poorly documented and no suitable guidance available, the material was evaluated at a site level during the phase of the pre-construction enabling works to foresee the likely weathering behaviour following exposure and design appropriate mitigation measures. Consequently, it was recognised that a rapid development of aggressive conditions was likely presenting significant detriment for the earthworks and slopes for which considered management for effective mitigation was developed. Comparison was made with the intermittent field analogues locally exposed along the adjacent north Somerset coast to help understand the rates and magnitude of change. Furthermore, a search of published work identified a paucity in studies relating to the engineering behaviour of the Westbury Formation. Therefore, findings of the targeted study better informed scheduling of earthworks and management of compaction works. The findings of the study are presented and discussed in this paper and offer brief insight into the behaviour of this fascinating pyritic mudstone to support ground engineering developments that may encounter Westbury strata. The studies concluded that the Westbury mudstone, which comprised the significant proportion of Westbury strata at the site is of very low durability and susceptible to rapid breakdown to a gravelly-silty-clay material. The Westbury deposits include significant material of high calcite content including subordinate lithologies of highly calcareous mudstone and argillaceous limestone that are of more favourable durability and break down over a slower period. The mudstone includes a high content of visible crystalline pyrite as well as the non-visible framboidal form. Although the visible form is generally considered as ‘the non-reactive’ form of pyrite, field evidence concluded that it was also susceptible to rapid oxidation. Based on the field studies, it was realised that the pyrite forms found in the Westbury strata present at the site were likely to oxidise rapidly following exposure. The acidity generated during the oxidation of pyrite would prove chemically aggressive to construction and natural ground material. Due to the high calcite content, reaction would occur with the acid producing selenite. At surface, much of the selenite would likely to be removed in solution due to a higher solubility brought about by the presence of chlorine in the marine environment but within disturbed and exposed strata and backfill heave could occur causing differential stressed and slope or foundation failures. Material storage would require timely management to avoid unnecessary exposure and limit degradation which was accomplished through an optimised compaction program determined by site trials on completion of the characterisation study. Slope and foundation excavations revealing Westbury strata necessitated treatment, which was provided with blinding concrete or shotcrete within 48 hours of exposure. Otherwise, material placed in loose stockpiles and allowed to weather would following rapid degradation and then be suitable for landscaping purposes and reuse as general platform construction fill, but that aggressive surface water flow from the stockpiled material would require appropriate containment and management.