| Abstract: |
The geotechnical ground investigation comprised excavation of trial pits and cablepercussion boreholes. A summary of interventions is given in Table 1 and the layout isillustrated in Figures 1 and 2. Three trial pits and one borehole at Greenwich originallydesignated for monitoring were descoped from the geotechnical works (BH-G-09, TPG-01 and TP-G-02). An additional borehole was recorded at Silvertown (BH-S-19)during standing time.The drilling of the cable percussion boreholes was initially intended to be monitoreddirectly onsite. However, the fieldwork in March 2020 coincided with theimplementation of COVID 19 Health and Safety restrictions. After an initial cessation,work resumed under a revised safe system of work. Consequently, the majority of theboreholes were recorded from bulk samples collected at the geotechnical contractor’sstore. No intact core samples were available for examination. U100 cores weregenerally sealed and sent offsite immediately for lab testing. The excavation of all trialpits was, however, monitored onsite.Bulk samples from deposits with the potential for heavy contamination (eg asbestos,hydrocarbons) as indicated from the drilling logs, particularly made ground depositsand the very upper levels of the alluvium, were stored separately, and were notexamined by the attending geoarchaeologist. A small number of sub-samples wererecovered on an opportunistic basis from peat deposits, but the broad nature of thebulk sample recovery and potential mixing of deposits from different levels limits theirusefulness for detailed palaeoenvironmental or dating work.The sediment was logged on standard proforma sheets in accordance with Jones et al.(1999) and Historic England guidelines for geoarchaeology (2015). The logs include adescription of colour, compaction, texture, sorting structure and inclusions. It was notpossible to log the contact between sediment due to the disaggregated nature of thesamples. These descriptions were integrated with the geotechnical logs allowingquality assurance of the descriptions and interpretations. The data was then enteredinto borehole modelling software (Rockworks17) for the correlation of stratigraphicunits and production of cross-sections.The interpreted digital stratigraphic data from the original deposit model (Quest 2015)was unavailable at the time of writing. Taking into account the relatively small numberof interventions covered by the watching brief, it was not considered an efficient useof resources to reinput the full dataset (500 datapoints) into Rockworks in order toupdate the full model at this stage. Consequently, a selection of datapoints (theoriginal logs being held in the BGS archive) from Transects A-D (Quest 2015, figs 10-13) have been included in two new cross sections for comparative purposes (Figs 4and 5). The full dataset derived from the watching brief is included in the appendices. Overall, the results of the watching brief in combination with the previous depositmodelling (Quest 2015) has served well in broadly characterising the sub-surfacestratigraphic architecture of the Holocene sequences that may be impacted byconstruction. However, it is clear there is much greater local complexity to thesequences, particularly at Silvertown associated with marginal channel locations andcycles of erosion, redeposition and stability associated with the shifting footprint ofthe River Lea at the Thames confluence. This complexity is set against the backdrop ofchanges within the wider estuary related to fluctuations in the rate of sea-level riseduring the Holocene. These changes incorporate episodes of marine incursion typifiedby deposition of silt, clay and sand with the development of mudflat, tidal creeks andsaltmarsh environments. |
