Geology and hydrogeology ... …
Report No. SRL/FP/004.1 (10/05/2006)
rely upon the presence of one form or another of geological stratum underneath the terrace gravels. Nearby, there was deeper excavation to lay the sewers for the estates built on Long Furlong Farm in the late 1980’s. There the Kimmeridge Clay was thicker, extending to a depth of a few metres, below which was a thin layer of Corallian Rag. The sewer had to pass through this layer of rag, somewhere in the vicinity of the Long Furlong Community Centre. From memory, the depth of the sewer was 7 metres and the layer of rag only a metre or two thick. But the cutting process was nonetheless extensive because the falls of the sewage pipe was designed such that it would work correctly and this
coincided approximately with the penetration occurred over quite an
strike slope of the underlying ragstone, so the
Thames water, also dug Explosives Division enabled well known to the residents
a water-main link. The blasting program devised by ICI the rag layer to be penetrated during about a week. This was of North and South Avenue because of the damage done to
their property when Kimmeridge Clay and
explosions base of the
rag is only about 7
Corallian m below
Rag lies under the surface, that
accommodating layers of terrace Corallian Rag remains close to
gravel, Kimmeridge Clay and Corallian Rag. Clearly the the surface in the South Avenue area because it was
closely enough connected to the houses above carefully designed explosives eastward along the overlying clays and gravel to the housing above.
that it could transmit the effect of the rag layer and then upwards through the Equally clearly the rag layer cannot be
much thicker effects of the
under South Avenue vibration would have
it is under Long dissipated.
These facts about the Kimmeridge Clay and the Corallian Rag in Abingdon, just 2 km from Lake E are to be contrasted with the statements made by RWE npower in their Environmental Statement.
Kimmeridge Clay “The thickness of this formation is considered to be approximately 30m”. [See page 2-3 of Appendix 7 of the ES] the authority for this information being the ES itself (!) presumably the entirely unattributed statement on page 137 which states “there is approximately 30 m of … Kimmeridge Clay”. They do not say who considers it to be so, why they do so, or what the consequences may be that they are wrong. And how approximately is “approximately”? It is a highly dangerous statement to be made about any rock layer which has a geological unconformity (as here) at its upper surface. Published geological cross-sections11 for Oxfordshire 16km to the southwest show the thickness of the intact Kimmeridge Clay layer to vary between 25m and 50m, so 30 metres might seem to be a reasonable guess. However the clay is much thinner than this in areas traversed by palaeo river courses, such areas delineated, as at Radley, by thick layers of overlain alluvium. Within such areas, palaeo channels, the remnants of ancient water courses, may create even deeper incisions into the clay. Boreholes sunk by RWE Npower only confirm the depth of the clay horizon in a few places. They give no indication of its thickness.
Corallian formation: “the thickness at Radley is not known but elsewhere can be up to 100m.”. What has that got to do with it? It is much thinner in North Abingdon (see above). There must be many other measurements better than an allusion to “great thickness”, probably unrelated to Radley. If RWE npower would like to find out more, I refer them to the drillers of the boreholes, which they present in the same Appendix 7 of the Environmental Statement.
Institute of Geological Sciences, 1971
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© SAVE RADLEY LAKES 2006