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diagrammatic vertical section through the site (Fig. 2). Access to the caverns was by two 2-m diameter drilled shafts, and the total excavation in each of the two caverns is 120,000 m3.
The storage galleries lie in the lower part of the Welton chalk with the sumps extending down into the top of the Ferriby chalk. The Welton and Ferriby chalks correlate approximately with the Middle and Lower chalks of southern England.
The photograph shows one of the nine storage galleries, each of which is 10 m wide and 10 m high, with a cross-section of 85 m2. The galleries were excavated in two stages: first a top heading and then a bench. The top heading was excavated partly by roadheader and partly by blasting; the bench was excavated by blasting. All blasted surfaces were trimmed by roadheader. The photograph shows the gallery after bulk excavation but before the bench sidewalls had been trimmed by roadheader (Fig. 4).
The galleries have required no rock support, except on a minor scale at gallery intersections. Some of the principal characteristics of the Welton chalk are listed in Table 1, and selected geophysical logs of the chalk at cavern level are
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