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Electric
Mountain
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Geology and rock support
The main caverns and access tunnels, the tailrace and high pressure tunnels, the lower half of the high pressure shaft and the diversion tunnel are excavated in rock of the main Cambrian Slate Belt. The upper works are constructed in a Lower Ordovician succession of grits and slates. The overall geology is shown on the geological section of the power station complex.
The geological succession involved in the works is as follows:-
| Main bedrock | � | � |
| Period | Name | Description |
| Ordovician | Maentwrog Slates | Finely interbedded slates and siltstones and fine grits. |
| � | Bronllwyd Grits | Siliceous grits with very subordinated slates. |
| � | Cambrian Slates | Green Slate Group
Spotted Upper Red Slate Group Mottled Grey/Blue Slate Group Garrat Interbanded Group Spotted and Striped Purple Group Red Slate Group Hard Striped Blue Group Lower Grit Group Lower Red and Blue Group |
| Intrusive rocks | ||
| Period | Name | Description |
| Tertiary | Dolerite | Scarce olivine-dolerite dykes |
| Ordovician | Dolerite | Abundant metadolerite dykes |
Generally, the main caverns lie in a folded and faulted anticline of purple, red and greyish blue Cambrian slate beds with the longitudinal axes of the caverns lying almost normal to the strike of the axial plane of the anticline. The slates are occasionally interbedded with grit bands and intruded by near vertical dolerite dykes up to 7m thick. The geology of the main caverns is shown on the geological plan and section of the caverns.
The stability of the rock, for both the underground excavations and surface rock slopes, is primarily determined by discontinuities of which there are five major types present in the slate beds; these are bedding surfaces, cleavage planes, dyke margins, joints and faults (minor and major) which form discrete blocks within the rock mass. The behaviour of individual blocks (and hence the overall mass) is governed by the properties of these discontinuities i.e. their orientation, spacing, continuity, nature and infill material, occurrence, and by any imposed loads. The cavern layout was dictated by plant requirements coupled with the need to limit certain rock excavation dimensions and to retain pillars and walls in key locations. Additional factors which also had to be considered were the state of stress within the rock mass due to the weight of overlying material, tectonic forces and changes imposed on this state of stress by the excavation of underground openings.
The methods used at
Dinorwig to ensure rock stability include:
| (1) | Careful control of excavation shapes and the methods of excavation. |
| (2) | Installation of rock reinforcement (dowels, bolts and anchors) which both �stitch together� adjacent blocks within the rock mass and, by applying tension to the reinforcing elements, provide a clamping action across joints. |
| (3) | Application of sprayed concrete, both with and without mesh reinforcement, to maintain the integrity of the excavated surface. |
| (4) | Provision of heavy supporting elements such as steel arch ribs or concrete linings. |
| (5) | Provision of drainage holes and channels to prevent the build up of water pressure within open discontinuities. |
Rock support
The influence of bedding and cleavage on the excavation work was considered
to be insignificant and the dyke margins presented difficult conditions
in localised areas. But from the geotechnical aspect, the joints and faults
were, by far, the most important discontinuities, which had a major influence
on the methods of excavation and rock support used in the affected areas.
Three basic types of mechanical rock reinforcement were used to secure
the integrity of rock formations affected by discontinuities and excavation
work:
| (1) | Rock anchors to provide primary reinforcement for overall stability. |
| (2) | Rock bolts to provide secondary reinforcement to secure small/medium-sized blocks near the surface. |
| (3) | Rock dowels to provide tertiary reinforcement to prevent surface loosening and to secure mesh. |
Sprayed concrete (�shotcrete�) was used extensively throughout to maintain the integrity of excavated surfaces. The thickness of shotcrete varies from 25mm to 300mm, applied in layers not exceeding 100mm, with the addition of an accelerator to promote rapid setting of the initial application immediately following excavation. In critical areas the shotcrete is reinforced with a steel mesh of various grades ranging from 2.6mm diameter wires on 70mm centres to 5mm diameter wires on 200mm centres.
The influence of geology also had a major effect on the drilling and blasting techniques that the contractors were able to use during excavation. Problems were encountered in the areas of cavern-to-cavern intersections, which required the designs for drilling and blasting to be compatible with the changing geology.