Totnes Hydro Scheme – Construction Progress Summary
Previous Week (22 June – present)
We encountered a couple of issues early in the week which has resulted in a one weeks delay to the construction program. The main issue was finding an outcrop of very hard limestone which extended into the working area. The rock has been trimmed back as far as possible with the breaker we have on site, to remove it in entirety would mean bringing in a larger breaker which would cause further delays. To overcome the issue we have revisited the design of the lower section and have introduced a step into the concrete base. This allows the slab to extend over the rock outcrop. Further design work has allowed us to regain the weeks delay. The powerhouse roof will now be entirely fabricated off site and lifted into position in four sections.
Further Cofferdam Works
Preparation of formation for bearing pile rig
- A large concrete beam was cast along the base of the cofferdam piles to provide a better seal against the rising tide
- Once the lower area has been excavated down to approximately 400 – 500mm below formation level, single sized stone will be placed over the base of the foundation and compacted using the large machine driving up and down. This will provide a stable uniform surface for the bearing pile rig to operate,
- Setting out of bearing piles (16 No. max)
Next Week (29 June – 3 July)
Installation of bearing piles
- Mobilisation of piling equipment. This will involve bringing all the equipment to site via the Dairy Crest site,
- Piling operation. A maximum number of 16 bearing piles are required to support the dead and imposed loads from the Archimedes screws and concrete support structure. The piles are strategically located along the downstream edge where the bedrock dips down to approximately 3 metres below the concrete support structure. The piles are installed using a mini pile drill rig (approx. 4.5 tonnes). The rig drills down into the underlying material and into the bedrock simultaneously inserting a casing which stops all the soft material entering the hole. When the required rock socket has been drilled the drill head is withdrawn. Grout (water and cement mix) is poured into the hole followed by a steel rod. Once the grout has cured the pile is complete,
- The operation will be carried out sequentially. The noisier drilling element will be carried out first for all the piles (perhaps 3 days), followed by grouting and insertion of the steel bars.