Holdfast Training Services Trial Blog

There has been some slow progress on site. Last week came to a grinding stop, as a conflict between the four parties (client, designer, contractor and subcontractor) forced all excavation to cease.

The access shaft is broken down into a number of components. The Secant piles have been designed under the contract C123, by Jacobs. The remainder of the shaft has been designed under the contract C121 by Mott Macdonald. This has caused several issues as there appears to have been very limited communication between the two designers.

Jacobs insisted that once the Secant piles had been driven into place, propping would be necessary during the excavation phase. Additionally they insisted that the shaft was excavated in quadrants, sprayed with a regulating layer before moving onto the next quadrant.

However Mott Macdonald, who is the designers for the Sprayed Concrete Lining (which covers the Secant piles), insisted that the shaft did not require propping. BFK, who have the construction contract (C300) to build the shaft, decided to excavate the shaft without propping. This was agreed by CRL and Mott Macdonald. However after 8m of excavation the project was stopped as Jacobs had not agreed with the decision not to prop the shaft. The site subsequently fell silent for a week until CRL finalised an agreement between both its designers.

As a result BFK have been able to continue the further 2m to the required Ring-beam depth. However this has forced Barhale (sub-contractor) and BFK (contractor) to issue a compensation notice.

The ordeal has not stopped there. We have now got to the Ring-beam depth of 10m and the next phase is to pour the concrete Ring-beam. This process involves excavating the shingle away from the bottom of the piles and behind the piles in order to create a cavity for the Ring-beam to be poured. Again the sub-contractors were under the impression they could construct the Ring-beam in one go. However the reinforcement bars from the piles does not allow for excavation of the shingle, as they protrude into the shingle. They had planned to cut the reinforcement bars away however the bars are required to provide shear reinforcement between the Secant Piles and the Ring-beam. Additionally the reinforcement bars that protrude from the piles and eventually into the Ring-beam are required to suspend the Ring-beam when the next phase of excavation takes place (Stage 6 in the sketch).

So it is now back with the designers, because if the sub-contractors have to cut each reinforcement bar in each pile, then attach them again it will take 3 weeks instead of 1 to construct the Ring-beam. The governing factor in my opinion will be how many bars will be required for the Shear reinforcement between the Secant Piles and the Ring-beam. The issue of suspending the Ring-beam in ‘stage 6’ of the excavation can be done by increasing the embedment depth or increasing the strength of the concrete (currently designed at 35N at 3 days), A decision needs to be made today or the project will go on hold again!!!!!

On another theme, I will be meeting Chris Sexton (Ex-RE, now Tech director of CRL) and the CGS on the 26th June. I’m sure to have a great time.