Cray Valley Foul Sewer, Kent

 

Client: J Murphy & Sons

The Cray Valley Foul Sewer was constructed by J Murphy & Sons (JMS) for Thames Water. JMS were using an Iseki ‘Unclemole’ tunnelling machine which was specifically designed to cope with crushing the anticipated coarse river gravels and chalk with flints which had been predicted from the site investigation.

On three occasions, the Unclemole became stuck and was unable to advance. JMS had to sink rescue shafts in order to free the machine, recover and refurbish it before the tunnel drive could be resumed. After the second blockage, JMS employed AGE to assist in determining the cause of the problem.

AGE logged the excavation of manhole shafts while tunnel construction continued. Many flint boulders were noted in the excavation; their presence had not been predicted from the site investigation. The presence of the boulders was carefully recorded and samples were taken and subjected to a programme of testing to determine their Unconfined Compressive Strength, Point Load Strength Index, Brazilian Disc Tensile Strength and Cerchar Abrasivity.

While this programme of logging and testing was continuing, the Unclemole became stuck for a third time. AGE logged the rescue shaft excavation from the surface and examined material in the face of the TBM as it was exposed.

It was clear that the ground included clusters of large, boulder-sized flints that had been the cause of the problem. The TBM was simply unable to crush such a large quantity of very hard particles at one time and the head became blocked.

The presence of these clusters of coarse particles was considered to be due to peri-glacial solifluction activity not shown on the local geological map, which had indicated that the site was underlain by River Gravels over the Upper Chalk.

AGE’s work enabled JMS to agree revised terms for the tunnel construction and the contract was completed without further difficulty.