A trunk foul sewer at Rue de Maupertuis on Jersey’s east coast collapsed causing a large depression to open up in the road in a densely populated urban area, posing a danger to property.
The sewer had originally been installed at a depth of 4 metres through farmland, prior to urban development in marshland, with a very high water table and an overburden of some 8m of fine alluvial deposits overlaying granite rock head.
Previous deep excavations in this area had been extremely problematic and demanded significant temporary works.
Brenwal were awarded the repair of the sewer by the States of Jersey Transport and Technical Services Department (TTS) on the basis of Brenwal’s proven ability to successfully undertake deep excavations in challenging ground conditions.
TTS had commissioned geotechnical reports that recommended that secant piled walls be installed either side of the 30m long sewer collapse in order to form a long cofferdam within which the sewer could be re-laid. It was considered that this was the only solution that could be installed due to the close proximity of houses and the risk associated with other, tradition methods involving driven sheet piles.
Brenwal offered an alternative to secant piles in the form of 9 meter long interlocking sheet piles installed completely without vibration by means of a pile press. This method provided a safe and effective method of ground support and prevented formation heave in the saturated fine sands and gravels.
The piles were installed in 10 days including mobilisation and demobilisation from the mainland. The sewer was replaced with a ductile iron pipe and the trench was completely backfilled with foam concrete.
Under the contract, the temporary works were installed, the sewer replaced and the road and surrounding damage to boundary walls was completely reinstated in a period of 4 weeks.
The installation of the sheet piles was carried out using a non-vibration method with a Giken silent piler along each side of the trench line pushed to a depth of 9 meters.
The excavation of the trench was carried out in sections of 7 meters long with the pipe installed on a concrete bed and full surround. The trench was backfilled with foam concrete to underside of tarmac level. The boundary wall was rebuilt and kerbs re-laid. The road was then resurfaced and reopened.