It’s now been just over two weeks since the decision was taken to re-flood the southern end of Model Boating Pond, due to a concern over the stability of the sheet pile dam. During regular inspections of the temporary structure, it became apparent that some of the piles in the middle section were leaning more than was expected. After the water pressure was equalized, the sheet piles stopped moving and in the last two weeks BAM Nuttall have been carrying out investigations. A Mackintosh probe was used to take samples along the length of the sheet metal dam. This probe works well in soft soils and is able to determine the silt depths with accuracy.
Using this method, BAM Nuttall have been able to determine the silt in the middle of the pond is deeper than previous studies (bathymetric and boreholing) have shown. As a result, the sheet piles in the center of the dam have not had enough resistance from stable clay to hold them in place.
The solution to this problem will be to prop the middle section of piles from the downstream side using 15 metre H section piles. This prop will support and stabilise the piles in the middle of the dam.
The H section piles will be installed using a crane on pontoons. At first, they will be vibrated into place but when they hit hard bed level, they will then be hammered in. The crane will be arriving on site in the week commencing 7 September and we expect the work to take around two weeks.
This issue has caused delay to the work at Model Boating Pond, which is around 12 weeks behind. The anticipated finish date at this pond is now late summer 2016. There should not be delay to the 18 month Ponds Project programme as a whole however, which is still scheduled to finish in October 2016.