River Crossing

In Situ carried out an over-water based ground investigation for the client. The aim of this investigation was to determine the ground conditions on the river bed, and the locations for the proposed bridge foundations.

For this project, the 3.5 tonne mini-crawler CPT unit was used; this was the appropriate rig to use for the project. It was mounted onto a floating pontoon which was held in position by a pair of spud legs. The rig was secured to the pontoon to give the initial reaction required to carry out the tests; 50 CPTs were carried out in total over a 4 week period. The depth of these tests ranged from 6-17 metres, depending on the depth of alluvium. Dissipations were also carried out at some of the locations to look at settlement characteristics. The mini CPT rig proved to be the perfect solution for working on a platform with limited space due to its compactness.

Problems were encountered on this project when working in the centre of the river and especially at high tide; this cause a 10m gap between the deck and the river bed. This lack of support was still evident and a problem even when casing was put in place. Also during the periods when the river current was very strong, the cone deviated a significant amount before penetrating the ground. In Situ overcame this problem by programming the tests at the centre of the river during periods of slack water at low tide.