Tremie Concrete Techniques Turn an Unstable Situation into a Strong Foundation

04.01.16   Robert C. Rabeler, PE | More by this Author

Tremie Concrete Techniques Turn an Unstable Situation into a Strong Foundation

A new pump station was constructed near a creek in a low-lying part of town. The groundwater level was high – just a few feet below the surface, and steel sheeting was being used for temporary earth retention. Dewatering wells were installed within and adjacent to the sheeting to dewater the excavation to allow for placement of the concrete foundation slab in dry conditions.

Temporary earth retention and dewatering

During construction, while excavating and dewatering, the contractor noticed that the water level within the sheeting was not being lowered sufficiently and sand boils were developing, making the excavation unstable. The contractor stopped work, and sought advice from SME on how to proceed.

Sand boils

After reviewing soil boring logs along with sheeting and dewatering well depths, SME identified the cause of the problem. The sheeting was not deep enough to cut off the water, and the dewatering wells were not installed deep enough to adequately lower the water level within the sheeting. As a result, water pressure was creating a “quicksand” effect within the sheeted excavation – a very unstable condition.

Once the problem was understood, SME’s team looked for a reasonable solution to solve the problem. Extending the sheeting deeper would have helped, but would have been costly and not practical. Extending the wells deeper was not practical either, and would have delayed the project and would have been costly to install.

SME concluded that tremie concrete techniques without further dewatering would quickly solve the problem. Tremie concrete was placed below the water in two lifts using a concrete pump to establish the base for the station. Once the concrete mat-type foundation/slab was poured and cured, the water was removed from within the excavation and the pump station was constructed. This solution eliminated construction delays and did not increase the construction costs.

Concrete being placed below the water using a concrete pump

For more information contact Bob Rabeler.


TAGS: Geotechnical

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