base & beam underpinning - a case study

Castle Lane, Eynesford, Kent

This project was originally specified as a mass concrete underpinning scheme with a required depth of 3.0 metres below local external ground level and transition bases being cast at the extremities of the scheme. However, once the work was started, it soon became obvious that the extra-ordinarily high water table would prohibit this scheme going ahead as planned and we were asked to consider alternative schemes.

Initially a piled solution was generally considered to be the most appropriate although as access was available from one side only, this really limited the solution to that of a canti-levered pile system. On further consideration, our Engineers felt that the ground conditions would not be appropriate for the provision of tension piles and we therefore put forward a base and beam underpinning solution for approval.

Clearly the high water table would still be a problem although by providing a series of remote well points, adequate de-watering was achieved although this had to be strictly controlled so as to avoid removing any of the fines from the ground. This allowed all of the bases to be excavated and concreted (after approval) in one operation and all that remained was the construction of a ground beam. This was placed beneath the existing foundation as this was at a relatively shallow depth. In some circumstances, it is possible to construct such a beam in the brickwork above the existing footing, but this is dependent on relative levels.

This project, despite the difficulties encountered, was successfully completed with a minimum contract over-run and to the entire satisfaction of both the client and the Engineer.

Base and beam Underpinning Schemes should be used when: -

• The imposed loads are relatively heavy allowing larger spans for the simply supported beams
• The ground conditions are such that the bases have to be relatively deep.
• Access is available one side only
• When a simple to construct and less technical option is preferred and to minimise costings

If you are considering specifying an underpinning scheme and are unsure of which option or underpinning system to utilise, please do not hesitate to contact one of our estimators or Engineers who would be more than pleased to provide any advice or budgetary options for your further consideration.

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