traditional underpinning - a case study

Shelley Grove, Loughton, Essex

We were instructed by the client's Engineer to carry out Traditional or Mass Concrete Underpinning to the right hand wall of the above named property to arrest ongoing settlement of these walls caused by insufficiently deep existing foundations bearing upon highly plastic and desiccated clays infiltrated by live tree roots from adjacent trees.

From the site investigation, it was necessary to extend the existing foundations to a depth of 3 metres below local ground level, onto a stratum beyond the zone of tree root activity. At this depth, and because the Engineer required to use this scheme as a preventative root barrier, Mass Concrete Underpinning was the preferred option. Additionally, as it is only necessary to access these works from one side of the wall only, by installing dust-proof and security screens, it was not necessary to move the occupier into temporary alternative accommodation. Consequently, this cost saving was one of the parameters the Engineer used to specify this option of underpinning.

Adjacent underpinning bays were connected utilising shear keys as opposed to steel dowel bars as these, whilst having the same technical effect, are far less prone to injure the operative constructing the adjacent base. Additionally, the Engineer specified a sequence of working, incorporating curing times of the work's phases, to ensure that these operations did not at any time affect the stability of the property.

These works were completed on programme and on budget and this system of working caused least disturbance to the occupiers of the property.

Mass Concrete Underpinning Schemes should be used when: -

• The imposed loads are heavy
• The ground conditions are such that the bases do not have to be too deep (< 3.50 - 4.00 metres)
• A continuous barrier to tree roots is required to prevent their future growth affecting other foundations
• Access is available one side only
• When a simple to construct and less technical option is preferred
• To form deep perimeter retaining walls when constructing a basement

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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