Thursday, November 26, 2015

Underpinning - General Precautions

Underpinning the main objective of most underpinning work is to transfer the load carried by a foundation from its existing bearing level to a new level at a lower depth. Underpinning techniques can also be used to replace an existing weak foundation. An underpinning operation may be necessary for one or more of the following reasons:

1. Uneven Settlement this could be caused by uneven loading of the building, unequal resistance of the soil action of tree roots or cohesive soil settlement.

2. Increase in Loading this could be due to the addition of an
extra storey or an increase in imposed loadings such as that
which may occur with a change of use.

3. Lowering of Adjacent Ground usually required when
constructing a basement adjacent to existing foundations.

General Precautions before any form of underpinning work iscommenced the following precautions should be taken:

1. Notify adjoining owners of proposed works giving full details and temporary shoring or tying.

2. Carry out a detailed survey of the site, the building to be underpinned and of any other adjoining or adjacent building or structures. A careful record of any defects found should be made and where possible agreed with the adjoining owner(s) before being lodged in a safe place.

3. Indicators or `tell tales' should be fixed over existing cracks so that any subsequent movements can be noted and monitored.

4. If settlement is the reason for the underpinning works a thorough investigation should be carried out to establish the cause and any necessary remedial work put in hand before any underpinning works are started.

5. Before any underpinning work is started the loads on the building to be underpinned should be reduced as much as possible by removing the imposed loads from the floors and installing any props and/or shoring which is required.

6. Any services which are in the vicinity of the proposed underpinning works should be identified, traced, carefully exposed, supported and protected as necessary.

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