UNDERPINNING IN CONSTRUCTION BASIC INFORMATION AND TUTORIALS

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 is commenced 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.

CONSTRUCTION SITE SURVEY AND ANALYSIS TIPS AND TECHNIQUES

Site Analysis † prior to purchasing a building site it is essential to conduct a thorough survey to ascertain whether the site characteristics suit the development concept. The following guidance forms a basic checklist:

* Refer to Ordnance Survey maps to determine adjacent features, location, roads, facilities, footpaths and rights of way.

* Conduct a measurement survey to establish site dimensions and levels.

* Observe surface characteristics, i.e. trees, steep slopes, existing buildings, rock outcrops, wells.

* Inquire of local authority whether preservation orders affect the site and if it forms part of a conservation area.

* Investigate subsoil. Use trial holes and borings to determine soil quality and water table level.

* Consider flood potential, possibilities for drainage of water table, capping of springs, filling of ponds, diversion of streams and rivers.

* Consult local utilities providers for underground and overhead services, proximity to site and whether they cross the site.

* Note suspicious factors such as filled ground, cracks in the ground, subsidence due to mining and any cracks in existing buildings.

* Regard neighbourhood scale and character of buildings with respect to proposed new development.

* Decide on best location for building (if space permits) with regard to `cut and fill', land slope, exposure to sun and prevailing conditions, practical use and access.

Site Investigation For New Works ~ the basic objective of this form of site investigation is to collect systematically and record all the necessary data which will be needed or will help in the design and construction processes of the proposed work.

The collected data should be presented in the form of fully annotated and dimensioned plans and sections. Anything on adjacent sites which may affect the proposed works or conversely anything appertaining to the proposed works which may affect an adjacent site should also be recorded.