MANUAL DESIGN & CONCRETE DETAILING BOOK DOWNLOADS
MANUAL DESIGN & CONCRETE DETAILING BOOK DOWNLOADS
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The specified formation level will normally be below the zone of cyclical shrinkage and swelling due to seasonal rainfall, and taking into account the significant effects of trees, shrubs and hedges. Site survey drawings normally indicate all past and existing trees and hedgerows. Account of these should have been taken in the design of foundations. Trees, stumps and root systems of dead trees can occur outside the site boundary, but close enough to affect new foundations. If old stumps or root systems are discovered or if there are any doubts about the possible effects of trees and hedgerows over the site boundary, the engineer should be consulted before proceeding.
The required depth of foundations as a safeguard against damaging movement caused by swelling or shrinking of clay soils in open ground or where influenced by growing or cut-down trees depends on three factors:
• the water demand of the clay
• the long-term rainfall conditions (climatic zone)
• the type and mature height of the tree or shrub.
The NHBC Standards, Chapter 4.2: ‘Building near trees’, provides detailed guidance on foundation depth requirements taking the above factors into account. Formation surfaces in clay should be protected from the softening effects of rain and drying shrinkage until inspected, then immediately blinded with 50 mm of 10 N/mm concrete, or otherwise cover in accordance with the specified requirements. Remove any soil inadvertently softened before blinding.
See Fig. 8.4a and b. Delay in concreting a strip foundation may cause problems, particularly in clay. Where compressible material or void formers are provided to the sides of trenches to absorb horizontal clay swelling they must be set truly vertical and secured against floating in the wet concrete.
Shoring and underpinning
This is required:
• to support a structure which is sinking or tilting due to ground subsidence or superstructure instability
• as a safeguard against the possible settlement of a structure when excavating close to and below its foundation level. The underpinning of a structure is required for the reasons given above and, in addition, to enable the foundations to be deepened for structural reasons, e.g. to construct a basement beneath a building.
t is also used:
• to increase the width of a foundation to permit heavier loads to be carried, e.g. when increasing the storey height of a building
• to enable a building to be moved bodily to a new site. Shoring and underpinning are highly skilled operations and must always be undertaken under the direction of the engineer responsible for designing the underpinning scheme. Sometimes specialist subcontractors are engaged.
The engineer must always produce details and sequencing of underpinning work and these must not be varied without his prior agreement.
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