Strip and trench fill foundations

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Excavations for strip and trench fill foundations shall:

  1. take account of the design dimensions
  2. take account of localised effects
  3. be compact, reasonably dry, even and correctly shaped.

Design dimensions

Inaccuracy may prevent walls and piers from being located centrally and therefore result in eccentric loading of foundations and possible foundation failure.

Excess excavation should be avoided. Accurate trench digging is particularly important where the width of the foundation is only slightly wider than the wall to be supported.

Acceptance from the foundation designer is required where the foundation design is modified.

Localised effects

At soft spots, excavations should be deepened to a sound bottom or the concrete should be reinforced. Hard spots should be removed.

Where roots are visible at the bottom or sides of trenches, especially in clay soils, excavations may need to be taken deeper, or special precautions determined by an engineer in accordance with Technical Requirement R5.

Compact, reasonably dry, even and correctly shaped

Unless otherwise designed by an engineer in accordance with Technical Requirement R5:

  • trench bottoms should be horizontal, with all loose material removed
  • trench sides and steps should be, as near as possible, vertical.

Trench bottoms affected by rain water, ground water or drying should be rebottomed to form a sound surface.