Problem Sites and Footings Performance Simplified
For builders & owners.
Most common onsite issues:
– Fill materials on site
– Existing building to be removed
– Trees on-site or adjacent properties
– Site cut deeper than 500 mm
Fill material
1) Loose materials on-site exceeds 600-800 mm of sand or 300-400 mm of clay. Suspend slab will be required with bored piers or piles system. Strip footing or pads must penetrate through the fill to reach the clay
2) Compacted materials under level 1 supervision with compaction report during subdivision construction.
From experience, fill compaction lacks consistency.
Stiffened raft to span over the soft spot.
We recommend bored piers under.
Existing building removed
• Dry soil under the building will heave due to exposure and change of allotment landscape
• An allowance must be made in the design of the new footing to resist additional loading created by the clay expansion ( Heave ) under the removed building
• Stump holes must be filled with clean compacted nonreactive soil
• Deepening of footings when crossing existing and service trenches
Trees on-site or adjacent properties
• Trees absorbs moisture from the soil causing shrinkage and settlement of footings
• Different trees have different moisture needs
• Distance from the buildings is crucial to keep trees away from buildings at least equivalent to the mature height of the tree
• Consult an arborist to comment on existing trees
• Cutting a tree will result in soil heave for 12 months, moisture going back into the clay soil
• An allowance must be made in the footing design to resist additional movement created by the trees growing or removal
Site Cut Deeper Than 500mm
May expose dryer soil to moisture, increasing the soil movement.
What is a problem site and how does it change the footing design?
- Standard class m site
- Site cut of 1500 mm
- Existing house removed
- Trees along the boundary
- Estimated surface movement for standard class m site 40 mm
- Estimated additional surface movement for site cut 20 mm
- Estimated additional surface movement for trees 20 mm
- Removal of the existing house 20 mm
- Total surface movement 100 mm against 40 mm from site
- Edge beam for m site brick veneer is 400 m deep with one layer of reinforcement
- The edge beam for the p problem site will be 800 to 1000 mm with two layers of reinforcement slab that must be stronger to withstand additional extreme soil movement