Information sourced from Queensland Building & Construction Commission.
Sections covered in this blog are :-
Home owners guide to planning landscaping and maintenance of foundations
Structural damage can result from movement in clay soils caused by varying moisture conditions around the perimeter of homes. The majority of Queensland homes are situated on what are termed reactive clay soils. These soils are subject to expansion and contraction depending on seasonal weather and site conditions. Sandy sites and rocky terrain are usually not prone to this expansion and contraction. In Queensland, engineers design footings to the Australian Standard AS2870 – 2011 Residential slabs and footings. Footings correctly designed to this standard are intended to accommodate the expected movements caused by seasonal volume changes in the soil (swelling when wet and shrinking when dry) - under normal conditions. The performance of footings under this standard requires normal conditions to be maintained around the house. This guideline identifies abnormal conditions that should be avoided and/or corrected. Dry seasons cause soils to dry out rapidly. Wet seasons cause soils to become wet quickly. This results in sudden extreme volume changes and movement in soil. Abnormal conditions (other than seasonal changes), may include the effect of trees, poor surface drainage and/or leaking plumbing. If not allowed for in the engineer’s design, these types of abnormal conditions may lead to movement and damage varying from minor to extreme.
How much can the soils under my house move?
The amount of movement that may occur depends on the site classification, which is defined under the Australian Standard AS2870. The relevant classifications and expected movements under normal site conditions and seasonal influences are
Description of type of clay and reactivity
Expected range of movement
Mostly sand and rock with little or no ground movement from moisture changes.
Slightly reactive clays that do not present significant trouble. Very limited damage could be expected in the life of the building.
0 to 20mm
Moderately reactive clays that may cause minor movement and damage in the life of the building. Reasonable care is required in planning the site
20 to 40mm
H - (H1 & H2)
Highly reactive clays have potential to move more, and react to variable moisture change conditions. Some minor damage may occur in the life of the building under normal conditions. More significant damage may occur where site maintenance conditions are a problem e.g. influence of trees or leaking underground plumbing or poor drainage. Particular care is required in planning the site
H1 40 to 60mm H2 60 to 75mm
Extremely reactive clays have the potential to react significantly to any variable moisture changes in the foundation clays and require significant attention to detail in planning the site works. Extreme movement and damage may occur if the site conditions and foundation maintenance requirements are not observed. Footing systems and site conditions on “E” sites require very detailed specification from an engineer
>75mm (Note: Movements on E sites have been known to move up to and in excess of 100 to 150mm in SE Qld)
Is it normal to expect cracking to occur in brickwork, walls and ceilings?
Yes, damage in varying degrees can be expected in the life of the building depending on the relevant site classification. If cracking becomes apparent the site maintenance conditions should be checked as noted under “key points to consider...” Corrective action should be carried out immediately and may include regrading surface drainage, moving gardens and trees or repairing leaks in water supply, stormwater and/or sewer drainage. A sound plan for a reactive clay site is to provide a consistent moisture regime around the building by installing paths and patios against the house. Locate lawns up against paths & patios. Garden beds, the most heavily watered parts of a garden, should be kept well away from the house. If gardens must be placed in close proximity to the house, they should be sealed with plastic and contain only a few small plants. Take care not to trap water against the building if using garden edging. Caution: Care should also be taken if placing filling against the house. Always ensure weep holes are not covered and that existing Termite Management Systems are not compromised
Key points to consider when planning landscaping to avoid structural damage.
• Plan type and location of gardens, paths, driveways, lawns, filling and retaining walls
• Take care in selection of trees and shrubs. Do not over plant next to the house
• Keep trees with high water demand well away from buildings in reactive clay areas .
• Avoid variable conditions around the house and maintain adequate moisture/watering. Do not over water and avoid the use of unregulated sprinkler systems.
• Locate ponds and water features away from the house.
• Direct surface water away from the house. Do not allow water to be trapped or pond near the house.
• Repair leaking pipes and taps. Note: these issues should be considered as part of planning and maintaining the home. Aim to provide a consistent moisture regime around the house. This will minimise soil moisture variations that may cause movement and result in structural damage.
Summary of owner responsibilities for houses under warranty
1. Maintain the site drainage at all times.
2.Do not alter the site drainage provided by the builder. Any changes to the site drainage should ensure that water will be directed away from the building and not pond adjacent to the footing and slab system. Care should also be taken to avoid directing water flow to adjoining properties.
3.Where possible on reactive clay sites (Type M,H,E Classifications), avoid placing gardens or installing garden edging, gravel pavements etc next to the building. This may cause water retention and/or promote a greater variation in moisture conditions around the building.
4.Installation of sprinkler systems next to dwellings on highly and extremely reactive sites (Type H & E Classifications) should be avoided. Adequate and consistent watering only is recommended. Over watering should be avoided.
5.Do not plant trees within a distance from a building that equates to their mature height. Always plant in accordance with the requirements for the relevant site classification.
6.Regularly check and maintain plumbing, drainage and stormwater systems by immediately carrying out repairs to leakages or breakages when observed (usually displayed by seepage and/or greener lawns etc.), or when minor damage or cracking exceeding 3-5mm appears in walls or ceilings. By observing these requirements, movement and damage which may be expected in the life of the building can be minimised and maintained within normal performance requirements