Using Topographic Maps
What was the area like before a house was built on it? That’s the question you need to ask to help determine the cause of slab heave.
Topographic photos can be sourced from local councils and websites. The information available depends on the resolution of the photos and the history of aerial surveys in the area.
Sources of Topographic Maps
Try these sources to find historic aerial photos of your investigation site:
- Google Maps
- Google Earth
- Local council mapping department or engineering department.
- Department of Natural Resources (Qld) or equivalent.
Google Maps will allow you to zoom on to a property and get a feel for the local topography. In city areas, the maps are updated frequently so it is unlikely you will find the type of historic topographic information that will help you solve a slab heave issue. However it is still a useful source to help you identify the overall topography of a site.
Google Earth has an excellent timeline feature to help you find historic aerial photos of sites – particularly house sites in city areas. Use this tutorial to learn how to view historical aerial photography in Google Earth.
Local Councils and Department of Natural Resources
Mapping Departments in local councils and government bodies may hold and sell historic aerial photography. Much of this data has now been scanned and digitised however smaller regional councils may still have maps hanging or laying in map drawers.
Don’t forget to ask if anyone remembers the area around your site before it was built. Local knowledge could fill in your missing gap of information.
What to Look For
You are looking for terrain features that have been covered up by the construction process such as:
- Water course (creeks and rivers)
- sloping land
- Large trees
- previous uses (rubbish tip, gravel pit, hardstand,)
Each of these features has a different effect on ground slabs. f you find an unusual terrain feature, then the treatment of that feature may become a prime part of your investigation into slab heave.
For example, if the land was recorded as sloping and the house allotment is flat, then determine check that the soil report records the presence of fill and then determine if that fill has been properly placed and prepared.
How was the Feature Treated
Removal of water courses presents a reasl concern in many subdivisions. Its difficult to capture and divert water courses. Some undetected underground flow might still be tracking along the original watercourse, Use this information to guide further geotechnical testing or relate the information back to the locations and levels of house damage.