The geotechnical (soil) report is probably the most underrated consultant report of the townhouse or unit development design process. While these reports have a small overall cost ($300-$500), their impact on the final design could save you tens of thousands of dollars.
The soil report is probably the least understood and most underrated part of the townhouse or unit development design process. While most novice developers treat it as a compliance issue, the findings of soil reports can vary depending on a range of factors from how familiar a geotechnical engineer is with the area, to how they decide certain types of trees will effect a site.
These recommendations have a flow on effect to the structural design of your development’s foundations and could cost you $30,000 or more in additional foundation design.
Why Soil Matters
Why do soil conditions matter? Just ask the designers of the Leaning Tower of Pisa! While most sites are covered with a layer of top soil, what lies beneath could be anything from sand, to silt, to clay, or rock.
Each type of different underlying condition comes with it different material characteristics and reactivities. On top of that, each particular site may have specific issues such as buried boulders or perched groundwater tables.
The main issue with soil conditions in most townhouse and unit developments is movement. Soils are made up of little grains that are stuck to each other at various levels of cohesion. Depending on environmental conditions (e.g. drought and rain) these soils will react to varying degrees. This reaction will cause the ground on your site to heave and depress to different levels. That’s right, the ground under your house is moving all the time.
Your structural engineer will design a slab and foundations that are strong enough to bridge the gap in these movements. The larger the anticipated movement, the stronger the foundations will need to be.
Foundations that are under-designed will allow the framing above the ground to move more than it is designed to and can often result in unsightly cracks in brickwork, plasterboard, and in extreme cases, serious structural damage.
The ABCs of Site Classification
The Australian Standards help to simplify the design for soils based on seasonal movement by assigning a letter classification to a site.
What we are most interested in is the characteristic movement that the site could experience on a yearly basis. For example, an M-class site can be expected to have local depressions of 20-40mm. This means that the structural slab on an M-class site needs to be strong enough to sustain pockets of movement of between 20-40mm on a yearly basis, and so on and so forth for each of the other classification types.
Now is probably a good time to mention the dreaded P-class site. We hear a lot of complaints from developers and builders when they find out that a site has been classified P-class. This is likely because the P stands for ‘Problem’ and is sometimes associated with more elaborate (and therefore more costly) slab designs. However, this is not always the case. Sometimes, all it means is that there is an existing house on the site that, when removed, will have an impact on the surrounding soils.
A $30,000 Saving
Your structural engineer will rely on the commissioned soil report to determine the size and extent of any basement, foundations and slab. For a standard three townhouse development, the cost difference between raft slabs of different classifications is around $2,000. The cost of bored piers can be in the order of $10,000-$12,000.
In one instance we had a developer come to us with a design that he suspected was too conservative. After looking at the structural design we found that it had been designed in accordance with the soil report. However, having designed other similar developments in the area we were surprised at those recommendations.
At our recommendation, the developer commissioned a new soil report from one of our preferred contractors and we re-designed the foundations based on these revised recommendations. Our new design had significantly smaller foundations and fewer bored piers, with the builder reporting back that the new design was a saving of over $30,000!
Low Cost, High Impact
Without a full understanding of how a soil report can impact the overall design of a project, it’s easy to understand why some developers would shop around for the lowest cost soil report.
However, invariably, the soil report is the most underrated consultant report of the design process as a $100 saving in consultant fees can end up costing you tens of thousands of dollars.
At Tingmore Structures, we maintain strong working relationships with a panel of knowledgeable and experienced geotechnical consultants that can provide you with prudent but not overly conservative soil test reports.
If you are looking for structural and drainage design engineers for your next unit, townhouse or multi-dwelling development, please get in contact with us so we can provide you with a comprehensive fee proposal for our services. We look forward to partnering with you on your next project.
