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If you’re an architect managing a client’s residential project or a homeowner undertaking an extension to your property, chances are you’re going to need a geotechnical report. If you’re excavating into a sloping site, or putting in a new swimming pool, or digging out a new basement level for your house, your structural engineer (that’s us!) is definitely going to need a geotechnical report. The purpose of this article is to outline why and when you’ll need a report; how you and your consultants (plus your builder) can benefit from it; and what to look for. Some of the descriptions and examples we’ll discuss below reference the suburbs and geography of Sydney, but the principles are the same Australia-wide.
On the surface (pun intended!) a geotechnical report might just seem like a ground investigation, but a good geotechnical report is so much more than that. A good geotechnical report won’t just tell you “what’s in the ground”, but it will make recommendations and address aspects relating to the footings; for the types of retaining walls required; which solutions are best suited to your site; how to shore the excavation; how best to construct the necessary building elements; which excavation methods to employ; vibration monitoring information to prevent damage to neighbouring houses; ground water seepage issues; the design data and properties of the underlying soils; and building settlement guidance. Yes, it’s a big list, and different geotechnical consultants charge different levels of fees, depending on their expertise, their capabilities, and what they’re asked to report on.
Some people are tempted to view geotechnical reports as an extravagance or an unnecessary expense. Nothing could be further from the truth! The construction industry is perhaps the best example of “you get what you pay for” and a good, descriptive, and fit-for-purpose geotech report should not be viewed as an expense. In reality, it is an investment in your building project that can pay huge dividends, and save you thousands. For the rest of this article, we’ll focus on houses, but what we’re going to discuss and demonstrate is also directly applicable to commercial, institutional, and industrial projects…
Houses get built from the ground up, so the first thing that needs to be established for any building is its foundations. Depending on where you live, the founding material your house is built on will generally be either sand, clay, or rock. Or your site might encounter a combination of these. You’ll appreciate that the footings of your house need to be designed so that they best respond to and cope with the behaviour of the soil – and so different soil types need different footing solutions. This is something an engineer needs to know immediately at the start of a project, so that s/he can design the size and shape of the footings accordingly. For example:
- Rock is strong and shouldn’t give you any building settlements, and so the footings for a house on rock are typically narrow and shallow.
- Sand is (obviously) significantly weaker than rock, but the material is at least non-reactive and so the footings are wide and shallow.
- Clay is typically a bit stronger than sand, but it can be reactive, meaning that its shape and volume shrinks and swells with changes in moisture conditions as the clay dries out or gets wet respectively. As a result of this, footings in clay could be thinner than they might otherwise have been in sand, but they are typically much deeper, because they have to be stiff enough to handle the rising and falling of the clay as its volume changes in response to seasonal and climatic changes.
Generally speaking, many of the houses in Sydney’s eastern suburbs encounter sand foundations, with rock usually a few metres lower down. However, once you head west of the CBD, the vast majority of Sydney’s houses will encounter clay in one form or another. The option of piering through the clay down to more stable rock below is sometimes an option, but you’ll need a geotechnical report to tell you if the rock is close enough to the surface to be worth chasing.
So – as a general rule – if your house is in the eastern suburbs (including the northern and southern beaches), your engineer needs to know if your house will be founded on sand, or whether rock will be part of the picture.
If your house is on the north shore, inner west, or far west, then it’s not enough to simply assume “clay”. Your engineer will need to know how reactive the clay is, and will require a Site Classification in accordance with AS2870. The three relevant classifications are S, M, and H for Slightly, Moderately, and Highly Reactive respectively, and as the reactivity of the clays increase, so too does the size of the footings.
For example, let’s consider a typical, single storey house with brick walls on a conventional concrete raft slab. If we were building the house on the sands of Sydney’s east, then the rib beams of our raft slab can be just 400mm deep. However, if we’re on clay, then…
- A Class S site, as you might typically find in Erskineville, would have a raft slab with 500mm deep rib beams
- A Class M site, as you might typically find in Wahroonga or Five Dock, would have a raft slab with 625mm deep rib beams
- A Class H site, as you might find around Burwood/Strathfield, would have a raft slab with 750mm deep rib beams.
So, you can see straight away how critical it is to accurately know the quality and properties of the soil you’re building on. It’s also important that your geotechnical engineer is instructed to carry out the appropriate sampling and testing on site so that the Site Classification is accurately established. The situation is illustrated below, which is an extract from AS2870. It shows a typical raft slab layout, with the depth of the rib beams noted “Depth”. Providing the Site Classification is known, the engineer can read off the required depth from the tables in the standard, but the information needs to be known, not guessed.
The consequences of getting it wrong can be disastrous! For example…
- If the clay on your site is only Class M, but the geotechnical consultant or design team conservatively assumes Class H, then your footings might be twice as big as they need to be. This can add tens of thousands of dollars to the construction cost unnecessarily by the time you account for the extra excavation for all the rib beams; the removal of the extra spoil; the extra cubic metres of concrete to be poured; and all the extra reinforcement.
- If the clay on your site is Class H, but the geotechnical consultant or design team assumes/guesses Class M, then your footings will be only half as big as they should be! This is likely to result in significant settlement and rotation of the footings, which can lead to nasty cracking in the brick walls; tearing of flashings and waterproofing; and rupturing of plumbing pipes.
Another good reason to commission a geotechnical report is to reduce the risk of unexpected surprises in the ground. Or, in other words, to give everyone a clear picture of what they’re likely to encounter when it comes to excavations. Swimming pools are a good example: Most pool builders assume that the excavation will be in “natural ground” (i.e. fill, soils, sands, clays) and they exclude rock excavation from their pricing. In the event that rock is discovered unexpectedly during the dig, you can be hit with huge variations and additional costs to excavate out the rock. However, a geotechnical investigation that drills a borehole in the location of the pool will readily identify if rock is close to the surface, and whether it needs to be allowed for. Builders can price for what they know. They can’t price for what they can’t see; they have to make generous assumptions for when they have to guess. A geotechnical report eliminates much of the guesswork.
It’s important when you commission a geotechnical report that the investigation and fieldwork is tailored for your project and the local conditions. Subsurface conditions below the ground are rarely uniform, and merely drilling one borehole or digging a test pit in one location is unlikely to capture an accurate model of the subsurface conditions. For example, the bedrock shelf is rarely flat and might not reflect the same topographic profile as the surface. The sketch below illustrates a typical condition that we frequently encounter with some of the larger houses we design. In the example below, knowing in advance the rock profile at the design stage meant we were able to design and document the piles for the left-hand end of the house where the rock falls away. This meant the piles were priced for by the builder, and we eliminated any variations and costs associated with re-design that would have occurred had the falling rock profile not been realised until after the builder started on site.
The depth that the geotechnical engineer investigates to should also be appropriate to the project. There’s no point drilling or probing to just three metres deep in sands if the building is four storeys high and should be founded on rock that was sitting at four metres below the surface. You need to make sure your geotech consultant is given appropriate instructions and has a good idea of what building works are proposed.
Similarly, the geotechnical engineer should be commissioned to give appropriate advice when it comes to shoring – that is, how to safely hold back the loose soils when you’re doing a large and deep excavation. We’ve seen a few geotech reports that stated that “shoring will be necessary”, but they’ve given no guidance or recommendations on how that might be safely and economically achieved. That’s the difference between a $2,000 geotech report and a $4,000 one. Remember – you get what you pay for.
The final thing we’ll say about ground investigations at this stage is to never assume things are constant and uniform. Just because your next door neighbour did a basement excavation and found rock at one metre down does not mean your site will have the same profile. We’ve seen too many projects where assumptions about the subsurface conditions were made based on “nearby” situations, only to discover that the rock dropped away at the boundary or disappeared altogether. Be sure to obtain information that is specific to your site and your project.
Geotechnical reports aren’t always cheap. But then good advice rarely is.