Depth to bedrock: how to read rock risk before you price the excavation

Rock is the variation nobody argues about and everybody pays for. The pier holes stop short, the excavator swaps to a hammer, the pool shell becomes a rock cut, and a week disappears out of the program. What makes it worse is that rock rarely announces itself: it turns up as refusal in one hole and clay to the toolbox depth in the next, ten metres away.

You cannot know the founding level from a desk. You can know, before anyone mobilises, whether shallow rock is a live risk on this ground, how variable it is likely to be, and what to ask the driller to find out. Here is the sequence.

Step 1: Ask what the rock is, not whether there is rock

Start with the mapped geological unit under the lot, because the parent material governs everything downstream. Sedimentary sequences, metamorphic units such as the phyllites of southwest Brisbane, coastal sands and volcanic units all behave differently in an excavation, and the reputation of a suburb is a poor substitute for the unit.

The gotcha: a suburb often sits across two units. If a mapped boundary crosses the site, expect the excavation to change character somewhere on the site, and expect the two halves to need different assumptions.

Step 2: Read the weathering profile, not a single depth

"Depth to rock" implies a surface. Weathered profiles do not work that way. Material grades from residual soil formed in place through progressively fresher rock, and the depth at which a bucket or auger stops depends on which grade sits under that spot.

How variable that can be is well documented. Working on the Precambrian rocks of Western Australia's Darling Range, Bulley and Masterson (2003) describe profiles where depth to bedrock swings from surface outcrop to weathered material extending more than 20 metres down, with corestones and floaters, isolated hard blocks left within the weathered material, producing uneven excavation and refusal depths. The principle travels: the same corestone problem shows up in weathered granites and dolerites wherever they occur.

The transition can also be abrupt rather than gradual. Marques and Williams (2015) studied the Bunya Phyllite in southwest Brisbane and found the zone between weathered rock grades to be very thin, with sharp contacts between them, driven by the structure of the rock mass itself. On that kind of ground, an extra half metre of digging can move you from something a bucket handles to something it does not.

In South East Queensland more broadly, Priddle et al. (2013) note a review of regional piling data suggesting extremely weathered rock is commonly met somewhere in the 10 to 15 metre range, which is deep for a house footing and shallow for a tower or a piled basement. Which of those you are is the question that decides how much the number matters.

Step 3: Use nearby borehole logs for refusal, and treat disagreement as the finding

Registered bore logs and geotechnical records near the site are the closest thing to ground truth available before drilling. Read them for three things: the depth at which each log reports refusal or rock, the material logged immediately above it, and how well the logs agree with each other.

Agreement is a result. If four logs within a few hundred metres all refuse between 2 and 3 metres, that is a defensible expectation. If one refuses at 1 metre and another reaches 12 metres in clay, the variability is the answer, and it should be priced or investigated rather than averaged away. The same records usually carry standing water levels, which is why they are worth reading properly; we covered how in reading groundwater and bore records.

Step 4: Look for shallow-rock signatures in the terrain

Terrain gives away rock that is close to the surface. Short steep sections in an otherwise even slope, benches and breaks that follow a bedding direction, an unusually thin soil cover on ridges and spurs, and outcrop in the road cut down the street are all indicators.

Historical aerial imagery adds the human evidence: an old quarry face, a driveway cut into rock, a batter that stands up steeply without retaining. Ground that stands unsupported at a steep angle for decades is not doing that in soil.

Step 5: Watch the opposite trap, where refusal never arrives

The mirror image of shallow rock costs just as much. Delaney (2005), reviewing the Newcastle region, describes both ends of it: some volcanic units in the area occur as solid rock masses close to the surface, hard enough that a sizeable cut may need drilling and blasting rather than machine excavation, while the region's drowned river valleys were cut deep and later filled with sediment, so depth to bedrock in the estuaries varies enormously. The same paper notes piles refusing prematurely on hard layers within sand profiles, short of their target depth.

Coastal and estuarine ground across Queensland, New South Wales and Victoria carries that same double risk: rock too shallow to dig, or so deep that the founding problem becomes soft ground instead.

Step 6: Turn it into a drilling brief and a tender qualification

The desktop work is finished when it has changed two documents.

The drilling brief should now say where the holes go and how deep, based on where the ground is expected to change, whether coring is needed if rock is likely to be met in the founding zone, and whether the investigation must prove the depth of a hard layer or just its presence. Placement is worth more than count, which is the point behind our six myths about borehole numbers and depth.

The tender or contract should say what the price assumes. An excavation rate that assumes soil to a given depth, a stated assumption about rock being encountered, and a clear position on who carries the risk if refusal comes higher than assumed. Contractors price uncertainty they cannot see, so handing over the ground model you have, labelled as inferred rather than measured, usually costs less than withholding it.

None of this replaces the investigation. A desktop review builds the expectation; the boreholes test it, and the engineer's logs are what a footing or an excavation gets designed against. What it buys is a price and a program built on the ground that is likely to be there, and a set of holes drilled in the places most likely to prove it wrong.

Before you price an excavation, generate a desktop report for the address to see the mapped geology, terrain and nearby bore records together, or see a sample report to check the evidence it pulls in.

LayeredGeo compiles geology, soils, groundwater and site data into an automated geotechnical desktop report for any address in Queensland, New South Wales or Victoria.

Sources

• Bruce Bulley and Stuart Masterson (2003). Precambrian rocks of the Darling Range. Australian Geomechanics, Volume 38, Number 4 (Dec 2003). geomechanics.org.au
• E. A. G. Marques and D. J. Williams (2015). Weathering Profiles of Bunya Phyllite in Southwest Brisbane - a Geotechnical Approach. Australian Geomechanics, Volume 50, Number 4 (Dec 2015). geomechanics.org.au
• Jared Priddle, David Lacey, Burt Look and Chaminda Gallage (2013). Residual soil properties of South East Queensland. Australian Geomechanics, Volume 48, Number 1 (Mar 2013). geomechanics.org.au
• M. G. Delaney (2005). An overview of engineering geology and geotechnical challenges in the Newcastle Region. Australian Geomechanics, Volume 40, Number 1 (Mar 2005). geomechanics.org.au

About this article. Published by LayeredGeo and written from the published research cited in the Sources section above. It is general information about how property and ground conditions are assessed in Australia, not engineering, planning, legal or financial advice, and it is not specific to any property. Check anything that matters against the source dataset or a suitably qualified professional before you rely on it. If you spot something wrong, tell us at hello@layeredgeo.com.au and we will fix it.

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