Depth to bedrock: how to check rock risk before excavation

Depth to bedrock decides how an excavation is priced: whether footings, piers, a pool or a basement can be dug with a bucket or need a hammer, rock saw or blasting. Only drilling measures it, but mapped geology, bore records and terrain show beforehand how likely shallow rock is and how much it may vary.

The variation is what costs money. At North Dandalup Dam in the Darling Range of Western Australia, the cut-off trench and foundation excavations on the lower left abutment ran into zones of corestones: rounded blocks of fresh to moderately weathered rock, from half a metre to more than 4 m across, sitting in extremely weathered material. They had to be broken out with local blasting and extra digging to reach the true rock surface, which undulated. The program slipped and the excavation went deeper than planned (Bulley and Masterson 2003).

Why is depth to rock a range, not a level?

Rock weathers from the top down, and unevenly, grading from soil formed in place through less and less weathered rock to fresh rock. Where an auger or bucket stops depends on which grade it meets, and that can change within a few metres.

• Corestones and uneven weathering. In the Darling Range, Bulley and Masterson describe depth to bedrock changing quickly from outcrop to weathered material more than 20 m deep. Corestones (floaters) form in weathered granite, up to a few metres across, and in dolerite, usually under half a metre. In gneiss, feldspar and mica-rich bands weather deeper than quartz-rich ones, so refusal depth jumps around.
• Sharp contacts. In cut slopes in Bunya Phyllite at St Lucia, Taringa and Indooroopilly in western Brisbane, Marques and Williams (2015) found thin soil cover, in places under 0.7 m, and abrupt changes from soil to rock controlled by the rock's structure. There, half a metre more depth can change the plant you need.
• Weathering can make rock harder. Delaney (2005), reviewing the Newcastle region, notes that some fresh conglomerates can be ripped with a dozer while moderately weathered ones often need blasting, hammering or saw-cutting: weak clay cements hold the fresh rock together, and partial weathering re-cements it with iron oxide.
• Deep profiles. For South East Queensland, Priddle et al. (2013) cite a review of piling data suggesting extremely weathered rock is typically reached at 10 to 15 m. That is deep for a house footing and shallow for a piled basement, which is why the question has to be asked against the structure.

Evidence	What it indicates	Limit
Mapped geology	Parent material and likely excavation behaviour	Unit boundaries are approximate
Registered bore and borehole logs	Depth to refusal, material above it, water levels	Point data of variable quality
Terrain and slope	Shallow rock, benches, thin soil on ridges	Indicative only
Historical aerial imagery	Quarry faces, rock cuts, unsupported batters, fill	Interpretation, not measurement
Site investigation	Depth, strength and excavatability at the holes	Cost and program; still point data

Which rock is under the site?

Start with the mapped geological unit; it is a better guide than the suburb. Where a mapped boundary crosses the site, expect the excavation to change character somewhere across it and treat the two parts as separate assumptions.

What do the nearby bore logs say, and do they agree?

Registered bore logs and geotechnical records near the site are the closest thing to ground truth before drilling. Take three things from each: where refusal or rock is recorded, the material just above it, and whether the logs agree.

Agreement is a result in itself. Four logs within a few hundred metres refusing between 2 and 3 m support a defensible expectation. One refusing at 1 m while another reaches 12 m in clay is variability to price or investigate, not to average. The same records usually carry standing water levels, covered in reading groundwater and bore records.

What do the terrain and old photos show?

Rock near the surface shows in the landform: short steep steps in an even slope, benches following the bedding, thin soil on ridges, outcrop in nearby road cuttings. Old aerial imagery adds quarry faces, driveways cut into rock, and batters that have stood steep without retaining for decades, which are more likely rock than soil.

What if the rock is too deep instead?

No rock within reach costs money too, and Delaney's Newcastle review shows both problems in one region. Some volcanic units sit as solid rock so close to the surface that a sizeable cut is likely to need drilling and blasting. Meanwhile old river valleys under the Hunter estuary were cut up to about 80 m deep and later filled with sediment, so the problem there is soft ground and settlement. Even in sand, a cemented layer often present in the top 5 m, known locally as coffee rock, has stopped piles short of their target depth.

What should the drilling brief and the tender say?

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

The drilling brief says where the holes go and how deep, based on where conditions are expected to change; whether to core if rock is likely within the founding zone; and whether the investigation has to prove the thickness of a hard layer or only find it. Placement matters more than count, as covered in how many boreholes a house block needs.

The tender or contract states what the price assumes: the excavation rate to a given depth, the assumption about rock, and who carries the cost if refusal comes higher than assumed. Contractors price the uncertainty they cannot see, so issuing the ground model, labelled as inferred rather than measured, usually costs less than withholding it.

Founding levels, rock strength and excavatability come from the site investigation, and the design from an engineer working from the field logs.

Common questions

How do I find out the depth to bedrock at a property?

Only drilling gives the depth at that point. Before drilling, mapped geology, registered bore and borehole logs near the site, terrain and historical imagery together indicate the likely range and how variable it is.

Why do two holes on the same block hit rock at different depths?

Because weathering profiles are uneven. Corestones and floaters, the thickness of the residual soil, and the structure of the rock mass all vary across short distances (Bulley and Masterson 2003; Marques and Williams 2015).

Does hitting rock always increase the cost?

Not always. Shallow rock can provide good founding conditions and reduce footing depth. It raises cost where excavation is required through it, which is why pools, basements, services trenches and deep beams are where rock is felt first.

To review mapped geology, terrain and nearby bore records for a site before an excavation is priced, generate a desktop report for the address, or see a sample report to check the evidence it compiles.

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

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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