Founding In Sydney Sandstone And Rock Sockets

Most of Sydney sits on Hawkesbury sandstone, and where it is sound it is one of the better founding materials available. Very high bearing capacity, almost no settlement, and predictable behaviour.

The complication is that its quality varies across a site, with depth, and between one borehole and the next. Which is why the question on a Sydney project is rarely whether to found on rock, and almost always how far down the sound rock actually is.

Rock is classified, not assumed

Sandstone is graded by its strength and its weathering, and the difference between the classes is large.

Fresh, high strength rock at depth carries enormous pressure. The same formation near the surface can be weathered to the point where it behaves closer to a dense sand than to rock, and it sits above the sound material in a transition that can be several metres thick.

Which means "we hit rock" is not a founding condition. What matters is the class of rock, confirmed by someone qualified looking at it, and the depth at which the required class is continuous rather than present as a band.

Sandstone also contains shale bands and clay seams, which are far weaker than the surrounding rock and can sit directly beneath an otherwise sound founding level. A footing bearing on good sandstone with a shale seam a metre below it is founded on the seam, not on the sandstone.

The investigation has to be honest about variability

A single borehole establishes the profile at one point. On a sandstone site the rock surface can be irregular, dipping steeply, or interrupted by an old channel filled with soil.

Which means the number and the position of the boreholes matters more here than the depth of any one of them, and the report's statement about variability is the part worth reading closely. The general approach is in [understanding geotechnical investigation reports](/blog/understanding-geotechnical-investigation-reports) and [site classification and soil investigation basics](/blog/site-classification-soil-investigation-basics).

The practical consequence is a footing design with a stated founding criterion rather than a stated depth: found in rock of this class or better, confirmed by inspection, at whatever depth that occurs. Which transfers the variability into an inspection and a variation rather than into a surprise.

Choosing the system from the depth

Rock at or near the surface. Pad or strip footings bearing directly on it, with a modest excavation. The cheapest outcome available and the reason some Sydney sites are far less expensive to build on than others. The comparison is in [foundation design, raft versus piled versus strip](/blog/foundation-design-raft-vs-piled-vs-strip).

Rock a few metres down. Bored piers taken down to it, which is the standard residential and small commercial answer across much of Sydney.

Rock at significant depth, or variable. Bored piles with rock sockets, or in some conditions driven piles. The selection logic is in [piling types and when each is used](/blog/piling-types-and-when-each-is-used).

Rock too deep to reach economically. A raft or a piled raft in the soil above, designed on settlement rather than on rock bearing, which brings it back to [bearing capacity versus settlement](/blog/bearing-capacity-versus-settlement).

Rock sockets

Where a pile continues into rock rather than stopping on top of it, the socket does two things.

Carries load in side friction along the socket wall, which on sound sandstone is substantial and frequently carries most of the load.

Carries load in end bearing at the base.

Which means a socketed pile is often far more efficient than a larger diameter pile bearing on the rock surface, because the socket mobilises friction over its full length.

Three things decide whether it performs.

A clean socket. Drilling spoil, slurry or softened material left in the base destroys the end bearing, and a smeared socket wall reduces the friction. Cleaning and inspecting the base before concrete is the critical hold point on a socketed pile, and it is invisible afterwards.

A rough socket wall, because the friction depends on the roughness. An over-polished socket from the wrong tooling performs worse than a rough one.

The right rock class at the socket level, confirmed from the drilling record and the returns rather than from the design depth.

Excavating in it is its own problem

Sandstone is good to found on and hard to remove.

Which means excavation for a basement is rock excavation: hammering, sawing, or in some conditions controlled blasting, each with its own noise, vibration and programme consequence. That is the work in [deep excavation solutions for Sydney geology](/blog/deep-excavation-solutions-sydney-geology) and [shoring and bulk excavation, a developer guide](/blog/shoring-bulk-excavation-developer-guide).

The vibration from hammering is the issue for the neighbours, and it is managed through the monitoring described in [vibration monitoring on construction sites](/blog/vibration-monitoring-on-construction-sites) with the baseline record in [the importance of dilapidation reports](/blog/importance-of-dilapidation-reports).

The compensation is that sandstone frequently stands unsupported to a significant height, which can remove the need for shoring over part of a basement. That depends on the joint pattern rather than on the rock strength, because sandstone fails along its bedding and its joints rather than through the intact material. Which is why a rock face is assessed by mapping its defects, and why a face that looks sound can have a wedge of it ready to release.

Water

Sandstone transmits water along its joints and bedding planes rather than through its body.

Which means groundwater inflow into a rock excavation is concentrated at specific seams rather than spread across the face, and it can be substantial from a single joint. That changes the dewatering approach described in [dewatering and groundwater control in excavations](/blog/dewatering-and-groundwater-control-in-excavations), and it drives the long term design in [basement tanking and hydrostatic uplift](/blog/basement-tanking-and-hydrostatic-uplift).

ACSES provides geotechnical engineering and foundation design across Sydney. Talk to us about a project.