Specifying rock products for civil works can be deceptively complex. The right rock product depends on size, shape, weight, durability and the specific job it has to do — and getting any of these wrong can mean rock that washes out in the first heavy rain, retaining structures that bulge, or batters that slump.
This guide explains the four most commonly used rock products in NSW civil construction — gabion rock and spalls, armour rock, 1 metre plus oversized rock and rail ballast — and where each one fits in real civil and infrastructure projects.
The Rock Products Used in Civil Works
| Rock product | Size | Common civil applications |
| Gabion rock and spalls | 100–200mm | Gabion baskets, drainage works, riverbank protection |
| Armour rock | 200–600mm | Erosion control, bank stabilisation, scour protection |
| 1m+ rock | 1000mm+ | Major civil works, breakwaters, large-scale erosion protection |
| Rail ballast | 32/65mm (ARTC-spec) | Mainline rail track construction and maintenance |
Gabion Rock and Spalls (100/200mm)
Gabion rock and spalls at 100 to 200mm are the workhorse rock product for smaller civil and drainage projects. The size suits hand-stacking and machine-filling of gabion baskets, and the angular shape interlocks well in drainage trenches and rock pitching.
Where gabion rock is used
- Gabion baskets — wire-mesh containers filled with gabion rock for retaining structures, channel linings and energy dissipation. Common in council drainage upgrades and bridge abutment works.
- Spall rock pitching — hand-placed rock layer along drainage channels, swales and table drains for erosion control without geotextile.
- Drainage backfill — around subsoil drains, culvert headwalls and stormwater pits where free drainage is required.
- Riverbank protection — smaller-scale bank stabilisation where armour rock would be oversized for the flow conditions.
What to look for in gabion rock
Look for angular crushed rock (not rounded river rock) for gabion fill — angular shapes interlock and hold pack density when the basket is filled. Reject rock products with significant flakiness or weak parent material; freeze-thaw and wet-dry cycling can break down poor-quality rock inside a gabion basket within a few seasons.
Armour Rock (200–600mm)
Armour rock at 200 to 600mm is the standard product for medium-scale erosion protection across NSW civil and council works. The larger size resists displacement by water flow, and the angular shape allows armour rock to interlock and resist removal under wave or current action.
Where armour rock is used
- Bank stabilisation — along creek lines, stormwater channels and minor watercourses where 100mm spalls would wash out under flood flow.
- Scour protection — around bridge piers, culvert outlets and pipe headwalls where high-velocity water can otherwise undercut foundations.
- Erosion control batters — along road embankments, dam spillways and floodway approaches.
- Breakwater and shore-protection works — smaller-scale marine and lake applications.
Sizing armour rock to your application
The right size of armour rock depends on the design flow velocity and impact loading. A common civil engineering rule of thumb is that the median (D50) rock size must be sufficient to resist the design flow — typically determined by Hudson’s equation or a similar method. As a rough guide, low-velocity drainage channels can use the smaller end of the 200/600 range; higher-velocity scour zones or rivers in flood need the larger end, or step up to 1m+ rock.
1 Metre Plus Rock — When Bigger Is the Only Answer
1m+ oversized rock is reserved for major civil works where smaller armour rock would still be displaced by design flow or impact loading. These large, individually-selected rocks weigh well over 1 tonne each and require specialised lifting equipment to place.
Where 1m+ rock is used
- Coastal and inland breakwater armour layers — primary armour on the seaward face of breakwaters and groynes.
- Large-scale river training works — main flow channel control on major waterways with high flood velocities.
- Mining hardstand and bund walls — large-scale erosion and stability works around tailings dams and large mine site water management infrastructure.
- Major civil scour protection — at large bridge crossings and major culvert outlets carrying significant flows.
The decision to step from armour rock up to 1m+ rock is normally driven by a civil engineering design — it’s rarely a contractor’s call. Once specified, the supply chain and placement logistics also change significantly: 1m+ rock typically ships one or two pieces per truck.
Rail Ballast (32/65mm) — A Specialised Rock Product
Rail ballast is a distinct rock product with its own gradation, durability and angular shape requirements. Hunter Blue Metal supplies 32/65mm rail ballast to the ARTC and TfNSW specifications used for mainline rail construction across NSW.
Rail ballast must meet stricter abrasion and breakdown tests than civil-grade rock products — the cyclic loading from passing trains can quickly degrade poor-quality ballast and require costly track maintenance. Read more in our dedicated guide on what rail ballast is and why it matters.
How to Choose the Right Rock Product
Use this short checklist when specifying rock for a civil project:
- Is the rock going inside a gabion basket? Yes → 100/200mm gabion rock. The basket size and wire spacing determine the maximum rock dimension.
- Will the rock be exposed to flowing water? Yes → calculate design flow velocity. Low-velocity drainage → 100/200mm; medium-velocity (creeks, stormwater) → 200/600 armour; high-velocity or large flood flow → step up to 1m+ rock.
- Is this a rail trackbed? Use only ARTC-spec rail ballast (32/65mm) — civil-grade rock will not meet rail durability requirements.
- Are you backfilling around drainage? Use gabion rock or smaller spalls; armour rock is too large to flow into drainage trenches.
- Need test data or certification? Request specifications and source certificates at the order stage. Larger projects often require sieve analysis and abrasion test results.
Quantities, Delivery and Placement
Rock products are sold by tonne. As a quick reference for site planning:
- Gabion rock — approximately 1.5 to 1.7 tonnes per cubic metre placed (excluding void space). A standard 1m³ gabion basket takes about 1.6 tonnes.
- Armour rock (200/600) — approximately 1.6 to 1.8 tonnes per cubic metre placed. Single tipper loads of ~17 tonne cover roughly 10m³ placed.
- 1m+ rock — placement is typically one rock per location, lifted by excavator with a rock grab. Allow extra plant and time per cubic metre.
Bulk delivery is available across the Hunter Region from our Merriwa quarry. For larger projects, scheduled delivery against a stockpile or progressive placement schedule reduces double-handling and site congestion. Read about our bulk delivery service for delivery options.
Sourcing Specialty Rock from Hunter Blue Metal
Hunter Blue Metal stocks gabion rock and spalls, armour rock, 1m+ oversized rock and 32/65mm rail ballast at our Merriwa quarry. When a project specifies a rock product we don’t quarry directly, our quarry material sourcing service draws on our network of partner quarries to find a compliant product across NSW.
Browse the full rock products range for specs, or call 1300 150 777 for a quote against your project specification.
Frequently Asked Questions
What is armour rock used for?
Armour rock (200–600mm) is used for erosion control, bank stabilisation, scour protection around bridge piers and culvert outlets, and small to medium breakwater works. The size and angular shape resist displacement under medium-velocity water flow.
What is gabion rock?
Gabion rock is 100 to 200mm angular crushed rock used to fill wire-mesh gabion baskets. It is also used as spalls for rock pitching, drainage backfill and small-scale bank protection where larger armour rock would be oversized.
What size rock do I need for bank protection?
The right rock size for bank protection depends on design flow velocity. Low-velocity drainage swales and small channels can use 100/200mm gabion rock or spalls. Medium-velocity creeks and stormwater channels typically need 200/600mm armour rock. High-velocity flood flow or major waterways need 1m+ rock — usually specified by a civil engineer using Hudson’s equation or similar.
Do you supply ARTC-specified rail ballast?
Yes. Hunter Blue Metal supplies 32/65mm rail ballast to ARTC and TfNSW specifications, used for mainline rail track construction and maintenance across NSW. Read our guide to rail ballast for technical details.
How much does armour rock weigh per cubic metre?
Placed armour rock weighs approximately 1.6 to 1.8 tonnes per cubic metre, depending on void space and packing density. A 17-tonne tipper load covers roughly 10 cubic metres placed.
Can you source unusual rock sizes for specialised projects?
Yes. Our quarry material sourcing service matches civil, mining and rail projects to specification-compliant rock products from our partner quarry network across NSW when the product isn’t part of our standard Merriwa range. Talk to our team with your spec.
