DGS40 Subbase: TfNSW 3051 Specification, Uses and NSW Supply

DGS40 is a 40mm dense graded subbase material specified by Transport for NSW (TfNSW, formerly RMS) for use as the subbase layer in road pavement construction. It sits directly below the base course layer (typically DGB20) and above the formation, distributing traffic loads across a wider area and protecting the subgrade below.

This guide explains what DGS40 is, the TfNSW specification requirements it meets, where it fits in a NSW road pavement, how to plan layer thickness and compaction, and how to source it in bulk across the Hunter Region and NSW.

What is DGS40?

DGS40 stands for Dense Graded Subbase, 40mm, a crushed rock product with a carefully controlled blend of particle sizes ranging from 40mm down to fines. The “dense graded” term refers to the gradation profile: enough smaller particles to fill the voids between larger ones, so the compacted material achieves high density with minimal air voids.

DGS40 is manufactured to meet TfNSW specification requirements (see specification section below) covering particle size distribution, plasticity, and California Bearing Ratio (CBR) strength. When a project spec calls for “DGS40” or “RMS-spec 40mm dense graded subbase”, this is the material required, and its supply comes with test certification available on request.

DGS40 is not a base course material; it is the layer that sits beneath the base course. For the primary load-bearing layer directly beneath the sealed surface, the corresponding product is DGB20 (Dense Graded Base, 20mm). See our DGB20 vs DGS40 vs unspec comparison guide for a full side-by-side.

DGS40 specification (TfNSW 3051)

DGS40 is manufactured to Transport for NSW specification 3051, which sets out the requirements for granular pavement materials used on NSW roads. The key parameters covered by the specification are:

• Particle size distribution: a defined gradation envelope from 40mm maximum size down to material passing 75 micron sieves. The gradation is controlled so that finer particles fill voids between larger stone, giving the “dense graded” behaviour.
• Plasticity index (PI): the fine fraction (material passing 425 micron) must have low plasticity to avoid movement under moisture change. Excessive PI is a common reason unspec materials fail to meet TfNSW 3051.
• California Bearing Ratio (CBR): the compacted material must achieve a specified minimum CBR strength after immersion. This is the direct measure of structural load-bearing capacity that flows through to pavement design.
• Los Angeles abrasion: the parent rock must resist breakdown under abrasion, so the material stays dimensionally stable under repeated traffic loading rather than crushing to fines.
• Wet strength and wet/dry variation: the material must retain adequate strength when wet and not lose more than a specified percentage of strength between dry and wet conditions.

Test certificates for these parameters are available on request when DGS40 is supplied from our Merriwa quarry. Always confirm the current version of TfNSW 3051 before ordering, because the specification is updated periodically and project specifications occasionally reference specific clauses.

Where DGS40 sits in a NSW road pavement

DGS40 is the subbase layer. In a typical NSW sealed road pavement, working from the natural ground upward, the layers are:

The subbase layer does two critical jobs. First, it distributes concentrated traffic loads over a wider area, reducing peak stress on the subgrade below. Second, it acts as a drainage layer, because the coarser 40mm gradation passes water more freely than the finer DGB20 above, protecting the base course and subgrade from moisture damage during construction and in service.

When to use DGS40

• All TfNSW-funded road construction and maintenance projects. Where the spec calls for DGS40, use DGS40 with no exceptions. Test certificates required.
• Council road works where the local authority follows TfNSW pavement standards (most NSW councils do for anything larger than driveways and minor works).
• Major commercial pavements (truck parks, distribution centres, industrial hardstands) where the pavement will carry heavy axle loads.
• Any project requiring test documentation for compliance handover to a client or authority.
• Where drainage matters. The DGS40 gradation passes moisture more freely than finer alternatives, protecting the pavement structure over time.

When you might use something else

For private driveways, farm roads and light-traffic private hardstands where no TfNSW certification is required, 40mm unspec road base offers similar gradation without the test paperwork, typically 15 to 30% cheaper per tonne and adequate performance for the application. Read our full DGB20 vs DGS40 vs unspec comparison guide for the decision framework.

Layer thickness and compaction

For most NSW pavement designs, DGS40 is laid 150 to 300mm thick as the subbase layer, compacted in lifts of no more than 150 to 200mm at a time to achieve the specified density. Actual thickness for a given project comes from the pavement design engineer, based on:

• Design traffic loading: heavier truck traffic requires thicker subbase.
• Subgrade CBR: weaker subgrade requires thicker subbase to distribute loads adequately.
• DGB20 thickness above: some designs use thicker DGS40 to allow thinner (less expensive) DGB20 above.
• Drainage requirements: where lateral drainage is a design consideration, thicker DGS40 may be specified.

Compaction is typically to 95 to 98% of maximum dry density, measured against a standard laboratory Proctor test. Correct moisture content at compaction is critical. Too dry and the material will not achieve target density; too wet and it will pump under the roller.

For deeper pavement designs on weak subgrades, a 65mm minus Select Material Zone (SMZ) commonly sits below the DGS40 subbase, providing a uniform working platform for the layers above.

Coverage, weight and delivery planning

DGS40 weighs approximately 2.1 to 2.3 tonnes per cubic metre compacted (roughly 1.8 to 2.0 tonnes per cubic metre loose). Quick reference for site planning:

• Coverage at 200mm compacted thickness: approximately 4.5 to 5 m² per tonne.
• Single tipper load (~17 tonnes): covers roughly 75 to 85 m² at 200mm thickness.
• Trailer combination (~30 tonnes): covers roughly 135 to 150 m² at 200mm thickness.
• B-double load (~40 tonnes): covers roughly 180 to 200 m² at 200mm thickness.

For larger projects, delivery is typically scheduled against your placement crew’s daily capacity to avoid double-handling. Talk to us at ordering stage if you need progressive delivery matched to construction sequence.

Sourcing DGS40 in NSW

Hunter Blue Metal stocks DGS40 at our Merriwa quarry in the Upper Hunter, alongside the full range of RMS-spec road base materials: DGB20 base course, 65mm minus formation fill, and unspec alternatives. Bulk delivery runs across the Hunter Valley, Newcastle, Central Coast and broader NSW, with next-day delivery on most orders.

For TfNSW-specified projects, test certificates for particle size distribution, plasticity, CBR and abrasion are available on request. If you need certificates delivered with each load rather than on request, mention this at ordering. We can arrange the paperwork flow to suit your project’s compliance requirements.

Browse our full road construction product range, read our companion buyer’s guide on which road base to specify, or call 1300 150 777 for a quote against your project specification.

Frequently asked questions

What is DGS40?

DGS40 is 40mm dense graded subbase, a RMS/TfNSW-specified crushed rock product used as the subbase layer in NSW road pavement construction. It sits beneath the base course (typically DGB20) and above the formation, distributing traffic loads and providing drainage. DGS40 must meet the requirements of TfNSW specification 3051 covering particle size distribution, plasticity, CBR strength and abrasion resistance.

What is the difference between DGS40 and DGB20?

DGS40 is the subbase layer, DGB20 is the base course layer. DGS40 is coarser (40mm top size, coarser gradation) and sits beneath DGB20 (20mm top size, finer gradation) in a road pavement. Both are RMS/TfNSW-specified. Most properly-designed sealed road pavements use both layers, with DGS40 distributing loads and providing drainage, and DGB20 providing the primary load-bearing surface below the sealed surface course.

How thick should the DGS40 subbase be?

For most NSW council and commercial pavements, DGS40 is laid 150 to 300mm thick, depending on design traffic loading and subgrade strength. The exact thickness comes from the pavement design engineer based on a CBR analysis of the subgrade and expected traffic loading. DGS40 is compacted in lifts of no more than 150 to 200mm at a time to achieve target density.

Can I use unspec 40mm road base instead of DGS40?

Only for projects that do not require RMS/TfNSW compliance, typically private driveways, farm roads and non-certified private works. For any TfNSW-funded, council or commercial project that will be audited, certified or handed over to a public authority, DGS40 with test certification is required. Unspec 40mm road base offers similar gradation at 15 to 30% lower cost, but does not carry the test paperwork.

What does DGS40 weigh per cubic metre?

DGS40 weighs approximately 2.1 to 2.3 tonnes per cubic metre compacted, or 1.8 to 2.0 tonnes per cubic metre loose. As a quick site-planning reference, a single tipper load of around 17 tonnes covers approximately 75 to 85 square metres at 200mm compacted thickness.

Do you supply test certificates for DGS40?

Yes. DGS40 supplied from our Merriwa quarry comes with test certification available on request, covering particle size distribution, plasticity index, CBR strength and abrasion resistance to meet TfNSW specification 3051 requirements. If your project requires certificates delivered with each load rather than on request, we can arrange the paperwork flow to suit, so talk to our team at ordering stage.

Do you deliver DGS40 across NSW?

Yes. Hunter Blue Metal delivers bulk DGS40 across the Hunter Valley, Newcastle, Central Coast, Upper Hunter, Singleton, Muswellbrook, Scone, Merriwa and surrounding NSW. Truck options scale from single tipper loads (around 17 tonnes) through to B-double loads for major projects. Most orders ship next-day. Call 1300 150 777 for pricing to your project address.

Written by

Blair Robert

Quarry Sales & Allocations Manager, Hunter Blue Metal

Blair has over 15 years in the quarry industry and works with civil, mining, farming and construction customers across the Hunter Region to specify and source the right materials for their projects.

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