DGB20 Base Course: TfNSW 3051 Specification, Uses and NSW Supply

DGB20 is a 20mm dense graded base course material specified by Transport for NSW (TfNSW, formerly RMS) for use as the primary structural layer in road pavement construction. It sits directly beneath the surface course (asphalt or sprayed seal) and above the DGS40 subbase, carrying the concentrated wheel loads from traffic and transferring them to the layers below.

This guide explains what DGB20 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 DGB20?

DGB20 stands for Dense Graded Base, 20mm, a crushed rock product with a carefully controlled blend of particle sizes ranging from 20mm 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 and a tight, closed surface suitable for supporting the sealed surface course directly above.

DGB20 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 “DGB20” or “RMS-spec 20mm dense graded base”, this is the material required, and its supply comes with test certification available on request.

DGB20 is not a subbase material. It is the primary structural layer that sits above the subbase, immediately below the sealed surface. For the coarser layer that distributes loads and provides drainage beneath the base course, the corresponding product is DGS40 (Dense Graded Subbase, 40mm). See our DGB20 vs DGS40 vs unspec comparison guide for a full side-by-side.

DGB20 specification (TfNSW 3051)

DGB20 is manufactured to Transport for NSW specification 3051, which sets out the requirements for granular pavement materials used on NSW roads. Because DGB20 is the base course layer directly below the sealed surface, its specification parameters are tighter than those for the subbase below. The key parameters covered by the specification are:

• Particle size distribution: a defined gradation envelope from 20mm maximum size down to material passing 75 micron sieves. The gradation is tighter than DGS40, giving a denser and smoother finish suitable for direct sealing.
• Plasticity index (PI): the fine fraction (material passing 425 micron) must have low plasticity. Because DGB20 carries the surface course directly, movement in the fines translates to surface deformation, so the PI limit is critical.
• California Bearing Ratio (CBR): the compacted material must achieve a specified minimum CBR strength after immersion. DGB20 typically carries a higher CBR requirement than DGS40 because it is the direct load-bearing layer.
• Los Angeles abrasion: the parent rock must resist breakdown under abrasion, so the base course stays dimensionally stable under repeated wheel loading rather than crushing to fines that would cause rutting.
• 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 DGB20 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 DGB20 sits in a NSW road pavement

DGB20 is the base course layer. In a typical NSW sealed road pavement, working from the natural ground upward, the layers are:

The base course does the hardest structural job in a flexible pavement. It carries concentrated wheel loads from traffic directly under the surface, spreads them out laterally, and transfers them to the subbase and subgrade below. Because it is the layer that most directly resists rutting and surface deformation, its density and gradation control are critical.

When to use DGB20

• All TfNSW-funded road construction and maintenance projects. Where the spec calls for DGB20, use DGB20 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, such as truck parks, distribution centres and industrial hardstands, where the pavement will carry heavy axle loads and be sealed.
• Any project requiring test documentation for compliance handover to a client or authority.
• Where a sealed surface is planned, whether asphalt or sprayed seal, DGB20 provides the tight, closed finish the surface course needs to bond to and perform on.

When you might use something else

For private driveways, farm roads and light-traffic private hardstands where no TfNSW certification is required, 20mm unspec road base offers similar gradation without the test paperwork, typically 15 to 30 percent 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, DGB20 is laid 100 to 200mm thick as the base course layer, compacted in lifts of no more than 150mm 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 a thicker base course.
• DGS40 subbase thickness below: some designs use thicker DGS40 to allow thinner (less expensive) DGB20 above; others balance the two.
• Subgrade CBR: weaker subgrade often means thicker layers overall to distribute loads adequately.
• Surface course type: asphalt overlays and sprayed seals have slightly different base course thickness recommendations under TfNSW pavement design guides.

Compaction is typically to 98 to 100 percent of maximum dry density for the base course layer, measured against a standard laboratory Proctor test. The compaction target is generally higher than DGS40 because DGB20 sits directly under the surface course and any under-compaction shows as surface deformation quickly.

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. On DGB20 the moisture window is narrower than DGS40 because the finer gradation is more sensitive to overwetting.

Coverage, weight and delivery planning

DGB20 weighs approximately 2.2 to 2.4 tonnes per cubic metre compacted (roughly 1.9 to 2.1 tonnes per cubic metre loose). Quick reference for site planning:

• Coverage at 150mm compacted thickness: approximately 5.5 to 6 square metres per tonne.
• Single tipper load (approx. 17 tonnes): covers roughly 95 to 105 square metres at 150mm thickness.
• Trailer combination (approx. 30 tonnes): covers roughly 170 to 185 square metres at 150mm thickness.
• B-double load (approx. 40 tonnes): covers roughly 225 to 245 square metres at 150mm 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, especially where DGB20 goes on top of freshly placed DGS40 subbase and the two need to be sequenced on the same day.

Sourcing DGB20 in NSW

Hunter Blue Metal stocks DGB20 at our Merriwa quarry in the Upper Hunter, alongside the full range of RMS-spec road base materials. That includes DGS40 subbase, 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 DGB20?

DGB20 is 20mm dense graded base course, a RMS/TfNSW-specified crushed rock product used as the primary structural layer in NSW road pavement construction. It sits directly beneath the sealed surface course (asphalt or sprayed seal) and above the DGS40 subbase, carrying the concentrated wheel loads from traffic. DGB20 must meet the requirements of TfNSW specification 3051 covering particle size distribution, plasticity, CBR strength and abrasion resistance.

What is the difference between DGB20 and DGS40?

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

How thick should the DGB20 base course be?

For most NSW council and commercial pavements, DGB20 is laid 100 to 200mm thick, depending on design traffic loading and the DGS40 subbase thickness below. The exact thickness comes from the pavement design engineer based on a CBR analysis of the subgrade and expected traffic loading. DGB20 is compacted in lifts of no more than 150mm at a time to achieve target density, typically to 98 to 100 percent of maximum dry density.

Can I use unspec 20mm road base instead of DGB20?

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, DGB20 with test certification is required. Unspec 20mm road base offers similar gradation at 15 to 30 percent lower cost, but does not carry the test paperwork.

What does DGB20 weigh per cubic metre?

DGB20 weighs approximately 2.2 to 2.4 tonnes per cubic metre compacted, or 1.9 to 2.1 tonnes per cubic metre loose. As a quick site-planning reference, a single tipper load of around 17 tonnes covers approximately 95 to 105 square metres at 150mm compacted thickness.

Do you supply test certificates for DGB20?

Yes. DGB20 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.

Do you deliver DGB20 across NSW?

Yes. Hunter Blue Metal delivers bulk DGB20 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.

Written by

Blair Robert

Quarry Sales and 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.

Read more about Blair →