Asphalt Base Preparation: Why Subgrade Stability, DGB20 Compaction & Drainage Matter Most

Introduction

There is an old saying among experienced road engineers: “The asphalt wearing course gets all the credit, but the roadbase does all the work.”

When a newly laid asphalt driveway, car park, or industrial yard develops sunken wheel tracks or crocodile cracking within two years, the customer naturally blames “bad asphalt.” In reality, 90% of premature flexible pavement failures have nothing to do with the quality of the hot mix coming out of the asphalt plant—they are caused by shortcuts taken in subgrade preparation, DGB20 roadbase compaction, drainage grading, or layer bonding before the paver ever arrived on site (Accurate Stabilising).

Here is how National Road Surfacing engineers foundations for engineered asphalt paving across Sydney’s diverse geological conditions.

1. Understanding Sydney Subgrade Soils & California Bearing Ratio (CBR)

The natural earth left after excavating topsoil is called the subgrade. In civil engineering, subgrade strength is measured by its California Bearing Ratio (CBR %)—the higher the CBR percentage, the stronger the soil:

• Coastal Sands & Hawkesbury Sandstone (Eastern Suburbs, Northern Beaches, Parts of North Shore & Sutherland): Typically exhibit high, free-draining CBR values (8% to 15%+), requiring moderate granular base thicknesses.
• Cumberland Plain Reactive Clays & Bringelly/Ashfield Shales (Western Sydney, Inner West, Canterbury-Bankstown, Liverpool, Macarthur): Often exhibit low soaked CBR values (2% to 4%). When dry, Sydney clay feels rock-hard; when saturated by groundwater or leaky stormwater pipes, it loses bearing capacity and turns plastic.

Why Proof-Rolling Is Non-Negotiable

Before spreading roadbase, we proof-roll the trimmed subgrade with a loaded heavy truck or roller. If a pocket of clay pumps or deflects like a sponge under the wheels, covering it with 100mm of gravel will not fix it. Soft pockets must be excavated (“bridged”) and replaced with compacted select rock, geofabric separation membrane (geotextile), or lime/cement stabilisation.

2. Choosing and Compacting the Right Roadbase (DGB20)

Above the prepared subgrade sits the unbound granular base course—the structural layer that absorbs and spreads wheel loads downward.

• What Is DGB20?: In NSW civil specifications (TfNSW / council specs), DGB20 stands for 20mm Densely Graded Base. It is a precisely crushed blend of 20mm hard quarry rock, smaller interlocking stones, and controlled stone dust fines with specified plasticity limits.
• Why Optimum Moisture Content (OMC) Matters: If DGB20 is rolled bone-dry, friction between the dry stones prevents them from sliding into a tight interlock, leaving air voids that collapse the first time rain hits. If rolled soaking wet, water trapped between the pores prevents compaction (“slurry wave”). Roadbase must be moisture-conditioned to its Optimum Moisture Content (~6%–8%), spread in lifts no thicker than 100mm–150mm compacted, and vibrated with heavy rollers until it rings solid like concrete under a hammer.

3. Surface & Subsoil Drainage: Keeping the Base Dry

Even the best compacted DGB20 loses strength if it sits below the water table:

• Surface Cross-Falls (2%–3%): Both the finished asphalt surface and the trimmed subgrade beneath the roadbase should slope at a minimum 1:50 (2%) to 1:33 (3%) grade toward gutters or grated pits.
• Subsoil Drainage (Ag-Lines): Along cut embankments, retaining walls, or low kerb lines where groundwater seeps horizontally under the road, slotted subsoil agricultural pipes wrapped in geotextile filter sock and clean 20mm drainage aggregate intercept water before it can wet the roadbase.

4. The Critical Interface: Prime Coats vs Tack Coats

As highlighted by Accurate Stabilising, the thin boundary between the roadbase and the asphalt wearing course is where shear forces peak under vehicle braking:

Treatment	Where It Is Applied	How It Works
Prime Coat / Primerbinder	Sprayed onto a freshly trimmed, swept unbound granular roadbase (DGB20) before laying asphalt or a spray seal.	Low-viscosity cutback bitumen or special emulsion that soaks 5mm–10mm deep into the pores of the roadbase, toughening the top crust, waterproofing the base during construction, and bonding to the asphalt above.
Tack Coat	Sprayed onto an existing asphalt layer, milled surface, concrete slab, or between two hot mix asphalt lifts (AC20 -> AC14).	Sticky cationic bitumen emulsion (CRS/CAM) that breaks from brown to shiny black as water evaporates, gluing the old and new asphalt layers into a single structural beam.

Recommended Roadbase & Asphalt Thickness Guide by Application

Site Application	Typical Subgrade Condition	Minimum Compacted DGB20 Roadbase	Minimum Compacted Asphalt Thickness
Pedestrian Footpath / Cycleway	Firm trimmed soil	100 mm	25 mm – 30 mm AC7
Residential Car Driveway	Firm clay/shale (CBR >= 4%)	100 mm – 150 mm	30 mm – 40 mm AC10
Strata Driveway / Commercial Car Park Bays	Proof-rolled subgrade	150 mm – 200 mm	40 mm – 45 mm AC10
Commercial Car Park Aisles / Light Trucks	Proof-rolled subgrade	200 mm	45 mm – 50 mm AC14
Heavy-Duty Industrial Hardstand	Proof-rolled / stabilised subgrade	200 mm – 300 mm+	75 mm–120 mm AC20 + 50 mm AC14/SMA

Frequently Asked Questions

Can recycled crushed concrete (RCC / DGB20 Recycled) be used as roadbase under asphalt?

Yes—quality-assured NSW EPA specification recycled roadbase (blended crushed concrete and brick free of contaminants) compacts extremely well and slightly self-cements over time due to residual calcium hydroxide in the crushed concrete. However, for high-specification council roads or wet conditions, quarry-crushed virgin DGB20 basalt/granite or cement-stabilised base is often specified by civil engineers.

Conclusion & Next Steps

Never accept a cheap asphalt quote that cuts corners on excavation depth, DGB20 roadbase thickness, or tack coating. To see what happens when base preparation fails, read our next diagnostic guide: Common Asphalt Pavement Problems: Causes & Repairs.

Want your pavement engineered properly from the ground up? Call National Road Surfacing on 0475 013 264 or request a full-depth surfacing quote online.