G1 to G7 Explained: The Layerworks Spec

G1 to G7 is a classification of graded crushed stone and gravel used in South African pavement layers. The number describes the material’s quality and its place in the stack — and the grade you buy means nothing until it is placed and compacted to specification.

G1 to G7 Explained: The Layerworks Spec — practical guidance from the team running geotechnical and layerworks on the Garden Route. Below is what each grade actually is, where it belongs, why the compaction number matters more than the material name, and the mistake that turns a properly specified layer into an expensive pile of stone.

What the G classification actually means

The G classification comes from the South African pavement design tradition — TRH14 and the COLTO specifications — and it grades granular materials from G1 at the top to G10 at the bottom. In practice, G1 to G7 covers almost everything you will meet on a road, a platform or a hardstanding.

The grade is not simply about stone size. It is a package of requirements: the parent rock quality, the particle size distribution, the plasticity of the fines, the strength of the compacted material, and the density that has to be achieved on site.

Two things follow from that, and they are the source of most confusion on site.

First, a G-grade is a specification, not a product. A quarry sells you material that is capable of meeting G5 when correctly placed. It becomes a G5 layer only when it has been laid at the right thickness, at the right moisture content, and compacted to the required density — and tested to prove it.

Second, the number runs backwards to intuition. G1 is the highest quality, G7 much lower. A lower number means a better material, higher up the pavement, doing more structural work.

Where the material comes from matters as much as what it is called. G1 is a crushed rock product from a commercial quarry and cannot be made from run-of-pit gravel however it is screened; G5 and below can often be won locally or from selected cut on site, which is why the economics of a layerworks design change with haul distance. Afrimat and the Southern Cape quarries are the usual regional source. What proves the layer is not the delivery note but the testing — grading, plasticity index and in-situ density against the specified percentage of modified AASHTO.

The grades, and where each one belongs

A pavement is a stack. Load is applied at the surface and spreads downward through progressively cheaper material until it reaches ground that can carry what is left of it. Each layer only has to be as good as the stress reaching it.

G1 is high-quality crushed rock from sound parent material, tightly graded, with very low plasticity and a demanding density requirement — typically 88 percent of apparent relative density, which is a genuinely difficult target. It is used as a base course directly beneath a surfacing on heavily trafficked roads. It is expensive and rarely justified on a residential or light commercial job.

G2 and G3 are crushed stone base materials with progressively relaxed requirements. G2 is still a proper base course; G3 sits between base and upper subbase depending on the design.

G4 is where natural gravels start to qualify alongside crushed material. A light base course or a strong subbase.

G5 is the workhorse of South African civils and the one everybody has heard of. A good subbase, and a base course on lightly trafficked surfaces. Well understood, widely available, and the default for driveways, parking and platform work.

G6 is a subbase or upper selected layer — lower quality, more plasticity allowed, cheaper.

G7 is a selected subgrade material. Its job is to give the layers above it a uniform, competent surface to be built on. It is not a structural layer in its own right.

Below G7 you are into subgrade — the natural ground, prepared and compacted as found.

Grade	Typically	Position in the stack	Common use
G1	High-quality crushed rock	Base course	Heavily trafficked roads
G2 / G3	Crushed stone	Base or upper subbase	Roads, industrial hardstanding
G4	Crushed or good natural gravel	Light base / strong subbase	Estate roads, yards
G5	Graded gravel	Subbase, light base	Driveways, parking, platforms
G6	Lower-grade gravel	Subbase / upper selected	Fill under subbase
G7	Selected material	Selected subgrade	Levelling and uniformity

Why compaction matters more than the grade

This is the part that separates a layer that works from a layer that looks the same and does not.

Every G-grade carries a density requirement expressed as a percentage — of modified AASHTO density for most grades, or apparent relative density for G1 and G2. That percentage is not advisory. It is the specification. G5 compacted to 90 percent is not G5; it is loose gravel that happens to have the right grading.

Achieving it depends on three things working together. Layer thickness — compaction energy only penetrates so far, so a layer placed too thick is compacted at the top and loose at the bottom. Layers are placed and compacted in defined lifts for that reason, not to be difficult.

Moisture content — every material has an optimum moisture content at which it compacts best. Too dry and the particles will not move into place; too wet and you get pumping, where the material moves under the roller like a sponge instead of densifying. On the Garden Route, where you can lose optimum to a dry southeaster in an afternoon or gain it from an overnight front, water management on a layerworks job is a real activity rather than an afterthought.

Compaction effort — the right plant, enough passes, in the right pattern.

And then it gets tested. Density testing at defined frequencies is what converts an assumption into a fact, and on any job of consequence it is what the engineer signs against. A layerworks job with no test results is a layerworks job with no evidence.

Choosing the stack for the job

You do not need a highway specification for a driveway, and using one is a waste of money. The design principle is that each layer only needs to handle the stress that reaches it.

A domestic driveway carrying cars: prepared subgrade, a selected layer, and a compacted G5 subbase under the surfacing. Straightforward, and the most common failure is skimping on the subgrade preparation rather than on the material grade.

A parking area or light commercial yard: the same approach with more thickness, and G4 or G5 base depending on the surfacing and the loads.

Delivery vehicles and light trucks change the picture significantly. Axle loads do not increase the demand on a pavement linearly — a heavy axle does disproportionately more damage than a car. A yard that will see a delivery truck twice a week is a different design from a domestic driveway, and building the first as though it were the second is a common and expensive mistake.

An estate road or anything adopted by the municipality will be designed to a specified standard and tested accordingly. That is not negotiable, and the specification will name the grades.

A building platform is a different problem again: you are creating uniform, competent ground for a structure rather than a trafficked surface, and the emphasis shifts to consistency and settlement rather than surface strength.

Garden Route ground, specifically

Local conditions change what is sensible here, and a specification imported from elsewhere often does not fit.

Material availability. Layerworks material is heavy and transport is priced by distance. What is economic here is what the local quarries and pits produce, and a specification calling for a grade that has to come a long way can quietly double the cost of a layer. It is worth designing around what is available before it is worth optimising the grade.

Water. Rain falls throughout the year here rather than in a defined season, and a compacted layer left open overnight before a front can be saturated by morning. Programme layerworks so layers are sealed or surfaced promptly, and expect to manage moisture actively.

High water tables. On low-lying erven around Wilderness, Sedgefield and the Great Brak flats, the winter water table can sit high enough to affect the subgrade. Layers built on a subgrade that is competent in February and saturated in July will fail in July. Subsoil drainage is part of the design, not an extra.

Clays inland of George. Expansive clay subgrades move seasonally, and a rigid pavement stack over moving ground cracks. That is dealt with in the design — a thicker selected layer, moisture control, or in some cases stabilisation — rather than by adding a better base course on top.

Sandy coastal profiles drain well but can be loose near the surface and need proper preparation rather than simply being rolled.

What goes wrong, and what to ask for

The failure modes on layerworks are consistent and every one of them is visible in the paperwork before it is visible in the surface.

Layers placed too thick and compacted from the top, leaving a loose bottom that consolidates later under traffic. Shows up as rutting or settlement within a season.

Wrong moisture content, giving a layer that never reached density however many passes the roller made.

Contaminated material. Layerworks material blended with site spoil, or placed on a subgrade that was not stripped of topsoil, is not the grade on the delivery note.

No separation. A granular layer placed directly on a fine-grained subgrade will contaminate from below over time as fines migrate up. A geotextile separation layer prevents it and costs very little.

No drainage. Water in a pavement layer is the single largest cause of premature failure. Subsoil drainage, and surfaces that actually shed to somewhere.

No testing. Without density results at defined frequencies, nobody knows whether any of the above happened.

What to ask for on a quote: the layer thicknesses, the grades, the compaction percentages, the testing frequency, whether a geotextile is included, and what the drainage arrangement is. A quote that gives you a rate per square metre and a material name is telling you the specification has not been thought about.

Frequently asked questions

What is G5 material?

A graded gravel meeting the South African G5 specification — a package covering parent material quality, particle size distribution, plasticity of the fines, strength and a required compacted density. It is the workhorse subbase material in South African civils and a base course on lightly trafficked surfaces. Important distinction: a supplier sells you material capable of meeting G5. It becomes a G5 layer only once it is placed at the correct thickness and moisture content and compacted to the specified density.

What is the difference between G5 and G7?

Quality and position in the stack. G5 is a better material — tighter grading, lower plasticity, higher strength — used as a subbase or a light base course. G7 is a selected subgrade material whose job is to provide a uniform competent surface for the layers above it, not to carry structural load itself. Lower numbers are better materials, higher up the pavement, doing more work.

Which is better, G4 or G5?

G4 is the higher-quality material, so in structural terms it is better — but better is not the same as correct. Each layer in a pavement only needs to be good enough for the stress that reaches it, and specifying G4 where G5 is adequate is money spent for no benefit. The right question is what the design calls for at that position given the traffic, not which grade is superior in the abstract.

How thick should a G5 layer be?

It depends on the pavement design — the traffic, the subgrade strength and what is above and below it. What is not negotiable is that the material is placed and compacted in defined lifts rather than in one deep layer, because compaction energy only penetrates so far. A thick layer rolled from the top is dense at the surface and loose underneath, and it will consolidate under traffic in the first season.

What compaction does G5 need?

A specified percentage of modified AASHTO density, and the figure comes from the specification for that layer and position. The percentage is the specification, not a target to approach — material at the right grading compacted below its required density is not that grade. Achieving it depends on layer thickness, moisture content at or near optimum, and adequate compaction effort, and it is verified by density testing at defined frequencies.

Do I need G5 under a driveway?

For a domestic driveway carrying cars, a properly prepared subgrade with a compacted G5 subbase under the surfacing is a sound and common specification. What matters at least as much is what is underneath: topsoil stripped, subgrade prepared, layers compacted in lifts, and drainage that takes water away. Most failed driveways failed in the preparation, not because the material grade was one step too low.

Can I use building rubble as fill?

Not as a pavement layer. Crushed concrete can be processed into a usable granular material where it is properly crushed, graded and tested — that is a manufactured product, not the rubble from a demolition. Unprocessed rubble has no grading, unknown contamination and no consistency, so it cannot be compacted to a reliable density. It will settle unevenly, and anything built on it will follow.

What is a geotextile for in layerworks?

Separation, mainly. Placed between a granular layer and a fine-grained subgrade, it stops fines migrating up into the stone over time, which would otherwise gradually contaminate the layer and destroy its strength. It also helps distribute load over soft spots. It costs very little relative to the layer above it and it is a common and cheap omission from quotes.

Related reading

• Building a road: the layerwork stack
• Earthworks: cut, fill, compact
• Paving vs asphalt: picking a surface
• Geotechnical and layerworks in George
• Civil engineering in George