Using CBR to design unsealed maintenance roads

Unsealed maintenance roads are essential for reaching drainage assets, utility corridors, construction sites, farms, forestry blocks and remote infrastructure. Their performance depends heavily on the strength of the natural ground beneath the gravel. A well-graded wearing course can still rut, pump or fail when it is placed over weak, wet subgrade.

The California Bearing Ratio (CBR) provides a practical way to measure subgrade and pavement-material strength and convert test results into an economical pavement thickness. Used with traffic estimates, drainage information, material grading and construction control, CBR helps engineers design access tracks that remain serviceable through Australia’s dry seasons, intense rainfall and repeated maintenance traffic.

What CBR tells the designer

CBR is an index of resistance. In a standard test, a plunger penetrates a compacted soil or pavement specimen, and the force required is compared with the force for a standard crushed-rock material. The result is expressed as a percentage. A higher CBR generally indicates stronger support, while a low value suggests that the subgrade may deform under wheel loads.

For an unsealed maintenance road, the relevant CBR may be measured for several layers: the in-situ subgrade, selected capping material, imported gravel and compacted pavement layers. The design value should represent the condition expected during service, rather than an unusually dry or highly compacted laboratory specimen. In many parts of Australia, the soaked CBR is particularly important because a clay subgrade can lose significant strength after prolonged rainfall.

CBR should not be treated as a complete description of soil behaviour. Two soils can have the same CBR but differ in shrink–swell potential, permeability, plasticity and susceptibility to pumping. The test is most useful when supported by particle-size distribution, Atterberg limits, moisture content, density, groundwater observations and site classification.

A disciplined testing process is also important where results will inform public works or commercial construction. For reference, the UP PPP testing unit operates laboratory and field investigation services under SNI ISO/IEC 17025 and ISO 45001, covering soil, asphalt, concrete, elevation and infrastructure assessment. Its Jakarta context differs from Australian practice, but the principle of traceable testing and documented quality assurance is directly relevant.

Building a reliable site investigation

The investigation should begin with the road’s intended function. A track used by a light council vehicle once a month requires a different standard from a route carrying water carts, graders, waste trucks or loaded maintenance vehicles every day. Record axle loads, traffic frequency, turning areas, gradients and locations where vehicles brake or accelerate. These zones often deteriorate earlier than straight sections.

Divide the alignment into reasonably consistent soil units rather than relying on one result for the whole road. Test locations should reflect changes in geology, cut and fill areas, low points, visible wet patches and transitions between natural ground and imported fill. Dynamic Cone Penetrometer testing can provide rapid field coverage, while laboratory CBR testing offers a controlled result for representative samples.

Sample preparation matters. The test report should state the compaction effort, moulding moisture, curing or soaking condition, surcharge and penetration result. A soaked value can be more appropriate for a road exposed to seasonal wetting, although an unsoaked result may help describe construction-stage behaviour. The designer should also identify the weakest credible sections instead of using only the average CBR.

Australia’s conditions make local interpretation essential. Black cracking clays around western and northern Sydney can move substantially with seasonal moisture changes. Lateritic and gravelly soils in parts of Western Australia may perform well when shaped and drained, yet weaken rapidly if fine material seals the surface. In tropical Queensland, high rainfall and wet-season access requirements can govern the design more than nominal dry-season strength.

Field observations should be retained with the test data. Note wheel ruts, standing water, seepage, soft spots, vegetation, exposed rock and evidence of previous repairs. A short geotechnical report that links each CBR result to a chainage, soil description and moisture condition is more useful than an unexplained list of percentages.

Converting CBR into pavement thickness

A CBR value becomes a design input only after it is combined with traffic and material properties. The basic pavement arrangement usually consists of the natural subgrade, a capping or improved layer where needed, a compacted base or subbase, and a wearing course. The thickness required increases as subgrade CBR decreases and as cumulative heavy-vehicle loading increases.

For low-volume maintenance roads, designers commonly use an unsealed pavement chart or a mechanistic-empirical procedure based on design traffic. The traffic category should account for cumulative equivalent standard axles where appropriate, but simple vehicle counts may be sufficient for lightly trafficked access routes if axle loads and wet-weather use are clearly defined. A road that carries only ten vehicles per week may still need a stronger design if those vehicles are loaded trucks.

The selected gravel must provide more than bulk thickness. It should resist crushing, produce a stable compacted layer and contain enough fines to bind the surface without becoming slippery or impermeable. Excessive plasticity can cause corrugation and rutting, while a shortage of fines may leave loose aggregate that migrates under traffic. Material specifications should address grading, plasticity, CBR, durability and oversize particles.

If the in-situ CBR is very low, increasing gravel thickness may be an inefficient response. Options include undercutting and replacing weak soil, installing a capping layer, improving the subgrade with lime or cement, using geotextiles or geogrids, or raising the formation above wet ground. These treatments should be selected from site evidence and constructability, not from CBR alone.

Planning and documentation are especially valuable when several parties are involved. A structured project assessment resource can help explain why investigation, measurement and verification should occur before construction decisions are fixed. For Australian projects, the same logic aligns with Austroads guidance, state road authority specifications and council asset-management requirements.

Making drainage part of the design

Water is usually the main cause of premature failure in an unsealed road. A strong pavement can lose much of its load-bearing capacity when water enters the subgrade, while an apparently weak soil may perform acceptably when it remains drained and well compacted. The crossfall, table drains, culverts, mitre drains and outlet protection therefore deserve the same attention as the CBR calculation.

The finished surface should shed water quickly without creating unsafe crossfall or erosion. Crowned profiles are common, but the correct shape depends on width, grade, material and traffic. Table drains must be sized and located to avoid saturating the road edge. At creek crossings and low-lying sections, the design may require flood assessment, raised formation, scour protection or a different pavement treatment.

Drainage details should reflect local weather patterns. A maintenance track outside Darwin may be inaccessible during the wet season unless culverts and floodways are designed for intense rainfall. In regional Victoria or Tasmania, prolonged winter moisture can keep clay subgrades weak for months. In the dry interior, a road may appear stable for long periods before a single storm exposes inadequate outlets and causes severe washouts.

Moisture conditioning and compaction during construction are equally important. Material placed too wet may rut under the roller, while material placed too dry may fail to achieve the target density. Proof rolling can identify yielding areas that were missed during sampling. Corrective work should be completed before the wearing course is spread, because a thin layer of gravel will not hide a moving formation for long.

Vegetation clearing should be controlled rather than excessive. Removing stable ground cover beside a track can increase erosion and sediment movement into waterways. At the same time, roots, organic soil and buried debris must be removed from the pavement footprint. Environmental approvals, Aboriginal heritage requirements and council drainage standards may affect the construction method and programme.

Verifying construction and planning maintenance

A CBR-based design only performs as intended when the built road matches the assumptions. During construction, inspect formation levels, moisture, layer thickness, compaction and material quality. Check that the road crown is continuous and that drains have positive grades. Density testing, proof rolling and level surveys provide evidence that weak areas and low points have been addressed.

The wearing course should be placed at a thickness that can be compacted effectively and maintained with available plant. Graders, rollers, water carts and trucks should be selected according to access constraints and material type. Remote Australian sites may face long haul distances for suitable gravel, so borrow-pit testing and haul planning can materially change the most economical design.

Maintenance should be planned from the beginning. Grading removes corrugations and restores crossfall, but aggressive grading can strip the binding fines and reduce pavement thickness. Re-sheeting is required when the gravel layer becomes too thin, contaminated or rounded. Drains should be cleared before the wet season, and potholes should be repaired after the underlying cause—usually water or inadequate compaction—has been corrected.

The local market includes councils, civil contractors, mining operators, agricultural businesses and infrastructure owners with different procurement systems. A council may require a specification aligned with state road authority practice, while a mine may impose stricter heavy-vehicle and safety controls. An experienced engineering and construction partner such as Matis Projects can help connect field conditions, project planning and delivery requirements.

The following comparison provides a starting point for selecting a treatment. It is not a substitute for site-specific testing, traffic assessment or drainage design.

Subgrade condition	Typical CBR indication	Suitable response	Main control
Strong, well-drained granular soil	Above 15%	Shape, compact and apply a suitable wearing course	Maintain crossfall and gravel grading
Moderate cohesive or mixed soil	7–15%	Use a compacted granular pavement with local undercutting	Control moisture and layer density
Weak clay, silty soil or variable fill	3–7%	Add capping, increase pavement thickness or improve the subgrade	Treat soft spots and provide drainage
Very weak or saturated formation	Below 3%	Undercut, stabilise, raise the formation or use reinforcement	Keep water out and verify support
Highly variable alignment	Mixed results	Divide into design sections and use targeted treatments	Map chainages and design for the weakest zones

Before opening the road to traffic, confirm that the as-constructed pavement thickness, surface levels and drainage arrangements meet the design intent. Keep test certificates, material source records, density results, photographs and maintenance observations with the asset file. These records make future rehabilitation decisions faster and help demonstrate that public funds and safety obligations have been managed responsibly.

A robust unsealed maintenance road starts with representative CBR testing, but it succeeds through integrated engineering judgment. Arrange laboratory and field investigations, define the actual vehicle demand, design for wet conditions, specify suitable gravel and verify every construction layer. For roads, bridges, drainage access and pedestrian-related infrastructure, technical testing support can provide the measured evidence needed to move from a preliminary concept to a defensible pavement design.