How To Identify Material & Conduct On-Site Subgrade Soil Analysis To Approximate California Bearing Ratio (CBR)
Subgrade CBR assessment is a critical part of pavement construction, because the ground beneath the pavement ultimately determines how well the completed construction will perform. The subgrade should be exposed and assessed to identify the soil or material present, whether that is bedrock, compacted stony ground, granular material, sand, silt or clay, and to establish whether there are any soft, variable or waterlogged areas.
Assessment can include visual inspection, simple site methods such as the boot test, and where necessary more formal testing such as CBR or in-situ stiffness testing. It is important to understand the difference between strength and stiffness: strength is the subgrade’s ability to resist failure under load, whereas stiffness is its resistance to deformation or deflection. A subgrade can appear firm at the surface but still have insufficient stiffness to adequately support the pavement. Correct assessment allows the required foundation thickness, improvement or capping measures to be determined before construction begins.
Bedrock
Hard rock and very dense materials that require a mechanical excavation plant typically exhibit high bearing resistance. Because they are difficult to break out or penetrate, they are generally associated with CBR values above 5%, making them suitable for standard pavement design assumptions without requiring increased sub-base thickness.
Compact Sand and Gravel (CBR > 5%)
Compact granular soils, such as dense sand and gravel, usually provide adequate structural support. A practical field indicator is where a 50 mm square peg is difficult to drive 150 mm into the surface. When well compacted and properly drained, these materials typically exhibit CBR values above 5% and perform reliably under domestic loading conditions.
Stiff Clay and Sandy Clay (CBR ≈ 2.5%–5%)
Cohesive soils require closer scrutiny. Stiff clay or sandy clay that cannot be moulded by hand and requires a pick for excavation generally falls within an approximate CBR range of 2.5% to 5%. While this may be acceptable for certain applications, these soils are highly moisture-sensitive. Seasonal wetting can significantly reduce effective bearing capacity, particularly during prolonged rainfall or winter conditions.
Firm Clay and Sandy Clay (CBR ≈ 2.5%–5%)
Firm clays, which can be moulded by hand and are excavated using a spade rather than a pick, also tend to fall within the 2.5% to 5% CBR range. These materials are commonly encountered on residential projects and often govern increased foundation or sub-base thickness in accordance with structural design guidance. Their performance is strongly influenced by moisture content.
Loose Sand, Silty Clay and Clayey Sand (CBR ≈ 2.5%)
Loose granular or mixed soils, such as loose sand, silty clay, or clayey sand, are typically identified by dry lumps that break down easily and by the relative ease with which a peg can be driven into the surface. These materials are commonly associated with CBR values around 2.5% and are highly susceptible to ground heave or settlement.
Where CBR is likely below 2.5%, BS 7533-101 requires formal structural design.
Solutions may include:
- Increased capping layers
- Ground improvement
- Stabilisation
- Engineered slab construction
Specialist advice is recommended.
Soft Silt and Clay (CBR < 2.5%)
Soft cohesive soils that can be easily moulded between the fingers generally exhibit CBR values below 2.5%. These materials present a high risk of long-term deformation, seasonal movement, and loss of support under load. They typically require either increased structural thickness or ground improvement.
Very Soft Silt and Clay (Specialist Assessment Required)
Where soil is very soft and exudes between the fingers when squeezed, its strength is likely to be highly variable and inadequate for conventional pavement construction without intervention. In such cases, specialist geotechnical assessment and engineered design solutions are advisable before proceeding
Strength is likely to be highly variable and inadequate for conventional pavement construction without intervention; in such cases, specialist geotechnical assessment and engineered design solutions are advisable.
Here Is An Approximate Guide On Required Base And Sub-base Depths
Once topsoil has been removed and the site excavated to a firm, ideally dry, or slightly moist ground, the subgrade type should be identified. Examples include:
Note: For bound construction, placing a lightweight stainless steel mesh in the laying course further helps mitigate failure and surface cracking.
- Hard subgrades (rock, dense sands and gravels): typically ≥ 5% CBR. Cannot be dug by hand; plant machinery, a pick, or a mattock is required. Generally 100mm compacted depth MOT type 1 for a patio, and 150mm type 1 is generally acceptable for a driveway.
- Stiff clays or sandy clays: typically 2.5–5% CBR. These subgrades range from difficult to hand-dig to easily forming a cube in your fingers. Highly recommend a minimum 150mm type 1 for a patio, including non woven geotextile membrane and geogrid, and a minimum 250mm for a driveway, again with membrane and geogrid. Where CBR approaches 2.5%, increased sub-base thickness or ground stabilisation (e.g., geogrid reinforcement) should be considered.
Patio Spec (Traffic Category 1)
- Base (unbound): ≥100 mm
- Sub-base (unbound): ≥100 mm
Driveway Spec (Traffic Category 3)
- Base (unbound): ≥150 mm
- Sub-base (unbound): ≥200 mm
Note: A non woven geotextile should be installed upon the compacted Subgrade, with a minimum of 300mm overlap.
We recommend the use of geogrid in general, but especially on weak subgrades below 5% CBR.
Weak silts or soft clays: < 2.5% CBR, requiring structural design to BS 7533-101. A putty like texture that squeezes through fingers when squeezing in to a ball. Ground improvement, capping layers, increased structural thickness or engineered slab solutions will be required refer to BS7533:101 design specification, or consult a professional.
