CBR testing, clearly specified.

Send us the specification or tell us what the result is for. In-situ, laboratory and derived CBR are not interchangeable, and the right choice depends on the material, access and how the result will be used.

A derived CBR can be calculated from plate bearing data where appropriate, but it is a different measurement to a direct plunger CBR and should be labelled as derived in the report.

Only where the specification calls for it. Soaking simulates worst-case moisture conditions and is agreed with you before testing begins.

No. They are different portable methods that can support a ground assessment. Whether they are suitable, and how any correlation to CBR is used, depends on the material and the specification.

Same-day and next-day testing dates are available in many areas. Use our regional coverage hub or call to check availability for your location and test requirements.

A CBR value measures how well the ground resists a plunger being pushed into it, expressed as a percentage of the resistance a standard crushed stone would give under the same load. Designers use it to assess the subgrade and set the thickness of the pavement foundation, meaning how much capping and subbase the ground needs above it. On highway schemes, DMRB CD 225 converts subgrade CBR into a subgrade surface modulus for foundation design.

There is no universal pass mark, because it depends entirely on what the ground is being designed to carry. As a rough guide, soft cohesive soils often return low single figures while well compacted granular material can give values in the tens. A low result is not a failure; it usually means the design needs more capping or subbase, or some form of ground improvement, and your designer decides which.

Usually not: the plunger method is intended for materials with a maximum particle size of around 20 mm, so coarse fill, crushed rock or stone in the surface will deflect the plunger and give a misleading result. Where the material is too coarse, plate bearing testing loads a much larger area and is normally the better route. The same 20 mm limit applies to a laboratory CBR, and the test is not applicable where too much of the material is retained on that sieve.

We need the test level exposed and trimmed flat, safe vehicle access to each position and a clear working area around it. The equipment is vehicle mounted and reacts against the vehicle's own weight, so confined positions, steep gradients or very soft ground can rule a location out. We'll confirm the working area and any reaction load needed before the visit. The plunger hole is backfilled before we leave.

The number is set by your specification or, where it is silent, by the size and variability of the area being assessed. Positions are usually spread across the area so that weaker ground shows up rather than being averaged away, and each test is carried out at the level the design relates to, which often means digging down to formation first. In-situ testing is commonly carried out in accordance with BS 1377-9.

Yes, significantly: CBR is strongly influenced by moisture content, so the same ground can return a noticeably lower value after heavy rain than it would in a dry spell. A result reflects the condition of the material on the day it was tested, which is worth remembering if the formation has been open to the weather. Where the design has to allow for the wettest likely condition, that is normally handled in the laboratory rather than by picking a dry day.

A bulk disturbed sample of the material, large enough to prepare a full specimen, together with the specification, the sample reference and where it was taken from. The specimen is remoulded into a 152 mm mould at the compaction and moisture condition the specification calls for, then tested with annular surcharge discs on top to represent the construction above it. Testing is commonly carried out in accordance with BS 1377-2, which replaced the withdrawn Part 4. Confirm sample quantity and containers with us before sampling.

Mainly the number of test positions, where the site is and how long the visit takes. Excavating down to the test level, working around traffic management or restricted access, standby time waiting for a working area to be handed over and any out-of-hours or weekend working all add to it, as does reinstatement beyond backfilling the plunger hole. Send us the postcode, the number of positions and your preferred dates and we'll quote.