In-situ CBR testing

How is an in-situ CBR test carried out on site?

A cylindrical plunger is driven into the prepared surface at a controlled rate and the force needed to push it in is recorded against the depth of penetration. The load reached at set penetrations is compared with the load a standard reference material would need at the same penetration, and the ratio is reported as a percentage. The equipment is mounted on our vehicle, so the test is self contained.

Which test method does an in-situ CBR follow?

The in-situ CBR test is normally carried out to the method given for it in BS 1377-9, with the plunger advanced at a controlled rate of about one millimetre per minute. Your specification may set the penetration the result is read at and the number of positions required. Where a specification only says CBR test, tell us what the value is being used for, because the in-situ, laboratory and derived methods are not interchangeable.

How is an in-situ CBR percentage used in pavement design?

It gives an indication of subgrade strength that pavement and platform thicknesses are designed from, with a higher percentage meaning less construction is needed above it. Highway foundation design in CD 225 of the Design Manual for Roads and Bridges converts CBR into a subgrade surface modulus using a relationship intended for values of roughly 2 to 12 per cent. The report gives the value at each position with the load and penetration data behind it.

Do we need to provide plant or a reaction load?

No. The apparatus is bolted to our vehicle and reacts against its weight, so you do not need to supply an excavator or any other dead weight. What we do need is enough room to bring the vehicle up to each test position and stand it safely on reasonably firm ground. Positions a vehicle cannot reach usually mean a different method, such as LWD or DCP testing.

What particle size rules out an in-situ CBR test?

Material containing particles larger than about 20 mm is generally unsuitable, because a single stone under the plunger controls the reading and the result stops representing the material as a whole. That makes the test best suited to clays, silts, sands and fine gravels. Where the subgrade contains coarser stone, a plate bearing test with an agreed derived CBR calculation is usually the more defensible route. Send a photograph of the exposed material if you are unsure.

What state does the formation need to be in for a valid test?

It must be flat, clean and at the level you actually want tested, with loose or disturbed material trimmed off first. The plunger bears on a small area, so a rutted, stony or churned surface gives readings that reflect the disturbance rather than the subgrade. Keep the area clear of standing water and protected from site traffic, and tell us if the formation has been open through heavy rain since it was prepared.

Does the test have to be at formation level, or can you test in a trial hole?

Testing can be done on an exposed formation or in a small excavated hole, and what matters is that the plunger bears on the level your design refers to. The result represents the material immediately beneath the plunger, not the ground several metres down, so reaching the right layer is the whole point. If you need us to test below existing ground level, have the hole open and safe on the day and allow for backfilling it afterwards.

How many CBR positions do we need across a site?

The number comes from your specification and the size and variability of the area rather than from a fixed rule. A common approach is to spread positions so that every material type and every part of the area is represented, with extra tests where the ground looks softer or the design is most sensitive. On a long linear route, positions are usually set at intervals along the alignment agreed with the designer.

How long does each test position take?

A single position usually takes around 20 to 30 minutes once the vehicle is set up over it, so several positions can be covered in a visit. Moving between positions, opening trial holes and waiting for plant to clear the area normally takes longer than the testing itself. Access is the main variable, and a site where the vehicle can drive between positions is far quicker than one where it cannot.

Why has our CBR come back lower than the design value?

Most often because the material was wetter when it was tested than the condition the design assumed. CBR is sensitive to moisture content and compaction, so a formation left open through rain, trafficked by plant or not yet compacted to the specified density will read low. The test reports the ground as it was on the day, so it is worth reading the result alongside the moisture condition and the compaction records.

Is the in-situ CBR method on your UKAS schedule?

Hixtra holds UKAS accreditation to ISO/IEC 17025 for the tests listed in its schedule of accreditation, number 8996. Accreditation is specific to the listed methods and scope rather than to everything we do, and a method can be listed for laboratory work but not for site work, or the other way round. Ask us to confirm the accreditation status of the in-situ CBR testing you need before you book.

What should we send you to get a price for in-situ CBR testing?

Send the site postcode, how many test positions you need, the level being tested, your preferred dates and the specification or testing schedule if you have one. Tell us whether the positions are on open ground a vehicle can reach or need trial holes, because that affects the time on site and whether the method is suitable at all. Travel, position numbers and any out of hours working are the main cost drivers.