Moisture Condition Value testing

What is an MCV test and how is it carried out?

It measures the compactive effort a soil needs to reach near-full compaction, which in practice tells you how wet and workable the material is. A 1.5 kg specimen of soil passing a 20 mm sieve is placed in a 100 mm mould and a 7 kg rammer is dropped 250 mm onto it, with penetration recorded as the blows build up. The change in penetration between blow counts is plotted and the MCV read off where that change falls to 5 mm.

What decisions does an MCV result let the site make on the day?

It is used to decide whether material can be placed, reworked or set aside, usually within minutes of the sample being taken. Earthworks specifications set acceptability limits for cohesive fill as an MCV range, alongside or instead of a moisture content limit, and the site tests against those limits as material comes out of the cut. Series 600 of the Specification for Highway Works works this way, with project limits given in Appendix 6/1.

Which method is MCV testing carried out to?

MCV is usually carried out to the method in BS 1377-2, which took over the withdrawn Part 4, with BS EN 13286-46 as the European equivalent for unbound and hydraulically bound mixtures. Earthworks contracts normally name the method and the acceptability limits in the specification, and a rapid assessment procedure may be permitted where the full determination is not needed. We work to the method your specification names rather than a house version of it.

What is a moisture condition calibration and when do we need one?

It is a relationship between MCV and moisture content, established by testing the same soil at a series of moisture contents. Once that relationship exists for a material, an MCV measured on site can be read across to a moisture content, so acceptability limits expressed either way can be checked with one quick test. It is required where the specification asks for it and has to be established separately for each distinct soil type.

What counts as an acceptable MCV?

There is no universal figure. The limits are set in the earthworks specification for each class of material. A low MCV means the soil is wet and will not carry plant or compact properly; a high MCV means it is dry and stiff, which can make it hard to compact to the required density. What is acceptable depends on the soil, the fill class and what the material is being used for, so send us the specification and we will confirm what applies.

How does MCV compare with a moisture content test or plasticity testing?

MCV measures how a soil behaves under compaction, whereas moisture content and plasticity describe what it is. A moisture content needs oven drying and a result the following day, and liquid and plastic limits are laboratory classification tests used to set the acceptability ratio in the first place. MCV gives a number on site in minutes, which is why it is the one used for daily control rather than for classifying material.

How quickly can you give us a result in the field?

Each specimen takes a matter of minutes once the sample has been prepared, so results are available while the plant is still at the face. That is the point of the test: it is a control tool for keeping muck shifting, not a laboratory exercise. Where a full moisture condition calibration is needed, that involves several specimens at different moisture contents and takes longer.

How much sample do you need, and how fresh does it have to be?

Each specimen uses 1.5 kg of soil passing a 20 mm sieve, and specifications commonly ask for at least two specimens from each sample with the values averaged, so allow a bulk sample of several kilograms. It has to be tested promptly and kept sealed until it is, because the whole result depends on the moisture condition at the moment of sampling. A sample left open in a wagon overnight is no longer representative.

Which soils is the test suitable for, and where does it not work?

It is designed for cohesive soils, which is where acceptability is governed by moisture condition. Only material passing a 20 mm sieve goes into the mould, so stony soils have to be screened and a very stony fill may not give a representative result at all. Free-draining granular fills, very soft slurried material and some silts are better assessed by other means.

How much does the weather move the result?

Enough to change a decision, and that is a feature of the test rather than a fault with it. MCV records the condition of the material as it is at that moment, so a cut that was acceptable before a night of rain may not be the next morning, and a stockpile can dry back into range over a hot week. Re-testing after a change in weather is normal practice on an earthworks job.

Is the MCV method within your accredited scope?

Ask us to confirm for the method your specification names, because accreditation is granted test by test. 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. It applies to site testing and laboratory testing separately, so it is worth checking both if your programme uses each.

How is MCV testing priced, and what should we send with an enquiry?

Cost depends on the number of tests, whether they are one-off visits or an engineer on site through the earthworks and whether a moisture condition calibration is needed for each soil type. Travel, standby time while material is being won and any out-of-hours working also count. Send the site postcode, the earthworks specification, the soil types, the expected number of tests per day and your preferred dates.