Chalk testing

Why does chalk need different tests from other fill materials?

Chalk is a weak porous rock that breaks down when it is worked, so grading and plasticity describe it poorly and the properties that actually control its behaviour are density and moisture. A dense intact chalk performs much like a rock fill, while a low density or weathered chalk holds a great deal of water inside its pores and can turn to slurry under plant. That is why intact dry density and saturation moisture content lead the programme.

What is intact dry density, and why does it matter for chalk?

Intact dry density is the dry density of an individual intact lump of chalk, measured before the material is broken down, and it is the best single indicator of how the chalk will behave as fill. A high intact dry density means low porosity, less water held in the pores and a material that stands up to handling and compaction. Series 600 of the Specification for Highway Works names it as an acceptability property for chalk general fill.

What is saturation moisture content, and what does a high value mean?

Saturation moisture content is the moisture content chalk would have if every pore in an intact lump were filled with water, and it follows directly from the intact dry density: the more porous the chalk, the higher the figure. A high value means the chalk can carry a lot of water, remoulds easily and is more likely to become unstable when worked. Like intact dry density it is measured on an intact lump, not on bagged bulk material.

What does the chalk crushing value test tell you?

The chalk crushing value measures how readily chalk breaks down when it is worked, which is what decides whether it survives placing and compaction or degrades into putty chalk. It is run on the 10 mm to 20 mm fraction using the moisture condition apparatus and is normally specified to BS 1377. Read alongside intact dry density and saturation moisture content it gives a reasonable picture of how the material will behave under plant.

Which specification usually governs chalk used as engineered fill?

On highway and most civils earthworks it is Series 600 of the Specification for Highway Works, where chalk is handled as Class 3 material. The acceptability properties listed for Class 3 general fill are moisture content and intact dry density, with the limits set in the contract's own appendix rather than in the specification itself. Class 3 also brings handling and compaction rules, so read those alongside the test requirements.

What sample do you need for chalk testing, and how should it be taken?

Intact lumps, not bagged and broken bulk material. Intact dry density and saturation moisture content both need a lump that can be measured whole, typically around a kilogram with a volume of a few hundred millilitres. Take it by hand from a fresh face or excavation rather than off a stockpile, wrap it so it neither dries out nor crumbles in transit, and send separate bulk material if crushing value or moisture content work is also wanted.

What happens if chalk is placed and compacted too wet?

It remoulds into a saturated putty that loses strength, will not carry plant and can leave the fill temporarily unstable. Chalk holds water inside the particles as well as between them, so once the lumps are crushed that water is released and the material behaves quite differently from the way it did in the cutting. Series 600 expects filling to stop until the chalk recovers sufficient strength, with the areas of instability recorded.

Can chalk be used in capping or granular fill as well as general fill?

It can where the specification permits it, but chalk used as a constituent of granular fill classes is normally subject to a saturation moisture content limit of 20 per cent, which rules out the more porous low density chalks. The higher the duty of the layer, the tighter the material requirements get. Test the chalk against the class you actually intend to use it in, because material acceptable as general fill may not pass as capping.

Does the time of year affect when chalk earthworks can be carried out?

Yes. Series 600 lets the contract define periods when earthworks in chalk and trafficking of chalk areas are not permitted, because wet ground and wet material are exactly the conditions that cause degradation. Chalk also cannot be stockpiled or double handled, so material has to move straight from the cutting to the fill. That ties earthworks and testing tightly together, and acceptability testing needs doing early enough to be useful.

How are chalk test results used on site?

They decide whether the chalk can be used as fill at all, and then how it has to be handled once it is. Intact dry density and saturation moisture content support the acceptability judgement against the specification, while moisture content is the day to day control test during placing. Results are reported with the sample reference, where it came from, the method used, the accreditation status and any deviation from the specified method.

How do we confirm which chalk tests are accredited?

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, and a chalk programme often mixes routine classification methods with more specialist ones, so the position can differ test by test within the same schedule. Send us the list of tests your specification calls for and we'll confirm the status of each before you book.

Why does the price of chalk testing vary between jobs?

The number of samples, how far the site is and whether we collect the material or you send it in. Intact lump work needs careful handling and preparation, so a programme combining intact dry density, saturation moisture content and crushing value costs more per sample than moisture content alone. Send the site postcode, the earthworks specification or relevant appendix and the dates results are needed by, and we'll price it against your programme.