Soil testing

What tests are in a standard soil classification suite?

A standard classification suite is usually moisture content, particle size distribution, the Atterberg limits and particle density. Moisture content tells you how wet the material is against its optimum, grading says what the soil is actually made of, the liquid and plastic limits describe how the fine fraction behaves with water, and particle density lets void ratio and air voids be calculated from any density result. Specifications often add organic content, sulfate and pH.

What does BS 1377 cover, and which part applies to our tests?

BS 1377 is the British Standard for testing soils for civil engineering purposes, and it has been restructured into Part 1 for general requirements and sample preparation, Part 2 for the laboratory classification and geotechnical property tests, Part 3 for chemical tests and Part 9 for the in-situ methods. Where a European method exists, such as the BS EN ISO 17892 series, it normally takes precedence. Older specifications still name the withdrawn Part 4 for compaction and CBR work, which now sits in Part 2.

Do I need disturbed or undisturbed samples?

It depends on the test. Grading, plasticity, compaction and remoulded CBR run perfectly well on disturbed bulk samples, while shear strength, consolidation and bulk density need an undisturbed sample that keeps the soil's structure intact. Disturbed samples are dug from a trial pit or taken off the excavation and bagged, undisturbed samples are driven or hand cut as a tube or block and sealed straight away. The investigation specification normally states the sample type required for each test.

How big does a soil sample need to be?

Big enough to represent the material and to make the specimens the test needs. A few hundred grams covers moisture content and the Atterberg limits, a few kilograms covers most classification work, and around 6 kg makes a single remoulded CBR specimen, rising to 15 kg or more where a compaction curve has to be established first. Coarse material needs far more, because the sample has to represent the largest particles present.

How should soil samples be packed and labelled before they come to us?

Disturbed material should go into sealed heavy duty bags or airtight tubs, filled well so little air is trapped and kept under about 25 kg a bag for safe handling. Undisturbed tubes should be sealed at both ends and carried upright, not dropped or left in the sun. Every container needs its own durable label giving the project, the pit or hole reference, the depth and the date.

How long does a soil sample stay usable once it has been taken?

Moisture sensitive work is the urgent part, so moisture content and anything depending on the as dug condition should be scheduled as soon as the sample reaches us, because even a well sealed bag loses water over time. Properly sealed material stays usable for grading, plasticity and particle density for a good deal longer. Undisturbed tube samples deteriorate faster and should be tested while the structure is still representative.

Can you take the samples as well as test them?

Yes. We can sample on site and carry the material through to the laboratory ourselves, which keeps the chain from sampling to result in one place and avoids any argument about how the sample was taken or stored. Site compaction monitoring can be combined with the laboratory work in the same way. If your own team is sampling, we'll set out quantities, containers and labelling before you start so nothing has to be repeated.

What is the difference between classification testing and performance testing?

Classification testing identifies what the soil is, performance testing measures how it behaves under a defined condition. Moisture content, grading, plasticity and particle density put the material into a recognised description a specification can be checked against. Compaction, CBR and shear strength go further and give a number a designer can use, but only for the density and moisture condition the specimen was prepared at, which is why that condition has to be agreed first.

What will we get back when the testing is finished?

A report for each test or group of tests, giving the sample references and where the material came from, the method used, the result and the accreditation status of that test. Classification results are normally presented as a table alongside grading curves and a plasticity chart where those tests were run. Any deviation from the specified method is stated. Reporting format and distribution are agreed before the work starts.

What makes a soil testing schedule take longer than expected?

The tests themselves are usually not the slow part, the conditioning is. Air drying a wet cohesive sample, a four day soak on a CBR, or a consolidation stage that has to run to the end of primary settlement all set their own pace and cannot be shortened. Incomplete sample references and a missing schedule hold work up just as effectively. We'll confirm the expected timing for your scope before the work is booked.

Is the soil testing we need covered by 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, so a schedule of soil tests will often contain some methods that sit inside it and some that do not, and the report states which is which. Ask us to confirm the accreditation status of the particular tests you need before you book.

How do you price a soil testing schedule?

By the number of samples and the preparation each test needs, with travel on top if we are collecting the material. Wet, stony or crushable soils take more preparation than a clean sample that arrives ready to test, and conditioning or soaking stages add laboratory time. Send the site postcode, the material type, the tests you need, rough sample numbers and your preferred dates. Call 01908 505833 or email enquiries@hixtra.co.uk.