Soil & aggregate sampling

What makes a sample representative?

A representative sample holds the same proportions of coarse and fine material, in the same moisture condition, as the material it came from. That is achieved by combining several increments taken from different positions and depths rather than one scoop, then reducing the combined sample to laboratory size by riffling or quartering. Aggregate sampling is commonly carried out to BS EN 932-1, with reduction to BS EN 932-2. A poor sample cannot be corrected later in the laboratory.

How do you take a representative sample from a stockpile or a delivered load?

By taking increments from several positions and depths once the disturbed surface material has been removed, and avoiding the toe of a stockpile where the coarse fraction collects. Stockpiles segregate as they are built, so a single sample off the surface usually reads finer or coarser than the material as a whole. Where it can be done safely, sampling the discharging stream or the conveyor gives a better sample, taking the full width each time.

How is a sample taken from a trial pit or a trench?

From the face or the base at a stated depth within a single stratum, rather than from mixed arisings on the spoil heap. Each sample is recorded with its depth, position and a description of the material, because a result without a reliable depth is difficult to use. Where strength or density has to be measured on undisturbed material, a tube or block sample is needed instead of a bulk bag, and that has to be planned before the excavation is dug.

How is a sample of fresh concrete taken from a pour?

A composite sample is built up from increments taken at several different points during discharge, passing the scoop through the full width and depth of the moving stream. Sampling fresh concrete is usually carried out to BS EN 12350-1, and the sample is remixed before any test or cube is made from it. The sample is deliberately larger than the tests require, commonly by about half again, so nothing has to be repeated.

How much material should we collect for the tests we have scheduled?

It depends on the test and on the largest particle size in the material, so the quantity is worth settling before anything is collected. A moisture content on a fine soil can need only a few hundred grams and classification tests a kilogram or two, while compaction work runs into tens of kilograms and reaches around 50 kg on coarse material. Send us the scheduled tests and we'll confirm the quantity and the container for each one.

What containers do samples need, and do they have to be sealed?

Anything tested for moisture has to go straight into an airtight container, sealed as it is filled, because a sample that dries out in an open bag gives a moisture content describing the bag rather than the ground. Bulk samples normally go into rigid tubs or double-bagged heavy-duty sacks that survive handling. Undisturbed tube samples are sealed at both ends and kept upright, and lime or cement treated material needs sealing immediately because it carries on reacting.

What has to go on a sample label?

A unique sample reference, the site, the position or chainage, the depth, the date taken and who took it, together with a short description of the material. Write it on the container itself as well as on any tag, in something that survives water and mud. Where the tests are already known, listing them on the label saves time at the laboratory. Missing or duplicated references are the most common reason a sample cannot be used.

What is chain of custody, and does it affect whether a result can be relied on?

Chain of custody is the documented record of who held a sample from the moment it was taken until it was tested, and it is what ties a result back to a specific position on a specific day. Without it a result still has a number, but nothing links that number to the ground it came from. It matters most when a result is challenged, when material is rejected or when testing supports a contractual decision.

How quickly does a sample need to reach the laboratory, and what changes on the way?

Samples for moisture-sensitive tests need to reach the laboratory quickly, while samples for classification and grading are far less urgent. Moisture content, moisture condition value and anything on lime or cement treated material are the time-critical group: moisture is lost through packaging and treated material continues to react in the tub. Heat, frost and rough handling also do damage, so collection or delivery arrangements are agreed when the sampling is booked.

If we take the samples ourselves, how does that show on the report?

The report records the sample as supplied by the client and states that the results relate to the sample as received, because we cannot vouch for how or where material was taken when we were not there. Sampling is a separate activity from testing and can carry its own accreditation entry, so a client-taken sample changes what the report is able to say. The testing itself is unaffected, provided the quantity, container and condition suit the scheduled tests.

Does accreditation cover sampling as well as testing?

Sampling and testing are assessed separately, so one can sit within an accredited scope while the other does not. Hixtra holds UKAS accreditation to ISO/IEC 17025 for the tests listed in its schedule of accreditation, number 8996, and accreditation is specific to the listed methods and scope. Site and laboratory activities carry different entries. Ask us to confirm whether the sampling and the testing you need are covered before booking.

What do you need to quote for sampling?

Send the site postcode, the material, how many samples are needed and where they are being taken from, the tests they are for and your preferred dates. Cost is driven by the number of positions and visits, travel, whether plant or an excavation is needed to reach the material, the containers required and how samples get to the laboratory. If a delivery is already booked and you need someone there, call 01908 505833.