Geotechnical laboratory testing

What does geotechnical laboratory testing cover, and how is it different from soil testing for earthworks?

It covers the same classification tests plus the measured parameters a design needs: shear strength, consolidation, permeability, particle density and compaction behaviour. The difference is what the numbers are for. Earthworks testing asks whether a material is acceptable against a specification, geotechnical testing asks what values a designer can safely assign to a stratum for a bearing capacity, settlement or stability calculation. The same test can serve both, but the sampling and scheduling differ.

Why does my testing schedule name both BS 1377 and BS EN ISO 17892 for the same test?

Because the two standards overlap and the schedule is covering itself. The BS EN ISO 17892 series was adopted in the UK for water content, density, grading, oedometer, triaxial, direct shear, permeability and the Atterberg limits, and where a 17892 method exists it normally takes precedence over the equivalent British Standard clause. BS 1377 still carries compaction, CBR, MCV, the chemical tests and the in-situ methods. We work to whichever your specification names.

What is an oedometer consolidation test used for?

It measures how much a cohesive soil compresses under load and how quickly that compression happens, which is what a settlement calculation needs. The specimen is confined in a ring and loaded in stages, each stage held until primary consolidation is complete, so the test gives both the magnitude of settlement and its rate. It needs an undisturbed specimen, because a remoulded one has already lost the structure the result depends on.

Do you carry out permeability testing, and which method should be specified?

Yes. The method follows how permeable the soil is: constant head suits free draining granular material, falling head suits lower permeability soils, and very low permeability clays are normally tested in a triaxial or hydraulic cell under back pressure. A laboratory test measures one small specimen in one direction, so it can read lower than the mass permeability of the ground, which matters if the figure feeds a drainage or dewatering design.

Do you carry out chemical testing for foundation and buried concrete design?

Yes. Chemical and electro-chemical testing on soils, typically pH, water soluble and acid soluble sulfate and organic matter, is usually specified to BS 1377-3, and it is what an aggressive ground assessment to BRE Special Digest 1 is built on. Those results feed the design sulfate and ACEC classification that sets the concrete specification for buried elements. Groundwater samples are often needed alongside the soil, so check what the specification asks for.

How do laboratory results become the design parameters an engineer actually uses?

The results are factual data, and the designer selects characteristic values from them, which Eurocode 7 describes as a cautious estimate of the value governing the limit state rather than a simple average. That is why a spread of results across a stratum is more useful than one number, and why every result has to stay tied to its exact sample, depth and test condition. Results can also be issued as an AGS data file.

How does laboratory data sit alongside SPT, CPT and other field testing?

They answer different halves of the same question. Field testing gives a near continuous profile that shows where the ground changes and how variable it is, but usually through correlations rather than a directly measured parameter. Laboratory testing gives measured values at discrete depths on identified material. Most ground models use the field work to decide where to test and the laboratory results to put numbers on the strata that matter.

Why don't two laboratories get exactly the same number on the same soil?

Some variation is normal, because soil is not uniform and no two specimens are identical even from the same bag. Preparation choices matter as much as the testing itself: how a sample is subdivided, how long it was air dried, which compaction method was used and the rate a test is run at all move the answer. That is why methods are defined so tightly, and why a result only means anything with its conditions reported beside it.

Why can't a consolidation or effective stress test be speeded up?

Because the soil sets the pace, not the laboratory. A consolidation stage has to run until primary consolidation is complete before the next load goes on, and an effective stress triaxial has to saturate, consolidate and then shear slowly enough for pore pressure to stay in equilibrium through the specimen. Rushing either gives a number that is simply wrong. Low permeability clays are the slowest, and we confirm expected timings before the work is booked.

What is the difference between a factual report and an interpretative report?

We produce factual test reports: the method, the conditions the test was run under and the measured result, with no interpretation of what it means for your design. An interpretative report goes further, selecting design parameters, assessing the ground model and making recommendations, and that is your geotechnical adviser's work rather than a testing laboratory's. The separation is deliberate, so the factual data stays independent of the conclusions drawn from it. We are happy to discuss a result with you.

Which of the geotechnical methods you offer are 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, so it is settled method by method rather than by service, and each report states the status of the result on it. Send the list of methods from your schedule and we'll confirm which of them are covered before anything is instructed.

How should we put a testing schedule together without paying for tests nobody uses?

Start from the design questions rather than the sample list, and test the strata those questions depend on. Schedule the classification work first and widely, then target the slower strength, consolidation and permeability tests at the horizons that matter once the strata are confirmed. Send the logs, the site postcode, sample numbers and your reporting date, and we'll comment on the draft schedule before it goes to price.