Shear strength testing

What does shear strength testing tell you about a soil?

It measures the resistance a soil offers before it shears, which is the strength figure a geotechnical designer works from. Depending on the method, the result is either an undrained shear strength, cu, or the effective stress parameters cohesion and angle of friction, c' and phi'. Those values feed slope stability, retaining wall and bearing capacity calculations. Which of them you need depends on what the design is checking.

Which shear strength test do I need: shear box, triaxial, vane or unconfined compression?

It depends on the soil and on whether the design needs short-term or long-term strength. A shear box suits granular soils and strength along a defined plane, and is the usual route to a residual value. Triaxial testing suits cohesive soils and is the only route to effective stress parameters. A laboratory vane gives a quick undrained strength in soft clay, and unconfined compression suits firm intact cohesive material.

What is the difference between undrained and drained shear strength, and which does my design need?

Undrained strength describes the soil immediately after loading, before pore water has had time to drain away, while drained strength describes it once those pore pressures have dissipated. Short-term cases such as a temporary cut face or a rapidly built embankment are normally checked undrained. Long-term slope stability and retaining wall design normally need effective stress parameters from consolidated triaxial testing. Plenty of designs check both conditions.

What do cohesion and friction angle actually mean to a designer?

They are the two parameters that describe how a soil's shear strength changes with the stress acting on it: cohesion is the strength available at zero normal stress, and the friction angle is how much further strength is gained as that stress increases. Designers put the pair into slope stability analyses, earth pressure calculations for retaining and embedded walls, and bearing capacity checks for foundations.

Which standards is shear strength testing usually carried out to?

Shear strength testing is now usually carried out to the BS EN ISO 17892 series: part 7 for unconfined compression, part 8 for the unconsolidated undrained triaxial test, part 9 for consolidated triaxial tests on saturated soils and part 10 for direct shear. These superseded the equivalent clauses of BS 1377 parts 7 and 8, which some specifications still name. The laboratory vane sits in BS 1377-2.

Why do undisturbed samples matter, and what should we send you?

Because the result only describes the soil in the condition the specimen was in, and disturbance during sampling destroys the structure that gives a cohesive soil much of its strength. Triaxial, unconfined compression and laboratory vane testing all need intact tube samples, typically 100 mm diameter, kept sealed, the right way up and tested before they dry. Granular soils that cannot be sampled intact are recompacted to an agreed condition instead.

How much sample do you need for each shear strength test?

As a guide, a triaxial set needs an intact tube sample with enough undisturbed length to cut three specimens, a 60 mm shear box needs only a kilogram or two of material, and a 300 mm large shear box needs around 35 kg. Coarse soils need more of everything. Confirm quantities, containers and delivery arrangements with us before you sample, because a sample that arrives short cannot be made to go further.

How long does shear strength testing take, and why is triaxial work slow?

An unconsolidated undrained triaxial set, a shear box on a granular soil or a laboratory vane can be turned round reasonably quickly, but effective stress triaxial testing cannot. Each specimen has to be saturated, consolidated and then sheared slowly enough for pore pressures to stay in equilibrium, so a low permeability clay can take one to two weeks per stage. We confirm the expected timing before the work is booked.

Can one specimen give both cohesion and friction angle, or do we need a set of three?

You need more than one point on the failure envelope, so those parameters come either from a set of three specimens tested at different confining pressures or from a multistage test on a single specimen. A set of three is generally preferred where there is enough intact material, because multistage relies on one specimen surviving successive stages. The report gives each specimen's result alongside the derived parameters and the failure plot.

Why has the measured shear strength come back lower than the design assumed?

The usual causes are sample disturbance and the condition of the specimen rather than the test itself. Cohesive samples lose strength if tubes are knocked, stored badly, allowed to dry or left too long before testing, and a recompacted specimen only reflects the density and moisture content it was prepared at. Fissures and laminations matter too, since a small specimen may miss or exaggerate them. Send the borehole records with the samples.

Is shear strength testing covered by your UKAS accreditation?

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, rather than to every service. Shear strength covers several distinct methods, specimen sizes and sample conditions, so ask us to confirm the status of the particular test you need before it is scheduled.

We have samples from a borehole. How do we get shear strength testing booked in?

Tell us what you are holding and what the design needs. Send the site postcode, the soil type, the tests and specimen sizes required, how many samples are involved and your preferred dates, with the specification or testing schedule if there is one. Tell us whether the design needs total stress or effective stress parameters, because that changes the programme considerably. Cost is driven mainly by specimen numbers, sample preparation and how long a consolidated test occupies a cell.