Triaxial Testing in Glasgow: Shear Strength Parameters for Foundation Design

A recent ground investigation on a brownfield site near the Clyde revealed a dense layer of glacial till overlying weathered sandstone. The contractor needed reliable effective stress parameters to model the excavation support system before a single pile was driven. Standard index tests gave an indication, but the design required the full Mohr-Coulomb envelope. That is where the triaxial test becomes essential. Glasgow's geology, with its mix of stiff boulder clays, silts, and variable bedrock, demands more than just SPT numbers. A CPT test can map the stratigraphy quickly, but for critical infrastructure or deep basements, you need the strength data from a consolidated undrained triaxial test to feed directly into the geotechnical model. We run these tests in our accredited lab, handling the complex stress paths that Glasgow's overconsolidated soils produce.

A triaxial test on Glasgow's glacial till typically shows an effective friction angle between 27 and 33 degrees, but the cohesion intercept can vary significantly with the clay fraction.

Process and scope

In Glasgow, many sites on the northern side of the city encounter the stiff, overconsolidated till known locally as 'boulder clay'. This material has a strong preconsolidation pressure from past glaciation. When we sample it, the clay often has fissures and occasional cobbles that make specimen preparation a skilled task. The triaxial test lets us measure the friction angle and cohesion under the site's specific stress conditions. We run multi-stage consolidated undrained tests with pore pressure measurement, following BS 5930:2015, to capture the full stress-strain behaviour. This data then feeds into a slope stability analysis if you are cutting into a natural hill in the West End, or into the design of retaining walls for a tight urban excavation. The test also helps confirm the undrained shear strength for short-term loading scenarios, which is critical for assessing base heave in deep digs.

Site-specific factors

The drift geology map of Glasgow shows a complex sequence of till, laminated clay, and sand and gravel lenses within the city centre. The laminated clay, often found near the Clyde corridor, can have a very low undrained shear strength and a high sensitivity. If you base a foundation design solely on borehole logs without a triaxial test, you risk underestimating the long-term settlement or the short-term stability during construction. An incorrect assessment of the effective cohesion in a fissured till can lead to a retaining wall design that is inadequate for the actual lateral earth pressures. We have seen projects where the design assumed a cohesion of 10 kPa, but the triaxial test on undisturbed samples gave a value closer to 3 kPa. That difference changes the factor of safety on a deep excavation from comfortable to critically low.

Typical values

Parameter	Typical value
Test Standard	BS 5930:2015, EN ISO 17892-8:2018
Sample Diameter	38 mm, 50 mm, 70 mm or 100 mm
Common Test Types	CIU, CID, UU (multi-stage)
Pore Pressure Measurement	Back pressure saturation with electronic transducers
Shear Rate (Undrained)	0.02 to 0.1 mm/min depending on soil permeability
Reported Parameters	c', φ', cu, φu, E', Af (pore pressure coefficient)

Complementary services

Consolidated Undrained (CIU) Triaxial Test

The standard test for determining the effective stress strength parameters (c' and φ') of saturated cohesive soils. Includes saturation, consolidation, and shearing stages with pore pressure measurement.

Multi-Stage Triaxial Test

A single specimen is sheared in multiple stages at different confining pressures. This is a practical alternative when only limited undisturbed sample material is available from a borehole.

Unconsolidated Undrained (UU) Triaxial Test

Provides the total stress parameters (cu and φu) for fine-grained soils. Used for short-term stability analysis where immediate loading is applied and no drainage occurs.

Stress Path Testing

Follows a specific stress path to simulate in-situ loading conditions, such as excavation unloading or embankment loading. Provides stiffness parameters for advanced numerical modelling.

Questions and answers

What is the typical cost of a triaxial test suite in Glasgow?

For a set of three specimens to define the Mohr-Coulomb envelope, the cost typically ranges from £1,440 to £2,300. The final figure depends on the soil type, specimen size, and whether you need a single-stage or multi-stage test. We provide a fixed-price quote after reviewing the sample condition and your project specification.

How long does it take to get the results from a consolidated undrained triaxial test?

The full test programme, including saturation, consolidation, and shearing of three specimens, typically takes seven to ten working days. Silt and clay samples from Glasgow sites often have low permeability, so the consolidation and saturation phases alone can take several days. We can provide preliminary results earlier if the shearing stage is complete.

What size of sample do you need for a triaxial test?

We can run tests on specimens from 38 mm up to 100 mm in diameter. For Glasgow's glacial till, which can contain fine gravel and cobbles, a larger 100 mm diameter specimen gives the most representative strength data. The sample must be undisturbed, preferably from a U100 or Shelby tube, to preserve the in-situ structure and water content.