triaxial test












        Shear Failure

Fine-grained soil is tested in compression.  Undisturbed specimens cut from  tube samples and disturbed specimens are loaded in compression, recording load and deflection measurements.

Laboratory strength tests of soil are similar to testing concrete cylinders, but can be performed with or without lateral confining pressures. The unconfined test uses axial loading without lateral confining pressures, making it the simplest and easiest laboratory method of estimating strength.

To more accurately simulate actual loading conditions in the field, lateral confining pressures can be applied using a triaxial test, which is a completely different apparatus.

Research has shown that the strength of a soil determined by compression testing varies with extremes of the length to diameter ratio and the rate of strain. It is generally accepted that ratios of length to diameter of 1.5 to 3.0 are satisfactory. Ratios of 2.0 and 2.5 are commonly employed. Similarly, satisfactory rates of strain are 0.5 to 2.0% per minute. For most samples 0.5 to 1.0% per minute is used most frequently.

Unconfined compressive strength is calculated the same as for any material, with an additional calculation of the area change from bulging.  The shear strength is defined as half the compressive strength.

Equation Set 1.1: Unconfined strength

Since soils tend to deform much more than concrete, the area of the specimen changes to maintain constant volume through the test (bulging).  Thus, the average cross sectional area at a particular deformation during the test is calculated using:




Unconfined compression machine
Wire saw
Vertical lathe
Mitre box
Caliper and ruler
Split cylindrical mold
Pocket penetrometer




Procedure and data sheets [right-click | "save target as" Excel Office 2000 ]


Al-Khafaji & Andersland (1992), pages 472-481.

Lambe (1951), 29 - 42.