AASHTO T 236-08 (2018) PDF Download

The AASHTO T 236-08 (2018) standard provides procedures for determining the consolidated drained shear strength of soil materials through direct shear testing. This test can be conducted in either a single shear or double shear configuration. The advantage of the direct shear test in a consolidated drained condition is that excess pore pressures can be dissipated relatively quickly due to the short drainage paths within the specimen.

The test can be carried out on all types of soil materials, including undisturbed or remolded samples. The results obtained from the direct shear test are applicable to field conditions where complete consolidation has occurred under the existing overburden, and failure occurs slowly, allowing excess pore pressures to dissipate. Additionally, the test is useful for determining the shearing resistance along weak planes within the soil material.

It is important to note that if the failure is forced to occur on or near a horizontal plane at the middle of the specimen, it may not necessarily happen along the weakest plane, which could lead to an overestimation of shear strength parameters. Only when weak planes or interfaces between dissimilar materials are being tested and are located within the forced failure zone can the shear resistance along these planes or interfaces be evaluated accurately.

However, the direct shear test is not suitable for developing precise stress-strain relationships or evaluating other associated quantities such as moduli within the test specimen. This limitation is due to the non-uniform distribution of shearing stresses and displacements and the slow rate of displacement, which allows for the dissipation of excess pore pressures but also permits plastic flow of soft cohesive soils. It is essential to ensure that the testing conditions represent the specific investigation being conducted.

The AASHTO T 236-08 (2018) standard is similar to ASTM D 3080-72 (2003), with the exception of the units of measurement used (SI units in the AASHTO standard).