The shear strength of soils is critical in geotechnical engineering in the analysis of the stability of the soils as well as the behavior of the soils under a load. The triaxial compression test is one of the most frequent laboratory tests to do so, with three significant forms being Unconsolidated Undrained (UU), Consolidated Undrained (CU), and Consolidated Drained (CD). The CD triaxial test is unique because it allows drainage at the consolidation and shearing stages of the test; that is, pore pressures do not build up in the specimen.
Key ASTM Standards
ASTM D7181 Standard Test Method for Consolidated Drained Triaxial Compression Test of Soils. The standard specifies the procedures to be followed in conducting the CD triaxial test, which provides the parameters of the drained shear strength of soil to be utilized in its design.
ASTM D4767 Standard Test Method of Consolidated Undrained Triaxial Compression Test of Cohesive Soils. The CU test can be regarded as a method of determining pore water pressures.
ASTM D2850 Standard Test Method of Unconsolidated Undrained Triaxial Compression Test on Cohesive Soils. Establishes the UU test, which is often used when a fast test is required, in which there is no drainage.
ASTM D2435/D2435M Standard Test Methods of One-Dimensional Consolidation Properties of Soils, Supplementary data on consolidation, which is used to support triaxial testing.
Methodology
The CD triaxial test is conducted in a system procedure to guarantee that the measurement of the drained shear strength is reliable. A confining pressure is applied all around a cylindrical soil specimen, typically encased in a rubber membrane in a triaxial chamber. In the stage of consolidation, drainage occurs until the total pressure of the pore water is completely dissipated, and stabilization of the volume change occurs. At the shearing stage, the specimen is compressed along an axial direction at a low strain rate, thus permitting uninterrupted drainage and eliminating pressure accumulation at the pore. Axial load, deformation, and change of volume are all measured during the test. Findings are presented in the form of the effective stress parameters, which are the parameters used to draw failure envelopes and identify the shear strength of soils under drained conditions.
Applications
The CD triaxial test is very common in geotechnical engineering, which requires long-term drained soil stability. It is used in foundation engineering to give the necessary information in designing shallow and deep foundations on sandy soils. In earth dam and embankment design, the test is used to determine that the soil can sustain long-term stress without undue deformation. It is also applied in the construction of tunnels and excavations in predicting soil behavior under the conditions of drained soil.
Importance
The CD test is unlike the undrained tests, which are only applicable in short-term loading conditions; it depicts the performance of soils in long-term drainage scenarios. Hence, it is invaluable in determining the long-term stability. Through ASTM standards, laboratories maintain consistency, repeatability, and precision of the results, which gives the engineers confidence in designing. Finally, the CD triaxial test is a foundation of soil mechanics and foundation engineering, which is directly related to the safety and longevity of civil infrastructure.
