Undrained Shear Strength Test
Undrained shear strength is a fundamental property of cohesive soils, such as clays, and is typically measured using an unconsolidated-undrained (UU) triaxial test or a vane shear test. The undrained shear strength of soil is an important parameter in geotechnical engineering, as it is used to calculate the bearing capacity of foundations, stability of slopes, and other design parameters. The undrained shear strength is typically denoted by the symbol Su and is expressed in units of stress, such as kPa or psi.
Additionally, the undrained shear strength of soil can also be influenced by the soil's mineralogy, as certain minerals may contribute to the soil's cohesion or inter-particle friction. The presence of organic matter or other contaminants in the soil can also affect its undrained shear strength.
During undrained shear testing, the shear stress is applied to the soil sample faster than the water can drain out, which leads to an increase in pore water pressure. This increase in pressure affects the effective stresses within the soil, reducing the shear strength. The undrained shear strength is typically lower than the drained shear strength, as the pore water pressure reduces the effective stresses and shear resistance within the soil.
Why test for undrained shear strength?
Liquefaction may occur in saturated, loose sands subjected to seismic loading conditions due to the rapid loading rate. An undrained loading situation might cause this.
When clays and silts are subjected to rapid static loads and earthquake loads, undrained conditions nearly always arise because of the low material permeability.
The undrained shear strength is stress-independent, in contrast to the drained shear strength. This is because when the soil is not drained, the pore water will carry all of the extra load from the outside because it is much stiffer.
Undrained shear strength is frequently easier and less expensive to evaluate than drained strength. It can be determined with a pocket penetrometer, a vane shear test, an unconfined compression test, and UU triaxial test.
The undrained shear strength is an essential parameter for designing and evaluating the stability of soil structures, and it is also crucial for assessing the liquefaction potential of soil during seismic events.