Undrained shear strength

Definition

Undrained shear strength is the measure of a soil's resistance to shear stress when it is saturated and pore water pressure is not allowed to dissipate. This property is crucial in assessing the behavior of cohesive soils during rapid loading conditions, such as during an earthquake, where immediate strength rather than drained conditions needs to be considered.

5 Must Know Facts For Your Next Test

1. Undrained shear strength is commonly denoted as 'Su' and is often determined using tests such as the unconfined compressive strength test or vane shear test.
2. In saturated clays, undrained shear strength remains relatively constant until a certain level of pore water pressure is reached, making it critical for short-term stability analysis during events like earthquakes.
3. Undrained shear strength is typically lower than drained shear strength because it accounts for excess pore water pressure that develops under rapid loading.
4. The total stress acting on saturated cohesive soils can be broken down into effective stress and pore water pressure, where undrained shear strength primarily relates to effective stress conditions during dynamic loading.
5. The relationship between undrained shear strength and soil properties can be expressed using empirical correlations, often linking it to the plasticity index of the soil.

Review Questions

• How does undrained shear strength influence the stability of structures during an earthquake?
  • Undrained shear strength plays a key role in determining the short-term stability of structures during an earthquake. Since soils may experience rapid loading without allowing for pore water pressure dissipation, understanding the undrained shear strength helps predict potential failure mechanisms. A lower undrained shear strength can lead to increased risks of liquefaction or slope failure, thereby affecting design decisions for earthquake-resistant structures.
• Discuss the factors that affect undrained shear strength in cohesive soils and how they relate to site classification.
  • Several factors affect undrained shear strength in cohesive soils, including moisture content, consolidation history, and soil composition. Site classification often incorporates these factors to categorize soils based on their engineering behavior during seismic events. For instance, heavily overconsolidated clays typically exhibit higher undrained shear strengths compared to normally consolidated clays, influencing the amplification factors used in seismic design.
• Evaluate the implications of neglecting undrained shear strength in geotechnical engineering when designing foundations in earthquake-prone areas.
  • Neglecting undrained shear strength can have serious implications for foundation design in earthquake-prone areas. It may lead to an underestimation of potential ground failure risks, resulting in inadequate structural support. The failure to consider this factor could result in catastrophic outcomes during seismic events due to excessive settlement or even collapse, highlighting the necessity for accurate assessments and appropriate design considerations based on undrained conditions.

Related terms

Cohesive Soil: Soil that has significant clay content and exhibits plasticity, meaning it can deform without cracking and has a higher undrained shear strength compared to non-cohesive soils.

Pore Water Pressure: The pressure exerted by water within the soil pores, which can influence effective stress and thus the undrained shear strength of saturated soils.

Effective Stress: The concept that describes the stress carried by the soil skeleton, which is equal to the total stress minus the pore water pressure; vital for understanding soil behavior under load.