VertekCPT: Do you know about Soil Shear Strength ? Sponsored

Shear strength is the ability of a material to resist shear forces - that is, forces that produce a sliding failure in the material parallel to the direction of the force. The diagram above demonstrates shear stress, along with tensional and compressional stress. (What's the difference between a stress and a force? Stress is defined as force per area.)

But, how is this relevant to soil testing? Consider a sliding failure in soil, for exaple as occurs along a fault plane in an earthquake. Shear strength indicates how the soil will behave under shear forces and during changes in stress, for example due to an earthquake or excavation. The In-Situ shear strength of soil is difficult to measure, and many methodologies for doing so have been proposed. In general, estimating undrained shear strength - that is, the shear strength of the soil with in-situ moisture - using the CPT is accomplished via the relationship between overburden stress and cone resistance, as shown in the equation below.

Where: 

• su = undrained shear strength (unitless)
• qc = cone resistance (psi)
• σvo = overburden stress (psi)
• Nk = empirical cone factor (a unitless constant) Nk is determined in the lab, for example via triaxial compression tests. The exact value varies based on the type of reference test used, so it is important to be consistent in this regard. Most test methods return values between 10 and 30, varying with factors such as OCR (over-consolidation ratio), pore pressure, and soil plasticity.

Read the entire article and find out several alternative methods that may be used to estimate undrained shear strength via CPT, depending on the test conditions and available data, in Vertek CPT University Page!