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## CPT Dictionary: Soil Shear Strength

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 at right 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.) How is this relevant to soil testing? Well, consider a sliding failure in soil, such as occurs along a fault plane in an earthquake. Shear strength tells us a great deal about 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. su = (qc – σvo)/Nk 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. Several alternative methods may be used to estimate undrained shear strength via CPT, depending on the test conditions and available data. One such method uses pore pressure at u2 (directly behind the cone) in place of overburden stress: su = (qc – u2)/Nk The disadvantage of this method is [...]

## CPT Dictionary: Soil Liquefaction

In our last blog, we discussed using the CPT to estimate the shear strength of soil, which helps gauge how soil will behave during changes in stress. One important application of this capability is the estimation of soil liquefaction potential, meaning the potential of soil to dramatically lose strength when subjected to changes in stress. Liquefaction is of particular concern in sandy, saturated soils. Shaking due to an earthquake or other sudden force causes the grains of loosely packed, sandy soils to settle into a denser configuration. If the soil is saturated and the loading is rapid, pore water does not have time to move out of the way of settling soil: pore water pressure rises, effectively pushing the soil grains apart and allowing them to move more freely relative to each other. At this point, the soil can shift and flow like a liquid—hence the name liquefaction. This dramatic reduction of soil stiffness and strength causes soil to shift under pre-existing forces—say, the pressure of a building’s foundation or the pull of gravity on a slope. The increased pore pressure also increases the force of the soil on in-ground structures such as retaining walls, dams, and bridge abutments. How can the potential for these effects be evaluated using the CPT? The subject is complex, as the wealth of research on the subject over several decades shows! Many approaches for determining cyclic liquefaction potential rely on the cyclic stress ratio (CSR), which requires a seismic analysis of the site. It expresses the ratio of the average cyclic shear stress in an earthquake of a given magnitude and the effective vertical overburden stress at the test site. CSR = 0.65(MWF)(amax/g)(σvo/σ′vo)rd Where: MWF = Magnitude Weighting Factor = (Magnitude)2.56/173 amax = maximum ground surface acceleration g = acceleration of gravity, 9.81m/s2 σvo [...]