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CPT Dictionary: Soil Liquefaction

- Posted by VertekCPT

Sand boils are a dramatic manifestation of soil liquefaction, shown here in a 2011 earthquake in Christchurch, New Zealand. Source: http://resources0.news.com.au/images/2011/02/23/1226010/776488-a-vehicle-stuck-in-the-liquification.jpg

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.

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Topics: Experienced, Cone Penetration, Soil Testing, CPT Dictionary

CPT Dictionary: Soil Shear Strength

- Posted by VertekCPT

Geological stresses (image source: http://earthquake.usgs.gov/learn/glossary/images/stress_types.gif)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. 

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Topics: Experienced, Soil Testing, CPT Dictionary

CPT Dictionary: Overburden Stress

- Posted by VertekCPT

in situ soil strataOverburden stress, also called vertical stress or overburden pressure, is the pressure imposed on a layer of soil by the weight of the layers on top of it. Overburden stress can cause errors or drift in CPT measurements, creating the need for correction factors in deeper tests depths and soft or fine-grained soils. However, overburden stress is also useful in determining the soil’s mechanical properties. In this blog, we’ll give an overview of the effect of overburden stress on CPT testing and what we can learn from it.

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Topics: Soil Testing, CPT Dictionary