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CPT University

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

Understanding the Relationship between SPT Data and CPT Data

- Posted by VertekCPT

Vertek CPT Data Acquisition System

As you know, Cone Penetration Testing is not the only method for determining the mechanical properties of soil. Another method is the Standard Penetration Test, or SPT: in this test, a borehole is drilled to a desired depth, then a hollow sampler is inserted and driven downwards with a hammer. The hammer blows are counted until the sampler travels the desired depth (usually 18”) – this number, denoted NSPT, indicates the mechanical properties of the soil. As with CPT data, a handful of corrections are commonly applied: for example, the N60 value indicates NSPT data corrected for the mechanical efficiency of a manual hammer, estimated at 60% at shallow overburden conditions.

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Topics: Soil Testing, CPT Data

Intro to Seismic CPT

- Posted by VertekCPT

TruckfromseismicgraphWhat is Seismic Cone Penetration Testing?

Seismic CPT or SCPT is a method of calculating the small strain shear modulus of the soil by measuring shear wave velocity through the soil. The small strain modulus is an important quantity for determining the dynamic response of soil during earthquakes, explosive detonations, vibrations from machinery, and during wave loading for offshore structures. The wave speeds and moduli derived from seismic CPT measurements aid in the determination of soil liquefaction potential and improve the interpretation of surface seismic surveys by providing wave speed profiles as a function of depth. Seismic waves from SCPT tests have been detected at depths of up to 300 feet.

How does it work?

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Topics: Introductory, Soil Testing

Beyond the Basics: Contamination Detection and Other Applications of CPT Equipment

- Posted by VertekCPT

Vertek modular soil samplerCone Penetration Testing equipment was originally designed – and is still most commonly used – to characterize subsurface soil behavior types. But when you invest in CPT equipment, you are getting the capability to do much more. A variety of sensors and in-situ samplers can be integrated into CPT modules, making CPT equipment a versatile and efficient choice for contamination detection, environmental site assessment, and other field applications.

CPT equipment has several advantages over conventional hollow stem auger drilling and percussion drilling based methods, especially in contaminated soils. Specialized CPT tests can identify contaminants and determine the physical extent of the contamination with minimal disturbance of the soil, thus avoiding costly disposal of drill cuttings and minimizing contact between field personnel and potentially hazardous materials.

Here’s an overview of some tests and technologies that you can harness via CPT equipment:

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Topics: Soil Testing, Geotechnical Services

Intro to CPTu: What Can You Learn From Pore Pressure Data?

- Posted by VertekCPT


Vertek CPTu Cone and Data PackThe most basic CPT tests classify soil based on tip resistance and sleeve friction measurements. In coarse soils and shallow testing depths, this data may be sufficient to accurately characterize the soil behavior. However, most modern CPT cones incorporate a third measurement: pore water pressure. What does this measurement mean and how can it add to our understanding of soil behavior?

Pore pressure is simply a measure of the in-situ groundwater pressure, i.e. the water pressure in the “pores” between soil grains. This data is used to determine the compressibility and permeability of the soil, as well as indicating groundwater conditions. It is used to correct or “normalize” the sleeve friction and tip resistance readings in the presence of in-situ moisture and overburden stress. This is especially important in soft, fine-grained soils where in-situ moisture takes longest to dissipate, and in tests at depths greater than 100 feet.

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Topics: Cone Penetration, Introductory, Soil Testing, CPT Data

How to Read a CPT Soil Behavior Type Chart

- Posted by VertekCPT

Normalized Soil Behavior Type Zone chart with sample dataAs you analyze your CPT data, you are likely to come across several different charts designed to classify soil type based on CPT results.If you are new to the field, these charts can be a bit confusing, so here’s a brief overview of one of the more common chart types.

Soil behavior classification via CPT is fast, efficient, and frequently automated via software. Still, understanding the classification method is important, as it will help you to recognize and determine the cause of any errors or irregularities in the data.

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Topics: Cone Penetration, Introductory, Soil Testing, CPT Data

Is a Sieve Analysis Accurate?

- Posted by VertekCPT

introductory

sieve-analysis

If you regularly use Cone Penetration Testing on the job, you probably already know that there are a number of alternative soil testing methods out there. Some of the more common procedures include the Standard Penetration Test, which has been covered before in this blog, and the sieve analysis, also known as the gradation test. Most commonly used in civil engineering, this basic soil testing method is used to assess the particle size distribution of soil and other granular material. 

But is sieve analysis accurate?

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Topics: Introductory, Soil Testing

How to Interpret Soil Test Results from CPT Testing

- Posted by VertekCPT

introductory

soil-test-results

Even if you already have a solid grasp of what Cone Penetration Testing is and how CPT rigs test soils, understanding soil test results is a bigger task. You likely already know that CPT rigs are equipped with automated interpretation programs, but that doesn't mean test results are easily readable right away. Fortunately, even if you aren't a technician, it is possible to gain some understanding into soil test results. Read on to find out how. 

The basics of soil test results

At the most basic level, the results of CPT testing are based on the relationship between cone bearing, sleeve friction and pore water pressure. With these three measurements, you can learn quite a bit about soil composition and conditions. For example, friction ratio measured by the sleeve is used to determine soil type. Soil is then classified according to the Unified Soil Classification System (USCS).

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Topics: Introductory, Soil Testing, CPT Data