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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

Using CPT Pore Pressure Dissipation Tests to Characterize Groundwater Conditions

- Posted by VertekCPT

In a previous blog, we talked about how pore pressure data is used to correct and adjust soil behavior type characterizations – but this is only one application of this important and revealing information. Pore pressure data can also be used to estimate the depth of the water table and the direction and rate of groundwater flow. This information is useful both for site characterization and for geo-environmental and remediation applications.

What is a Pore Pressure Dissipation Test?

As a CPT cone is pushed into saturated subsurface soil, it creates a localized increase in pore pressure (denoted excess pore pressure, ui) as groundwater is pushed out of the way of the cone. In a pore pressure dissipation test, the downward movement of the cone is paused and the time it takes for the pore pressure to stabilize is measured. This stable pore pressure is called equilibrium pore pressure, uo. This information allows the user to identify important hydrogeologic features:

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Topics: Cone Penetration, Introductory

What is DCP testing, and how does it compare to CPT?

- Posted by VertekCPT

Vertek DCP and Data Acquisition UnitDynamic Cone Penetration (DCP) testing is used to measure the strength of in-situ soil and the thickness and location of subsurface soil layers.

It is similar to CPT in that a metal cone is advanced into the ground to continuously characterize soil behavior. However, unlike in CPT, where the cone is driven into the ground at a constant rate by varying amounts of force, in DCP, the cone is driven by a standard amount of force from a hammer, and how far the cone moves with each blow is used to determine the soil density and properties at that level.

In DCP testing, the pushing force is applied by manually dropping a single or dual mass weight (called the hammer) from a fixed height onto the push cone unit. The resulting downward movement is then measured. Unlike CPT systems, basic DCP equipment is hand-portable and may be limited to test depths of 3-4 feet: this makes it a good choice for shallow testing applications such as road bed construction and maintenance. Since DCP is essentially hand-powered, it is cheaper and more portable than CPT equipment, but the possibility of human error makes it trickier to obtain consistent and accurate data.

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Topics: Cone Penetration, Introductory, DCP

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

CPT 101: Determining Soil Profiles from CPT Data

- Posted by VertekCPT

CPT Cones and Data Acquisition SystemCone Penetration Testing allows the tester to identify the nature and sequence of subsurface soil types and to learn the physical and mechanical characteristics of the soil – without necessarily taking a soil sample.

How does it work?

During a CPT test, a hardened cone is driven vertically into the ground at a fixed rate, while electrical sensors on the cone measure the forces exerted on it. The zone behavior type of the subsurface layers can be extrapolated from two basic readings: cone or tip resistance and sleeve friction. 

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Topics: Cone Penetration, Introductory, CPT Data

What Can You Reveal Using Fluorescence Detection?

- Posted by VertekCPT

introductory

fluorescence-detection

Even if you use CPT technology daily to test soil, you may not be aware of the further advantages CPT testing has to offer beyond its more commonly used or basic geotechnical functions. Take fluorescence detection, for example. Fluorescence detection records a fluorescent response to a specific excitation of automatic carbons in a chemical. This excitation is caused by an ultraviolet light source. But you're probably wondering how fluorescence detection can help you. Read on to find out!

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Topics: Cone Penetration, Introductory, Soil Testing

What is Triaxial Testing and is it the Best Method for Testing Soil?

- Posted by VertekCPT

introductory

triaxial-testing-and-testing-soilThose familiar with soil testing probably already know that there are a number of ways to test soil. One of the most common methods is the Standard Penetration Test, which is best known for its simplicity and versatility, but is held back by its lack of accuracy compared to more advanced options. More advanced methods include, of course, Cone Penetration Testing and Mud Rotary Drilling, both of which are common. Another common method is Triaxial Testing. 

What is Triaxial Testing?

In order to conduct Triaxial Testing, you need a Triaxial Apparatus, which is made up of a Triaxial cell, universal testing machine and pressure control panel. For testing soil and other loose granular materials like sand and gravel, the material is placed in a cylindrical latex sleeve and submerged into a bath of water, or another liquid, which puts pressure on the sides of the cylinder. A circular metal plate at the top of the cylinder, called a platen, then squeezes the material. The distance the platen travels is measured, along with the net change in volume of the material.

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Topics: Cone Penetration, Introductory, Soil Testing

Cone Penetration Testing Glossary of Terms

- Posted by VertekCPT

introductory

This brief glossary contains some of the most frequently used terms related to CPT/CPTU. These are presented in alphabetical order.

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Topics: Cone Penetration, Introductory

Building a CPT Truck in Less Than 3 Minutes (Video)

- Posted by VertekCPT

introductory

This short video compresses about a month of construction time on a 20 Ton CPT Truck built by Vertek CPT at our Vermont facility during the summer of 2014. CPT Trucks are popular for those looking for maximum push force and all-in-one mobility. This truck will be delivered to the customer upon completion of the internal components.

Large trucks provide greater push force and improved working environs while smaller trucks provide greater mobility in tight spaces. Contact us to see which is best for you. Our new S4 Push System also provides a path to entering the CPT market with limited investment.

 

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Topics: Cone Penetration, Vertek Partners, Video Posts