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Using CPT Pore Pressure Dissipation Tests

March 19, 2015 by Vertek Team

Using CPT Pore Pressure Dissipation Tests to Characterize Groundwater Conditions

Pore Pressure Dissipation Test

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?

Dissipation tests measure the decay of pore water pressure over a specified amount of time. With the advancing CPT push, excess pore water pressure builds up around the cone tip, measured via the filter element. 

During a dissipation test, the CPT is paused, and the pore water pressure decay readings are recorded in-situ.

The test is usually run until 50% of the pore water has dissipated (t50); this allows for the calculation of geotechnical parameters to assess the hydraulic conductivity for use in understanding the material’s settlement properties.

Dissipations are useful in helping understand how the soil will behave with heavy surface loads.

A dissipation test can be carried out during any CPTu sounding and does not require any additional set up. Furthermore, the results are instantaneous, allowing for quick decisions on site.

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:

The water table (or phreatic surface) depth is defined as the distance below the soil surface at which pore pressure is equal to atmospheric pressure. This can be roughly visualized as the level below which subsurface materials are fully saturated with groundwater.

  •  Especially in fine-grained soils, estimating the water table can be more complex than simply detecting moisture, since surface tension draws groundwater upwards, creating negative pore pressures. This is effect is called capillary rise.
    • Very low or negative pressures can be difficult to measure precisely with the piezocone, which is primarily designed to measure high pressures below the water table. In this case, the water table depth can be calculated by the following formula:

      dwater = dcone – hw

      dwater = water table depth

      dcone = depth of piezocone
      hw = water head

      The water head, hw, is the height of the water above the cone, which can be calculated based on the pore pressure and the unit weight of water:

      hw = u/ɤw + z

       

       

      hw  = water head
      uo = equilibrium pore pressure
      ɤw = unit weight of water
      z = distance, if any, between pressure sensor location and depth reference point on the piezocone

       

       

  • The rate of dissipation indicates the permeability or hydraulic conductivity of the soil – that is, the tendency of the soil to allow or resist the flow of groundwater.
     
    • A rapidly dissipating pore pressure indicates the presence of an aquifer (a porous region where groundwater tends to flow), while a slowly dissipating pore pressure indicates an aquitard (a compacted region that resists the flow of groundwater).

Multiple CPTu soundings can be used to generate a three-dimensional model of subsurface hydrogeologic conditions, including a method to accurately predict the localized groundwater table, estimate the direction and velocity of groundwater flow, and estimate the distribution of hydraulic conductivity.

These parameters can then be used to predict the characteristics and spread of shallow aquifers: this information is critical to deep foundation design, site investigations, and remediation design.

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Filed Under: CPT

Seismic CPT: Intro to Seismic Cone Penetration Testing

March 10, 2015 by Vertek Team

What is Seismic CPT (Cone Penetration Testing)?

Truck seismic graph

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 Seismic Cone Penetration Testing Work?

CPT cone CPTU cone

SCPT testing is performed as part of a normal CPT or CPTU test. Equipment consists of a CPT rig, push system, and:

  • SCPT Cone: The SCPT cone is a CPT or CPTU cone that is equipped with one or more geophone sensors. These sensors measure the magnitude and arrival time of seismic shear and compression waves.
  • Wave Generator:  Seismic shear waves are generated at the soil surface in one of two ways:
    • The simplest method is to press a steel bar onto the ground lengthwise using the weight of the CPT rig, then strike the end of the bar with a large hammer. An electronic trigger attached either to the hammer or the bar records the exact time of the strike.
    • Another method uses an electronic wave generator attached to the CPT rig. This method increases repeatability and reduces physical strain and testing time for the field team.

The CPT test must be paused briefly at the desired intervals to perform the wave generation and data collection. These pauses may be used to conduct a pore pressure dissipation test as well.

  • Data Acquisition System: As seismic waves are registered by the geophone sensors, data is transferred from the cone to the soil surface by wires that run through the pushrods. The SCPT data acquisition system logs this data and analyzes it to determine the speed of the waves based on their arrival time and the distance between the wave generator and the sensors. Finally, the wave speed is used to calculate shear modulus, soil liquefaction risk, and other parameters.
SCPT tests

Adding seismic testing to your CPT test is simple and cost-effective if you have the proper equipment. Seismic technology is an integral part of Vertek’s CPT systems: all our digital CPT cones contain integrated dual-axial or tri-axial geophone sensors, and the Vertek DataPack 2000 data acquisition system is fully compatible with seismic data.

Thus, seismic testing requires minimal set-up time beyond what is needed for a normal CPT test. If you outfit your CPT rig with an electronic wave generator, collecting seismic data is as simple as pressing a button.




CPT platforms designed for Cone Penetration Testing.

Vertek CPT has one goal. To make your business a success.

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Filed Under: CPT

CPT 102: Common Corrections in CPT Data Analysis

February 23, 2015 by Vertek Team

cpt report vertek

In a previous blog, we discussed the pore pressure sensor that is common to most modern CPT cones and briefly introduced why this reading is helpful in soil profiling. Today we’ll take a closer look at how pore pressure data is used to correct and analyze CPT data.

Pore pressure data is used to correct or “normalize” sleeve friction and cone 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. Corrections based on pore pressure data also help standardize soil behavior type characterizations when CPT cones of different shapes and sizes are used.

How are these corrections calculated, and how do they work?  

Correction of cone resistance data:

soil behavior type horizontal

The corrected cone resistance, qt, correct the cone resistance for pore water pressure effects.

qt = qc + u2(1 – a)

qc = cone resistance
u2 = pore pressure measured directly behind the cone
a = cone area ratio (this value is dependent on the design and geometry of the cone, and is determined via lab calibration)

Corrected cone resistance is used in calculating the normalized cone resistance, Qt, which indicates the cone resistance as a dimensionless ratio while taking into account the in-situ stress:

Qt = (qt – σ­vo)/ σ′­vo

σ­vo = total vertical stress
σ′­vo = effective vertical stress (the stress in the solid portion of the soil – in other words, the total vertical stress minus the stress due to in-situ water and air)

Some geologic knowledge of the test site – for example soil unit weight and groundwater conditions – is necessary to estimate σ­vo and σ′­vo.

Correction of sleeve friction data:

Sleeve friction data is sometimes corrected for the effects of excess pore pressure – that is, pore pressure that is generated in front of the cone as it is pushed into the ground. Excess pore pressures are usually different at the top of the cone (the pore pressure measurement denoted u3) and the bottom of the cone (the pore pressure measurement denoted u2). The corrected sleeve friction, ft, is calculated from the difference between the two measurements:

ft = fs – (u2 ∙ Asb – u3 ∙ Ast)/As

fs = sleeve friction
Ast = cross-sectional area of the top of the cone
Asb = cross-sectional area of the bottom of the cone (often the same as Ast)
As = surface area of the friction sleeve
CPT data acquisition system

This correction is not always possible, since many CPT cones have only one pore pressure sensor, usually located at u2. When u3 pore pressure data is available, the corrected sleeve friction is used to calculate the normalized friction ratio, Fr:

Fr = ft/(qt – σ­vo)

qt = corrected cone resistance
σ­vo = total vertical stress     

If u3 pore pressure data is not available, the uncorrected sleeve friction fs is substituted for ft.

How long is this going to take?

If you’re not looking forward to all this number-crunching, don’t worry – a good CPT data acquisition system will make these corrections for you. Vertek’s HT DataPack is an all-in-one unit for CPT data collection, analysis and plotting. Cone calibration information is stored in nonvolatile memory, and available corrections can be automatically applied to the data. You will be able to quickly analyze your data and even generate report-quality plots in the field. To learn more, download our catalog or check out our website!

Filed Under: CPT

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

February 16, 2015 by Vertek Team

Cone 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: [Read more…]

Filed Under: CPT

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

February 9, 2015 by Vertek Team

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

Dynamic Cone Penetration

It is similar to CPT (cone penetration testing) 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.

dcp data

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 roadbed 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.

Historically, one of the largest difficulties associated with DCP has been obtaining accurate depth difference measurements with a hand rule after each blow of the hammer.

As you can imagine, taking these measurements by sight and recording them by hand can be slow, finicky work.

Plus, to measure the total depth, the sum of these measurements is calculated, so it is easy to accumulate a troublesome amount of error if each measurement is even slightly off.

Fortunately, handheld electronics technology has alleviated these issues to a great extent. Vertek’s Handheld DCP System uses a smartphone app and a laser rangefinder to automatically count blows and measure, record, and plot depth.

From the smartphone, this data can be easily graphed in the field and transferred to a computer or client for reporting and analysis.

Automatic data collection saves time, increases accuracy, and means that the test can be efficiently completed by one person.

adcp solutions

To further increase the repeatability and efficiency of DCP testing, fully automated (ADCP) systems are available. ADCP rigs can be mounted on lightweight trailers, commercially available trucks, or ATVs.

Automated tests are both faster and more consistent than their manual counterparts, and also decrease physical labor for the operator.

DCP and ADCP testing and data analysis is a broad subject, so we’ll return to it in another blog.

In the meantime, be sure to check out the DCP testing information and video demonstrations on our site! You can also download our catalog to see the full specifications and capabilities of Vertek’s DCP and ADCP equipment.


General FAQs


What is a dynamic cone penetrometer?

The Dynamic Cone Penetrometer (DCP) is used to determine underlying soil strength by measuring the device’s penetration into the soil after each hammer blow.

How does a cone penetrometer work?

A cone penetration test rig pushes steel cone into the ground, generally up to 20m below the surface or until the cone reaches a hard layer. The steel cone contains an electronic measuring system that records tip resistance and sleeve friction.

What is DCP testing?

Dynamic Cone Penetration (DCP) testing is used to measure in-situ soil’s strength 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 characterize soil behavior continuously.

What is DCP in the construction industry?

Dynamic Cone Penetrometer, or DCP, is a tool used for evaluating the strength of soils on site. It also helps with monitoring the condition of granular layers and subgrade soils in pavement sections over time.


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Work with the world leader in the development and manufacturing of advanced in-situ soil testing apparatus.

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Filed Under: CPT

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