CPT

It is critical to understand the difference between SPT data and CPT data.

SPT data and CPT data are both important and as you know, Cone Penetration Testing (CPT) 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 N_SPT, indicates the mechanical properties of the soil.

As with CPT data, a handful of corrections are commonly applied: for example, the N₆₀ value indicates N_SPT data corrected for the mechanical efficiency of a manual hammer, estimated at 60% at shallow overburden conditions.

Since SPT is one of the most common in-situ soil testing methods, you may find it necessary to compare information from both SPT and CPT tests, or convert from one set of parameters to the other, for example from SPT N₆₀ values to CPT tip resistance values.

Several methods have been proposed for calculating this relationship. Below are two of the most frequently used:

• Robertson and Campanella:

This method for correlating SPT and CPT data uses the following relationship between SPT N₆₀ data and CPT tip resistance:

(q_c/p_a)/N₆₀ 

q_c = tip resistance (psi)
p_a = atmospheric pressure (psi)

Soil behavior type can be determined from this equation based on the following table:

This is perhaps the simplest method for relating the results of these two tests, but it can cause some confusion when the results fall on the border of two soil behavior type zones, or in situations where the ratio of CPT to SPT data could indicate one of several different soil types.

• Jefferies and Davies:

This is a more robust method for determining SPT N-values based on CPT data, or vice versa.  It avoids the discontinuities of the first method, but it is not applicable to soil behavior type zones 1 (sensitive fine grained soils), 8 (very stiff sand to clayey sand), and 9 (very stiff fine grained soils).

First, one calculates the Soil Behavior Type Index, I_c, from the CPT data:

I_c = ((3.47 – log Q_t)² + (log F_r + 1.22)²)^½

Q_t = normalized tip resistance (dimensionless)
F_r = normalized friction ratio (%)

Next, the following equation relates the soil behavior type index to the SPT N₆₀ value:

(q_c/p_a)/N₆₀ = 8.5(1 – I_c/4.6)

Using these equations, one can easily work back and forth from one set of parameters to the other, or estimate soil behavior type directly from the calculation of I_c, per the table below.

Which ASTM Standard Cone Penetrometer Sizes is Best for Your Application?

The ASTM Standard Cone Penetrometer sizes depend on your application.

CPT cones are available in multiple sizes, but the 10 cm² cone is the industry standard. Other sizes, the most common of which is the 15 cm² cone, are essentially scale models of the 10 cm² cone, having the same proportions as specified by the ASTM Standard for CPT testing.

What factors determine what cone size you should use?

Most CPT cones range from 5 cm² to 15 cm² in cross-sectional area, though smaller cones (down to 1 cm²) are used in specialized lab or research applications. Different cone sizes have different advantages depending on the testing situation:

• The larger 15 cm² size is more robust and gives more accurate cone resistance values in very soft soils. Additionally, it has more room inside for additional sensors.

• Smaller piezocones have faster pore pressure sensor response and thus are better suited for characterizing very thin layers of soil.

• The 10 cm² cone is suitable for most applications. It is the industry standard and considered the reference penetrometer for field testing.

Cones in the 5 cm² to 15 cm² range have been shown to produce consistent data in most soils, so corrections for different sizes are generally not needed.

When using a cone outside this size range, corrections may be necessary to ensure that results are consistent with the body of CPT data: for example, very small cones tend to produce higher cone resistances than standard-size cones.

If there are questions as to the effect of scaling the penetrometer to either larger or smaller size, a 10 cm² penetrometer should be used in the same soils so that the results can be compared.

Penetrometers are made of high strength steel and designed to resist abrasion by soil, but over time, normal wear and tear may blunt the cone and affect the accuracy of the data.

Since the proportions of all penetrometer sizes are held to an exact standard, it is important to measure them daily to ensure that they remain within the ASTM standard tolerances. An excessively blunted or asymmetrically worn cone tip or sleeve should be replaced. 

It’s imperative to field check with calipers and document the diameter of the peizocone tip and sleeve before each sounding to validate that the diameter of the tip is smaller than the sleeve. 

Vertek can provide electronic documentation to help track your daily readings that can be stored on your field PC as well as easy-to-use field gauges, calipers, and load cells for more stringent field verifications.      

Minimum and Maximum Cone Measurements

Using CPT Pore Pressure Dissipation Tests to Characterize Groundwater Conditions

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, u_i) 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, u_o. 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:

    d_water = d_cone – h_w

    d_water = water table depth

    d_cone = depth of piezocone
    h_w = water head The water head, h_w, is the height of the water above the cone, which can be calculated based on the pore pressure and the unit weight of water:

    h_w = u/ɤ_w + z
    h_w  = water head
    u_o = equilibrium pore pressure
    ɤ_w = 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.

What is Seismic CPT (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 Seismic Cone Penetration Testing Work?

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.

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.

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:

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

q_t = q_c + u₂(1 – a)

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

Q_t = (q_t – σ­_vo)/ σ′­_vo

Some geologic knowledge of the test site – for example soil unit weight and groundwater conditions – is necessary to estimate σ­_vo 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, f_t, is calculated from the difference between the two measurements:

f_t = f_s – (u₂ ∙ A_sb – u₃ ∙ A_st)/A_s

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, F_r:

F_r = f_t/(q_t – σ­_vo)

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

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!

What can you learn from pore pressure data?

The 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.

A CPT cone that is equipped with one or more pore pressure sensors is called a piezocone, and a CPT test using a piezocone is often indicated with the abbreviation CPTu. Piezocones may have between one and three pore pressure sensors, located on the cone (denoted u₁), directly behind the cone (u₂), or at the top of the friction sleeve (u₃). Most piezocones for everyday applications use one sensor located at u₂ (see image below). 

The pore pressure sensor consists of a porous filter (usually made of plastic resin), a small cavity of incompressible, low-viscosity fluid, and a pressure transducer. The filter and tubing between the filter and transducer must be fully saturated with fluid, usually glycerin or silicon oil, to ensure fast and accurate readings. The filter must be replaced frequently so that it does not become clogged with soil.

The procedure for the CPTu test is slightly different than the basic CPT test, where tip resistance and sleeve friction data are taken continuously as the cone is pushed into the ground at a constant rate. To collect pore pressure data, the pushing of the cone must be paused long enough to take an initial reading, then allow pore pressure around the cone to dissipate as subsequent readings are taken. 

When the cone is pushed into the ground, pore pressure builds up around it until the in-situ moisture dissipates into the surrounding soil. The rate of dissipation depends upon the soil’s coefficient of consolidation, which indicates the compressibility and permeability of the soil. 

A fixed period of dissipation may be used, or in other cases dissipation is allowed to continue until reaching a specific value of U (usually 50%), calculated by the following ratio:

In addition to correcting against the effects of in-situ moisture and overburden stress, pore pressure data helps to ensure consistent measurements and soil classifications among different types and sizes of CPT cones.

The process of pore pressure data analysis involves a handful of new terms and formulas, so we’ll discuss it in depth in our next blog. In the meantime, you can get the specifics on Vertek’s full line of CPTu cones on our website, or by downloading our Geotechnical and Environmental Products Catalog.

As you analyze your CPT test data and CPT area, 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.

First of all, it is important to note that, since a traditional CPT test does not involve a soil sample, these charts are not designed to tell you the exact makeup of the soil. Instead, CPT tests indicate the soil’s physical and mechanical properties, or how it behaves. Hence, a CPT soil classification chart is technically referred to as a Soil Behavior Type (SBT) chart. 

Most CPT soil charts are derived from tip resistance (or normalized tip resistance, Qt) and friction ratio data.

• The tip resistance is measured in some unit of pressure (Bars, Pa, PSI, etc) and is usually plotted on the vertical axis. This axis is logarithmic, meaning it increases by orders of magnitude rather than linearly as it gets further from the origin. Thus you will see units of 10, 100 and 1000 marked an equal distance apart.
• The friction ratio is given on the horizontal axis. It is the ratio of the sleeve friction divided by the tip resistance: the two units of pressure cancel, so this unitless ratio is multiplied by 100 and given as a percent. This percentage is generally low: 10% would be considered a high friction ratio, since the CPT cone experiences greater pressure on its tip due to the shear strength of the soil than on its sleeve due to friction. Occasionally, this data is plotted logarithmically as well, as in the example chart above. 

The area of the chart is divided into a number of Soil Behavior Type zones (usually 9 or 12). Simply overlaying your data on the chart gives you an idea of the soil’s mechanical properties. Depending on the depth of your soil test and the type of soil you will be testing (i.e. fine-grained or coarse-grained), additional data and mathematical corrections may be used to account for the effects of overburden stress, pore water pressure, and other factors.

Thanks to the continuous nature of CPT data collection, soil behavior can be determined at any depth, creating a detailed soil stratigraphy profile.

This may sound like it involves a lot of computation, but with the appropriate tools, data analysis, and visualization are quick and simple.

With Vertek’s all-in-one CPT data acquisition and analysis unit, report-quality plots can even be generated in the field on a Windows-based laptop. .

CPT platforms designed for Cone Penetration Testing.

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Cone Penetration Testing (CPT) 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.

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. 

Cone Resistance represents the ratio of the measured force on the cone tip and the area of the normal projection of the cone tip. The cone resistance indicates the undrained (i.e., including in-situ moisture) shear strength of the soil. Sleeve Friction is the friction force acting on the sleeve divided by its surface area.

The relationship between these two measurements is expressed in the Friction Ratio, denoted R_f  and given as a percent. It is the ratio of the sleeve friction to the cone resistance. High friction ratios (high friction, low cone resistance) indicate clayey soils, while low friction ratios indicate sandy soils.

The relationship between friction ratio and cone resistance is the simplest method of identifying soil strata with a CPT system, and is especially convenient because the soil behavior type can be extrapolated immediately as the data is collected. An example soil classification chart is given below (though this example uses the corrected cone ratio q_t, which we’ll discuss in another blog).

As you can imagine, several factors can affect the accuracy of these predictions, for example:

• Overburden Stress: the pressure exerted on a substrate by the weight of the overlying material
• Pore Water Pressure: the pressure of the groundwater in the gaps between soil or rock particles

Fortunately, most modern CPT cones are equipped with pore pressure sensors: corrections based on this data provide more accurate and repeatable classifications, especially in soft, fine-grained soils. Mathematical corrections along with some geologic knowledge of the test site can resolve many of the other factors.

And yes, CPT systems can be used to take soil samples – and groundwater and soil gas samples, as well.

We will discuss the collection and interpretation of CPT data in more detail soon! To see Vertek’s full line of CPT cones, push systems, carriers, and data acquisition systems, check out our Product Catalog.