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Cone Penetrometer Testing - Video

Articles > Cone Penetrometer Testing - Video

Welcome to my series of geotechnical engineering webcasts

This web cast is an introduction to cone penetrometer testing

In this webcast, I'll provide a short overview of the cone penetration test commonly called

the CPT.

My objective here is to provide a simple introduction to this test method, explaining how the test

is performed I'll describe the measurements taken during

the test and explain how those measurements are made

Finally, I'll discuss some of the applications of the CPT test

I will not be discussing details of how to analyze CPT data

CPT testing is most often performed using large truck mounted rigs.

These rigs have a hydraulic jack system to raise and level the truck.

The weight of the truck is used a as a reaction mass when pushing the CPT probe into the ground.

For sites that cannot be accessed by a large truck, smaller track mount rigs are available

Very small track mounted rigs or rigs that fit on skid loaders also available for sites

with very limited access These smaller rigs are light weight and must

have an anchor system to hold them down since the mass of the rig itself is not sufficient

to provide the reaction needed to push the CPT probe.

The red oval is highlighting one of the anchors for the track mounted rig.

I'll be using the truck mounted rig to illustrate the CPT testing process in this webcast, since

it's by far the most common rig.

However, the testing process is essentially the same regardless of what type of rig is

used.

The cone penetration test starts my locating the rig on site at the desired location

The rig is then leveled using its external hydraulic jacks

The test consists of pushing an instrumented probe into the ground and measuring the forces

the soil applies to the probe as it penetrates the ground.

The probe is pushed into the soil using a hydraulic jack system located inside the truck

The jack system allows the probe to be pushed in at a steady rate of approximately 20 mm/s

As the probe is pushed into the ground, the jack is cycled up and down and additional

rods are added until the probe has reached the desired depth of investigation or until

it can no longer be advanced.

When the probe can no longer be pushed is has reached a point we call refusal

For most CPT rigs, the limiting factor on how deep the probe can be pushed is the mass

of truck against which the jack system pushes to advance the probe.

Now let's take a more detailed look at the CPT probe itself.

The CPT probe consists three separate parts A 60 degree conical tip which measure the

end bearing resistance to penetration.

A friction sleeve which measures the side friction.

Between the cone tip and the friction sleeve is a flexible porous donut shaped element

through which pore pressure measurements are made.

Rods are attached at the top of the probe and used to push the probe into the soil.

The earliest CPT probes did not contain the pore pressure element and measured only tip

resistance and side friction.

The probe I'm showing you here is called a piezocone, because it measures pore pressure

in addition to tip resistance and side friction.

When we use this probe, the test called the CPTU test, with the U standing for pore pressure.

Today, nearly all CPT probes are peizocones and the test is simply referred to as the

CPT or CPT test.

There are a number of different sized CPT cones as shown in this photo.

All four of the cones shown here consist of a 60 degree conical tip, a friction sleeve,

and a pore pressure element located just between the cone and the friction sleeve.

The smallest cone shown here has a cross sectional area of 2 cm2 which corresponds to a diameter

of just 1.6 cm.

Because of its small size, this cone can be used to delineate very thin soil layers.

However it has a relatively low load capacity due to its small size and is generally used

only in soft to medium clays or silts.

The largest cone shown here with a cross-sectional area of 40 cm2 is normally used in situations

where medium sized gravel is expected.

The most common sized cone used for testing typical soils is the 10 cm2 outlined in red.

Let's take a look at a cutaway section of the piezocone to see how the measurements

are made.

Inside the probe there is an inner rod that passes through the friction sleeve to the

cone element.

The cone is attached to the inner rod through threaded connection.

The inner rod is machined with a square shoulder part way down the shaft.

The friction sleeve has a matching shoulder designed such that the friction sleeve bears

on the inner rod at the shoulder.

There are two load cells measuring vertical forces in the probe.

The lower load cell measures the vertical force below the shoulder.

The upper load cell measures the vertical force above the shoulder.

In addition to the load cells, the probe contains a pressure transducer to measure pore pressure.

When the probe is pushed into the ground, the cone must push the soil out of the way

as the probe advances.

The soil imparts a vertical load on the end of the cone as it is pushed out of the way.

The cone transfers this end bearing load to the inner rod.

Because the load is applied at the end of the inner rod, both load cell measure this

end bearing force.

The soil at the side of the probe imparts a shear force along the outside of the friction

sleeve.

The friction sleeve transfers this side friction to the inner rod at the shoulder where the

friction sleeve and the inner rod meet.

Therefore the side friction force is measured by only the upper load cell.

By simultaneously reading both the upper and lower load cells it is possible to separate

the end bearing load measured by both load cells

from the side friction force which is measured by only

the upper load cell The end bearing is reported as a stress called

the cone resistance or tip resistance.

The symbol q-sub-c is used to represent the tip resistance and is equal to the measured

end bearing load, F-sub-c divided by the cross-sectional area of the cone, A-sub-c.

Q-sub-c is normally reported in units of tons per square foot or kilograms per square centimeter.

The side friction is reported as a stress called the cone side friction or simply side

friction.

The symbol f-sub-sc is used to represent the side friction and is equal to the side friction

force, F-sub-s divided by the outside area of the friction sleeve, A-sub-s.

It is common to express the side friction by the friction ratio, R-sub-f which is simply

the side friction, F-sub-sc divided by the tip resistance, q-sub-c.

The friction ratio is normally presented as a percentage.

The pore pressure transducer is hydraulically connected to the flexible porous element sandwiched

between the cone and friction sleeve.

It measures the pore pressure directly behind the cone as the probe is pushed into the soil.

The pore pressure is reported in the same units as the tip resistance and side friction.

Some CPT probe measure the pore pressure at locations other than directly behind the cone

as shown here.

The symbol u-sub-2 is used to designate the pore pressure measured at this location, directly

behind the cone.

This is by far the most common location for measuring pore water pressure.

So the measurements we get from the piezocone are

tip resistance friction ratio and

pore pressure As you can see in this animation, these three

parameters are measured continuously as the probe is pushed into the soil

Because the CPT provides a continuous profile of measurements, it is very useful in delineating

different soil layers.

Note that the CPT readings above the green line are notably different from those below

the line The soil above the green line produced a relatively

high tip resistance, a low friction ratio and a very low pore pressure

In contrast, the soil below the green line produced a relatively low tip resistance,

a high friction ratio, and a high pore pressure We can use the CPT measurements to classify

the soil based on its behavior.

This is done using the chart developed by Robertson and Campanella in 1983.

As you can see, this chart has different soil classification regions based on the measured

friction ratio and tip resistance.

The area in the upper left-hand corner of the classification chart is an area of high

tip resistance and low friction ratio.

Note that this area is labeled as a region of sands and gravels.

Sands and gravels have high frictional resistance but no cohesion.

Therefore, when the CPT probe is pushed into sands and gravels they produce relatively

high tip resistance as the soil shears around the tip of the probe.

However, once the cone tip has pushed the sand or gravel out of the way, the soil provides

very little side friction on the probe because these soils have no cohesion.

For this reason soils exhibiting high tip resistance and low friction ratio are generally

sands and gravels as the chart indicates.

Conversely, clay soils generally have lower frictional resistance and provide less tip

resistance during CPT testing.

However, due to the significant cohesion of such soils, they stick to the side of the

friction sleeve and generate a higher friction ratio.

Therefore if we follow a line from the area of high tip resistance and low friction ratio,

in the upper left-hand corner of the classification chart, down and to the right, to an area of

low tip resistance and high friction ratio, the soil classification changes from gravel

and sand, to sitly sand, to sandy silt, to silty clay and finally to clay.

Although our illustration does not have numerical values for the CPT results,

we can see that the upper soil layer has high tip resistance with low friction ratio, indicating

that it is a sandy soil The lower soil layer, by contrast, is characteristic

of a clayey soil with low tip resistance and high friction ratio

Furthermore, we observe a large positive jump in pore pressure as the probe enters the second

layer.

This is an excess pore pressure generated during shearing of the soil as it passes around

the cone.

Because of this positive shear induced excess pore pressure, we can deduce that the lower

soil layer is probably a normally consolidated soil.

It is important to note that this classification process is known as soil behavior type classification.

We call it behavior type classification, because it depends on the soil's behavior during SPT

testing not on the physical properties of grain size distribution and Atterberg limits

used in the traditional Unified Soil Classification System.

In order to have a definitive classification of these soils according to the Unified Soil

Classification System, we would have to take samples and perform the proper lab tests to

determine grain size distribution and Atterberg limits.

Another common variant of CPT probe is the seismic cone.

Like the piezocone, this CPT probe measures tip resistance, side friction and pore pressure.

It is designated SCPTu, with the S standing for seismic

This probe contains a 3-axis geophone in addition to the load cells and pore pressure transducer

in the piezocone.

The geophone allows the probe to detect seismic waves in.

Using the geophones and a shear wave source located at the ground surface, we can determine

the shear wave velocity of the soil as a function of depth.

The shear wave velocity is a particularly important variable in determining the liquefaction

potential for cohesionless soils.

To make shear wave measurements, the CPT probe is pushed into the soil and tip resistance,

side friction, and pore pressure measurements are taken as I've already described.

However at intervals where we want to measure the shear wave velocity, the probe is stopped.

While the probe is stopped a shear wave is generated at the ground surface.

The shear wave source can be as simple at using a mallet to hit one of the truck supports,

though it is very common today to use an automated shear wave generator which employs a solenoid

attached permanently to the truck supports.

The shear wave generated by the source propagates through the soil and is picked up by the geophones

in the probe.

Knowing the distance the wave travels, ls,1, and the arrival time of the wave from the

source, ts,1, we can compute the average shear wave velocity for the soil in this region,

vs,1, as ls,1 divided by ts,1.

After completing the shear wave measurement, which takes only a few moments.

The cone is advanced again and collects tip resistance and side friction data.

We stop the cone at some deeper depth, generate another shear wave, and repeat the measurements,

this time determining ls,2 and ts,2.

This time, we can compute the average shear wave velocity for the soil between the first

measurement point and the second, that is vs,2, as the difference in the two travel

distances divided by the difference in arrival times, as I've shown.

The actual computational method is slightly more complicated than this, but conceptually

it is the same.

The process is repeated by stopping the cone at successive depths and generating shear

waves until the CPT sounding is complete.

Using the data collected, we can then develop a plot of shear wave velocity as a function

of depth.

One of the characteristics of the CPT is that it does not take soil samples.

At sites where disposal of drilling cuttings is a problem, either because of the location

of the site, or because the cutting may be contaminated, this can be an advantage.

However, in general, this is a short coming of the CPT test.

However this problem can be mitigated.

While the CPT probe itself cannot take soil samples.

It is possible to attach a separate sampling device on the standard CPT push rods and retrieve

a soil sample using the same equipment used to push the CPT probe.

Samples are collected using a CPT push sampler which is approximately the same diameter as

the CPT probe but longer As shown in this cutaway drawing, the sampler

consists of an inner rod and piston

an outer cylinder and a locking mechanism

When the locking mechanism is engaged, the inner rod and piston move together with the

outer cylinder.

When the locking mechanism is released, the inner rod and piston move independently.

By using locking mechanism to lock or release the inner and outer parts of the sampler,

it is possible to retrieve samples as I'll show you next.

I'll use this cutaway illustration to show how the push sampler retrieves samples

With the locking mechanism engaged, the sampler is pushed down to just above the location

where we want to take a sample.

We then release the locking mechanism and push the outer cylinder is down.


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