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CPTU - "the sound of Music for the Geotechnical Engineer"
lecture held by Kjell Elmgren ( Kolkata 2007, Tampa 2008 ).

Background and History.
Since ancient times, and especially with the construction of our first railways, engineers have had the need for probing the ground before laying out their construction plans. At first, the way of doing this was to simply push an iron bar into the ground and trying to ”sense” the bearing capacity of the soil.
One way to measure was to take record of if the bar was sinking by the weight of one man or two men. The first mechanical CPT ( Cone Penetration Test ) was introduced around 1932 in Holland. Pushing a cone of a certain size at the end of the iron bar, which is now a steel tube, means that one is able to measure the resistance from the soil without influence of side friction on the tubes. In order to do this, it was necessary to have an inner rod in the tubes so that one can push the cone only and the tubes are standing still. The force needed for pushing was in some way measured and recorded. Picture 1.
The mechanical CPT is, for different reasons, not as accurate as is required by todays standards. To solve this problem, the ELECTRIC CPT was introduced around 1955, originally measuring Cone resistance and Local friction, but from 1975 also measuring the Pore water pressure. Now, the geotechnical designer had a tool that he could really call his ”Sound of Music”.

CPTU - adding Pore pressure measurement
Adding the measurement of generated pore pressure by 1975 opened up a number of new possibilties to make a more advanced interpretation of the CPT results, now designated CPTU, the U being the pore pressure parameter. When the cone is penetrating coarse material like sand, the pore pressure more or less follows the static pressure. When penetrating clay, a great over-pressure is generated and in silty soils, both over- and under-pressure will be generated.
By the combination of these three parameters, science is now at a stage where a number of basic soil character parameters can be found by just pushing a single CPTU probe into the ground. In the following, I will try to clearify which parameters and how.

Soil parameters that can be found by interpretation of CPTU:
Soil Stratigraphy Undrained shear strength Effective stress Sensitivity Overconsolidation ratio Relative density
Friction angle Stiffness ratio Module
Permeability

1. Soil Stratigraphy
Obtaining information about the different layers of soil having different properties, is the first object with the CPTU sounding. The CPTU result in Picture 2. Shows 35 meters of soft clay with a few sand layers at 21 and 31 meters depth.

Another result from CPTU performed on the sea bed of the Red Sea in Sudan.
The sounding starts 1 meter down from the actual sea bed.
1 – 2.3 meters is very soft material and 2.3 to 3 meters, the cone has penetrated a coral reef.
At 3 to 5.5 again the soft material and eventually the limestone begins getting successively harder.
It is clear that any building structure must be placed at the hard material at 6 meters and the coral reef must be removed.

Pore pressure Point resistance Friction Friction ratio Soil type

2.Undrained shear strength.
The undrained shear strength is the most important parameter when establishing slope stability and excavated walls stability. There are several ways to get information about shear strength. By taking samples of the soil, one can shear off the sample using a shear machine in the laboratory. More often, the triaxial test is used. These are quite accurate but it incorporates the risk of sample disturbance.

The in – situ methods are Field Vane test and the CPTU. The former is an accurate direct, however time consuming, method and not continious. The CPTU provides a continious information and is fast and efficient.
Picture 4 shows a comparison between undrained shear strength obtained from CPTU with values from triaxial tests on samples. ( SGI )

3. Effective stress
An effective stress interpretation method has been developed in Norway by Sennerset, Janbu and Sandven. A bearing capacity formula can be expressed as

It includes a soil attraction parameter which can be found in a soil type table by the same authors.

4. Sensitivity:
The sensitivity ( St ) is defined as the ratio of undisturbed undrained shear strength to totally remoulded undrained shear strength.
The soil around the friction sleeve can be regarded as remoulded and the point resistance can be regarded as the resistance from the soil before failure.
Schmertman therefore suggested that sensitivity can be estimated from the friction ratio Rf in percent as:

S = __Ns____
Rf

The value of the constant has later been found to vary between 5 and 10 with 7.5 as the mean value. The N will vary with OCR and mineralogy. A way to find the value for a local soil would be to calibrate against the Field Shear Vane test.

5.Overconsolidation ratio (OCR)

The Overconsolidation ratio ( OCR ) is to some extent possible to obtain from CPTU results but must be seen as complimentary to oedometer or triaxial tests.

In clays or silty clay, OCR can be obtained using the net cone resistance using the formula in Picture 6.
This requires however knowledge about the liquid limit ( WL ). The Initial vertical stress has first to be calculated and then the OCR can be found using the formula:

6. Relative density

Based on tests in sweden and Norway, Larsson and Mulabdic, Rad and Lunne and others proposed this chart for obtaining a rough estimate of soil density in clays.
An interation is necessary since soil density is also needed for computation of net cone resistance.

Another way of estimating soil density is to use the soil classification of Robertson & campanella 1986:

1)	Sensitive fine grained	17.5
2)	Organic material	12.5
3)	Clay	17.5
4)	Silty clay to clay	18
5)	Clayey silt to silty clay	18
6)	Sandy silt to clayey silt	18
7)	Silty sand to sandy silt	18.5
8)	Sand to silty sand	19
9)	Sand	19.5
10)	Gravelly sand to sand	20
11)	Very stiff fine grained	20.5
12)	Cemented sand	19

7. Friction angle.
In an approach to find relationships between CPT and strength caracteristics of sand, full scale experiments with a tank, filled with sand, has been performed.
Lunne and Christophersen, Robertson and Campanella have suggested the relations as shown in Picture 8.

The diagram to the rightis valid for silica sands, it tends however to give too low friction angles for highly compressible sands.

8. Stiffness ratio.
The undrained Youngs Modulus ( E ) is an important parameter when calculating initial settlements. For sand and coarse silt, ( E ) can be calcultaed as:

0.93
E = 4.3 qt if under 90 MPa.

9. Constrained Modulus ( M ).
An interpretation of the M modulus has, among others been suggested by Kulhawy and Mayne using the net cone resistance. Ref. to picture 9.

Compression properties of clays should normally not be evaluated from CPTU tests since this test is done under totally undrained conditions.
It is however possible to obtain valuable information about the permeability of clays by performing the so called Dissipation test. Ref. to following chapter 10.

10. Permeability.
When the generated pore pressure around the CPT was first measured by Torstensson, Vlasbloom and Elmgren around 1974, it was observed that clay generated very high pressures. The reason for this is that by introducing a new volume of firm material ( the CPT cone ), the soil around the cone is stressed to failure which results in a very high pore water pressure. This pressure can dissipate quickly in sand but very slow in clay.
Torstensson suggested that the rate of decline of this over-pressure could be a measure of the permeability of the soil.
1977 Torstensson also suggested two models for calculating the coefficient of consolidation ( ch ), one based on spherical expansion and one based on cylindrical expansion.
Since then, a variety of models of interpretation have been developed and refined.

Dissipation test.
To perform a dissipation test is very simple. When your CPTU cone is penetrating a clay layer, ( can be identified by the the high pore pressure reading ) you simply stop the penetration and record the pressure values against periods of elapsed time. In the beginning, you take note of the pressure every 10 seconds for the first 2 minutes, thereafter less frequent. In modern dataloggers, this taken care of automatically.

The usual way of testing is to wait until the excess pore pressure ( generated pressure minus static pressure ) has fallen by 50 %. This is called T50.
When the pressure has fallen below 50%, you just continue the penetration. If you want to make another dissipation test, you stop again and repeat the sequence.

The time for 50% dissipation can vary extensively from 10 seconds in silt to several hours in clay. Since the in-situ measurement of permeability is quite a dificult task,, the simple use of CPTU dissipation tests can give valuable information indeed. It can however not be considered as more exact than about one order of magnitude but it will, of course clearly show variations of permeability.

Note:
In some dense fine or silty sand, negative ( lower than static ) pore pressures may be recorded.
This is due to dilatancy effects in the soil structure.

CPTU - "the sound of Music for the Geotechnical Engineer" lecture held by Kjell Elmgren ( Kolkata 2007, Tampa 2008 ).

CPTU - "the sound of Music for the Geotechnical Engineer"
lecture held by Kjell Elmgren ( Kolkata 2007, Tampa 2008 ).