Static Cone Penetration Test

Cone Penetration test (CPT) is a type of in-situ test which is carried out to determine different geotechnical properties of soil. CPT is of two types:

Static Cone Penetration Test (SCPT)
Dynamic Cone Penetration Test (DCPT)

We have already discussed the dynamic cone penetration test in the previous blog. If you are curious about DCPT, then you can find it on the link given below:

DCP test – Dynamic cone penetration test Principle, Procedure, Calculation

Today, we will be talking about the static cone penetration test.

The static cone penetration test is also known as the Dutch cone test. This test is standardized by, “IS: 4968 (Part III)- 1976– Method for subsurface sounding for soils- Part III static cone penetration test”.

The static cone penetration test is a type of cone penetration test in which cone is used to penetrate the ground surface. In SCPT, we do not apply any hammer blow as in DCPT.

We push the hammer into the soil at a particular rate of 1 cm/sec. In the case of SCPT, we will be measuring continuous data so we will get continuous measurements of the resistance to penetration of the cone and a surface sleeve.

The static cone penetration test is done with the help of static cone penetrometer. Static cone penetrometer consists of a steel cone, a friction jacket, sounding rod, mantle tube, a driving mechanism and measuring equipment.

Description of the equipment:

The cone has an apex angle of 60º±15’ and an overall base diameter of 35.7 mm and gives a cross-sectional area of 10 mm².
The friction sleeve has an area of 150 cm² as per standard practice.
The sounding road is a steel rod. It has a diameter of 15 mm which can be extended with additional rods of 1m length each so you can conduct the test for up to the required length.
For manual operation, the driving mechanism must have a capacity of 20 to 30 KN and for mechanical operation, the driving mechanism must have a capacity of 100 KN.

This test does not require to bore a hole but takes a sample in with the help of split spoon cone.

Why static cone penetration test needed?

The static cone penetration test is carried out to determine the characteristics of specific soils (soft sensitive silt, sand, clay and some coarse cohesionless soil) such as:

Soil type
Relative soil density
In-situ stress conditions
Shear strength parameters

After performing the test, the data obtained can be used to determine the soil type and design the shallow foundation in the cohesive soil. It can be used directly for the design for pile foundation in sand and gravel and indirectly for shear strength for piles in clay.

Principle of Static Cone Penetration Test

A static cone penetration test involves the penetration of a cone inside the ground and measures the cone resistance, friction resistance, friction ratio and pore water pressure.

Apparatus required

A steel cone
A friction jacket
Sounding rod
Mantle tube
A driving mechanism
Measuring equipment

Procedure

Step 1: The cone and friction jacket assembly is put vertically touching the ground.

Step 2: The cone is then pushed down at a constant speed of penetration ranging from 1.5 to 2.5cm/sec to a depth a (say 40 mm). Then, cone tip resistance (q_c) is recorded.

Step 3: Now the sounding rod is pushed further to a depth b (say 40 mm). Then, sleeve friction (q_f) is recorded.

Step 4: Then, the mantle tube is pushed down to a+b=80mm depth. This brings the cone and jacket assembly to step 1.

Step 5: Then the friction ratio is calculated using the formula:

Calculation formula and table

Different correlations have been developed for Static cone penetration test.

In clays,

In sand,

S.N.	Depth (mm)	Cone resistance (MPa)	Friction resistance (Mpa)	Friction ratio (Rs)
1	0
2	40
3	40
	Until required depth is reached

Now, constructing a graph between cone resistance in the x-axis and vertical effective stress in y-axis we can find the relative density of the soil.

Now, if we plot a graph between cone resistance in y-axis and friction ratio in the x-axis, we can determine what type of soil is that where we are conducting test and define other characteristics otherwise.

Advantages

SCPT is a simple and rapid test.
This test is economical to perform.
This test helps to identify the problem of soils.
This test provides us with detailed and precise data.
This test helps us with real-time measurement.

Disadvantages

The soil sample is not obtained in this test.
The depth till which we can carry out the test is limited.
The cone cannot be penetrated through the graveled surface.
This test requires special equipment and skilled labor.

Conclusion

This test helps to save the money for the owner by identifying the geological strata more accurately. This test also helps in seismic evaluation to satisfy the new building code requirements with the measurement of the low strain for shear wave velocity.

This test can further be used to predict the pile capacity and also provides good estimates of settlement for the shallow foundation.

Precaution

The test should be conducted carefully and data should be recorded precisely.
A large number of the test should be carried out for more accuracy.
The test data can be influenced by various factors such as pore water pressure, humidity, the temperature so they should be managed as per the standards.