Cone Penetration Test

Cone penetration test is used to determine the geotechnical properties of soil and its stratification. One of the better advantages of this test is boreholes are not required to conduct the test. Unlike the standard Penetration Test, however, soil samples cannot be recovered for record and laboratory tests. For performing this test, basically two types of cone penetrometer are available:

• Static Cone Penetration Test
• Dynamic Cone Penetration Test

Static Cone Penetration Test(SCPT):

It is also known as Dutch Cone Test. It is used to record variation in the in-situ penetration resistance of the soils when-

• The in-situ density of soil is disturbed by boring method
• Standard penetration test is not reliable below the groundwater table.
• It is useful for soft clays, soft silts, medium sand, and fine sand.

Procedure:

• The standard cone (Apex angle: 60⁰, Overall diameter: 35.7 mm and End area: 10 cm²) is connected to the drilling rod.
• The standard cone is pushed into the soil at the rate of 10 mm/sec.
• The friction is noted by which strength of the soil is determined.
• For finding combine cone resistance, the shearing strength of the soil q_s, and tip resistance q_c is noted in gauge & added to get the total strength.

Limitations of SCPT:

• Unsuitable for gravelly soil.
• Unsuitable for soil having standard penetration value greater than 50.
• Unsuitable for field operation

Ratios of Static cone penetration Test: These are the various formula which we use in cone penetration test-

1. Friction Ratio(f_r):

                    f_r=  (qs/qc)*100

         where,

         q_s-measured site/slip friction

         q_c-tip/point resistance

2. Sensitivity of soil (S_t):

                                      S_t=10/f_r

3. Undrained Cohesive Strength (for cohesive soil):

                                       S_u=(q_c-p_o)/N_k

Where,

P₀ = overburden pressure = ꭚ  z

N_k = cone factor = 15 to 20

Dynamic Cone Penetration Test: This test is done for cohesionless soil when static penetration test is difficult to perform and soils are of special interest. It is done to get the dynamic properties of soil.

Procedure:

These are the various steps which we follow to do the DCPT-

• The standard cone (Apex angle: 60⁰, Overall diameter: 35.7 mm and End area: 10 cm²) is connected to the drilling rod.
• Afterward, the assembly is kept vertically on the ground in the position that the cone of the drill should be resting on the ground.
• For the circulation of the bentonite slurry, the pumping unit of the bentonite slurry is properly connected to the guide. Bentonite is used to get rid of friction from the rods.
• The cone is driven into the soil by a 65 Kg hammer falling from a height of 750 mm.
• The blows are counted for every 30 cm penetration in the ground by the cone.

Advantages:

• It doesn’t need a borehole. It can be done quickly to cover a large area economically.
• The test helps to identify variability of subsoil profile and to locate soft pockets such as filled up ponds.
• When DCPT is carried out close to a few boreholes, suitable corrections may be obtained for a particular site and the number of boreholes can be reduced.

Disadvantages:

• The test is normally not suitable for cohesive soils or very loose cohesionless soils. It is normally not possible to evaluate the mechanical properties of the soil at great depths when the friction