Standard penetration test,Cone penetration tests and vane shear test

 

Standard penetration test

The standard penetration test is the most commonly used in situ test, especially for cohesionless soil which cannot be easily sampled.

The test is extremely useful for determining the relative density and the angle of bearing resistance of cohesionless soils. It can also be used to determine the unconfined compressive strength of cohesive soils.

Procedure-

The standard penetration test is conducted in a bore hole using a standard split spoon sampler, when the bore hole has been drilled to the desired depth, the drilling tools are removed, and the sampler is lowered to the bottom of hole.

The sampler is driven into the soil by a drop hammer of 63.5 kg mass falling through a height of 750 mm at the rate of 30 blows per minute (IS: 2131-1963). The number of hammer blows require to drive 150 mm of the sample is counted.  The sampler is further driven by 150 mm and the number of blows recorded. Likewise, the sampler is once again further driven by 150 mm and the number of blows recorded.

Fig: Standard Penetration Test

The number of blows recorded for the first 150 mm is disregarded. The number of blows recorded for the last two 150 mm intervals are added to give the standard penetration number (N). In other words, the standard penetration number is equal to the number of blows required for 300 mm of penetration beyond a sitting drive of 150 mm.

If the number of blows for 150 mm drive exceeds 50, it takes as refusal, and the test is discontinued.

Correction for N value -

The standard penetration number is corrected by-

1. Dilatancy correction

2. Overburden correction

 

Dilatancy Correction - Silty fine sands below the water table develop pore pressure which is not easily dissipated. The pore pressure increases the resistance of the soil and hence the penetration number (N).

Terzaghi and Peck (1967) recommend the following correction in the case of silty fine sands when the observed value of N exceeds 15.

The corrected penetration number, N_c = 15 + ½ (N_R – 15)

 Where,

     N_R  is the recorded value, and 
     N_c is the corrected value.

If    N_R ≤ 15,   then,     N_{c =} N_R    

 

Overburden Pressure Correction - In granular soils, the overburden pressure affects the penetration resistance. If the two soils having same relative density but different confining pressures are tested, the one with a higher confining pressure gives a higher penetration number. As the confining pressure in cohesionless soils increases with the depth, the penetration number for soils at shallow depths is underestimated and that at greater depths is overestimated.

For uniformity, the N-values obtained from field tests under different effective overburden pressures are corrected to a standard effective overburden pressure.

Gibbs and Holtz (1957) recommend the use of the following equation for dry or moist clean sand.

               N_c = N_R x 350 / (σ_o + 70)

 Where,

    N_R = Observed N-value,

    N_c = Corrected N-value

    σ_o = effective overburden pressure (kN/m²)

It is applicable for overline σ_o  ≤  280 kN/m².

The ratio (N_c / N_R) should lie between 0.45 and 2.0.

If (N_c / N_R) ratio is greater than 2.0, N_c should be divided by 2.0 to obtain the design value used in finding the bearing capacity of the soil.

Cone penetration tests

The cone test was developed by the Dutch Government, Soil Mechanics Laboratory at Delft and is, therefore, also known as Dutch cone Test. The test is conducted either by the static method or by dynamic method.

(a) Static Cone penetration test -

The Dutch cone has an apex angle of 60ᵒ and an overall diameter of 35.7 mm, giving an end area of 10 cm².

For obtaining the cone resistance, the cone is pushed downward at a steady rate of 10 mm/sec through a depth of 35 mm each time. The cone is pushed by applying thrust and not by driving.

Fig: Static Cone Penetration Test

After the cone resistance has been determined, the cone is withdrawn. The sleeve is pushed on to the cone and both are driven together into the soil and the combined resistance is also determined. The resistance of the sleeve alone is obtained by subtracting the cone resistance from the combined resistance.

(b) Dynamic cone penetration Test - 

The test is conducted by driving the cone by blows of a hammer. The number of blows for driving the cone through a specified distance is a measure of the dynamic cone resistance.

Dynamic cone tests are performed either by using a 50 mm cone without bentonite slurry or by using a 65 mm cone with bentonite slurry (IS: 4968-part I and II - 1976 ) . The driving energy is given by a 65 kg-hammer falling through a height of 75 cm. The number of blows required for 30 cm of penetration is taken as the dynamic cone resistance (N_cbr ). If the skin friction is to be eliminated, the test is conducted in a cased bore hole.

Fig: Dynamic Cone Penetration test

When a 65 mm cone with bentonite slurry is used, the set-up should have arrangements for circulating slurry so that the friction on the driving rod is eliminated.

In-situ vane shear test -

In-situ vane-shear test is conducted to determine the shear strength of a cohesive soil in its natural condition.

It consists of four blades, 100 mm (or 150 mm or 200 mm) long, attached at right angles to a steel rod. The steel rod has a torque-measuring device at its top. The height-diameter ratio (H/D) of the apparatus is generally equal to 2.

Fig: In situ Vane Shear Test

For conducting the test, the shear-vane is pushed into the ground at the bottom of the bore hole. When a torque is applied through the handle at the top of the rod, the soil is sheared along a cylindrical surface. The torque required to shear the cylinder of the soil is measured by means of a spring balance. The undrained shear strengths s_u of the soil is determined from the equation -

                        S_u = T / (π (D²H / 2 + D³ / 6))

Where,  

T = torque applied,

H = height of the vane,

D = diameter of the soil cylinder sheared.

The vane-shear test is extremely useful for determining the in-situ shear strength of very soft and sensitive clays, for which it is difficult to obtain undisturbed samples. The test can also be used even for determining the shear strength of stiff, fissured clays. However, the method cannot be used for sandy soils.