Geotechnical Engineering

DHARM

SOIL EXPLORATION 739

where N′ = observed SPT value,

N = corrected SPT value, and
σ = effective overburden pressure in KN/m^2 , not exceeding 280 KN/m^2. This implies
that no correction is required if the effective overburden pressure is 280 kN/m^2.

Terzaghi and Peck also give the following correlation between SPT value, Dr, and φ:

Table 18.4 Correlation between N, Dr and φ

S. No. Condition N Dr φ

1. Very loose 0 – 4 0 – 15% Less than 28°


2. Loose 4 – 10 15 – 35% 28° – 30°


3. Medium 10 – 30 35 – 65% 30° – 36°
   4. Dense 30 – 50 65 – 85% 36° – 42°
   5. Very dense Greater Greater Greater than 42°
   than 50 than 85%

For clays the following data are given:

Table 18.5 Correlation between N and qu

S. No. Consistency N qu (kN/m^2 )

1. Very soft 0 – 2 Less than 25


2. Soft 2 – 4 25 – 50


3. Medium 4 – 8 50 – 100


4. Stiff 8 – 15 100 – 200


5. Very stiff 15 – 30 200 – 400


6. Hard Greater than 30 Greater than 400

The correlation for clays is rather unreliable. Hence, vane shear test is recommended
for more reliable information.

18.5.2Static Cone Penetration Test (Dutch Cone Test)

The Static cone penetration test, which is also known as Dutch Cone test, has been standard-
ised by the ISI and given in “IS: 4968 (Part-III)-1976—Method for subsurface sounding for
soils—Part III Static cone penetration test”.

Among the field sounding tests the static cone tests in a valuable method of recording
variation in the in-situ penetration resistance of soils, in cases where the in-situ density is
disturbed by boring operations, thus making the standard penetration test unreliable espe-
cially under water. The results of the test are also useful in determining the bearing capacity
of the soil at various depths below the ground level. In addition to bearing capacity values, it is
also possible to determine by this test the skin friction values used for the determination of the
required lengths of piles in a given situation. The static cone test is most successful in soft or
loose soils like silty sands, loose sands, layered deposits of sands, silts and clays as well as in
clayey deposits. In areas where some information regarding the foundation strata is already