Geotechnical Engineering

DHARM

SOIL EXPLORATION 753

Example 18.3: A SPT is conducted in fine sand below water table and a value of 25 is obtained

for N. What is the corrected value of N?

Corrected N = 15 1

2

+′−()N 15

Here N′ = 25

∴ N =^15

1

2

+−()25 15 = 20.

Example 18.4: A SPT was conducted in a dense sand deposit at a depth of 22 m, and a value

of 48 was observed for N. The density of the sand was 15 kN/m^2. What is the value of N,

corrected for overburden pressure?

N′ = 48

N = N′.()+

350

σ 70

where σ = overburden pressure in kN/m^2.

Here, σ = 22 × 15 = 330 kN/m^3

∴ Corrected N =^48

350

330 70

48 350

400

×

+

= ×
() = 42.
Example 18.5: A vane, used to test a deposit of soft alluvial clay, required a torque 72 metre-
newtons. The vane dimensions are D = 100 mm, and H = 200 mm. Determine a value for the
undrained shear strength of the clay.

D = 100 mm H = 200 mm = 2D

Torque, T = 72 m N = 72,000 mm N.

τ =

T

πD^2 HD

26

F +

HG

I

KJ

=

6

7 3

T

πD,

for H = 2D

∴τ =

6 72000

7 100 100 100

×

××××π

N/mm^2

= 19.64 × 10–3 N/mm^2

=

19 64 10 1000 1000

1000

. ×× ×−^3
kN/m^2

= 19.64 kN/m^2.

Example 18.6: A seismic refraction study of an area has given the following data:

Distance from impact 15 30 60 90 120

point to geophone (m)

Time to receive wave (s) 0.025 0.05 0.10 0.11 0.12

(a) Plot the time travel data and determine the seismic velocity for the surface layer

and underlying layer.

(b) Determine the thickness of the upper layer.

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