Geotechnical Engineering

DHARM

278 GEOTECHNICAL ENGINEERING

Alternatively, a loading frame with proving ring and a dial gauge for measuring the

axial compression of the specimen may also be used. The maximum compressive stress is that

at the peak of the stress-strain curve. If the peak is not well-defined, an arbitrary strain value

such as 20% is taken to represent failure.

Mohr’s Circle for Unconfined Compression Test

The Mohr’s circles for the unconfined compression test are shown in Fig. 8.18. From

Eq. 8.36, recognising that σ 3 = 0

σ 1 = 2c tan (45° + φ/2) ...(Eq. 8.43)

c

f

s 3 =0

a

s 1 s

2 = 90° +af

Failure plane

t

Failure

plane

s 1

a

s 1

c

s 3 =b s 1 s

t

a= 45° 2 = 90°a

Failure plane

f=0

(a) For a c- soilf (b) For a pure clay ( = 0)f
Fig. 8.18 Mohr’s circles for unconfined compression test
The two unknowns–c and φ–cannot be solved since any number of unconfined compres-
sion tests would give only one value for σ 1. Therefore, the unconfined compression test is
mostly found useful in the determination of the shearing strength of saturated clays for which
φ is negligible or zero, under undrained conditions. In such a case, Eq. 8.43 reduces to
σ 1 = φu = 2c ...(Eq. 8.44)
where φu is the unconfined compression strength.
Thus, the shearing strength or cohesion value for a saturated clay from unconfined
compression test is taken to be half the unconfined compression strength.

8.8.4 Vane Shear Test

If suitable undisturbed for remoulded samples cannot be got for conducting triaxial or unconfined

compression tests, the shear strength is determined by a device called the Shear Vane.

The vane shear test may also conducted in the laboratory. The laboratory shear vane

will be usually smaller in size as compared to the field vane.

The shear vane usually consists of four steel plates welded orthogonally to a steel rod,

as shown in Fig. 8.19.

The applied torque is measured by a calibrated torsion spring, the angle of twist being

read on a special gauge. A uniform rotation of about 1° per minute is used. The vane is forced

into the soil specimen or into the undisturbed soil at the bottom of a bore-hole in a gentle

manner and torque is applied. The torque is computed by multiplying the angle of twist by the

spring constant.

The shear strength s of the clay is given by

s =

T

πDH^2 (/ 26 +D/)

...(Eq. 8.45)