Geotechnical Engineering

DHARM

SOIL STABILISATION 713

as per IS: 2720 (Part-II)-1973. The presence of any oversize particles shall be verified which
may affect the results if they happen to be located directly below the penetration plunger.

The penetration test may be repeated for the reverse end of the sample as a check. The
set-up is shown schematically in Fig. 17.2.

Applied load

Proving ring

for measuring load

CBR Mould

Surcharge

weight

Penetration

plunger

Dial gauge for

measuring penetration

Base of the

loading machine

Soil specimen

Fig. 17.2 Schematic of the set-up for CBR test

Load-penetration curve

Load vs penetration curve is plotted. This curve will be mainly convex upwards although the
initial portion of the curve may be concave upwards due to surface irregularities. A correction
shall then be applied by drawing a tangent to the upper curve at the point of contraflexure.
The corrected curve shall be taken to be this tangent plus the convex portion of the original
curve with the origin of penetrations shifted to the point where the tangent cuts the horizontal
penetration axis as illustrated in Fig. 17.3.

CBR value

Corresponding to the penetration value at which the CBR is desired, corrected load shall be
taken from the load penetration curve and the CBR calculated as follows:

CBR =

P

P

T

s

× 100 ...(Eq. 17.4)