Geotechnical Engineering

DHARM

INDEX PROPERTIES AND CLASSIFICATION TESTS 63

graph paper, with the logarithm of the number of blows on the x-axis, and the moisture con-

tents on the y-axis. The graph thus obtained, i.e., the best fit straight line, is referred to as the

‘Flow-graph’ or ‘Flow curve’. (Fig. 3.18).

Adjusting screws

Brass cup

125 mm

Brass cup 27 mm

(^54) mm
Cam
60
mm
Adjusting screws
50
mm
150 mm
Hard
rubber
base
Liquid limit device
20
20
12
50
8
50 2 11
20
45°
1.6 mm
22 R
10
75 15
Soil cake after test
Type B-ASTM grooving tool
Soil cake before test
Type A-Casagrande grooving tool
(all dimensions are in mm)
13.5
2 60 10
Fig. 3.17 Liquid limit apparatus
The moisture content corresponding to 25 blows from the flow curve is taken as the
liquid limit of the soil. This is the practical definition of this limit with specific reference to the
liquid limit apparatus and the standard procedure recommended. Experience indicates that
such as curve is actually a straight line.
The equation to this straight line will be
(w 2 – w 1 ) = If log 10
N
N
1
2
...(Eq. 3.42)
where w 1 and w 2 are the water contents corresponding to the number of blows N 1 and N 2 and
If is the slope of the flow curve, called the ‘flow index’.