Determining the Unconfined Compressive Strength of Cohesive Soil - Soil Properties: Testing, Measurement, and Evaluation - page 328

[B] Compression Calculations

For the first applied load in this test, the deformation dial H and

the proving ring dial readings were 0.025 and 0.0024 in., respectively.
The axial strain

Δ

for this particular applied load can be computed using

Eq. (21-1):

¢ H
H 0

(21-1) e

0.025
5.98

0.004

e

The corresponding cross-sectional area can be computed using
Eq. (21-2):

A 0

(21-2) A

1

e

4.91

4.93 in. 2

A

0.004 1

The applied axial load P can be computed by multiplying the proving
ring dial reading by the proving ring calibration factor:

P 0.0024 6,000 14.4 lb 1 21 2

The load per unit area for this particular applied load is therefore
14.4/4.93, or 2.92 lb/in 2 . This converts to 420 lb/ft 2 .
The foregoing data furnish the values to fill in the second row of the
form on page 330. Similar computations provide values needed to fill in
the remaining rows.

[C] Graph

The required graph showing the relationship between load per unit area
and unit strain can be obtained by plotting values of load per unit area
(the last column in the form on page 330) as the ordinate and unit strain
(the second column in the form on page 330) as the abscissa. The graph
for this example is shown in Figure 21-3. From this graph, the uncon-
fined compressive strength q u , which is the maximum value of load per
unit area or the load per unit area at 15% strain, whichever occurs first,
is 3,930 lb/ft 2 . The cohesion c , which is half the unconfined compressive
strength (i.e., c = q u /2), is therefore 3,930/2, or 1,965 lb/ft 2 .