Direct Shear Test - Soil Properties: Testing, Measurement, and Evaluation - page 382

The wet unit weight g wet of the soil specimen at the beginning of the
test can be determined next:

a 161.52
4.91

ba 1,728
453.6

ft 3

125.3 lb

g wet b >

The initial water content may be computed as follows (see Chapter 4):

248.43

216.72

w

100 18.5%

45.30

216.72

With water content known, dry unit weight

γ dry can be determined next:

125.3
18.5

g wet

ft 3

100 100 105.7 lb

g dry >

w 100 100

The final water content may also be computed:

226.20

182.58

w

100 32.0%

46.28

182.58

[B] Shear Stress

The normal load ( 20.6 lb ) divided by the initial area of the specimen
(4.91 in. 2 ) gives a normal stress of 4.20 lb/in. 2 , or 604 lb/ft 2 . The first
proving ring dial reading ( 0.0030 in. ) multiplied by the proving ring
calibration ( 3,125 lb/in. ) gives a corresponding horizontal shear force
of 9.4 lb. Dividing this horizontal shear force by the initial area of the
specimen (4.91 in. 2 ) and multiplying by 144 to convert square inches
to square feet gives a corresponding shear stress of 276 lb/ft 2 .
The foregoing values fill in the second row of the form on page 388.
Similar calculations furnish required values for succeeding given dial
readings and fill in the remaining rows on the form.
It should be emphasized that one or two additional evaluations
would be required for testing additional specimens at different normal
loads. Such evaluations are not included here.

[C] Curves of Shear Stress and Specimen Thickness Change
versus Shear Displacement

Curves of shear stress and specimen thickness change versus shear dis-
placement are shown in Figure 23-6. The maximum shear stresses for
the specimens tested are determined from the graph of shear stress
versus shear (horizontal) displacement as follows:

For specimen no. 1, with a normal stress 604 lb/ft 2 , maximum

shear stress 1,522 lb/ft 2

For specimen no. 2, with a normal stress 926 lb/ft 2 , maximum

shear stress 1,605 lb/ft 2

For specimen no. 3, with a normal stress 1,248 lb/ft 2 , maximum

shear stress 1,720 lb/ft 2