Geotechnical Engineering

DHARM

SHEARING STRENGTH OF SOILS 277

it is often used as a field test, besides being used in the laboratory. The failure plane is not
predetermined and failure takes place along the weakest plane.

The test specimen is loaded through a calibrated spring by a simple manually operated
screw jack at the top of the machine. Different springs with stiffness values ranging from 2 to
20 N/mm may be used to test soils of varying strengths. The graph of load versus deformation
is traced directly on a sheet of paper by means of an autographic recording arm. For any
vertical or axial strain, the corrected area can be computed, assuming no change in volume.
The axial stress is got by dividing the load by the corrected area. The apparatus is shown in
Fig. 8.17.

Rotating handle for

applying compression

Screw

Spring for

measuring

load

Supporting

plate

Upper

plate

Metal

Cones

Lower

plate

Soilsample

Pivot for

recording

arm

Weighted

arm

Auto

graphic

recording

Chart

plate

Auto

graphic

recording

(a) Side view (b) Front view

Arm

Fig. 8.17 Unconfined compression apparatus
The specimen is placed between two metal cones attached to two horizontal plates, the
upper plate being fixed and the lower one sliding on vertical rods. The spring is supported by
a plate and a screw on either side. The plate is capable of being raised by turning a handle so
as to apply a compressive load on the soil specimen.

The stress-strain diagram is plotted autographically. The vertical movement of the pen
relative to the chart is equal to the extension of the spring, and hence, is proportional to the
load. As the lower plate moves upwards, the upper one swings sideways, the weighted arms
bearing on a stop. The lateral movement of the pen is thus proportional to the axial strain of
the soil specimen. The area of cross-section increases as the specimen gets compressed. A
transparent calibrated mask is used to read the stress direct from the chart.