Engineering Properties of Soils Based on Laboratory Testing
Prof. Krishna Reddy, UIC
145
EXPERIMENT 12
UNCONFINED COMPRESSION (UC) TEST
Purpose:
The primary purpose of this test is to determine the unconfined compressive
strength, which is then used to calculate the unconsolidated undrained shear
strength of the clay under unconfined condi tions. According to the ASTM standard,
the unconfined compressive strength (qu) is defined as the compressive stress at
which an unconfined cylindrical specimen of soil will fail in a simple compression
test. In addition, in this test method, the unconfined compressive strength is taken
as the maximum load attained per unit area, or the load per unit area at 15% axial
strain, whichever occurs first during the performance of a test.
Standard Reference:
ASTM D 2166 - Standard Test Method for Unconfined Compressive Strength
of Cohesive Soil
Significance:
For soils, the undrained shear strength (su) is necessary for the
determination of the bearing capacity of foundations, dams, etc. The undrained
shear strength (su) of clays is commonly determined from an unconfined
compression test. The undrained shear strength (su) of a cohesive soil is equal to
one-half the unconfined compressive strength (qu) when the soil is under the f = 0
condition (f = the angle of internal friction). The most critical condition for the soil
usually occurs immediately after construction, which represents undrained
conditions, when the undrained shear strength is basically equal to the cohesion
(c). This is expressed as:
Engineering Properties of Soils Based on Laboratory Testing
Prof. Krishna Reddy, UIC
146
su = c = 2
u
q
Then, as time passes, the pore water in the soil slowly dissipates, and the
intergranular stress increases, so that the drained shear strength (s), given by s = c
+ s‘tan f , must be used. Where s‘ = intergranular pressure acting perpendicular to
the shear plane; and s‘ = (s - u), s = total pressure, and u = por