The objective of the unconfined
compression test is to determine the UU (unconsolidated, undrained) strength
of a cohesive soil in an inexpensive manner.
Fine-grained soil is tested in compression. Undisturbed specimens
cut from tube samples and disturbed specimens are loaded in compression, recording
load and deflection measurements.
Laboratory strength tests of soil are similar to
testing concrete cylinders, but can be performed with or without lateral confining
pressures. The unconfined test uses axial loading without lateral confining pressures,
making it the simplest and easiest laboratory method of estimating strength.
To more accurately simulate actual loading conditions in the field, lateral confining
pressures can be applied using a triaxial test, which is a completely different apparatus.
Research has shown that the strength of a soil determined by compression testing varies
with extremes of the length to diameter ratio and the rate of strain. It is generally
accepted that ratios of length to diameter of 1.5 to 3.0 are satisfactory. Ratios of 2.0
and 2.5 are commonly employed. Similarly, satisfactory rates of strain are 0.5 to 2.0% per
minute. For most samples 0.5 to 1.0% per minute is used.
Unconfined compressive strength is calculated the same as for any
material, with an additional calculation of the area change from bulging.
The shear strength is defined as half the compressive strength.
Equation Set 1.1: Unconfined strength
Since soils tend to deform much more than concrete, the area of the specimen changes to
maintain constant volume through the test (bulging). Thus, the average cross
sectional area at a particular deformation during the test is calculated using:
Writing Points for the report
Weights for the water contents were made to 0.01 gm or 4 significant
figures for weights greater than 10 gm. Thus, the water content and dry
density could be determined to 4 significant figures. [However, if water
contents were made with samples less than 10 gms, there would only be three
significant figures in the water content and dry density.]
The accuracy of the computed unit weights were limited by the weight of
soil which was measured to the nearest 0.1 gm (0.002 lb or four significant
figures for the weights obtained) and the measured volume of the sample.
Although dimensions for the latter were measured to the nearest 0.001
in.(four significant figures), the accuracy was less (probably 3 significant
figures) because of slight irregularities in the shape of the sample. Water
content (3 or 4 significant figures) is needed to compute dry unit weight.
Thus, the computed unit weights are accurate to 3 significant figures.
The saturation is computed from unit weights and water content. The
former is limited to three significant figures the latter is 3 or 4, so the
saturation is limited to three significant figures. This assumes that the
assumed specific gravity is correct.
Measurements of axial deformation were made to the nearest 0.001 in. so
the computed strain was accurate to 3 significant figures for deformations
greater than 0.1 in. Sample length is accurate to at least 3 significant
figures. Thus, the axial strain is accurate to 3 significant figures.
Axial load was determined by taking measurements of the deformation of a
proving ring and converting this to the load in lbs using the conversion
factor that was provided. For the undisturbed sample the largest
deformations had only two significant figures (maybe 3 depending on
magnitude of load) so the axial load and axial stress had only two (maybe 3)
significant figures. Thus, the compressive and shear strengths for the
undisturbed samples are accurate to only two significant figures. For the
remolded sample, deformation of the proving ring was so small that only one
significant figure could be obtained. (or possibly too low to measure)
There are several sources of operational error
in this test: (1) loading rate not constant; (2) sample ends not
perpendicular to the sides; (3) sample cross section not uniform, esp. for
remolded sample; (4) evaporation during test