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Unconfined Compression: Soil Mechanics Laboratory

background information

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

basic unconfined strength equation

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:

area correction

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


Get a bigger hammer


Unconfined compression machine
Wire saw
Vertical lathe
Mitre box
Caliper and ruler
Split cylindrical mold
Pocket penetrometer
Lab Vane apparatus

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Procedure / Data Sheets [right-click | "save target as" xlsx]


Holtz, Kovaks and Sheahan (2011), page 515

Holtz and Kovaks (1981), pages 566-574, chapters 10 and 11.

Das (1998), sections 9.10, starting on page 402.

Gorrill (1998), pages 21-23, 60.

Bowles (1986), pages 129-136.

ASTM D2166 (unconfined compression)

AASHTO T208 (unconfined compression)

ASTM D4648 (Laboratory vane shear)

[full citations]

crestlogotiny.jpg (2k) Manion, William P. (wmanion@(nospam)maine.edu "Soil Mechanics Laboratory Course CIE 366." University of Maine, Civil and Environmental Engineering Department, Orono, Maine.  04 January 2011 02:33 PM.  http://www.civil.maine.edu/cie366/.