Unconfined Compression Procedure

8/7/2019 Unconfined Compression Procedure

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Revised 2009, WKS Datasheet No.10

MOHAWK COLLEGE OF APPLIED ARTS AND TECHNOLOGY

BUILDING AND CONSTRUCTION SCIENCES DEPARTMENT

Unconfined Compression Test

REFERENCES:

Bowles, J. E. Engineering Properties of Soils and Their Measurement

INTRODUCTION

The unconfined compression test is still used as a means of rapidly evaluating theshear strength of cohesive soils although it is gradually losing favour. The unconfinedcompressive strength is the maximum load attained per unit area or the load at 15%axial strain, whichever occurs first on a cylindrical sample tested in compression. Theshear strength is taken as equal to 1/2 the unconfined compressive strength. Inpractice the test specimens are usually obtained by extruding thin-wall tube samples.The length to diameter ratio of the specimen should be between 2 and 3.

Two types of loading devices are permitted, either strain controlled or stress con-trolled. In practice strain controlled loading is now almost universally used for thistest. In the strain controlled test the rate of loading is controlled so as to produce auniform axial strain. In the stress controlled test the rate of loading is controlled soas to produce a uniform increase in axial load. In this laboratory, strain control will beused and the compression machines will be adjusted to a rate of loading which willprovide 15% axial strain on the sample in approximately 10 minutes.

Sensitivity of a cohesive soil is defined as the undisturbed strength of the soildivided by the remolded strength. The sensitivity cannot be less than 1. For mostlocal clays, the sensitivity is between 1 and 3. For some highly sensitive clays it canexceed 10 and, in some cases, be so high that it cannot even be accurately measured.The most accurate method of determining sensitivity involves use of the laboratoryvane shear apparatus.

Two devices used to check the unconfined strength or shear strength in the field arethe Torvaneand the Pocket Penetrometrer. You will use both and compare the resultswith that obtained from the unconfined test. The results of this lab will also becompared to the results of a triaxial compression test performed in a later lab. It istherefore necessary to retain a copy of all the results from this lab.


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APPARATUS:

Compression testing machine with proving ring and dial gauge to measure deformation(0.01 m x 25 mm).Vernier caliper for measuring sample.Miscellaneous apparatus: Spatulas, wire saws, balances, moisture tins, etc.

PROCEDURE:

1. The loading device should have been adjusted so that the loading rate will beapproximately that which will produce 15% strain in 10 minutes. (Check with theinstructor that this has been done).

2. Readings of the load and deformation should be taken at increments of 0.5 mm

strain.3. If a wax coating is on the specimens, carefully remove it. Using a mitre box trimthe ends square and to a length of between 100 and 104 mm; determine and record onDS 10 the exact length and the average diameter (to 0.1 mm) of the specimen and itsmass to 0.1 g.

Davg = (Dt + 2Dm + Db)/4

where:

Davg = Average diameter of the specimenDt = Average diameter at the top of the specimen

Dm = Average diameter at the middle of the specimen

Db = Average diameter at the bottom of the specimen

4. Based on the specimen length, determine the strain dial reading that wouldcorrespond to 15% strain and highlight on DS 10.

5. Insert the specimen in the machine and bring the top platen in contact with the topof the specimen using the manual feed. Zero the deformation and load dials andcommence the test. Using motor operation, take readings of the load and deformationdial readings at the specified intervals (0.5 mm).

6. Continue to take dial readings until the loads fall off in two successive readings ortwo readings beyond the 15% strain value.


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7. Upon completion cut the specimen in two and carry out:

a) a Torvanetest in each new face of the specimen and record the average shearstrength results in kg/cm2.

b) several Pocket Penetrometertests in the faces and record the average unconfined

compressive strength equivalents in kg/cm2. (Note: the unconfined equivalents will be

approximately double the shear readings).

8. Weigh the specimen again for a percent moisture determination, recording massesto 0.1 g on DS 10.

CALCULATIONS AND REPORT:

1. Calculate the axial strain, a for each reading as follows:

0a L

L

where:

L = change in length of the specimen read from the dial gauge

L0 = initial length of the test specimen2. Calculate the average cross-sectional area, A', for each reading as follows:

a0

-1

AA'

where:

A0 = the original average area of the specimen in mm2

3. Calculate the load per unit area (unconfined compressive stress), qu for eachreading as follows:

1000A'

Pqu in kPa

where:


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P = applied axial load in N

4. Prepare a graph showing stress (y-axis) vs. percent strain (x-axis) for the test. Theunconfined compressive strength, quf is determined as the maximum value of load perunit area in kPa if before 15% strain or otherwise that at 15% strain.

5. The shear strength is equal to half the unconfined compressive strength:

2

quff

where:

f = shear strength

quf = unconfined compressive strength

6. Calculate the results of the Torvanerests and the PocketPenetrometerreadings inkPa (1 kg/cm2 = 98.1 kPa). Compare the shear strength determined from the"Torvane" test with that obtained by halving both the "Pocket Penetrometer"readings and the maximum unconfined compression test result. NOTE: The maximumpossible Torvane reading is 1 kg/cm2!

7. Using the moisture content found, the pre-test mass of the specimen and assuminga specific gravity of 2.750, estimate the degree of saturation of the original

specimen. If this assumption appears to produce a negative volume of air in thespecimen then report the specific gravity value obtained by assuming saturation.Report this together with the moisture content and density in kg/m3.

8. Include labelled sketches of the "Pocket Penetrometer" and the "Torvane" showingthe scales and units of measurement as well as any pertinent dimensions.

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Unconfined Compression Procedure