Albert Nerken School of Engineering
Soil Mechanics Laboratory
Experiment No. 10 - Unconfined Compression Test
To determine the compressive strength of a cohesive soil sample.


1) 'Soil Testing for Engineers' by T. W. Lambe; Chapter XII.

2) "Engineering Properties of Soils and Their Measurements", 4th ed. by Joseph E. Bowles; Experiment No. 14.

3) A.S.T.M. Standards, 1997; (4.08) Designation D2166-91.


1) Unconfined Compression machine

2) Evaporating dish

3) Triple beam balance (sensitive to 0.1 gm)

4) Strain gauge (sensitive to 0.001 inch/division)

5) Miscellaneous apparatus:

  1. Calipers
  2. Oven


1) The soil to be tested is a pure clay, Kaolinite.  It was made in the laboratory by mixing dry clay with distilled water, curing it for about 1 day to ensure that all particles were wet, and then run through the extruding machine.   Plugs 2.5 in. in diameter and about 8 in. in length were obtained.  A water content of about 40% was planned: 10,000g of dry clay were used to 4,000g of distilled water.  The sample was wrapped in plastic wrap to avoid a loss of moisture.

2) Unconfined compression samples should have a length to diameter ratio of 2.0 to 2.5.   Therefore, for a diameter of 2.5 inches, the length should range from 5.0 to 6.25 inches.

3) Meaure the length of the sample, L0, in centimeters.   Record this on Data Sheet A.

4) Measure the diameter of the sample in three diffrerent places along its length in millimeters.  Take their avarage and record this as d0 on Data Sheet A, and determine the sample's average cross-sectional area, A0, in square inches. (A0 = pd02/4)   Measurements should be done with calipers at the approximate top, middle, and bottom of the sample.

5) Weigh the sample in an evaporating dish on a triple beam balance, to the nearest 0.1g.  Record this value as the intial wet weight on Data Sheet A.

6) Place two plastic disks (1.25cm thick) on the top and the bottom of the sample.  Place it in the Unconfined Compression Machine.

7) When the sample is in the machine, place the loading plate on to the top plastic plate of the sample.  Place a level bubble on top of this and see if the sample is level.  Remove bubble and raise loading until the ball bearing in the loading block is just in contact with the load ring.  Run the test at a constant rate of strain of 0.02 inches per minute.

8) Zero the load ring & strain gauge.  Record the load ring number on Data Sheet A.  The test begins when the load ring dial begins to move.   Since the strain gauge dial may begin moving before the load ring dial note the reading on the strain gauge dial when the load ring moves.  This is the new zero.   Readings will be taken on both gauges, with the controlling gauge being the strain gauge.  Take readings at every 0.02 inches of movment on the strain gauge.  Its calibration is 0.001 inched per division.

9) Run the test until cracks develop or about 12% strain.

10) Remove the sample from the machine and measure again with the calipers the diameter of the sample in three places, top middle and bottom in millimeters.   Also, measure the height of the sample in three places, and determine the average of both numbers.

11) Weigh the entire sample (less the plastic plates) again in the evaporating dish from step 5.  Record this as final wet weight on Data Sheet A.   This is for an end-of-test water content.

12) Place the sample in an oven to dry until the next lab period.   Remove from the oven and weigh the sample and evaporating dish on the triple beam balance to the nearest 0.1g.


1) Calculate the water contents, Axial load (P = Load Ring Reading x [Calibration of load ring] ), strain (ez = DL/L0 x 100%), instantaneous area (Ai = A0/(1-ez), where ez is in decimal format), and s1 = P/A.


1) Plot a graph of s1 or the deviator stress as ordinate vs. ez or vertical strain in % as abcissa on Cartesian graph paper.  Define qu at failure; this is s1 (peak).



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