Principle
In the unconfined compression test a cylindrical specimen of cohesive soil is subjected to a steadily increasing axial compression until failure occurs. The axial force is the only force applied to the specimen. The test is normally carried out on cylindrical specimens of a length equal to about twice the diameter. Nominal diameter range from 38 mm to 100 mm. The specimen length should be as close to twice the diameter as the nature of the soil and the end preparation will permit. The length may vary from 8 % under-size to 12 % over-size without significantly affecting the results. The test provides an immediate approximate value of the compressive strength of the soil, either in the undisturbed or the remoulded condition, it is carried out within a short enough time to ensure that no drainage of water is permitted into or out of the specimen. It is suitable only for saturated, non-fissured cohesive soils.
Failure criteria
The maximum value of the compressive force per unit area which the specimen can sustain is referred to as the unconfined compressive strength of the soil. In very plastic soils in which the axial stress does not readily reach a maximum value, an axial strain of 20 % is used as the criterion of failure.
Types of Test
Two methods are given for determining the unconfined compressive strength. The first is the definitive method using a load frame, in which specimens of any suitable diameter can be tested. The second makes use of an autographic apparatus. Our post would further discuss the load frame method
Apparatus
1. Hand-operated or motorized machine, capable of applying axial compression to the specimen at a suitable rate of displacement. The actual rate of platen displacement shall not vary by more than ± 20 % of the application rate. The machine shall be capable of providing an axial compression of about one-third of the height of the specimen tested.
NOTE: An alternative arrangement to the UCS machine would be to mount the specimen in a triaxial cell, without the addition of pressurizing fluid to the cell, provided that piston friction can be taken into account.
2. Calibrated means of measuring the axial compression of the specimen, readable to 0.01 mm, such as a suitably mounted micrometer dial gauge (the axial strain gauge).
3. Calibrated force-measuring device
NOTE: The force-measuring device may be a load ring or force transducer. Several devices of this kind, of various capacities, should be available, and the one selected for test should be appropriate to the strength of the soil specimen. The device shall be supported by the crosshead of the compression machine so as to prevent its own weight being transferred to the test specimen.
4. Two flat smooth solid platens of the same diameter of the test specimen or larger, through which the axial force is transmitted.
NOTE: Plastics end caps of not less than 20 mm thickness are usually satisfactory for specimens up to 50 mm diameter for soft or very soft soils. Metal end caps 10 mm to 20 mm thick are preferable for stiff soils. The platens may be attached to the compression machine.
5. Timer, readable to 1 s.
6. Balance, readable to 0.1 g.
7. Apparatus for determination of moisture content
8.Calibrated means of measuring the specimen dimensions, to an accuracy of 0.5 %.
Test procedure
Step 1: Determine the mass of the prepared test specimen to the nearest 0.1 g.
Step 2: Make at least three measurements of the length and of the diameter of the specimen to the nearest 0.1 mm, and determine the average dimensions.
Step 3: Place the specimen centrally on the pedestal of the compression machine between the upper and lower platens. Avoid disturbance especially if the specimen is soft, and avoid loss of moisture from the soil.
Step 4: Adjust the machine so that contact is just made between the specimen, upper platen and the force measuring device.
NOTE: A small seating force indicated by the force-measuring device confirms when contact is made. This force is included as part of the force applied to the specimen.
Step 5: Adjust the axial deformation gauge to read zero or a convenient initial reading.
Step 6: Record the initial readings of the force and compression gauges.
Step 7: Select a rate of axial deformation such that the rate of axial strain does not exceed 2 %/min.
NOTE: Stiff soils which fail at small deformations should be tested at a lower rate of strain than soft soils which require large deformations to produce failure. A suitable rate of strain usually lies between 0.5 % and 2 %/min.
Step 8: Apply compression to the specimen at the selected rate and record simultaneous readings of the force-measuring device and the axial deformation gauges at regular intervals of compression, e.g. corresponding to each 0.5 % strain. Obtain at least 12 sets of readings in order to define the stress-strain curve.
NOTE: Stiff and brittle soils fail at small strains, therefore readings of deformation should be recorded at frequent intervals of force, otherwise not enough readings will be taken before failure.
Step 9: Continue the test until the maximum value of the axial stress (calculated as in step 3) has been passed, or the axial strain reaches 20 %.
NOTE: It is convenient to make a plot representing axial stress versus compressive strain as the test proceeds, to enable the point at which failure occurs to be seen without the need for calculations.
Step 10: Remove the load from the specimen and record the final reading of the force measuring gauge as a check on the initial reading.
Step 11: Make a sketch of the test specimen to indicate its mode of failure.
Step 12: Remove the whole of the specimen from the apparatus, and determine its moisture content. Alternatively, for a large specimen take selected
portions of material for this purpose.
Calculations and Plotting (see attached form)
Step 1: Calculate the axial strain, ɛ, of the specimen for each set of readings from the equation
Where,
ΔL is the change in length of the specimen as indicated by the axial deformation gauge
(in mm);
Lo is the initial length of the specimen (in mm).
Step 2: Calculate the force, P (in N), applied to the specimen for each set of readings by multiplying the change in reading of the force-measuring device from zero load (in divisions of digits) by the relevant load calibration factor (in N/division or N/digit).
Step 3: Calculate the axial compressive stress, σ1 (in kPa), in the specimen for each set of readings, on the assumption that the specimen deforms as a right cylinder, from the equation below
Where,
Ao is the initial cross-sectional area of the specimen (in mm2).
Step 4: Plot calculated values of compressive stress as ordinates against corresponding values of strain (expressed as a percentage) as abscissae, and draw the stress-strain curve through the points.
Step 5: Ascertain the point on the graph representing the failure condition, which is the point at which the maximum compressive stress sustained by the specimen occurs, or the point corresponding to a strain of 20 % if that occurs first.
NOTE: If either of these points lies between two sets of observed readings their values should be interpolated from the graph.
Step 6: Using that point, determine the compressive stress in the specimen at failure, referred to as the unconfined compressive strength, qu (in kPa).
Step 7: Determine the axial strain of the specimen at failure.
Step 8: Calculate the moisture content, bulk density and dry density of the test specimen.
Test Report
The test report shall state that the test was carried out in accordance with section 7.2 of BS 1377-7:1990. It shall contain the following, in addition to the information listed in clause 9 of BS 1377-1:1990.
a) statement of the method used, i.e. determination of the unconfined compressive strength using the load frame method;
b) the unconfined compressive strength of the specimen, qu (in kPa), to two significant figures;
c) the strain at failure (in %), to two significant figures;
d) rate of strain (in %/min) applied;
e) dimensions of the test specimen;
f) moisture content (in %) and bulk density (in Mg/m3) of the test specimen;
g) sketch indicating the mode of failure of the specimen;
h) method of preparation of the test specimen;
i) if required, the stress/strain curve.
