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 carried out on 50 mm diameter by 100mm height specimens.
The test provides an immediate approximate value of the compressive strength of the soil in the remoulded condition, it is carried out within a short time to ensure that no drainage of water is permitted into or out of the specimen.
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.
Preparation of compacted and remoulded specimens
General. The specimen shall have a height equal to about twice the diameter, with plane ends normal to the axis.The size of the largest soil particle shall not be greater than 10mm.
Specimens may be of undisturbed soil, or of disturbed soil that has been prepared under specified conditions.
1. Specimens of a compacted soil are normally prepared in the laboratory by compaction into a mould.
2. The soil is compacted at a specified moisture content into a mould under the application of a specified compactive effort. The same compactive effort as the California Bearing Ratio compaction is to be apply as below:-
i. Dynamic compaction by hand or mechanical rammer is used to achieve a specified compactive effort in this testing method.
ii. The specified effort of compaction shall correspond to the 4.5 kg rammer method. The soil is placed and compacted in five layers.
iii. Stand the mould assembly on a solid base, e.g. a concrete floor or plinth.
iv. Place the first portion of soil into the mould and compact it, so that after 62 blows of the appropriate rammer the layer occupies about or a little more than one-fifth of the height of the mould. Ensure that the blows are evenly distributed over the surface. Alternatively the mechanical compacting apparatus may be used.
v. Repeat the previous stepusing the other four portions of soil in turn, so that the final level of the soil surface is not more than 6 mm above the top of the mould body. Preliminary trials may be necessary to judge the amount of soil required for each layer.
vi. Remove the collar and trim the soil flush with the top of the mould with the scraper, checking with the steel straightedge.
vii. A core cutter is to be used to retrieve the cylindrical sample of 50 mm diameter by 100mm height.
viii. The sample is to be wrap in plastic foil, to prevent lost in moisture content.
ix. After wrapping the sample is then stored in a cool and dry place within room temperature for 7days of curing.
x. The sample is then ready for testing after 7 days.
1. Hand-operated or motorized machine,capable of applying axial compression to thespecimen at a suitable rate of displacement. The actual rate of platen displacement shall not vary by more than ± 20 % of the rate specified as below.Select a rate of axial deformation such that the rate of axial strain does not exceed 1%/min. The machine shall be capable of providing an axial compression of about 40mm of thespecimen tested. Essential requirements of the compression test apparatus are shown in Figure 1.
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.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.
4. 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.
5. Two flat smooth solid platensof the same diameter of the test specimen or larger, through which the axial force is transmitted.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.
6. The platens may be attached to the compression machine.
7. Timer, readable to 1 s.
8. Balance, readable to 0.1 g.
9. Apparatus for determination of moisture content in accordance with BS 1377-2:1990.
10. Calibrated means of measuring the specimen dimensions, to an accuracy of 0.5 %.
1. Determine the mass of the prepared test specimen to the nearest 0.1 g.
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.
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.
4. Adjust the machine so that contact is just made between the specimen, upper platen and the force measuring device.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.
5. Adjust the axial deformation gauge to read zero or a convenient initial reading.
6. Record the initial readings of the force and compression gauges.
7. Select a rate of axial deformation such that the rate of axial strain does not exceed 1%/min.
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.
9. Continue the test until the maximum value of the axial stress has been passed, or the axial strain reaches 20 %.
Figure 1 Typical Load Frame for Unconfined Compressive Strength Test