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 specimens compacted with constant compactive effort
- 1. Specimens may be of undisturbed soil, or of disturbed soil that has been prepared under specified conditions.
- 2. Sieve the mixed material on the 5 mm test sieve for fine-grained materials or the 20 mm test sieve for medium-grained materials and use only the fraction passing the appropriate sieve for the preparation of the specimens.
For fine-grained materials
- Insert the appropriate plug into the bottom of the mould, i.e. the larger diameter uppermost, and add to the mould a quantity of the material sufficient to give a specimen length of 100 mm to 110 mm after compaction, i.e. about 450 g for most naturally occurring materials.
- Tamp the material gently and uniformly with the tamping rod during the operation.
- Insert the other plug into the mould, and compact the material by 15 blows of the rammer dropped from a height of 300 mm on to the plug.
- Invert the mould containing the specimen and replace the uppermost plug by the plunger.
- The material shall then be further compacted by 15 blows from the rammer applied to the plunger.
- Reject the compacted specimen for subsequent testing if its length exceeds 110 mm. Insert the plunger into the end of the mould having the smaller diameter and release the specimen from the taper by gentle hammering or pressure.
- Remove the specimen from the mould and determine its mass (m2) to the nearest 1
For medium-grained materials
- Insert the appropriate plug into the bottom of the mould, i.e. the larger diameter uppermost, and compact a quantity of material sufficient to give a specimen of 200 mm to 210 mm after compaction, i.e. about 3.5 kg for most naturally occurring materials, into the mould in six equal layers.
- Give each layer 25 blows of the rammer dropped from a height of 300 mm above the stabilized material.
- Distribute the blows uniformly over the surface of each layer, and scarify with the palette knife before the next layer is added.
- Reject the compacted specimen for subsequent testing if its length exceeds 210 mm.
- Insert the plunger into the end of the mould having the smaller diameter and release the specimen from the taper by gentle hammering or pressure. Extrude any length in excess of 200 mm beyond the mould.
- Gauge this length by placing the 200 mm long displacing collars on the plunger.
- Carefully level off the face of the specimen to the end of the mould using the straightedge and fill any irregularities with fine material from the same sample.
- Remove the specimen from the mould and determine its mass (m2) to the nearest 1 g.
- 3. After the specimens have been removed from the moulds they have to be allowed to cure for a sufficient time for strength to develop. Tests are therefore normally carried out on specimens that have been stored for various periods to determine whether or not the strength is increasing satisfactorily.
- 4. After removing the specimens from the moulds coat them with wax to maintain them at their specified moisture content.
- 5. Take care not to leave holes in the wax film and to complete the coating as quickly as possible to prevent absorption of the wax.
- 6. After coating with wax determine the mass (m3) to the nearest 1 g and store until required at a constant temperature of 20 ± 2 °C.
- 1. After the curing period and before testing weigh each specimen and record its mass (m4) to the nearest 1 g. Discard any specimen 100 mm high × 50 mm in diameter which has lost more than 4 g in mass and any specimen 200 mm high × 100 mm in diameter which has lost more than 12 g in mass during the curing period.
- 2. After weighing carefully remove the wax from the specimen; take care not to damage the surface. Keep the wax for subsequent re-use.
- 3. Measure and record the length (L) of the specimen to the nearest 0.1 mm using the calipers and the steel rule.
- 4. Place the specimen centrally on the lower platen of the compression testing machine and apply the force to the ends of the specimen so that the rate of deformation is uniform and approximately 1 mm/min.
- 5. Record the maximum force (ρ, in N) exerted by the machine during the test.
- 6. If required, determine the moisture content (ω2 ) of a representative sample of fragments taken from the interior of the specimen.
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