Unconfined Compressive Strength (UCS)


Sr. Dial Guage Reading (mm) Proving Ring Reading (Division)
Sr. Dial Guage Reading (mm) Proving Ring Reading (Division)

What Unconfined Compressive Strength?

A measure of a material’s strength. The unconfined compressive strength (UCS) is the maximum axial compressive stress that a right-cylindrical sample of material can withstand under unconfined conditions—the confining stress is zero. It is also known as the uniaxial compressive strength of a material because the application of compressive stress is only along one axis—the longitudinal axis—of the sample.

Alternate Form: UCS, uniaxial compressive strength.

The unconfined compressive strength (qu) is the load per unit area at which the cylindrical specimen of a cohesive soil falls in compression

Note: If the unit axial compression force per unit area has not reached a maximum value up to 20 percent, axial strain, shall be considered the value obtained at 20 percent axial strain.

  1. Compression Device The compression device may be any of the following types:

    1. Platform weighing scale equipped with a screw jack activated yoke,
    2. Hydraulic loading device,
    3. Screw jack with spring load measuring device,
    4. Any ot her loading device.

    All these loading devices shall have sufficient capacity and strain control, and shall be suitably calibrated.
    Note: The calibration of spring load measuring device especially should be checked frequently.

    The selection of the apparatus shall depend on the following:

    1. For relatively weak soil with qu less than I*kg/cm2, load shall be measurable to 0.01 kg/cm2. For soils with qu equal to or greater than 1 kg/cm2, load shall be measurable to the nearest 0.05 kg/cm2; and
    2. The axial deformation shall be measurable to 0.01 mm with a convenient dial indicator or a suitable measuring device.

  2. Sample Ejecter: When samples are pushed from the drive sampling to be, the ejecting device shall be capable of ejecting the soil core from the sampling tube in the same direction of travel in which the sample entered the tube and with negligible disturbance of the sample. Conditions at the time of removal of the sample may dictate the diretion of removal, but the principal concern should be to keep the degree of disturbance negligible.

  3. Deformation Dial Gauge a dial gauge with 0.01 mm graduations and specific travel to permit 20 percent axial strain.
    Note:A dial gauge of 20 mm travel will suffice for the test on 38 mm diameter specimen.

  4. Calipers suitable to measure physical dimensions of the test specimen to the nearest 0.1 mm.

  5. Timer timing device to indicate the elapsed testing time to the nearest second shall be used for establishing the rate of strain application prescribed in 5 and 6.

  6. Oven thermostatically controlled, with interior of non-corroding material, capable of maintaining the temperature at 110 m 5° C.

  7. Balances suitable for weighing soil specimens specially. Specimens of less than 100 g shall be weighed to the nearest 0.01 g, whereas specimens of 100 g or larger shall be weighed to the nearest 0.1 g.

  8. Miscellaneous Equipment Specimen trimming and carving tools, remoulding apparatus, water content cans, data sheets, etc. as required.

  1. The initial length, diameter and weight of the specimen shall be measured and the specimen placed on the bottom plate of the loading device.

  2. The deformation dial gauge shall be adjusted to zero. Force shall be applied so as to produce axial strain at a rate of l/2 to 2 percent per minute. Force and deformation readings shall be recorded at suitable intervals, the frequency of the readings shall be more at the initial stages, the frequency may be reduced at higher percentage of strain (see Note).
    Note:Up to 6 percent strain, the readings may be taken at every 30 s, after 6 percent, the frequency may be halved and beyond 12 percent, it may be decreased further.

  3. The specimen shall be compressed until failure surfaces have definitely developed or the stress- strain curve is well past its peak or until an axial strain of 20 percent is reached.

  4. The failure pattern shall be sketched carefully and shown on the data sheet or on the sheet presenting the stress-strain plot. The angle between the failure surface and the horizontal may be measured if possible, and reported.

The Unconfined Compressive Strength is calculated as

  1. Initial Area Of Specimen (A0) = π4D02

  2. Change in Length or Deformetaion L = Dail Reading in mm

  3. Axial Strain () = LL0

  4. Load (Kg) = Proving Ring Reading * Proving Ring Constant

  5. Corrected Area = A01-cm2

  6. Stress (δ) = LoadCorrected Area Kg/cm2

Result of Test
UCS (qu) = Maximum Value From Stress (δ)

Cohesion = qu2 Kg/cm2