Unconfined Compression Test
Concept of the Test
The Unconfined Compression Test (UCT) is a simple and widely used geotechnical procedure to determine the unconfined compressive strength of cohesive soil, particularly clays. The goal of the test is to measure the maximum axial compressive stress that a soil specimen can withstand without lateral confinement. This test provides valuable information about the soil's shear strength parameters, which are critical for designing foundations, embankments, and other soil structures. The standards governing the Unconfined Compression Test include ASTM D2166 and ISO 17892-7.
Descriptions and Procedure
Apparatus Required
- Loading frame or uniaxial compression device with a constant strain rate
- Load cell or proving ring
- Dial gauge or displacement transducer
- Soil trimmer or lathe
- Caliper or ruler
- Balance with an accuracy of 0.01 g
- Sample molds (if required)
- Moisture content determination tools (oven, containers)
Procedure
1. Sample Preparation
- Undisturbed Samples: Obtain undisturbed soil samples from the field, typically using Shelby tubes or similar devices. Trim the soil sample to a cylindrical shape with a height-to-diameter ratio of approximately 2:1 (e.g., 76 mm height and 38 mm diameter).
- Remolded Samples: For remolded samples, prepare the soil at the desired moisture content and compact it into a mold to the required density. Extract the sample from the mold and trim it to the required dimensions.
2. Specimen Measurement
- Measure the initial dimensions of the specimen (height and diameter) using a caliper or ruler. Record these measurements for use in calculations.
- Weigh the specimen to determine its mass and, subsequently, its bulk density.
3. Setup in Loading Frame
- Place the specimen on the bottom platen of the loading frame.
- Adjust the top platen so it just makes contact with the specimen without applying any initial load.
4. Loading the Specimen
- Apply an axial load to the specimen at a constant strain rate, typically 1% to 2% per minute. The strain rate should be slow enough to ensure that pore pressures have time to dissipate, simulating drained conditions.
- Continuously record the applied load and the corresponding axial deformation using the load cell and dial gauge or displacement transducer.
5. Failure and Data Recording
- Continue loading until the specimen fails, indicated by a peak in the stress-strain curve followed by a drop in stress, or until a strain of 15% to 20% is reached if no clear failure point is observed.
- Record the maximum load (P) applied to the specimen.
6. Moisture Content Determination
- After the test, determine the moisture content of the specimen by drying a small portion in an oven at 110 ± 5°C to a constant mass.
Calculations
Unconfined Compressive Strength (qu)
Calculate the unconfined compressive strength (qu) using the formula:
𝑞𝑢=𝑃𝐴
where 𝑃 is the maximum load applied and 𝐴 is the initial cross-sectional area of the specimen.
Cross-Sectional Area (A)
The initial cross-sectional area 𝐴 is calculated as:
𝐴=𝜋(𝑑/2)2
where 𝑑 is the initial diameter of the specimen.
Shear Strength (su)
The undrained shear strength (su) is half of the unconfined compressive strength:
𝑠𝑢=2𝑞𝑢
Data Interpretation
Stress-Strain Curve
- Plot the axial stress versus axial strain to obtain the stress-strain curve. The peak stress on this curve represents the unconfined compressive strength (qu).
Moisture Content
- The moisture content helps in correlating the compressive strength with the soil's consistency and comparing with other soil properties.
Summary Table
The Unconfined Compression Test is a straightforward yet essential method for assessing the shear strength of cohesive soils, providing crucial data for the design and stability analysis of geotechnical structures.