Consolidation Test
Concept of the Test
The Consolidation Test, also known as the One-Dimensional Consolidation Test or the Oedometer Test, is a geotechnical procedure used to determine the rate and magnitude of soil consolidation under applied loads. The goal of the test is to measure the soil's compressibility and estimate its settlement characteristics when subjected to static loads over time. This information is critical for predicting settlement in structures and designing foundations. The test is particularly important for fine-grained soils, such as clays, which are highly susceptible to consolidation. Standards governing the consolidation test include ASTM D2435 and ISO 17892-5.
Descriptions and Procedure
Apparatus Required
- Consolidation cell (oedometer)
- Load frame with incremental loading mechanism
- Porous stones or filter paper
- Dial gauge or displacement transducer
- Water reservoir
- Balance with an accuracy of 0.01 g
- Specimen trimming tools
Procedure
1.Sample Preparation
- Specimen Trimming: Obtain an undisturbed soil sample, typically from a borehole. Trim the soil specimen to the required dimensions, usually a cylindrical sample with a diameter of 50-75 mm and a height of 20-25 mm.
- Saturation: Place the specimen in the consolidation cell between two porous stones or filter papers to ensure even drainage and saturation. Saturate the sample by allowing water to percolate through it under a small load.
2.Assembly of Consolidation Cell
- Position the specimen centrally in the consolidation cell.
- Assemble the cell, ensuring the porous stones or filter papers are in contact with the soil sample.
- Attach the cell to the load frame and set up the dial gauge or displacement transducer to measure vertical displacement.
3.Application of Initial Load
- Apply a small seating load to the specimen to establish initial contact and seating. Record the initial dial gauge reading.
- Allow the specimen to consolidate under this initial load until primary consolidation is complete. Primary consolidation is generally considered complete when the rate of settlement decreases to a negligible value.
4.Incremental Loading
- Apply subsequent loads in increments, usually doubling the previous load, following a geometric progression (e.g., 12.5, 25, 50, 100, 200 kPa).
- After each load increment, allow the specimen to consolidate fully. Record the dial gauge readings at specific time intervals (e.g., 0.1, 0.25, 0.5, 1, 2, 4, 8, 15, 30 minutes, 1, 2, 4, 8, 24 hours).
- Continue this process until the maximum desired load is reached.
5.Unloading
- After the maximum load, reduce the load in decrements (usually by halving the previous load) and allow the soil to rebound.
- Record the dial gauge readings during the unloading phase to observe any hysteresis or secondary compression effects.
6.Calculations
Primary Data
- Initial Height of Specimen (H0): Measure the initial height of the soil specimen.
- Change in Height (ΔH): Calculate the change in specimen height for each load increment from the dial gauge readings.
Key Parameters
- Void Ratio (e): Calculate the initial void ratio and subsequent void ratios after each load increment using:
𝑒=𝑉𝑣𝑉𝑠
where 𝑉𝑣 is the volume of voids and 𝑉𝑠 is the volume of solids.
- Coefficient of Consolidation (Cv): Calculate 𝐶𝑣 using the time-settlement data from the test. One common method is the Casagrande method, which involves plotting the square root of time versus settlement.
- Compression Index (Cc): Determine 𝐶𝑐 from the slope of the linear portion of the e-log σ' curve (void ratio vs. log of effective stress).
- Preconsolidation Pressure (σ'p): Identify the maximum past pressure the soil has experienced using the Casagrande method or the strain energy method.
Data Interpretation
Consolidation Curve
- Plot the void ratio (e) versus the logarithm of effective stress (σ').
- Identify key parameters like the Compression Index (Cc), Recompression Index (Cr), and Pre-consolidation Pressure (σ'p).
Settlement Prediction
Use the test data to predict settlement under future loads using consolidation equations, such as:
𝑆=𝐻0Δ𝑒1+𝑒0
where 𝑆 is the settlement, 𝐻0 is the initial height, Δ𝑒 is the change in void ratio, and 𝑒0 is the initial void ratio.
Coefficient of Consolidation (Cv)
Estimate the time rate of consolidation using the Cv values, which helps in predicting how quickly a soil layer will settle under a given load.
Summary Table
The Consolidation Test is essential for understanding soil compressibility and predicting settlement behavior under static loads, making it a critical component in the design and construction of foundations and other geotechnical structures.