The Consolidation Test is an important tool for measuring the rate and magnitude of soil settlement. This test focuses on primary consolidation, which accounts for approximately 90% of total consolidation. By analyzing the settlement values obtained from the Consolidation Test, engineers can design strong and stable foundations for structures. The information provided by this test is crucial for ensuring the safety and longevity of buildings and other infrastructure. Overall, the Consolidation Test is a key aspect of the foundation design process, and its results play a critical role in creating strong and stable structures.
Apparatus Required for Consolidation Test
In geotechnical engineering, the consolidation behavior of soil is an important factor to consider in designing structures. To determine this, two common laboratory tests are carried out using a consolidation ring, two porous stones, two filter papers, a loading pad, a dial gauge with an accuracy of 0.002mm, a stopwatch, a knife or spatula or fine metal wires, a weighing balance with an accuracy of 0.01g, vernier calipers, an oven, and a water reservoir.
The first test is the consolidation test using a consolidometer. In this test, a soil sample is placed in the consolidation ring with a porous stone at the top and bottom, and filter papers on either side of the sample. The sample is then loaded using the loading pad, and the dial gauge is used to measure the deformation of the sample over time. The stopwatch is used to record the time intervals, and the knife or spatula or fine metal wires are used to ensure the filter papers are in place. The weight of the sample is also measured using the weighing balance before and after the test, and the thickness of the sample is measured using vernier calipers. The oven is used to dry the sample before and after the test.
The second test is the oedometer test. This test is similar to the consolidation test, but the sample is placed in an oedometer instead of a consolidometer. The sample is then loaded and the deformation is measured using the dial gauge, stopwatch, knife or spatula or fine metal wires, and weighing balance. The water reservoir is used to maintain a constant water content in the sample throughout the test.
Overall, these tests are crucial in determining the compressibility and settlement characteristics of soil, which are important factors in designing structures that can withstand the weight and forces exerted on them by the soil.
Fig 1: Parts of Consolidometer
Consolidation Test Procedure
The consolidation test procedure for soil begins with collecting a soil specimen using a clean and dried consolidation metal ring. The weight, inner diameter, and height of the ring are measured using a weighing balance and calipers. The metal ring is then pressed into the soil sample by hand, and the soil specimen is taken out with the ring. The soil specimen should project about 10 mm on either side of the metal ring. Excess soil content on the top and bottom of the ring is trimmed using a knife, spatula, or fine metal wires. This excess soil can be used to measure the water content of the soil sample. It is important to ensure that the outer part of the ring does not contain any soil, and the metal ring with the soil specimen is weighed. Two porous stones are then saturated by boiling them for 15 minutes or by submerging them in distilled water for 4 to 8 hours. The consolidometer is assembled by placing the parts from bottom to top in the following order: bottom porous stone, filter paper, specimen ring, filter paper, and top porous stone.
Fig 2: Arrangement of Consolidometer Parts
To begin, place the loading pad on top of the porous stone and securely lock the consolidometer using the provided metal screws. Next, mount the entire assembly onto the loading frame and ensure that it is centered so that the load applied is axial, i.e., directly in line with the center of the consolidometer. Lastly, arrange the dial gauge in a position that allows for sufficient space for swelling of the soil specimen, taking into consideration the potential expansion of the soil during the testing process.
Fig 3: Dial Gauge Position
To saturate the soil, the water reservoir is connected to the mounted assembly, and the water level in the reservoir is adjusted to be at the same level as the soil specimen. An initial trial load is then applied, which should not cause any swelling in the soil. Typically, for ordinary soils, an initial load of 5 kN/m2 is applied, while for very soft soils, a load of 2.5 kN/m2 is applied.
The load is left applied until there is no change in the dial gauge reading or for a duration of 24 hours, and the final reading of the dial gauge for the initial load is noted. Following this, the first load increment of 10 kN/m2 is applied, and the stopwatch is started immediately. The readings of the dial gauge are noted at various time intervals, such as 0.25, 1, 2.5, 4, 6.25, 9, 16, 25, 30 minutes, 1, 2, 4, 8, and 24 hours.
In general, it is observed that primary consolidation of the soil, which accounts for 90% of consolidation, is reached within 24 hours. Therefore, the readings are noted up to 24 hours to capture the primary consolidation characteristics of the soil.
Fig 4: Applying Loads on Consolidometers
The next step is to apply a second load increment of 20 kN/m2 and repeat the procedure as mentioned in the 14th step. Similarly, apply load increments of 50, 100, 200, 400, and 800 kN/m2, and repeat the same procedure, noting down the readings at each increment. Once the values of the last load increment are recorded, the load is reduced to ΒΌ of the last load value and left for 24 hours. At this point, the dial gauge reading is noted down. The load is then further reduced and the procedure repeated until the load reaches 10 kN/m2, with the final gauge readings noted at each point.
After completing the load reduction steps, the assembly is removed from the loading frame and dismantled. The specimen ring is taken out and excess water wiped off, and the specimen ring is weighed and recorded. Finally, the specimen is placed in an oven to determine its dry weight.
Observations for Consolidation Test of Soil
The consolidation test is a common laboratory test used to determine the rate and magnitude of settlement of a soil specimen under load. During the test, several observations are made to understand the soil behavior.
The height of the ring is measured using a ruler, and it represents the distance between the top of the ring and the base.
The diameter of the ring is also measured using a ruler, and it represents the distance across the top of the ring.
The area of the ring is calculated using the formula for the area of a circle, which is pi times the radius squared, where the radius is half the diameter.
The volume of the ring is calculated by multiplying the area of the ring by the height of the ring.
The weight of the ring is measured using a balance, and it represents the mass of the ring.
The specific gravity of solids, G, is determined by dividing the weight of the ring by the weight of an equal volume of water.
The weight of the ring and soil specimen is also measured using a balance.
The initial water content is determined by weighing the soil specimen before and after drying it in an oven.
The initial height of the specimen, H, is measured before the application of the load.
The final water content is determined by weighing the soil specimen after completion of the test.
The final weight of the soil specimen is also measured after the application of the load and the completion of the test.
Table 1: Dial gauge readings for different loads at different times
| Intensity of load (kN/m2) Horizontal | 10 | 20 | 50 | 100 | 200 | 400 | 800 |
| Time Interval (vertical) | |||||||
| 0 minutes | |||||||
| 0.25 minutes | |||||||
| 1.0 minutes | |||||||
| 2.5 minutes | |||||||
| 4 minutes | |||||||
| 6.25 minutes | |||||||
| 9 minutes | |||||||
| 16 minutes | |||||||
| 25 minutes | |||||||
| 30 minutes | |||||||
| 1 hour | |||||||
| 2 hours | |||||||
| 4 hours | |||||||
| 8 hours | |||||||
| 24 hours |
Calculations for Consolidation Test of Soil
The height of solids, denoted as Hs, refers to the vertical distance occupied by solid particles in a particular substance or material. It is typically measured from a reference point to the highest point of the solid particles in the material. On the other hand, the height of voids, denoted as Hv, represents the vertical distance between the reference point and the highest point of the void spaces within the material. In other words, Hv can be calculated by subtracting Hs from the overall height, H, of the material.
The void ratio, denoted as e, is a crucial parameter used to describe the relationship between the height of voids and the height of solids. It is defined as the ratio of Hv to Hs, expressed as e = Hv/Hs. The void ratio provides insights into the porosity and compactness of the material. A higher void ratio indicates a higher volume of void spaces relative to the volume of solid particles, which implies a higher porosity and lower compactness of the material. Conversely, a lower void ratio signifies a lower volume of void spaces and higher compactness of the material.
Table 2: Void ratio calculation for different pressure intensities
| Intensity Pressure ( kN/m2) | Initial Dial Reading | Final Dial Reading | Specimen height, H | Height of solids, Hs | Height of voids, Hv | Void Ratio, e |
| 10 | ||||||
| 20 | ||||||
| 50 | ||||||
| 100 | ||||||
| 200 | ||||||
| 400 | ||||||
| 800 |
Graphs to be Plotted
In order to determine the coefficient of consolidation (Cv), a graph can be plotted with dial gauge reading on the y-axis and the logarithm of time on the x-axis. This graph helps in analyzing the consolidation behavior of a soil sample. By monitoring the settlement of the soil sample using a dial gauge and recording the corresponding time, the logarithm of time is used to compress the time scale and make it more manageable for plotting on a graph. The slope of the resulting graph represents the coefficient of consolidation (Cv), which is a measure of how quickly a soil sample undergoes consolidation under an applied load. This method provides a visual representation of the consolidation process and allows engineers and geotechnical experts to analyze the behavior of soils and estimate their consolidation characteristics.
The relationship between dial gauge readings and the square root of time is commonly used in geotechnical engineering to determine the coefficient of consolidation (Cv) of soil. The coefficient of consolidation is a measure of how fast a soil consolidates, or settles, under an applied load. By plotting the dial gauge readings against the square root of time, engineers can analyze the settlement behavior of soil and estimate its coefficient of consolidation.
The dial gauge readings, which measure the settlement of the soil specimen, are plotted on the y-axis of the graph, while the square root of time, which represents the duration of consolidation, is plotted on the x-axis. As time progresses, the soil undergoes consolidation, and the settlement decreases. The rate of settlement can be observed from the slope of the curve on the graph.
The coefficient of consolidation (Cv) is calculated using the slope of the graph, which represents the rate of settlement with respect to time. A steeper slope indicates faster settlement, and thus a higher Cv value, while a shallower slope indicates slower settlement and a lower Cv value. By analyzing the relationship between the dial gauge readings and the square root of time, engineers can determine the coefficient of consolidation of the soil, which is a crucial parameter in designing foundations, embankments, and other geotechnical structures.
Fig 5: Dial gauge Vs Time and Void Ratio Vs Effective Stress Graphs
The relationship between final void ratio and effective stress can be used to determine the coefficient of compressibility (av) and the coefficient of volume change (mv). Final void ratio refers to the ratio of volume of voids to the volume of solids at the end of a soil consolidation test, while effective stress is the stress that is transmitted through the solid particles in the soil. By analyzing the changes in final void ratio and effective stress during soil consolidation, the coefficients of compressibility (av) and volume change (mv) can be determined. This information is crucial in understanding how soils behave under different loading conditions and can be used in geotechnical engineering to design foundations, slopes, and other structures. The relationship between final void ratio and effective stress provides valuable insights into the compressibility and volume change characteristics of soils, allowing engineers to make informed decisions in their design and construction processes.
The Compression Index (Cc) is determined by comparing the Final Void Ratio (e) with the logarithm of the Effective Stress. This relationship is used in geotechnical engineering to assess the compressibility of soils. The Final Void Ratio is a measure of the change in volume that a soil undergoes when subjected to an increase in effective stress, while the logarithm of the Effective Stress reflects the magnitude of applied pressure on the soil. By plotting the Final Void Ratio against the logarithm of the Effective Stress, a straight-line relationship can be observed, and the slope of this line represents the Compression Index (Cc). The Compression Index is a key parameter used in soil consolidation analysis and provides valuable information on the consolidation characteristics of a soil, including its compressibility and settlement potential.
Fig 6: Void Ratio Vs logarithmic Effective Stress Graphs
Results of Consolidation Test of Soil
The consolidation test is a common laboratory test used to determine the settlement characteristics of soil. The test provides valuable information on the compressibility and volume change characteristics of soils, which are essential for predicting soil behavior under different loading conditions.
In this particular consolidation test, four important parameters were determined. The coefficient of compressibility (av) is a measure of how much a soil will compress under a given load. It is calculated as the change in void ratio divided by the change in effective stress.
The coefficient of volume change (mv), on the other hand, is a measure of the change in volume of a soil due to changes in effective stress. It is calculated as the change in volume divided by the initial volume.
The compression index (Cc) is a measure of the compression behavior of soils. It is the slope of the line drawn between the void ratio and the logarithm of the effective stress, for stresses corresponding to the normal range of pressures.
Finally, the coefficient of consolidation (Cv) is a measure of the rate at which a soil will consolidate under load. It is determined by measuring the time it takes for a soil to achieve a certain degree of consolidation under a given load. The Cv is calculated as the product of the coefficient of permeability and the compressibility of the soil.
These four parameters are all important in determining the behavior of soils under different loading conditions, and are essential for designing safe and stable foundations for various structures.