The variable head permeability test is a method used to determine the permeability of soil. This technique is particularly useful in assessing the permeability of soil that has not been previously evaluated. Permeability refers to the soil’s ability to allow water to flow through its pores or voids.
Geotechnical engineers place great importance on soil permeability as it directly affects various aspects of engineering projects. For example, the rate of settlement of saturated soil under loads is impacted by permeability. The design of earth dams is often based on the permeability of the soil used. Permeability also significantly influences the stability of retaining structures and slopes.
In addition to these factors, soil permeability is crucial for estimating underground seepage and analyzing the stability of earth structures that are affected by seepage forces. Therefore, accurate assessment of soil permeability is essential for ensuring the safety and stability of engineering projects.
Apparatus
1. Mould Assembly
The paragraph describes a mould assembly that consists of a drainage base and a drainage cap. The assembly needs to conform to IS: 11209-1985, which is likely a standard or specification related to the design, quality, or performance of mould assemblies.
It is unclear what the purpose of the mould assembly is or in what context it is being used. However, based on the names of the components (drainage base and drainage cap), it could be assumed that the mould assembly is designed for use in a drainage system or in a process that involves the management or transfer of liquids.
2. Compaction Hammer
3. Set of Stand Pipes
Glass stand pipes with diameters ranging from 5 to 20 mm can be mounted on a stand or fixed onto a wall for various purposes. These pipes are made of glass and are designed to be used in applications where a visual indication of liquid level is required. They are typically used in laboratory setups or in industrial processes where monitoring of liquid levels is necessary.
The glass stand pipes are mounted on a stand or wall using appropriate fixtures and are secured firmly to prevent them from falling or breaking. They are manufactured to high standards of precision to ensure that the diameter is uniform along the entire length of the pipe. This ensures accurate measurement of liquid levels and prevents errors in readings.
The stand pipes are made of glass due to its transparency and durability. Glass allows for easy visual inspection of liquid levels and is resistant to corrosion and chemical reactions. The varying diameters of the pipes cater to different liquid volume requirements and enable the pipes to be used in a wide range of applications.
Overall, the glass stand pipes with diameters ranging from 5 to 20 mm are versatile and reliable tools for measuring liquid levels in various settings. Their durability, precision, and transparency make them an ideal choice for laboratory and industrial applications.
4. Constant Head Tank
To conduct a constant head test using a permeameter, a water reservoir that can continuously supply water to the permeameter is necessary. The reservoir should have the capacity to maintain a constant head of water throughout the testing period. This is crucial to ensure accurate and reliable test results. Therefore, selecting a suitable water reservoir is essential for conducting constant head tests effectively.
4. Miscellaneous Apparatus
The equipment required for a particular task includes various items such as IS sieves, a mixing pan, a graduated cylinder, a meter scale, a stop watch, a 75 micron wire gauge, a thermometer, and a source of de-aired water. These tools are essential for the successful completion of the task and ensure accurate and precise measurements. The IS sieves are used for particle size analysis, while the mixing pan is used for preparing samples. The graduated cylinder and meter scale are used to measure volumes and lengths respectively, and the stop watch is used for timing various processes. The 75 micron wire gauge helps in determining the particle size distribution, and the thermometer is used to measure the temperature accurately. Lastly, a source of de-aired water is used to provide a constant and standardized environment for conducting the experiment. All these tools and equipment are necessary for carrying out the experiment effectively and obtaining reliable results.
Soil Specimen Preparation
Disturbed Soil Sample
To evaluate the moisture content of a 2.5-kg soil sample, start by taking a thoroughly mixed air-dried or oven-dried material. Then, remove the collar of the mould and measure its internal dimensions. Weigh the mould with a dummy plate to the nearest gram and apply a little grease to the inside of the mould.
Next, clamp the mould between the base plate and the extension collar and place the assembly on a solid base. After that, place the soil specimen in the mould and compact it at the required dry density using a suitable compacting device.
Take a small specimen of the soil in a container for the water content determination. Remove the collar and base plate, and trim the excess soil level with the top of the mould. Clean the outside of the mould and the dummy plate, and find the mass of the soil in the mould.
Finally, place the mould with the sample over the permeameter, which has drainage and cap discs properly saturated. This will allow for the determination of the soil’s moisture content.
Undisturbed Soil Sample
To prepare the specimen, it should be trimmed into a cylindrical shape with a maximum diameter of 85cm and a height equal to that of the mould. Once the specimen has been cut, it should be placed over a porous disc located at the bottom of the drainage base which has been fixed to the mould.
Next, an impervious material such as cement slurry should be used to fill the space between the mould and the specimen. This will ensure that the specimen remains in place and will not move during the testing process. Finally, the drainage cap should be fixed over the top of the mould, completing the preparation of the specimen for testing.
By following these steps, the specimen will be properly prepared and ready for testing to ensure accurate and reliable results. It is important to follow these steps carefully to ensure that the testing process is performed correctly and that accurate data is obtained.
Procedure
To conduct the test, it is necessary to connect the specimen to the stand-pipe through the top inlet. Once this is done, the bottom outlet should be opened, and the time interval required for the water level to fall from a known initial head to a known final head should be recorded. The measurement of the head should be taken above the center of the outlet.
The next step is to refill the stand-pipe with water and repeat the test until three successive observations give nearly the same time interval. The time intervals should be recorded for the drop in head from the same initial to final values, as in the first determination.
Alternatively, it is possible to select the suitable initial and final heads, h1, and h2, respectively. Then, observe the time intervals for the head to fall from h1 to the square root of (h1h2), and similarly from the square root of (h1h2) to h2. In this case, the time intervals should be the same. If they are not, the observation shall be repeated after refilling the stand-pipe.
Data Sheet
In a variable head permeability test, a data sheet is used to record several key values related to the test specimen and the testing apparatus. One such value is the length of the specimen (L), which is the distance along the longitudinal axis of the specimen being tested. Another important value is the diameter of the specimen (D), which is the width of the specimen perpendicular to its longitudinal axis.
The volume of the specimen (V) is also recorded on the data sheet, which is the total amount of space occupied by the specimen. The water content (W) is another variable that is noted, which is the amount of water present in the specimen expressed as a percentage of the total weight of the specimen.
The diameter of the stand pipe (d) is also recorded on the data sheet, which is the width of the pipe used to collect water that flows through the specimen during the test. The area of the stand pipe (a) is also noted, which is the cross-sectional area of the pipe perpendicular to its longitudinal axis.
Finally, the specific gravity of solids is also recorded, which is the ratio of the weight of a given volume of solids to the weight of an equal volume of water. This value helps to determine the porosity of the specimen, which is the percentage of the total volume of the specimen that is occupied by voids or open spaces.
Table 1 Typical table for recording observations and calculations
| Sl. No. | Observations and Calculations | No. of Trials | ||
| 1 | 2 | 3 | ||
| Observation | ||||
| 1 | Mass of empty mould with base plate | |||
| 2 | Mass of mould, soil and base plate | |||
| 3 | Initial head h1 | |||
| 4 | Final head h2 | |||
| 5 | Head (h1*h2)^0.5 | |||
| 6 | Time interval | |||
| h1 to (h1*h2)^0.5 | ||||
| (h1*h2)^0.5 to h2 | ||||
| h1 to h2 | ||||
| Calculations | ||||
| 7 | Mass of soil = (2) – (1) | |||
| 8 | Bulk Density = mass/volume | |||
| 9 | Dry density (Pd)= P/(1+W) | |||
| 10 | Void ratio (e)= (Pw G)/ Pd | |||
| 11 | k=(QL)/(Aht) |
Calculations
The equation given is used to calculate the permeability, represented as kT, of water at a certain temperature T. This permeability value is determined based on the initial and final heads, h1 and h2 respectively, and the time interval t.
The equation includes other variables as well, such as the cross-sectional area of the liquid stand pipe (a), the cross-sectional area of the specimen (A), and the length of the specimen (L). These variables are used to calculate the permeability based on the given formula.
Overall, this equation provides a way to calculate the permeability of water at a certain temperature, given the relevant variables.
Result
The given information pertains to the permeability values of a sample at two different temperatures, namely T and 27°C. These values are expressed as numerical quantities with corresponding units of cm/s. Additionally, the state of the sample is described using three parameters, namely water content, void ratio, and degree of saturation. These parameters are indicative of the conditions under which the sample exists.
Permeability is a measure of a material’s ability to allow fluids to pass through it. In this case, the permeability values are reported for a sample at two distinct temperatures, T and 27°C. Both values are expressed in terms of the units of cm/s, indicating the rate at which fluid can pass through the sample at each temperature.
The state of the sample is described using three parameters, namely water content, void ratio, and degree of saturation. Water content is a measure of the amount of water present in the sample relative to its dry weight. Void ratio is the ratio of the volume of voids in the sample to the volume of solids. Degree of saturation is the ratio of the volume of water in the sample to the total volume of voids.
Together, this information provides a comprehensive understanding of the permeability of the sample, as well as the conditions under which it exists. By measuring these parameters at different temperatures, it is possible to understand how the permeability of the sample changes with temperature. Additionally, by understanding the sample’s water content, void ratio, and degree of saturation, it is possible to make inferences about the material’s properties and behavior under different conditions.