The vane shear test is a method used to determine the undrained shear strength of soils, particularly soft clays. This type of test can be conducted either in a laboratory setting or directly on the ground in the field. The vane shear test is especially useful for soils with low shear strength, measuring less than 0.3 kg/cm2.
The results obtained from the vane shear test are considered to be highly accurate. This makes it an important tool for assessing soil strength and stability in various applications, such as geotechnical engineering projects. The test involves rotating a vane blade attached to a rod, buried inside the soil to be tested. The force required to rotate the blade is then measured, providing a measurement of the soil’s strength. Overall, the vane shear test is an effective method for determining the undrained shear strength of soils, particularly for those with low shear strength.
Shear Strength of Soil by Vane Shear Test
Apparatus
The apparatus needed for a vane shear test includes a vane shear apparatus, a soil specimen container, and vernier calipers. The vane shear apparatus is used to apply rotational force to the soil specimen, allowing for the measurement of its shear strength. The soil specimen container is used to hold the soil sample during the test, providing stability and ensuring accurate results. Vernier calipers are used to measure the dimensions of the soil specimen, such as its diameter and height, which are important parameters in calculating shear strength. These three components work together to facilitate the vane shear test, a common method for determining the shear strength of soils in geotechnical engineering and soil mechanics.
Fig 1: Vane Shear Apparatus
The Vane Shear Apparatus is equipped with a high tensile steel rod that has four steel blades (vanes) attached to it. These vanes are fixed at right angles to each other and are positioned at the bottom of the rod. The steel rod is made of durable high tensile steel material, ensuring its strength and reliability during testing. The vanes are also made of sturdy steel, designed to withstand the pressures and forces applied during shear testing. The arrangement of the vanes at right angles to each other allows for accurate measurement of soil shear strength and provides consistent results. Overall, the Vane Shear Apparatus is a robust and reliable tool for measuring soil properties and conducting geotechnical tests.
Fig 2: Steel Rod with Vanes
Procedure of Vane Shear Test
The vane shear test procedure begins with cleaning the vane shear apparatus and applying grease to the lead screw to ensure smooth movement of handles. Next, a soil specimen is prepared in a container that is typically 75 mm in height and 37.5 mm in diameter. The soil surface is leveled on the top of the container, and the container is then mounted on the base of the vane shear test apparatus using provided screws. The vane is gradually lowered into the soil specimen until the top of the vane is positioned at a depth of 10 to 20 mm below the top of the soil specimen.
Fig 3: Lowering Vane into the Soil Specimen
The initial reading of the pointer on the circular graduated scale is noted before starting the test. The vane inside the soil specimen is then rotated using a torque applying handle at a rate of 0.1 degrees per second. The test is stopped when the specimen fails and the final reading of the pointer, which indicates the angle of torque at failure, is noted as well. This procedure is repeated on two more soil specimens. The difference between the initial and final readings of the pointer gives the angle of torque at failure for each specimen.
The diameter and height of the vane are measured using Vernier callipers. To determine the sensitivity of the soil sample, the test procedure is repeated on remolded soil, which is obtained after rapid stirring of the vane in the above test. The sensitivity of the soil is calculated as the ratio of undisturbed shear strength to remolded shear strength.
Observations and Calculations of Vane Shear Test
The shear strength of a given soil sample is determined using the following observations: the diameter of the vane, denoted by D, and the height of the vane, denoted by H. In addition, the torque, denoted by T, is calculated using a spring constant and the initial and final readings. The formula used to calculate the shear strength of the soil sample is not provided in the given context.
| S.No | Initial Reading | Final Reading | (Initial Reading – final reading) | Torque (T) | Shear strength (S) |
| 1 | |||||
| 2 | |||||
| 3 |
Result of Vane Shear Test
Shear strength of soil specimen = ________ kg/cm2.
Advantages of Vane Shear Test
The vane shear test is a popular testing method for assessing soil strength properties. One of its major advantages is its simplicity and speed, which make it an efficient way to obtain valuable information about soil characteristics. Additionally, this test can be carried out either in the laboratory or directly on the ground, making it a versatile tool for a range of applications.
The in-situ vane shear test is particularly useful for determining the undrained shear strength of non-fissured, fully saturated clay. This is because the test involves applying a twisting force to a cylindrical specimen of soil and measuring the torque required to cause it to fail. By analyzing these measurements, engineers can obtain accurate data on the shear strength of the soil, which is critical for designing safe and stable structures.
Another advantage of the vane shear test is its ability to determine the shear strength of soft clays at greater depths. This is important because these soils can be challenging to test using other methods, such as direct shear or triaxial tests. By using the vane shear test, engineers can obtain data on soil strength properties that would otherwise be difficult to obtain.
Finally, the sensitivity of soil can also be determined using vane shear test results of undisturbed and remolded soil samples. This is because the test measures the resistance of the soil to deformation, which can be affected by the degree of disturbance. By comparing the shear strength properties of undisturbed and remolded soil samples, engineers can gain insights into the sensitivity of the soil and its potential for settlement or other types of deformation.
Drawbacks of Vane Shear Test
The vane shear test is a widely used method for determining the shear strength parameters of cohesive soils. However, this test has some drawbacks that should be considered. One of the limitations of the vane shear test is that it may not be suitable for clays that have sand or silt laminations within them. This is because the test is designed to measure the shear strength of cohesive soils, and the presence of other materials may interfere with the accuracy of the results.
Another drawback of the vane shear test is that it cannot be conducted on fissured clay. Fissured clay refers to clay that has cracks or fractures in it, which can affect the results of the test. Since the vane shear test relies on the assumption of homogeneous and isotropic material, any heterogeneity or anisotropy in the clay could lead to inaccurate results.
Additionally, the vane shear test may not provide accurate results if the failure envelope is not horizontal. The failure envelope represents the relationship between the shear stress and normal stress during the test, and if it deviates from a horizontal line, the results may not be reliable. Therefore, it is important to consider these limitations when using the vane shear test to determine the shear strength parameters of cohesive soils.