Field Methods for Soil Parameter Testing
Pocket shear vane device and pocket penetrometer
In safe design and execution of various interventions in the soil, such as building foundations, dams, embankments, tunnels and retaining structures, understanding the properties of the soil plays a significant role. Not only is it crucial to understand the soil properties but also to implement testing methods and later process and interpret the results obtained. One of the essential soil properties is its strength. Soil strength represents the ability of the soil to bear loads without losing its bearing capacity or experiencing failure. Soil strength depends on its structure, type, moisture content, density and other factors.
When discussing soil strength, it is important to distinguish between the unconfined compressive strength and the undrained shear strength of the soil. Unconfined compressive strength qu (kPa) is the maximum compressive strength of the soil, i.e., the greatest compressive stress that the soil can endure before failure, when the stress/force is applied along one axis, in one direction, and drainage of water from the soil is allowed. On the other hand, undrained shear strength cu (kPa) refers to the maximum shear stress that the soil can withstand under triaxial stress conditions in an undrained state, i.e., when drainage of water from the soil is not allowed. Both the unconfined compressive strength and the undrained shear strength can be determined through field testing. Common devices used in geotechnical soil testing include the pocket shear vane device and the pocket penetrometer. These instruments allow for quick tests while providing highly useful data for assessing soil strength and general soil structure without the need for complex laboratory testing.
Pocket shear vane
The pocket shear vane device is a handheld instrument used to measure the approximate values of the peak and residual undrained shear strength of cohesive soils in the field. It is important to note that peak strength refers to the maximum stress the soil can withstand before it begins to experience permanent deformation, sliding or failure. Residual strength, on the other hand, refers to the strength that remains after the soil undergoes sliding or begins to permanently deform. The instrument consists of a housing with an indicator and a measuring scale to which an attachment with vanes is fixed on one side. Measurement is done by pushing the vane attachment into the soil and rotating the housing clockwise until failure occurs. Depending on the consistency of the soil, one of the three available attachments is used. The largest attachment has a measuring range of 0–20 kPa and is used for soft soils. The medium attachment has a measuring range of 0–100 kPa, while the smallest attachment has a measuring range of 0–250 kPa, typically used for hard clays.
To conduct the test, one hand is used to hold the pocket shear vane device, while the other hand is used to rotate the housing clockwise. Then, the rotational moment is released so that the reference mark on the housing rotates counterclockwise on the scale. Axial pressure must be maintained while slowly rotating the housing to apply an increasing rotational moment at a constant speed. The testing speed should be adjusted to achieve failure within 5–10 seconds. After the soil fails, the pocket shear vane device is withdrawn and the reference value is recorded. The measurement obtained corresponds to the torque needed to rotate the vanes inserted into the soil to cause failure of the cylindrical section of undisturbed cohesive soil.
Pocket penetrometer
The pocket penetrometer is a simple, portable and manual instrument used to measure the approximate value of the unconfined compressive strength of cohesive soils in the field. The pocket penetrometer consists of a housing with a measuring scale and an indicator inside of which is a calibrated spring. The test is performed by pressing the penetrometer’s measuring probe into the soil to a depth of 6.4 mm, after which the value is read from the scale. When the pocket penetrometer is pressed into the soil, the force required for its penetration is measured, which provides information about the soil’s strength. The measuring range is typically from 0 to a maximum of 450 kPa.
Conclusion
Field tests, such as those conducted with the pocket shear vane device and the pocket penetrometer, are essential tools for geotechnical investigation and testing because they allow for the quick and effective collection of data about the mechanical properties of soil in the field.
Both the pocket shear vane device and the pocket penetrometer are fundamental tools in field geotechnical testing because of their practicality, speed, cost-effectiveness and ability to provide essential but highly valuable data about soil strength. While these devices do not replace more sophisticated laboratory tests, they are indispensable in the preliminary investigation phase. They allow for fast, informed decisions about the type and depth of foundations, slope stability and other critical factors that influence the safety and long-term success of construction projects.
