Field vane shear is the appropriate field test for obtaining the shear strength of which of the following?
Clay
Understanding the Field Vane Shear Test for Soil Strength
The Field Vane Shear test is a widely used in-situ test in geotechnical engineering. Its primary purpose is to determine the undrained shear strength ($\tau_u$) of soft to medium-stiff cohesive soils.
The test involves pushing a four-bladed vane into the soil at the bottom of a borehole or directly into the ground surface. Once the vane is at the desired depth, it is rotated at a controlled speed. The torque required to shear the soil cylinder formed by the rotating vane is measured. This measured torque is then used to calculate the undrained shear strength of the soil using a specific formula:
\(\tau_u = \frac{T}{K}\)
Where:
- \(\tau_u\) is the undrained shear strength
- \(T\) is the maximum torque recorded during the test
- \(K\) is a vane constant that depends on the dimensions and shape of the vane
Why Field Vane Shear is Appropriate for Clay
The Field Vane Shear test is particularly appropriate for clay soils because clay is a cohesive soil. Cohesive soils derive their strength primarily from inter-particle forces (cohesion), especially under undrained conditions (where water cannot drain quickly during shearing). The vane effectively shears a cylindrical surface within this cohesive matrix.
Key reasons for its suitability in clay:
- Cohesion: The test directly measures the cohesive strength component which is dominant in clay, especially in its saturated, undrained state.
- Soft to Medium Stiff Soils: The vane can be easily pushed into these types of soils without excessive disturbance, which might occur with other testing methods.
- Undrained Strength: The test is typically performed rapidly, simulating undrained conditions which are crucial for evaluating the stability of structures built on clay immediately after construction or during rapid loading events.
Limitations of Field Vane Shear in Other Soil Types
Let's consider why the Field Vane Shear test is not appropriate for the other options:
- Sand and Gravel: These are cohesionless soils. Their strength is primarily due to friction between particles. The vane would simply displace the particles in sand and gravel rather than shearing a defined cylinder of soil. The test method relies on measuring resistance to shearing through cohesion, which is negligible in these soil types.
- Weathered Rock: Rock, even if weathered, is a much harder and more rigid material than soil. The vane cannot be easily pushed into weathered rock, and attempting to rotate it would likely result in damaging the equipment rather than obtaining a meaningful shear strength value for the rock mass. Shear strength of rock is determined using different methods suitable for hard materials.
Comparing Field Tests for Soil Shear Strength
Different field tests are suitable for different soil types:
| Field Test | Appropriate Soil Type(s) | Property Measured |
|---|---|---|
| Field Vane Shear Test | Soft to medium-stiff clays, silts | Undrained shear strength |
| Standard Penetration Test (SPT) | Sands, silts, gravels, stiff clays | Relative density (for sands), Consistency (for clays), provides empirical correlation for strength |
| Cone Penetration Test (CPT) | Sands, silts, clays | Cone resistance, sleeve friction, pore pressure (provides correlations for strength and soil type) |
| Plate Load Test | All soil types, especially useful for bearing capacity | Bearing capacity, settlement characteristics |
Conclusion: Field Vane Shear Test Suitability
Based on the mechanism of the Field Vane Shear test and the properties of different soil types, it is clear that the test is specifically designed and appropriate for determining the shear strength of cohesive soils, particularly soft to medium-stiff clay.
Revision Table: Field Vane Shear Test Summary
| Aspect | Description |
|---|---|
| Purpose | Measure in-situ undrained shear strength ($\tau_u$) |
| Appropriate Soils | Soft to medium-stiff clays and silts |
| How it Works | Rotating vane shears a cylinder of soil, torque measured |
| Why not Sand/Gravel | Cohesionless, vane displaces particles, doesn't shear |
| Why not Weathered Rock | Too hard and rigid, vane cannot penetrate/shear effectively |
Additional Information: Factors Affecting Soil Shear Strength
Soil shear strength is a critical parameter in geotechnical design, influencing bearing capacity, slope stability, and retaining wall design. It is influenced by several factors:
- Effective Stress: For cohesionless soils (sand, gravel), strength is directly proportional to effective stress. For cohesive soils (clay), effective stress also plays a role.
- Soil Type: Different soils have inherent strength characteristics (e.g., angle of internal friction $\phi'$ for granular soils, cohesion \(c\) for cohesive soils).
- Drainage Conditions: Strength can be significantly different under drained (slow loading, water can escape) and undrained (rapid loading, water trapped) conditions, especially in cohesive soils. Vane shear measures undrained strength.
- Overconsolidation Ratio (OCR): Clays that have been subjected to higher past pressures (overconsolidated clays) are generally stronger than normally consolidated clays.
- Soil Structure and Fabric: The arrangement and orientation of soil particles affect strength.
- Strain Rate: The speed at which shear occurs can influence measured strength, particularly in clays. Vane shear uses a controlled, relatively slow rotation rate compared to some lab tests.
Important Questions from Shear Strength
Quick sand condition occurs when:
Which of following tests is conducted to assess shear strength parameter of the soil.
Assertion (A): In the box shear test, the failure plane is predetermined and horizontal.
Reason (R): The shear stress is applied in the vertical direction.
Select the correct answer from the following:
In a direct shear test, the soil load is subjected to more stress at the _______.
Read the following statements and choose the CORRECT answer.
(i)The unconfined compression test is widely used for determining the consistency of saturated clays and other cohesive soils.
(ii) Unconfined compressive strength is thrice the value of the shear strength of clay soil under drained conditions.
