Understanding Unconfined Compression Tests and Soil Strength

This question asks us to evaluate two statements related to the unconfined compression test and the strength of clay soils. Let's analyze each statement carefully to determine its correctness.

Analysis of Statement (i): Unconfined Compression Test Use

Statement (i) says: "The unconfined compression test is widely used for determining the consistency of saturated clays and other cohesive soils."

The unconfined compression test (UCT) is a common laboratory test performed on cohesive soil samples, such as saturated clays or compacted silts, which can stand vertically without lateral support. In this test, an axial compressive load is applied to a cylindrical soil specimen until failure occurs. The maximum axial stress at failure is called the unconfined compressive strength ($q_u$).

• Cohesive soils (like clay) have internal bonding that allows them to maintain shape when unconfined.
• The UCT is ideal for these soils because it doesn't require confining pressure.
• The unconfined compressive strength ($q_u$) is a measure of the soil's shear strength under undrained conditions.
• Soil consistency (e.g., soft, medium, stiff) is directly related to its shear strength. Therefore, the UCT helps determine the consistency of saturated clays.

Based on the properties and applications of the unconfined compression test, Statement (i) is TRUE.

Analysis of Statement (ii): Strength Relationship Under Drained Conditions

Statement (ii) says: "Unconfined compressive strength is thrice the value of the shear strength of clay soil under drained conditions."

Let's examine the relationship between unconfined compressive strength ($q_u$) and shear strength ($\tau$ or $c$) for cohesive soils.

• The unconfined compression test is typically conducted under undrained conditions, especially on saturated clays.
• For saturated clays under undrained conditions, the shear strength ($c_u$) is generally considered to be half of the unconfined compressive strength. This is based on the Mohr-Coulomb failure criterion assuming $\phi_u = 0$. At failure in a UCT, the major principal stress is $\sigma_1 = q_u$ and the minor principal stress is $\sigma_3 = 0$. The shear strength $\tau_f$ is given by the radius of the Mohr's circle, which is $(\sigma_1 - \sigma_3)/2$. Thus, $\tau_f = (q_u - 0)/2 = q_u/2$. So, $q_u = 2 \tau_f$ (or $q_u = 2c_u$).
• The statement claims $q_u$ is *thrice* the shear strength ($q_u = 3\tau$). This factor of 3 is incorrect; the correct factor is typically 2 for undrained conditions in saturated clay.
• Furthermore, the statement refers to shear strength under *drained* conditions. Drained shear strength parameters for clay include both cohesion ($c'$) and friction ($\phi'$). The unconfined compression test is an undrained test and is not used to determine drained strength parameters ($c'$ and $\phi'$). Therefore, relating $q_u$ from an undrained test directly to drained shear strength in a simple factor relationship like this is inappropriate and incorrect.

Based on the standard relationship between unconfined compressive strength and shear strength for cohesive soils under typical test conditions, Statement (ii) is FALSE.

Conclusion on Statements

Statement (i) is TRUE because the unconfined compression test is widely used for assessing the consistency (which relates to strength) of saturated clays and other cohesive soils.

Statement (ii) is FALSE because the unconfined compressive strength ($q_u$) of saturated clay under undrained conditions is approximately twice (not thrice) its shear strength ($c_u$), and the statement incorrectly specifies drained conditions.

Therefore, the correct answer is that Statement (i) is TRUE and Statement (ii) is FALSE.

Revision Table: Unconfined Compression Test Concepts

Additional Information on Soil Strength Tests

Understanding soil strength is crucial in geotechnical engineering. Various tests are used to determine the shear strength parameters of soils ($c$ and $\phi$).

• Triaxial Compression Test: A versatile test where a cylindrical soil sample is subjected to confining pressure and then axial stress. It can be performed under different drainage conditions (Undrained, Consolidated-Undrained, Drained) to determine both total stress and effective stress shear strength parameters.
• Direct Shear Test: A test where a soil sample is placed in a shear box and subjected to a normal force and a shear force. It is commonly used for both sands and clays, though it forces failure on a predetermined plane.
• Vane Shear Test: An in-situ or laboratory test specifically used to determine the undrained shear strength ($c_u$) of soft to medium stiff cohesive soils, particularly sensitive clays. A vane is inserted into the soil, and torque is applied to rotate it, shearing the soil along a cylindrical surface.

The unconfined compression test is a simpler, quicker, and less expensive test compared to the triaxial test, but its application is limited to cohesive soils and it primarily provides the undrained shear strength ($c_u$).