Identifying the Field Test for Soil Shear Strength

This question asks us to pinpoint which of the listed tests is exclusively performed in the field to determine the shear strength of soil. Shear strength is a critical property that defines how much stress a soil can withstand before failing (shearing). Let's break down each option:

Understanding the Direct Shear Test

• Test Type: This is a classic laboratory test.
• Function: It directly measures the shear strength of soil. The soil sample is placed in a shear box, and a normal load is applied. Then, a shear force is applied horizontally until the sample fails along a predetermined horizontal plane.
• Field Applicability: Not a field test; requires a controlled lab environment.

Understanding the Triaxial Compression Test

• Test Type: This is a highly versatile laboratory test.
• Function: It assesses the shear strength of soil under various confining pressures, simulating different stress conditions found underground. It helps determine key strength parameters like cohesion ($c$) and the angle of internal friction ($\phi$).
• Field Applicability: Primarily conducted in a laboratory setting.

Understanding the Unconfined Compression Test

• Test Type: This is also a laboratory test.
• Function: It's used specifically for cohesive soils (like clays) that have significant strength even without confining pressure. It measures the maximum axial stress a soil sample can withstand before failing, which directly relates to its undrained shear strength ($c_u$).
• Field Applicability: Performed in a lab, not in the field.

Understanding the Penetration Test

• Test Type: These are field tests. Common examples include the Standard Penetration Test (SPT) and the Cone Penetration Test (CPT).
• Function: Penetration tests involve driving or pushing a standardized probe (like a split-spoon sampler or a cone penetrometer) into the ground. The resistance encountered during penetration is measured. This resistance (e.g., SPT N-value, CPT cone tip resistance $q_c$) is then used, often through empirical correlations, to estimate the soil's shear strength parameters ($c$ and $\phi$). CPT provides more direct measures (cone resistance and sleeve friction) related to shear strength compared to SPT.
• Field Applicability: These tests are performed directly on-site, making them field tests.
• Exclusivity for Shear Strength: While these tests measure resistance to penetration, their primary use in geotechnical engineering includes estimating or determining the shear strength of the soil in its natural state. Crucially, unlike the other options, they are conducted in the field.

Final Determination of the Field Test

Comparing the options, the Direct Shear Test, Triaxial Compression Test, and Unconfined Compression Test are all standard methods performed in a laboratory to measure shear strength. The Penetration Test, however, is conducted in the field and is widely utilized to infer or determine the shear strength of soil layers. Therefore, the penetration test fits the description of being exclusively a field test used for determining shear strength among the choices provided.