Question

23. Which of the following method is used for in-situ unit weight determination of a soil deposit
(A). Pycnometer method
(B). Oven dry method
(C). Core-cutter method
(D). Sand replacement method
(E). Water replacement method
Choose the correct answer from the options given below:

• A. (A), (B) and (C) only
• B. (B), (C) and (D) only
• C. (C), (D) and (E) only
• D. (A), (E) and (D) only

Hint:

Remember that laboratory tests (pycnometer, oven drying) require taking a disturbed sample to a lab, whereas in-situ methods (core-cutter, sand replacement) measure bulk volume directly in the field.
Choose core-cutter for soft cohesive soils and sand replacement for hard, gravelly soils.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
In-situ unit weight (field density) refers to the density of soil in its natural, undisturbed state directly at the field site.
Determining field density is essential for monitoring soil compaction during road construction, foundation design, and slope stability analyses.
Field methods must either extract an undisturbed sample of known volume or directly measure the volume of a manually excavated hole in the ground.

Step 2: Detailed Explanation:
Let us analyze each of the given soil testing methods:
(A) Pycnometer method: This is a laboratory-based test used to determine the specific gravity and water content of coarse-grained soils, not bulk density.
(B) Oven dry method: This is the standard laboratory method used to determine the moisture content of a soil sample by drying it in an oven at \(105^{\circ}\text{C}\) to \(110^{\circ}\text{C}\).
(C) Core-cutter method: This is a field test where a cylindrical metal cutter of a known inner volume is driven into the soil deposit to extract an undisturbed sample.
The in-situ bulk density is calculated by dividing the mass of the extracted soil by the volume of the cutter, making this method ideal for soft, cohesive clays.
(D) Sand replacement method: This is a field test used in gravelly, sandy, or hard soils where dry, calibrated sand of known density is poured into an excavated hole to measure its exact volume.
(E) Water replacement method: This is a field test suitable for highly cohesive soils where a small hole is dug, lined with a thin plastic sheet, and filled with water to determine the volume of the hole.
Therefore, only methods (C), (D), and (E) are used for in-situ unit weight determination.

Step 3: Final Answer:
The in-situ unit weight of a soil deposit is determined using the Core-cutter, Sand replacement, and Water replacement methods, corresponding to option (C), (D) and (E) only.