Geotechnical Laboratory Practice – Purpose of the Pycnometer (Soil Testing) In routine soil mechanics testing, a glass pycnometer (or density bottle) is primarily used to determine which property from a soil sample, especially for coarse-grained soils where rapid determination is helpful?

Introduction / Context:
The pycnometer is a simple but powerful tool in geotechnical laboratories. While it is widely known for aiding measurements related to water and volume, students often confuse what exact index property it directly determines versus what requires additional tests. This question checks whether you can distinguish the pycnometer’s primary role in routine moisture determination, especially for coarse soils.

Given Data / Assumptions:

• Standard geotechnical lab equipment includes a pycnometer (density bottle) with stopper.
• Tests follow conventional procedures used for index properties.
• Focus is on what the pycnometer is primarily used to determine directly and efficiently.

Concept / Approach:

A pycnometer allows rapid and fairly accurate calculation of the water (moisture) content of coarse-grained soils by mass–volume bookkeeping. The wet sample is placed in the bottle and topped with water, and companion readings (including the bottle filled only with water) enable separation of the mass of solids and mass of water within the specimen. Although a pycnometer also assists in determining the specific gravity of soil solids, among the options listed here the direct, routine outcome emphasized is the moisture content percentage, especially when oven-drying is impractical for large coarse particles.

Step-by-Step Solution:

Verification / Alternative check:

Oven-dry determination (105–110 °C) remains the reference method; pycnometer results agree closely if air is fully expelled and the procedure is followed carefully.

Why Other Options Are Wrong:

Voids ratio and density index require additional gradation/volume data. Dry density requires field/lab compaction or core cutter data, not just a pycnometer reading.

Common Pitfalls:

Trapped air bubbles, incorrect temperature assumptions for water density, and poor cleaning/drying of the bottle lead to errors.

Final Answer:

Water (moisture) content (%)