Understanding Soil Specific Gravity Measurement

The specific gravity of soil is a fundamental physical property that plays a crucial role in various geotechnical engineering calculations. It is defined as the ratio of the mass of soil solids to the mass of an equal volume of water at a standard temperature (usually $4^\circ \text{C}$). This value is essential for determining relationships between mass, volume, and density of soil components (solids, water, air).

To accurately determine the specific gravity of soil solids, a specific laboratory apparatus is required. The question asks to identify this apparatus from the given options.

Analyzing the Apparatus Options

Let's examine each option to understand what apparatus they represent and what they are used for:

• Penetrometer: A penetrometer is a device used to measure the resistance of soil to penetration. It is used to assess the strength, stiffness, or compaction of soil in situ (in place) or in the laboratory. Examples include the Standard Penetration Test (SPT) sampler, cone penetrometers, and Proctor penetrometers. It is not used for determining specific gravity.
• Tachometer: A tachometer is an instrument used to measure the rotational speed of a shaft or disk. It is commonly used in vehicles or machinery to indicate engine revolutions per minute (RPM). It is completely unrelated to soil properties or specific gravity.
• Pycnometer: A pycnometer is a glass bottle or flask with a stopper having a capillary tube, used to determine the density or specific gravity of a liquid or a finely divided solid. When used for soil solids, a known mass of dry soil is placed in the pycnometer, water is added, and the total mass and volume (using water density) are measured to calculate the specific gravity of the solid particles. This is the standard laboratory method for determining the specific gravity of soil solids.
• Graphometer: A graphometer is an old surveying instrument used to measure angles. It consists of a semicircle or full circle with sights and a alidade. It is used for topographical surveys and mapping and has no application in determining soil specific gravity.

Identifying the Correct Apparatus for Soil Specific Gravity

Based on the analysis of each apparatus, the pycnometer is specifically designed and used for determining the specific gravity of materials, including soil solids. The procedure involves using the pycnometer, distilled water, a vacuum pump (to remove air bubbles), and a balance to accurately measure masses and calculate the specific gravity.

Therefore, the apparatus used to determine the specific gravity of the soil is the Pycnometer.

Revision Table: Soil Testing Apparatus

Additional Information on Soil Specific Gravity

Specific gravity of soil solids ($G_s$) is a dimensionless quantity. For most mineral soils, $G_s$ typically ranges between 2.65 and 2.80. Organic soils, due to the lower density of organic matter, tend to have lower specific gravity values, sometimes less than 2.0.

The specific gravity of soil is used in many important formulas in soil mechanics, such as calculating void ratio ($e$), porosity ($n$), degree of saturation ($S$), and dry density ($\rho_d$). The relationship between many of these phase relationships often involves $G_s$. For example, the dry density can be related to water content ($w$), specific gravity ($G_s$), and degree of saturation ($S$) by the formula:

$\rho_d = \frac{(1+w)G_s \rho_w}{1 + e} = \frac{G_s \rho_w}{1 + e}$ (when considering dry density related to void ratio)

or using void ratio and degree of saturation:

$e \cdot S = w \cdot G_s$

Knowing the specific gravity of soil solids is therefore fundamental for characterizing soil behavior and performing various geotechnical analyses and designs.