What are the Properties of Granular Soil?

Granular soils, commonly encountered as sand, gravel, and mixtures, are fundamental geomaterials and construction materials across the globe. Their characteristics significantly influence their behavior under various loads and environmental conditions. Understanding the properties of granular soil is crucial for numerous engineering applications, from building foundations to road construction and seismic design.

Defining Granular Soils

Granular soils are distinct from cohesive soils (like clay and silt) primarily by their particle size and lack of plasticity. They consist of individual mineral grains that do not stick together significantly when dry. Their behavior is largely governed by interparticle friction and interlocking, rather than cohesive bonds.

Key types include:

• Gravel: Larger particles, typically visible to the naked eye.
• Sand: Finer particles than gravel, but still coarse enough to feel gritty.
• Non-plastic Silts: Some silts exhibit granular behavior if they lack sufficient clay content to be plastic.

Essential Properties of Granular Soil

The properties of granular soil can be broadly categorized into physical, hydraulic, mechanical, and dynamic characteristics.

1. Physical Properties

These describe the basic makeup of the soil.

• Particle Size Distribution: This is perhaps the most fundamental property, determined by sieve analysis. It defines the proportions of different grain sizes present, influencing density, permeability, and strength.
  • Example: Well-graded soils (wide range of particle sizes) can often compact to higher densities than uniformly graded soils (narrow range).
• Particle Shape: Angular, sub-angular, sub-rounded, and rounded shapes affect interlocking and friction. Angular particles tend to interlock better, contributing to higher strength.
• Specific Gravity (Gs): The ratio of the density of soil solids to the density of water. This is needed for calculating phase relationships (void ratio, porosity).
• Relative Density (Dr): A measure of how dense the soil is compared to its maximum and minimum possible densities. High relative density indicates a dense soil, which is generally stronger and less compressible.

2. Hydraulic Properties

These relate to the flow of water through the soil.

• Permeability (k): The ability of water to flow through the interconnected pore spaces. Granular soils, with their relatively large pores, are typically highly permeable compared to cohesive soils.
  • Practical Insight: High permeability in sand makes it suitable for drainage layers but susceptible to liquefaction under seismic loading if saturated.

3. Mechanical Properties

These describe the soil's response to stress and strain.

• Shear Strength: The soil's resistance to shearing or sliding along a plane. In granular soils, shear strength is primarily due to internal friction and interlocking between particles. It increases with confining pressure and relative density.
• Compressibility: Granular soils are generally less compressible than cohesive soils under static loads, especially when dense. Settlement under load occurs mainly due to particle rearrangement.
• Stiffness: The resistance to deformation under load. Stiffness increases with confining pressure and relative density.

4. Dynamic Properties

These properties are crucial for analyzing soil behavior under cyclic or transient loads, such as those from machinery or earthquakes.

As noted in the reference, the dynamic properties of granular soils, including secant shear modulus G and damping ratio D, are essential in engineering analyses involving dynamic loads.

• Secant Shear Modulus (G): A measure of the soil's stiffness under shear stress during dynamic loading. It represents the relationship between shear stress and shear strain. G is not constant but varies significantly with strain amplitude, confining pressure, and soil density.
  • Relevance: Essential for predicting how much soil will deform under dynamic shaking.
• Damping Ratio (D): Represents the energy dissipation characteristics of the soil during cyclic loading. It quantifies how quickly vibrations decay. Damping occurs due to friction between particles and viscous effects. Like G, D also varies with strain amplitude.
  • Relevance: Crucial for evaluating the attenuation of seismic waves as they pass through the soil layer.

These dynamic properties are vital for critical analyses like:

• Machine foundations design: Predicting vibration amplitudes.
• Earthquake ground response analysis: Evaluating how seismic waves are amplified or attenuated by the soil layers, affecting ground surface motion.

Summary Table of Granular Soil Properties

Understanding these properties, especially the dynamic characteristics Secant Shear Modulus G and Damping Ratio D, is fundamental for safe and effective design in geotechnical engineering, particularly when dealing with dynamic loading scenarios.