Soil lesson 2

• Soil is composed of solids, liquids, and gases
• The solid phase of soil may be mineral, organic matter, or both
• The spaces between the solids (soil particles) are called voids
• If all the voids are filled with water, the soil is saturated
• If all the voids are filled with air, the soil is unsaturated
• Water is often the predominant liquid and air is the predominant gas in soil
• Three Phases in Soils
• Void Ratio (e): The ratio of the volume of voids (Vv) to soil volume (Vs)
• Void ratios of real coarse-grained soils vary between 1 and 0.3, greater than 1 for clay soils
• Porosity (n): The ratio of the volume of voids (Vv) to total volume (V), 0 ≤ n ≤ 1
• Relationship between void ratio and porosity: e = n/(1-n), n = e/(1+e)
• Water Content (ω): The ratio of the amount of water (Ww) in the soil (Ws) expressed as a percentage
• Degree of Saturation (S): The ratio of the volume of water (Vw) to volume of voids (Vv) expressed as a percentage, 0% ≤ S ≤ 100%
• Completely dry soil: S = 0%
• Completely saturated soil: S = 100% or 1
• Unsaturated soil (partially saturated soil): S = Vw / Vv = (ω Gs)/e or Se = ω Gs
• Unit Weight (Ɣ): The ratio of weight to volume
• Specific Gravity (Gs): The ratio of unit weight of soil to unit weight of water
• Relative Density (Dr): Dr = (Gs - 1)/(emax - e), where emin ≤ e ≤ emax
• Particle Size Distribution
• Particle size distribution curve is a representation of the various grain sizes in a soil determined through sieving and sedimentation
• Sieve Analysis: Used for particle sizes > 0.075 mm in diameter
• Hydrometer Analysis: Used for particle sizes < 0.075 mm in diameter
• Effective size (D10): Diameter corresponding to 10% finer in the particle size distribution curve
• Uniformity Coefficient (Cu): Cu = D60/D10, a well-graded soil will have Cu > 4 for gravel and Cu > 6 for sand
• Coefficient of Curvature (Cc): Cc = (D30)²/(D60*D10), D30 is the diameter corresponding to 30% finer
• Clay Fraction (CF): Percentage by dry mass of particles smaller than 0.002 mm (2 μm)
• Well-Graded Material: Contains particles of a wide range of sizes, has greater strength and lower permeability
• Poorly-Graded Material: Contains a large portion of uniformly sized particles, has larger voids and poor strength with high permeability
• Special Cases for Unit Weight
• Saturated unit weight (S=1): γsat = [(Gs + e)/(1 + e)] γw
• Dry unit weight (S=0): γd = Ws/V = [Gs /(1 + e)] γw
• Effective or Buoyant unit weight: γ’ = γsat – γw = [(Gs - 1)/(1 + e)] γw