Unit
- Three phases: Soil consists of solid particles, water, and air. The solid phase is mineral grains, the liquid phase is typically water filling some voids, and the gas phase is air or other gases in the remaining voids.
- Phase diagram: A schematic representation separates the total volume into volume of solids (
V_s), volume of voids (V_v), volume of water (V_w), and volume of air (V_a). This diagram aids in deriving relationships between weight and volume. - Void ratio: Defined as the ratio of volume of voids to volume of solids (
e = V_v / V_s). It is a key index for compressibility and is dimensionless, typically ranging from 0.3 to 1.5 for natural soils. - Porosity: The ratio of volume of voids to total volume (
n = V_v / V_t), expressed as a percentage. Porosity is related to void ratio byn = e / (1 + e). - Degree of saturation: The fraction of void volume occupied by water (
S = V_w / V_v). A value of 0 means dry soil; 1 indicates fully saturated soil. It influences strength and permeability. - Water content: The ratio of weight of water to weight of solids (
w = W_w / W_s), often expressed as a percentage. It is measured in the laboratory by oven drying. - Unit weights: Bulk unit weight (
gamma) is total weight per total volume. Dry unit weight (gamma_d) is weight of solids per total volume. Saturated unit weight (gamma_sat) applies when voids are fully filled with water. - Specific gravity: The ratio of weight of solids to weight of an equal volume of water (
G_s). Typical values range from 2.60 to 2.80 for common soil minerals like quartz and feldspar. - Relative density: Used for granular soils to compare natural void ratio to maximum and minimum void ratios (
Dr = (e_max - e) / (e_max - e_min)). Values range from 0 (very loose) to 1 (very dense). It indicates compressibility and shear strength. - Air void ratio: Also known as air content, it is the ratio of volume of air to volume of voids (
A = V_a / V_v). It is useful in compaction control and describes the amount of air in partially saturated soils. Typically expressed as a percentage.
Code Example
Python
def compute_phase_relationships(V_t, W_t, W_s, G_s):
# V_t: total volume (cm3), W_t: total weight (g), W_s: solid weight (g), G_s: specific gravity
V_s = W_s / (G_s * 1.0) # density of water = 1 g/cm3
V_v = V_t - V_s
e = V_v / V_s
n = V_v / V_t
w = (W_t - W_s) / W_s
# assume water density = 1 for simplicity
V_w = W_t - W_s
S = V_w / V_v
gamma = W_t / V_t
gamma_d = W_s / V_t
return {'void_ratio': e, 'porosity': n, 'water_content': w, 'degree_saturation': S, 'bulk_unit_weight': gamma, 'dry_unit_weight': gamma_d}The function calculates key phase parameters from measured weight and volume. It assumes water density of 1 g/cm3. Void ratio and porosity reflect packing density; water content and saturation indicate moisture state.
Quick Revision
- Soil is a three-phase system: solids, water, air.
- Void ratio and porosity define void space relative to solids or total volume.
- Degree of saturation and water content describe moisture conditions.
- Unit weights (bulk, dry, saturated) are critical for stress calculations.