Consolidation Test (ASTM D-2435) - Theory, Data & Step-by-Step Calculation

Welcome, future geotechnical engineers! In this deep-dive video, we're exploring one of the most fundamental and important tests in soil mechanics: The One-Dimensional Consolidation Test, governed by ASTM D-2435.

Understanding consolidation is crucial for predicting how much a soil layer will settle over time under a structural load, like a building or an embankment. Get this wrong, and you risk major structural damage. Get it right, and you build on a solid foundation!
ere is a summary of the step-by-step calculation we performed in the video.

Given Data:

Initial Height of Specimen, H₀ = 20.0 mm

Specific Gravity of Solids, Gs = 2.70

Initial Water Content, w = 25%

Diameter of Specimen = 63.5 mm (Area, A = 31.67 cm²)

Dry Mass of Solids, Ms = 92.5 g

Step 1: Calculate Initial Void Ratio (e₀)

Volume of Solids, Vs = Ms / (Gs * ρw) = 92.5 g / (2.70 * 1 g/cm³) = 34.26 cm³

Initial Total Volume, V₀ = A * H₀ = 31.67 cm² * 2.0 cm = 63.34 cm³

Volume of Voids, Vv = V₀ - Vs = 63.34 - 34.26 = 29.08 cm³

Initial Void Ratio, e₀ = Vv / Vs = 29.08 / 34.26 = 0.849
tep 2: Calculate Void Ratio (e) at each Pressure Increment
We use the change in height to find the change in void ratio.

Formula: e = e₀ - (ΔH / H₀) * (1 + e₀)

Example for a load where ΔH = 0.45 mm:

e = 0.849 - (0.45 / 20.0) * (1 + 0.849)

e = 0.849 - (0.0225) * (1.849)

e = 0.849 - 0.0416 = 0.807
Step 3: Plot e vs. log(σ') and Find Preconsolidation Pressure (σ'p)

Plot the calculated e values against the log of the applied pressure (σ').

Use the Casagrande method (as shown in the video) to graphically determine the Preconsolidation Pressure. Let's assume we found σ'p = 120 kN/m².

Step 4: Determine Compression Index (Cc) and Recompression Index (Cr)

Recompression Index (Cr): Slope of the unloading/reloading loop.

From the plot, take two points on the reloading line: (σ'₁=50 kN/m², e₁=0.790) and (σ'₂=100 kN/m², e₂=0.780).

Cr = (e₁ - e₂) / log₁₀(σ'₂/σ'₁) = (0.790 - 0.780) / log₁₀(100/50) = 0.01 / 0.301 = 0.033

Compression Index (Cc): Slope of the virgin compression line.

Take two points on the straight-line portion: (σ'₁=200 kN/m², e₁=0.740) and (σ'₂=400 kN/m², e₂=0.650).

Cc = (e₁ - e₂) / log₁₀(σ'₂/σ'₁) = (0.740 - 0.650) / log₁₀(400/200) = 0.09 / 0.301 = 0.299
Calculate Coefficient of Consolidation (cv)
Key Takeaways:
Cc (Compression Index): High Cc means the soil is highly compressible (like soft clay).

σ'p (Preconsolidation Pressure): The maximum past pressure the soil has experienced. If your design load is below this, settlement will be small.

cv (Coefficient of Consolidation): Indicates how fast consolidation occurs. A high cv means settlement happens quickly.