Experiment # 6 Consolidation ASTM D 1883 Soil Mechanics Lab CE 350

Consolidation It is the process of gradual dissipation of excess pore water pressure under static load which is accompanied by gradual decrease in volume of voids (increase in settlement). Soil Mechanics Lab CE 350

Settlement Immediate Settlement Consolidation Settlement settlement time settlement time settlement Soil Mechanics Lab CE 350

The Consolidation Process @t=o σ=P/A Total stress u=P/A Pore water pressure σ’=0 Strength of the spring @t=t σ=P/A u=P/A-Δσ σ’= Δσ water spring @t=∞ σ=P/A u=0 σ’=σ Soil Mechanics Lab CE 350

The Parameters Compression Index, Cc used to determine the amount of consolidation. Coefficient of Consolidation, Cv used to determine the rate of consolidation. Soil Mechanics Lab CE 350

During consolidation…  remains the same (=q) during consolidation. u decreases (due to drainage). ’ increases which is the effective stress carried by the soil particles, transferring the load from water to the soil. Due to a surcharge q applied at the GL, the stresses and pore pressures are increased at A. q kPa GL saturated clay q kPa A GL saturated clay σ u σ’ σ=q u= σ -Δσ σ’= Δσ A Soil Mechanics Lab CE 350

Consolidation Test field lab 9P P = 9P/A ≈ 20kpa For the first day dial gauge 9P GL Plate load porous stone undisturbed soil specimen Dia = 75 mm Height = 20 mm metal ring (oedometer) P = 9P/A ≈ 20kpa For the first day Soil Mechanics Lab CE 350

Date, pressure Time Elapsed time, min. Dial reading, div. Temp. C Remarks: Soil Mechanics Lab CE 350

Soil Mechanics Lab CE 350

Calculate Cv50 Tv = time factor = 0.197 for 50% consolidation Hd =average length of largest drainage path during the given loading increment. = H for one way drainage. = H/2 for 2-way drainage. H = H0 – d50*C t50 = time elapsed corresponding to d50 Soil Mechanics Lab CE 350

Soil Mechanics Lab CE 350

Calculate Cv90 Tv = time factor = 0.848 for 90% consolidation Hd =average length of largest drainage path during the given loading increment. = H for one way drainage. = H/2 for 2-way drainage. H = H0 – d90*C t90 = time elapsed corresponding to d90 Soil Mechanics Lab CE 350

To determine Cc ΔH Water Vv Water Vv Solid Vs Solid Vs Soil Mechanics Lab CE 350

Change in thickness of specimen, H (mm) Pressure, p (kpa) Initial deformation dial reading at beginning of first loading, d0 (div.) Deformation dial reading representing 100% primary consolidation, d100 (div.) Change in thickness of specimen, H (mm) Change in void ratio e Void ratio, e [e = e0  e] (1) (2) (3) (4)=[(3)(2)] 0.002 (5)=(4)/Hs (6) = e0 –(5) 20 40 80 160 320 R320 R160 R80 R40 Loading Unloading Soil Mechanics Lab CE 350

Void Ratio Vs. Log Pressure Cc Cs Soil Mechanics Lab CE 350

Applications Settlement under foundations Soil Stabilization Dealing with ground water control Soil Mechanics Lab CE 350