Experiment No. 5 Part A Permeability By Constant Head Method Soil Mechanics Lab CE350 Permeability 1 Content Definition of Permeability Darcy’s Low Apparatus Required Measurements Calculation Soil Mechanics Lab CE350 Permeability 2 Permeability It is the measure of the soil's ability to permit water to flow through its pores or voids. Objective To determine the coefficient of permeability of a cohesion less (granular) soil. Soil Mechanics Lab CE350 Permeability 3 Darcy's Law The coefficient of permeability, k, is a product of Darcy’s Law. In 1856, Darcy established an empirical relationship for the discharge velocity of water through saturated soils. v ki v discharge velocity k coefficien t of permeabili ty i hydraulic gradient h / L Soil Mechanics Lab CE350 Permeability 4 If the cross sectional area, A, through which flow is occurring is known, the flow rate, q, can be determined as: q = kiA If the time, t, over which flow occurs is also known, the total quantity of water, Q, flowing during this period can be found: Q = kiAt Soil Mechanics Lab CE350 Permeability 5 Apparatus Permeameter cylinder Perforated metal or plastic disks and circular wire screens A device for maintaining a constant-head water supply Timing device Graduated cylinder, 100 ml capacity Scale, graduated in centimeters Balance, sensitive to 0.1 g. Oven. Centigrade thermometer, range 0 to 50C Soil Mechanics Lab CE350 Permeability 6 REQUIRED MEASUREMENTS Wet weight of soil specimen, Wwet. Moisture content of soil specimen, w. Diameter of soil specimen, D. Length of soil specimen, L. Reading of first piezometer, H1. Temperature of test water, T. Elapsed time of test, t. Weight of beaker and captured water, Wbw Reading of second piezometer, H2. Soil Mechanics Lab CE350 Permeability 7 Void Ratio Determination Wt. of dry soil + can Diameter of specimen, D Wt. of can Area of specimen, A Wt. of dry soil Ws Initial height of specimen, L Sp. Gravity, Gs Initial volume of specimen, V Vol. of solid (Vs = Ws/Gs) Initial void ratio, e Soil Mechanics Lab CE350 Permeability 8 Coefficient of Permeability Test No. Initial height of specimen Void ratio, e = (AL – Vs)/Vs Red. lower piezometer, h1 Red. upper piezometer, h2 Head loss, h Quantity of flow, Q Elapsed time, t Water temperature, C Viscosity correction factor, RT Distance between piezometers, LT Soil Mechanics Lab CE350 Coefficient Permeability of permeability, K 20 9 Correction Factor, RT, for Viscosity of Water Soil Mechanics Lab CE350 Permeability 10 POSSIBLE SOURCES OF ERRORS Non-uniform compaction of cohesionless soil, specially when there is an accumulation of fines on each compacted layer. Incomplete initial saturation of the specimen. Excessive hydraulic gradient that might cause the flow not to be laminar. The presence of fine bubbles in the system. Leakage along the circumference of the specimen in the permeameter. Soil Mechanics Lab CE350 Permeability 11
