1. Permeability By Constant Head Method
Experiment # 5 Part a
Permeability By
Constant Head Method
Soil Mechanics lab CE 350/01

2. Permeability Objective
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 CE 350/01

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.
Soil Mechanics lab CE 350/01

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 CE 350/01

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 50C
Soil Mechanics lab CE 350/01

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 CE 350/01

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 CE 350/01

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
Coefficient of permeability, K20
Soil Mechanics lab CE 350/01

9. Correction Factor, RT, for Viscosity of Water
Soil Mechanics lab CE 350/01

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 CE 350/01