The derivation of the coefficient of permeability is based on the assumption of the validity of the Dairy’s law to the flow of water in soil. The term coefficient of permeability implies the velocity of flow of water through the oil under unit hydraulic gradient, and consequently has the same units as that of velocity. A. Variable head test: The variable head test is used for fine grained soils like silts and silts clays. For the Variable head test the following formula is applicable: Where, k = Coefficient of permeability at To C (CM/sec). A = Cross Sectional area of stand pipe (com).
L = Length of soil specimen (CM) A = Cross-sectional area of soil sample inside the mould (com) t=(TTL – to) = Time interval for the head to fall from Hal to h2o. Hal = Initial head of water at time TTL in the pipe, measured above the outlet. h2o = Final head of eater at time to in the pipe, measured above the outlet. B. Constant head test: The Constant head test is suitable for coarse grained soils like sands, sandy silts. For the Constant head test the following formula is applicable: if Q is the total quantity of flow in a time interval t, we have from Darers low, Where, k = Coefficient of permeability at To C (CM/sec).
L = Length of soil specimen (CM) A Total cross-sectional area of soil sample (com) I hydraulic gradients. Q Quantity of water collected in measuring jar. T = total time required for collecting ‘Q’ quantity of water. H = Difference in the water levels of the overhead and bottom tank. APPLICATION: Water flowing through soil exerts considerable seepage force which has direct effect on the safety of hydraulic structures. The rate of settlement of compressible clay layer under load depends on its permeability.
The quantity of water escaping through and beneath the earthen dam depends on the permeability of the embankments and its foundations respectively. The rate of discharge through wells and excavated foundation pits depends on the coefficient of permeability of the soils. Shear strength of soils also depends indirectly on its permeability, cause dissipation of pore pressure is controlled by its permeability. The table below gives rough values of the coefficient of permeability of various soils: Type of soil Gravel Sand Silt Clay Value of permeability (CM/sec) 103 to 1. 1. 0 to 10-3 10-3 to 10-6 less than 10-3 According to U. S Bureau of Reclamations, soil are classified as follows: Impervious Semi-previous Previous k less than 10-6 CM/sec k between 10-6 to 10-4 CM/sec k greater than 10-4 CM/sec PROCEDURE: a) Preparation of remodeled soil specimen: Weight the required quantity of oven dried soil sample. Evenly sprinkle the lactated quantity of water corresponding to the OMG. Mix the soil sample thoroughly. Clean the mould and apply a small portion of grease inside the mould and around the porous stones in the base plate.
Weight the mould and attach the collar to it. Fix the mould on the compaction base plate. Keep the apparatus on solid base. The soil sample is placed inside the mould, and is compacted by the standard Proctor compaction tools, to achieve a dry density equal to the pre-determine MAD. Weight the mould along with the compacted soil. Saturate the porous stones. Place the filter papers on both ands of the soil specimen in the mould. Attach the mould with the drainage base and cap having saturated porous stones. B) Saturation of soil specimen: I.
Connect the water reservoir to the outlet at the bottom of the mould and allow the water to flow in the soil. Wait till the water has been able to travel up and saturate the sample. Allow about 1 CM depth of free water to collect on the top of the sample. Fill the remaining portion of cylinder with De-aired water without disturbing the surface of soil. Fix the cover plate over the collar and tighten the nuts in the rods. C) Constant head test: I. I. Place the mould assembly in the bottom tank and fill the bottom tank with water up to the outlet.
Connect the outlet tube with constant head tank to the inlet nozzle of the parameter, after removing the air in flexible rubber tubing connecting the tube. Adjust the hydraulic head by either adjusting the relative height of the parameter mould and constant head tank or by rising or lowering the air intake tube with in the head tank. Start the Stop watch and at the same time put a bucket under the outlet of the bottom tank, run the test for same convenient time interval and assure. Repeat the test twice more, under the same head and for the same time interval. ) Variable head permeability test method: I. Disconnect the water reservoir from the outlet at the bottom and connect the stand pipe to the inlet at the top plate. V. Vi. Vii. Fill the stand pipe with water. Open the stop cook at the top and allow water to flow out so that all the air in the cylinder is removed. Fix the height Hal and h2o on the stand pipe from the centre of the outlet such that (Hal -? h2o) is about 30 CM to 40 CM. When all the air has escaped, close the stop clock and allow he water from the pipe to flow through the soil and establish a steady flow.