PERMEABILITY TEST

 

TYPES OF TEST

Definition of coefficient of permeability

The rate of flow under laminar flow conditions through a unit cross sectional are of porous medium under unit hydraulic gradient is defined as coefficient of permeability.

 

need and Scope

Permeability is useful in solving problems involving yield of water bearing strata, seepage through earthen dams, stability of earthen dams, and embankments of canal bank affected by seepage, settlement etc.

The falling head method of determining permeability is used for soil with low discharge, where as the constant head permeability test is used for coarse-grained soils with a reasonable discharge in a given time. For very fine-grained soil, capillarity permeability test is recommended.

 

Preparation of the Specimen

The preparation of the specimen for this test is important. There are two types of specimen, the undisturbed soil sample and the disturbed or made up soil sample.

A. Undisturbed soil specimen

1.        Note down-sample no., borehole no., depth at which sample is taken.

2.        Remove the protective cover (wax) from the sampling tube.

3.        Place the sampling tube in the sample extract or and push the plunger to get a cylindrical shaped specimen not larger than 85 mm diameter and height equal to that of the mould.

4.        This specimen is placed centrally over the drainage disc of base plate.

5.        The annular space in between the mould and specimen is filled with an impervious material like cement slurry to block the side leakage of the specimen. (Protect the porous disc when cement slurry is poured.)

6.        Compact the slurry with a small tamper.

7.        The drainage cap is also fixed over the top of the mould.

8.        The specimen is now ready for test.

B. Disturbed Specimen

The disturbed specimen can be prepared by static compaction or by dynamic compaction.

(a) Preparation of statically Compacted (disturbed) specimen.

1.        Take 800 to 1000 gms of representative soil and mix with water to O.M.C determined by I.S Light Compaction test. Then leave the mix for 24 hours in an airtight container.

2.        Find weight W of soil mix for the given volume of the mould and hence find
the dry density
gd = W / [V.(1 + w)] . Here w = water content of the soil mix.

3.        Now, assemble the permeameter for static compaction. Attach the 3 cm collar to the bottom end of 0.3 liters mould and the 2 cm collar to the top end. Support the mould assembly over 2.5 cm end plug, with 2.5 cm collar resting on the split collar kept around the 2.5 cm- end plug. The inside of the 0.3 lit. Mould is lightly greased.

4.        Put the weighed soil into the mould. Insert the top 3 cm end plug into the top collar, tamping the soil with hand.

5.        Keep, now the entire assembly on a compressive machine and remove the split collar. Apply the compressive force until the flange of both end plugs touch the corresponding collars. Maintain this load for 1 min and then release it.

6.        Then remove the top 3 cm plug and collar place a filter paper on fine wire mesh on the top of the specimen and fix the perforated base plate.

7.        Turn the mould assembly upside down and remove the 2.5 cm end plug and collar. Place the top perforated plate on the top of the soil specimen and fix the top cap on it, after inserting the seating gasket.

8.        Now the specimen is ready for test.

(b) Preparation of Dynamically Compacted Disturbed sample:

1.        Take 800 to 1000 gms of representative soil and mix it with water to get O.M.C, if necessary. Have the mix in airtight container for 24 hours.

2.        Assemble the permeameter for dynamic compaction. Grease the inside of the mould and place it upside down on the dynamic compaction base. Weigh the assembly correct to a gm (w). Put the 3 cm collar to the other end.

3.        Now, compact the wet soil in 2 layers with 15 blows to each layer with a 2.5 kg dynamic tool. Remove the collar and then trim off the excess. Weigh the mould assembly with the soil (W2).

4.        Place the filter paper or fine wore mesh on the top of the soil specimen and fix the perforated base plate on it.

5.        Turn the assembly upside down and remove the compaction plate. Insert the sealing gasket and place the top perforated plate on the top of soil specimen. And fix the top cap.

6.        Now, the specimen is ready for test.