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Constant Head Permeability TestThe constant head permeability test is a laboratory experiment conducted to determine the permeability of soil. The soils that are suitable for this tests are sand and gravels. Soils with silt content cannot be tested with this method.The test can be employed to test granular soils either reconstituted or disturbed.
Objective and ScopeThe objective of constant head permeability test is to determine the coefficient of permeability of a soil. Coefficient of permeability helps in solving issues related to:
- Yield of water bearing strata
- Stability of earthen dams
- Embankments of canal bank
- Seepage in earthen dams
- Settlement Issues
What is Coefficient of Permeability?The coefficient of permeability, k is defined as the rate of flow of water under laminar flow conditions through a porous medium area of unit cross section under unit hydraulic gradient. The coefficient of permeability (k) is obtained from the relation Where q= discharge, Q=total volume of water, t=time period, h=head causing flow, L= length of specimen, A= cross-sectional area.
Apparatus for Constant Head Permeability Test
- Permeameter mould, internal diameter = 100mm, effective height =127.3 mm, capacity = 1000ml.
- Detachable collar, 100mm diameter, 60mm height
- Dummy plate, 108 mm diameter, 12mm thick,
- Drainage base, having porous disc
- Drainage cap having porous disc with a spring attached to the top.
- Compaction equipment such as Proctor’s rammer or a static compaction equipment, as specified in IS:2720 (Part VII)-1965.
- Constant head water supply reservoir
- Vacuum pump
- Constant head collecting chamber
- Stop watch
- Large funnel
- Weighing balance accuracy 0.1g
- Filter paper.
- Remove the collar of the mould. Measure the internal dimensions of the mould. Weigh the mould with dummy plate to the nearest gram.
- Apply a little grease on the inside to the mould. Clamp the mould between the base plate and the extension collar and place the assembly on a solid base.
- Take about 2.5kg of the soil sample, from a thoroughly mixed wet soil, in the mould. Compact the soil at the required dry density using a suitable compacting device.
- Take a small specimen of the soil in a container for the water content determination.
- Remove the collar and base plate. Trim the excess soil level with the top of the mould.
- Clean the outside of the mould and the dummy plate. Find the mass of the soil in the mould.
- The mould with the sample is now placed over the permeameter. This will have drainage and cap discs properly saturated
- Through the top inlet of the constant head reservoir, the specimen is connected.
- The bottom outlet is opened and a steady flow is established
- For a particular time interval, the quantity of flow can be collected.
- Measure the difference of head (h) in levels between the constant head reservoir and the outlet in the base.
- For the same interval, this is repeated three times.
Observation and CalculationsInitially, the flow is very slow. It later increases and will become constant. The constant head permeability test is best for cohesionless soils. Observations and Computation in Constant Head Pemeameter The data that is obtained directly from the tests are:
- Length (L) in cm
- Area (A) in cm2
- Constant Head (H) in cm
- Discharge (q) cm3/sec
Table.1: Observations and Calculations
|Sl. No.||Observations||Determination No.|
|1||Mass of empty mould with base plate|
|2||Mass of mould, soil and base plate|
|3||Hydraulic head (h)|
|4||Time interval (t)|
|5||Quantity of flow (Q)|
|(a) First time in period t|
|(b) Second time in period t|
|(c) Third time in period t|
|6||Mass of soil = (2) – (1)|
|8||Water content w|
|11||Coefficient of permeability Where q= dischargeQ=total volume of watert=time periodh=head causing flow L= length of specimen A= cross-sectional area.|