Permeability is the ease with which water can flow through any medium. It is a very important property of soil. Knowledge of permeability is essential to solve many soil engineering problems such as settlement of buidling, yield of wells, seepage through the earth structures etc. Therefore, it is necessary to determine the coefficient of Permeabilty. Several methods can be used to determine this property and one of those tests is Constant Head Permeability Test which is used for granular soil.
Constant head Permeability Test is based on IS: 2720 Part - 36:1987
Equipments:
- Constant Head Permeameter
- The Permeameter shall have a specimen cylinder with minimum diameter 8 to 12 times than maximum particle size whose covered at bottom with a porous disc and the opening in the disc shall not be more than 10% of the size of the particle to prevent the movement of the particle.
- A porous disc or suitable reinforced screen with spring attached to the top or any other device for applying 2 to 4 KG total load when the top plate is placed in place. This arrangement will help in maintaining the placement density and volume of the soil sample during the saturation of the sample and to satisfy the requirement of the test there should not be any change in the volume of the soil sample while performing the test.
Source: IS: 2720 - Part 36
- Constant Head Filter Tank - To supply water, shall be fitted with suitable control valves to prevent air bubbles.
- Large Funnels - These shall be fitted with special cylinderical spout, 25 mm diameter for 10 mm maximum size particles and 13 mm in diameter for 2 mm maximum size particles. The length of the spout should be greater than the full length of the permeability chamber by atleast 160 mm.
- Compaction Equipment - Suitable compaction equipment shall be used.
- Vacuum Pump or Water Faucet Aspirator - For evacuating and for saturating soil specimen under full vacuum.
- Balance - It should be sensitive to 1 gram.
- Scoop - with a capacity of 100 gram of soil.
Sampling and preparation of Specimen
- An air-dried granular sample containing less than 10% of particle less than 75 micron shall satisfy the following:
- A Sieve Analysis (as per IS: 2720 part - 4) shall be done on the sample prior to permeability test and all the particles greater than 20 mm shall be removed using sieve. These oversized particles shall not be used in the test. The percentage of these particles shall recorded and shall be used in grading of sand or gravel filter to be used at the bottom anda top of the sample in the permeameter.
- From the Sample left in the previous step, a sample size of approximately twice the required for filling the permeameater chamber shall be selected by the method of quartering of sample.
- The size of permeameter shall be used as:
Maximum Particle
Size between IS sieve
Opening
(mm)
Maximum Cylinder Diameter
(mm)
Not More than 35 % of
Total soil retained on
sieve opening
More than 35% of
Total soil retained
on sieve opening
2 mm
10 mm
2 mm
10 mm
2 mm and 10 mm
80
-
120
-
10 mm and 20 mm
-
160
-
230
- The following initial measurements shall be made:
- The inside diameter, $'D'$, the length $'L'$ between manometer outlets and the depth $'H_1'$ measured at four symmetrically spaced points from the upper surface of the top plate of the permeability cylinder to the top of the upper porous stone or screen temporarily placed on the lower porous plate or screen, this will automatically deduct the thickness of the upper plate or screen from the height measurements used to determine the volume of the soil placed in the permeameter cylinder.
- A duplicate top plate containing four larged symmetrically spaced openings through which the necessary can be made, shall be employed to determine the average value for $'H_1'$.
- Cross-sectional Area $A$ of the specimen shall be calculated.
- A small portion of specimen selected in step 2 and 3 shall be taken for water content determination and weight $'W_1'$ of the remaining air-dried sample shall be recorded for unit weight determination.
- If the maximum size of particles is 10 mm or less, place an appropriate size of funnel in the permeability device with the spout in contact with the lower porous plate or screen or previously formed layer, and fill the funnel with sufficient soil to form a layer, taking soil from different areas of the sample in the pan. Lift the funnel 15 mm or approximately unconsolidated layer of thickness to be formed, spread the soil with slow spiral motion, working from the perimeter of the device towards the centre so that a uniform layer is formed. Remix the soil in the pan for each successive layer to reduce the segregation caused by taking soil from the pan.
- For soils with a maximum size greater than 10.00 mm, spread the soil from a scoop. Uniform spreading can be obtained by sliding a scoopful of soil in a nearly horizontal position down along the inside surface of the device to the bottom or to the formed layer, then tilting the scoop and drawing it towards the centre with a single slow motion, this allows the soil to run smoothly from the scoop in a windrow without segregation. Turn the permeability cylinder sufficiently for the next scoopful, thus progressing around the inside perimeter to form a uniform compacted layer of a thickness equal to the maximum particle size.
- Compact successive layers of soil to the desired relative density by appropriate procedure, as follows, to a height of about 20-mm above the upper manometer outlet.
- Minimum Density (Zero percent relative density): Continue placing layers of soil in succession as described until the device is filled to the proper level.
- Maximum Density (100 percent relative density): Compact the sample by vibrating tamper, sliding weight tamker or any other approved methods such as deposition under water by vibratory packer equipment where care is taken to obtain a uniform specimen without segregation of particle sizes.
- Relative Density Intermediate Between zero and 100 Percent: By trial in a separate container of the same diameter as the permeability cylinder, adjust the compaction to obtain reproducible values of relative density. Compact the soil in the permeability cylinder by these procedures in thin layers to a height of about 20 mm above the upper manometer outlet.
Preparation of Specimen for Permeability Test
- Level the upper surface of the soil by placing the upper porous plate or screen in position and by rotating it gently clockwise and anti-clockwise.
- Measure and record the final height of specimen, $H_1 - H_2$, by measuring the depth, $H_2$, from the upper surface of the perforated top plate employed to measure $H_1$ to the top of the upper porous plate or screen at four symmetrically spaced points after compressing the spring lightly to seat the porous plate or screen during the measurements; the final weight of air-dried soil used in the test $(W_1-W_2)$ by weighing the remainder of soil $W_2$ left in the pan. Compute and record the unit weights, void ratio, and relative density of the test specimen.
- To satisfy no volume change condition, make an airtight seal by pressing down the top plate against the spring and attach it securely to the top of the permeameter cylinder.
- To satisfy the condition of laminar flow through saturated soil voids, connect the inlet tube of the top plate of the permeameter to a vacuum pump or suitable aspirator capable of evacuating the air content from the specimen and outlet to the water container as shown in figure below. Close the manometer outlets and the outlet valve at the base plate of the permeameter. Using a vacuum pump or aspirator, evacuate the specimen under 500 mm Hg, minimum for 15 minutes to remove air adhering to soil particles and from the voids. Follow the evacuation by a slow saturation of the specimen from the bottom upward under full vacuum in order to force any remaining air in the specimen. Continued saturation of the specimen can be maintained more adequately by the use of de-aired water, or water maintained at an in-flow temperature sufficiently high to cause a decreasing temperature gradient in the specimen during the test.
Source: IS: 2720 - Part 36
- After the specimen has been saturated and the permeameter is full of water, close the bottom val?ze on the outlet tube (See above Fig) and disconnect the vacuum. Care should be taken to ensure that the permeability flow system and the manometer system are free of air and are working satisfactorily. Fill the inlet tube with water from the constant-head tank by slightly opening the filter tank valve. Then connect the inlet tube to the top of the permeameter, open the inlet valve slightly and open the manometer outlet cocks slightly, to allow water to flow, thus freeing them of air. Connect the water manometer tubes to the manometer outlets and fill with water to remove the air. Close the inlet valve and open the outlet valve to allow the water in the manometer tubes to reach their stable water level under zero head.
Procedure:
- Open the inlet valve from the filter tank slightly for the first run, delay measurements of quantity of flow and head until a stable head condition without appreciable drift in water manometer level is attained. Measure and record the time $'t'$, head $'h'$ (the difference in level in the manometers), quantity of of flow $'Q'$, and water temperature $'T'$.
- Repeat the test runs at heads, increasing by 5 mm in order to establish accurately the region of laminar flow with velocity $'u'$ (where $u = Q/At$), directly proportional to hydraulic gradient $'i'$ (where $i = h/L$). When departures from the linear relation become apparent, indicating the initiation of turbulent flow conditions, 10 mm intervals, of head may be used to carry the test run sufficiently along in the region of turbulent flow to define this region if it is significant for field conditions.
NOTE - Much lower values of hydraulic gradient h/l. are required than generally recognized, in order to ensure laminar flow conditions. The following values are suggested:
Loose compactness ratings $h/L$ from $0.2$ to $0.3$ and
Dense compactness ratings $h/L$ from $0.3$ to $0.5$; the lower values of $h/L$ apply to coarser soils and the higher values to finer soils.
- At the completion of the permeability test, drain the specimen and inspect it to establish whether it was essentially homogenous and isotropic in character. Any light and dark alternating horizontal streaks or layers are evidence of segregation of fines.
Calculations:
Permeability $K_T$ at temperature $T$ is calculated by:
$K_T = \frac {Q}{A \times i \times T}$
and permeability at 27°C by using the expression,
$K_{27}= K_T - \frac {\mu_T}{\mu_{27}}$
where,
$\mu_T=$ Coefficient of viscosity at $T°C$, and
$\mu_{27}=$ Coefficient of viscosity at $27°C$.
Void ratio $e$ is calculated as
$e= \frac {G_s \times \gamma_w}{\gamma}-1$
where,
$\gamma_w =$ density of water $= 1 g/cm^3$
$\gamma=$ dry unit weight of specimen
$\gamma_s =$ specific Gravity.
- The Permeameter shall have a specimen cylinder with minimum diameter 8 to 12 times than maximum particle size whose covered at bottom with a porous disc and the opening in the disc shall not be more than 10% of the size of the particle to prevent the movement of the particle.
- A porous disc or suitable reinforced screen with spring attached to the top or any other device for applying 2 to 4 KG total load when the top plate is placed in place. This arrangement will help in maintaining the placement density and volume of the soil sample during the saturation of the sample and to satisfy the requirement of the test there should not be any change in the volume of the soil sample while performing the test.
- A Sieve Analysis (as per IS: 2720 part - 4) shall be done on the sample prior to permeability test and all the particles greater than 20 mm shall be removed using sieve. These oversized particles shall not be used in the test. The percentage of these particles shall recorded and shall be used in grading of sand or gravel filter to be used at the bottom anda top of the sample in the permeameter.
- From the Sample left in the previous step, a sample size of approximately twice the required for filling the permeameater chamber shall be selected by the method of quartering of sample.
| Maximum Particle Size between IS sieve Opening (mm) |
Maximum Cylinder Diameter (mm) |
|||
|---|---|---|---|---|
| Not More than 35 % of Total soil retained on sieve opening |
More than 35% of Total soil retained on sieve opening |
|||
| 2 mm | 10 mm | 2 mm | 10 mm | |
| 2 mm and 10 mm | 80 | - | 120 | - |
| 10 mm and 20 mm | - | 160 | - | 230 |
- The inside diameter, $'D'$, the length $'L'$ between manometer outlets and the depth $'H_1'$ measured at four symmetrically spaced points from the upper surface of the top plate of the permeability cylinder to the top of the upper porous stone or screen temporarily placed on the lower porous plate or screen, this will automatically deduct the thickness of the upper plate or screen from the height measurements used to determine the volume of the soil placed in the permeameter cylinder.
- A duplicate top plate containing four larged symmetrically spaced openings through which the necessary can be made, shall be employed to determine the average value for $'H_1'$.
- Cross-sectional Area $A$ of the specimen shall be calculated.
- A small portion of specimen selected in step 2 and 3 shall be taken for water content determination and weight $'W_1'$ of the remaining air-dried sample shall be recorded for unit weight determination.
- Minimum Density (Zero percent relative density): Continue placing layers of soil in succession as described until the device is filled to the proper level.
- Maximum Density (100 percent relative density): Compact the sample by vibrating tamper, sliding weight tamker or any other approved methods such as deposition under water by vibratory packer equipment where care is taken to obtain a uniform specimen without segregation of particle sizes.
- Relative Density Intermediate Between zero and 100 Percent: By trial in a separate container of the same diameter as the permeability cylinder, adjust the compaction to obtain reproducible values of relative density. Compact the soil in the permeability cylinder by these procedures in thin layers to a height of about 20 mm above the upper manometer outlet.
NOTE - Much lower values of hydraulic gradient h/l. are required than generally recognized, in order to ensure laminar flow conditions. The following values are suggested:
Loose compactness ratings $h/L$ from $0.2$ to $0.3$ and
Dense compactness ratings $h/L$ from $0.3$ to $0.5$; the lower values of $h/L$ apply to coarser soils and the higher values to finer soils.
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