How to determine the volume of air in the pipes for Method A unsat
This guide explains how to, for unsaturated tests using Method A, calculate the volume capacity of the pressure line connecting an air pressure controller to the specimen. This will include the volume capacity of the pipes in addition to the volume capacity of any valves used in that line
Instructions
This procedure should be done before the start of the test, with no sample in the system.
Set up the Pressure Line that connects the air pressure controller to the top cap. This is usually composed of
Line A 1/8'' pipe connecting the pressure controller to a valve at the cell base,
Line B the valve and the hydraulic channel inside the cell base, connecting the valve to the cell base fitting used for pore air pressure,
Line C the 1/8'' pipe connecting that fitting to the top cap, and the hydraulic line connecting the fitting on the top cap to the specimen or porous disc.
Fill a water pressure controller with deaired water. The backpressure controller can be used for this, as it will probably already have deaired water for the test. The controller does not need to be completely full, but it should have at least 50 000mm3 of water available. Place the water pressure controller at a slightly lower level than the cell base.
Ensure that the controller has no air inside and prop the end of controller up so that it is tilted with the water ports raised, or alternatively set the controller in a vertical position with the 1/8'' port facing up
With the 8mm port closed (if there is one) and the 1/8'' port open, empty the controller until the water fully fills the 1/8th fitting and is on the verge of overflowing.
Disconnect the pore air pressure line from the air pressure controller, make sure the whole line is empty, and then connect it to the water pressure controller. Set its volume reading to zero and apply a pressure offset, so that pressure reads 0 kPa.
Hold the top cap so that the drainage hole is facing up and forms the topmost end of the air pressure line, with the 1/8'' output from the controller being the lower point of the pressure line. This will ensure that water will slowly rise through the whole pressure line until it reaches the drainage port of the top cap.
Maintaining the pressure line in this position target 20kPa on the water pressure controller. Water will slowly rise through the pressure line. Stop the controller immediately as soon as the water reaches the drainage port of the top cap. Note down the volume reading on the controller, this is the total volume capacity of the air pressure line, which is equal to the Total Pressure Line Volume of air in that line at atmospheric pressure.
Disconnect the air pressure line from the controller and make sure the water pressure controller remains in the same state as in point 4. Zero its volume reading and apply a pressure offset.
Disconnect the pipe connecting the top cap to the cell base from the cell base. Make sure both pipe and top cap are completely dry before proceeding.
You now have the top cap with the pore air pressure pipe attached. Connect it to the water pressure controller and repeat steps 6 and 7. The volume reading at the end will be the volume capacity of the pore air pressure pipe and top cap drainage port - Line C
Disconnect the air pressure line from the controller and make sure the water pressure controller remains in the same state as in point 4. Zero its volume reading and apply a pressure offset.
Thoroughly dry the whole air pressure line then connect the pipe used to connect the air pressure controller to the cell base, to the saturated water pressure controller. Hold the end of the pipe high to make sure water will rise smoothly through it.
Repeat step 7, stoping the controller as soon as the water reaches the end of the pipe. The volume reading of the controller will be the volume capacity of Line A
The volume of Line B can be calculated by V Line B = V Pressure Line - V Line A - V Line C