Soils in many cases are unsaturated in thei rnatural state because of locating above of ground water level. It means that the pores are filled not only with water (liquid) but also partly with air(gas). The volume change measurement for this type of specimens is more complicated than the saturated specimens because of air inside of the specimen and the behaviour of air characteristics like as compressibility. 

The Existence of air and water inside the unsaturated specimen generates the soil suction (when the pore-water pressure is less than the ambient air pressure). Soil suction influences the mechanical behaviour of unsaturated soils, including the shear and the compression properties. Accordingly, mechanical behaviour of unsaturated soils under triaxial stress-states shall be assessed under controlled-suction condition.

There are different techniques for unsaturated testing which poular ones for lab are:

• Axis translation technique (ATT)

• Vapor equilibrium method

• Osmotic method

The axis translation technique is commonly used method in triaxial testing. It involves imposing and controlling positive air and water pressure in the soil sample (higher than atmosphere pressure) and allows controlling the matric suction in the soil. This technique is particularly useful for testing specimens with suction values greater than 100 kPa avoiding problems associated with cavitation.

In this Method for triaxial tests, a high air entry value (HAEV) ceramic disc which allows passage of water and not air below a given air pressure is placed bottom of the specimen and the normal porous stone is placed at top. Air and water pressure are independently applied and controlled in the sample the pore air pressure is applied to the top of sample via a normal porousstone while the pore water pressure is applied to the bottom of sample via a HAEV porous disc.

The main required components to run anunsaturated test are:

• High Air Entry Porous Disc

• Air Pressure Controller to applypore air pressure (APC)

• Unsaturated Software Module for GEOsys

• Measuring set up for total volumechange measurement

High Air Entry Porous Disc

There is a need to separate the pore water and pore air and measure themduring unsaturated test and for this purpose we use a High air entry porousdisc which is properly saturated. High air entry value (HAEV) ceramic disc allowspassage of water, and not air below a given air pressure which is always higherthan water pressure thus water cannot pass to the airline due to higher pressureof the air. It has variety of suction pressure from 100kPa to 1500kPa in differentspecimen sizes from 38mm to 100mm.

Air Pressure Controller (APC)

This microprocessor closed-loop pressurecontroller regulates  Air pressure withdifferent pressure ranges for different testapplications (e.g. confining pressure, pore pressureand unsaturated tests) up to 2000kPa. There isno need for a separate datalogger to transfermeasurement from device to PC.The device worksunder closed-Loop control by an integrated dataacquisition and control unit. It Includes a highaccuracy pressure transducer to measure thepressure. The device can be directly operated bya touch sensitive keypad or via PC with controllingsoftware and has a displayer in front panel.

Methods Of Measurement Of The Total Volume Change

In triaxial drained tests on saturated soil, the volume change of the sample is usually monitored bypore-water volume exchange. However, for unsaturated soils, the total volume change of the specimenis no longer equal to the pore-water volume change; therefore, the conventional method is no longerapplicable. The total volume change generally comprises the volume of the air and changes in thevolume of the water in the void spaces. Constant temperature conditions are important for any volumechange measurement methods with water as media.

Method 1: On-Sample Strain Measurement

In this method local strain measuring units which are directlymounted on sample measure axial and radial deformations of thespecimen and the total volume change would be calculated. Strainmeasuring devices can work up to high pressures depends on theirtype and can cover all pressure testing range of the users. 

Submersible Local Circumferential Strain measuring device is usedfor specimen-diameter from 38 up to 100mm and includes a highaccuracy Lvdt transducer and does the measurement of the averageradial strain of specimen during the test. The chain consists of anumber of small chain-segments, which roll independently fromeach other over the sample and can be configured for differentsample sizes.

This method is for small deformation tests and provides a good value of Low strain(Volume changes) but requires a fairly rigid specimen. Also, when a specimen tends to deform into a barrel shape, a single measurement of the radial changes in the middle of the specimen will not be representative enough for tests that demand high accuracy. This method can be combined with double wall cell method or with advanced VPC for pore air pressure to obtain.

Required Components For This Method:

• HAVE disc  • Air pressure controller to apply pore air pressure  • Radial strain Measuring unit  • Unsaturated software module

Method 2: Double Wall

The double wall triaxial cell includes of two inner and outer cells which have the same cell pressure during the test. This provides the samepressure inside and outside the inner cell and the expansion of the inner cell would be mizedand the volume of inner cell stays constant.

The volume change of the confining fluid in a triaxial cell under loading is mostly the sum of the soil specimen volume change; the volume changesof the fluid, the volume change of the cell and alsothe tubing and etc. As we eliminate the expansionproblem of inner cell then it allows us to havethe accurate measurement of the total volumechange of the specimen by monitoring the flowof water into or out of the inner triaxial cell usingrequired component (Advanced Volume pressurecontroller or APC and automatic burette).

Another advantage of the double wall is the sample positioning. When the specimen is ready and covered by membrane, the cell is lifted down and fastened in designed place with the specimen disturbance.The combination of this method with an onsample strain measuring method gives the high  accuracy for unsaturated tests.

Required Components For This Method:

• Double wall cell  • HAVE disc  • Volume pressure controller for inner cell  • Pressure controller for outer cell  • Air pressure controller to apply pore air pressure  • Unsaturated software module

Method 3:Direct Measurement Of The Pore-Water And Pore-Air Volume Changewith Volume Pressure Controllers (VPC)

In this method we use two volume pressure controllers (VPC)a VPC which one of them controlPore water pressure. We fill first the cylinder of this VPC with de-gassed water and connect it to the bottom of test specimen to control the porewater (back pressure) and also measure the porewater volume change automatically.

Another VPC is used to control pore air pressure.This one is connected to the top of the test specimen and enables us to control the pore air and also measure the pore air volume change in high resolution. The cylinder of this VPC is filled by Air. 

As mentioned before the pore air pressure always be higher than pore water pressure thus the total volume change of the specimen can be obtained by using a simple equation between porewater volume changes and pore air volume changes.

Automatic Electromechanical pressure volume controllers made by Wille Geotechnik® are syringe pulse-less pumps which generate and regulate pressure and provide flow control in a variety range for different applications. This device is used for application srelated to the control of pressure or flow rate for various fluids and gases in laboratory tests like water, oil or air pressure and also available for aggressive media.

Required Components For This Method:

• Volume pressure controller toapply pore air pressure  • Volume pressure controller for water  • HAVE disc  • Unsaturated software module

Method 4:Differential Pressure Transducer (Dpt) Measure The Water Head In Anadditional Internal Cell

Another method of measurement the total volume change of the specimen is using differential pressure transducer (DPT) in combination with an inner open-ended cell and measuring the water head in the inner cell. The differential pressure transducer (DPT) is connected to both water cellse.g. water surrounding the soil in inner cell andthe water in reference cell.

The inner cell in the double cell is a simple open-ended cell. The differential pressure between water inside the reference cell and inner cell is measured and monitored by a DPT. The changes in the water level of inner cell which is caused by volume change of the specimen gives us the required data to calculate the volume change ofspecimen.

Geosys software controls the cell pressure in inner and also outer cell and also the pore air pressure inside the specimen.High accuracy and resolution can be obtained by accurate calibration of all transducers and using de-gassed water inside the inner cell.

Required Components For This Method:

•  Inner cell  • Low range/High Accuracy DifferentialPressure Transducer  • HAVE disc  • Air pressure controller toapply pore air pressure  • Unsaturated software module