GDS TTS

The GDS Triaxial Testing System Hardware Handbook (Advanced/Standard)

About this Manual

The GDSTTS users manual describes the GDS Triaxial Testing System hardware.  Please refer to the GDSLAB software manual for information about setting up and running software for your GDSTTS system.  This manual is divided into logical chapters.  Where necessary at the start of each chapter is a contents page showing in detail the contents of the chapter.

System Overview

The study and measurement of soil properties in the Triaxial Test

Typically, soil comprises a skeleton of soil grains in frictional contact with each other forming an open-packed structure (loose/soft) or close-packed structure (dense/hard). The soil particles may be microscopic in the case of clays, just visible in the case of silts, and clearly visible in the case of sands and the larger particle sized gravels.  The soil skeleton, which can also be cemented, forms an interstitial system of connecting spaces or pores.  The pores in the soil will usually contain some moisture even in unsaturated soils.  The flow of pore water can be restricted by the small size of the pores thus giving rise to low permeability particularly in clays. During construction the change in load or total stress is shared between the soil structure and the pore pressure.  The time-dependent flow of water in soil under applied load is referred to as consolidation and is the means whereby the load is transferred from pore pressure to structural loading or so-called effective stress.  It is this time-dependency which gives clays their unique behaviour whereby they have a short term or undrained strength which is different from the long term or drained strength. The ability of the soil skeleton to support load is called the shear strength of the soil.  This strength depends on the frictional nature of the interparticle contact and is measured by the coefficient of friction or angle of shearing resistance which depends on normal effective stress, and by the constant, cohesion.  The deformability of the soil skeleton is measured by various elastic theory deformation moduli such as Youngs modulus and Poissons ratio.

            Soil is often geologically prestressed to a maximum past pressure or preconsolidation pressure. This prestress constitutes a yield point. At stresses less than yield the soil behaves like an elastic solid i.e. the strains are nearly recoverable.  At stresses more than yield the soil behaves like a plastic material i.e. the strains are not recoverable and the mathematical theory of plasticity is sometimes used to describe soil behaviour.  Stress distributions, however, can be generally described using the mathematical theory of elasticity.

            These properties and characteristics can be studied and measured in the triaxial test. 

            In the triaxial test, a right cylindrical specimen of soil is sheathed in a rubber membrane and placed in a cell filled with pressurised water.  Drainage is provided between the pores and the outside of the cell via a porous stone interface and a pore water duct.  Through this duct pore pressure can be measured and pore water can flow during consolidation and swelling. Axial stress is applied by a piston moving through the top or base of the cell. The piston is usually actuated by an electric motor and gearbox turning a screw or sometimes by hydraulic means. 

            The New PC-Controlled Stress Path Triaxial Testing System enables the classic triaxial test to be computer-automated.  In addition, complex loadings can be applied which follow real geological, construction and in-service conditions where vertical and horizontal stresses both change at the same time.  This is important because soil is a loading-dependent material i.e. the deformation moduli are not unique material constants but are related to the actual loading pattern or stress path.

The GDS PC-controlled Stress Path Triaxial Testing System

Using our existing advanced technology, we have developed our new PC-controlled Stress Path Triaxial Testing System.  The system uses our proven software, the classic Bishop & Wesley hydraulic stress path triaxial cell, and is based on a special version of our new low cost pressure controller (STDTTS) or the GDS advanced controllers (ADVTTS).  Three of these pressure controllers link the computer to the test cell as follows:

• one for axial stress and axial displacement control, 

• one for cell pressure control, 

• one for setting back pressure and measuring volume change. 

            All data logging is built into the system.  The system provides test control with on-line graphics.  Data presentation is via the software.  Alternatively, saved data can be presented by Excel or some other spreadsheet.

System Elements

The system comprises the following elements:

• the classic Bishop & Wesley 7kN/1700kPa/38mm/50mm Stress Path triaxial cell with both 38mm and 50mm diameter pedestals and top caps including extension top caps, one internal submersible load cell (the user can choose from 1kN, 2kN or 4kN ranges when placing their order), 2000kPa range pore pressure transducer and 50mm range displacement transducer,  OR

• The GDS motorised stress path cell.  This advanced cell based on the concept of the classic hydraulic Bishop & Wesley (B&W) stress path cell.  The GDS 7kN-38/50mm Motorised Cell (GDSMC) replaces the hydraulic actuator in the lower chamber of the B&W cell with a direct screw drive to actuate the base pedestal through the bottom of the cell.  In addition, the GDS cell has the top and base of the cell rigidly connected together by internal  rods.  The cell chamber is an outer cell that is easily raised and lowered with the help of a counter-weight.  The drive of the cell is interchangeable with the drive of the Standard 3MPa/200cc pressure/volume controller.  By simply unscrewing the motor lead of the pressure controller  and screwing in the motor lead of the GDSMC the pressure controller becomes the axial force/displacement controller for the GDSMC.   

• An axial load/displacement controller.  For the Bishop and Wesley-type cell this controller acts as a pressure controller.  Then the axial force is generated by the pressure in the lower chamber in the cell.  The controller may have an RFM (Remote Feedback Module) to control the axial load via the internal submersible load cell. 

• A cell pressure controller.  This controller may have an RFM which is connected to the axial displacement transducer.  The axial displacement transducer will normally be of the potentiometric type. 

• A back pressure controller.  This controller may have an RFM which is connected to the pore pressure transducer. 

• GDS RS232 MUX.  This multiplexer allows up to four devices to be connected to a single RS232 communications port on the PC. 

• 8 channel data acquisition pad to take readings from the transducers (instead of using RFM’s).

• controlling software written in Visual Basic for the IBM PC XT/AT or 100% compatible. 

• NOTE:  an GDSTTS system may be configured with an 8 channel data acquisition pad or RFM's providing a single channel of data acquisition on each controller.

• 8 channel data acquisition pad to take readings from the transducers (instead of using RFM’s).

• controlling software written in Visual Basic for the IBM PC XT/AT or 100% compatible. 

• NOTE:  an GDSTTS system may be configured with an 8 channel data acquisition pad or RFM's providing a single channel of data acquisition on each controller.

Typical Test menu

The test menu is as follows:

            B CHECK

            SATURATION RAMPS

            ISOTROPIC CONSOLIDATION

            ANISOTROPIC CONSOLIDATION

            UNCONSOLIDATED-UNDRAINED

            CONSOLIDATED-UNDRAINED WITH PORE PRESSURE MEASUREMENT

            CONSOLIDATED-DRAINED WITH VOLUME CHANGE MEASUREMENT

            STRESS PATHS

            LOW SPEED CYCLIC LOADING

System features

Automatic area correction.  The system automatically uses volume change and axial displacement to compute the current average area of the test specimen.  This is used in all control calculations.  The average area is defined as the cross-sectional area of the volumetrically equivalent right cylinder of the same height.  The resolution of volume change is 1cu.mm. 

• Pore pressure is measured at the base pedestal using a rigid pore pressure sensor. 

• Volume change measurement is resolved to one cubic millimeter. 

• Friction effects on load measurement are eliminated by measuring axial force with a submersible load cell inside the triaxial chamber. 

• Axial displacement is measured by two independent means - directly by transducer and indirectly by volume change into the lower chamber. 

• The classic U-U, C-U and C-D tests can be both strain and stress controlled in both compression and extension. 

• The system allows a single continuous linear stress paths to be programmed in stress space.  Any number of continuous linear paths can be made with user-intervention at the end of each path. 

• The system can also perform constant rate of strain consolidation testing using the Rowe & Barden-type hydraulic oedometer.  This option is available by changing the software and test cell. 

Remote Feedback Module (RFM) Option

The pressure controller precisely regulates pressure by closed loop servo control from the on-board pressure transducer.  The Remote Feedback Module (RFM) allows additional logging and alternative control from a single external transducer connected to the pressure controller. In the PC-controlled Stress Path Triaxial Testing System, axial force is measured by the load cell.  The output of the load cell goes into the axial pressure RFM.  This enables the piston movement in the axial pressure controller to be regulated by servo control from the load cell instead of the on-board pressure transducer.  The axial pressure controller is no longer regulating axial pressure - it is now regulating and displaying axial force applied in the triaxial cell.  Axial pressure is generated (but not regulated), measured and displayed also.  The cell pressure and back pressure RFMs are used as hosts to provide data logging from the pore pressure and axial displacement transducers.  In this way data logging is neatly built into the system via the pressure controllers which are linked to the PC by RS232 or IEEE cables.

            The STDTTS system may also be configured with an 8 channel data acquisition device instead of RFM's. 

            The Standard system is distinct and separate from the GDS Advanced Stress Path Triaxial Testing System which is a research tool with many further advanced features.  The further advanced features available on the Advanced system include: greater accuracy for research purposes, local strain measurement, mid-plane pore pressure measurement,  k-zero consolidation, permeability.

System advantages

The new GDS PC-controlled Stress Path Triaxial Testing System has many advantages including:

• very simple to set up and use, 

• cabling and piping is reduced to an absolute minimum, 

• the whole system including PC can be laid out on a lab bench area 1.4m by 0.9m, 

• the electrical and hydraulic inter-connections of the system are open and easy to see and not hidden behind wall panels, 

• tests can be carried out manually to give "hands-on" experience, or under direct PC control for automated testing, 

• ideal for teaching modern triaxial testing in colleges and universities,

• designed for effective stress and stress path testing for commercial and public works laboratories.

Accuracy and resolution of measurement and control

In general, all accuracies within STDTTS are better than 0.5 % of full scale values, within ADVTTS controller accuracy is better than 0.1%.  For the data acquisition system all measured values have a resolution that is better than 0.1 % of full scale values.

            The resolution of control is as follows:

• All pressure control has a resolution of 1 kPa 

• Axial displacement control for the Bishop & Wesley cell is made through the change in volume of the lower chamber.  The pressure in the lower chamber varies as the test proceeds and so the accuracy of displacement  control varies according to the bulk modulus of the water being  compressed.  Although the axial displacement for this cell is controlled  by volume change, axial displacement is also measured through the external displacement  transducer.  The control of axial displacement is better than 3 % of the full range value. The accuracy of measurement of axial displacement through the external transducer is better than 0.5 % of the full scale value. 

• Where a target rate has been selected e.g. a target strain rate, this is achieved to better than 0.5 % of the desired value.

GDSTTS Hardware

Hardware overview 

The system consists of a triaxial cell, three computer controlled pressure sources, an 8 channel data acquisition pad or three RFM’s attached to the controllers, an RS232 Mux or IEEE interface card and a computer (see Figures 2.1 and 2.3).  The computer controls the system allowing a simple interface providing full system control, data logging and calculation of required parameters.

            Due to the flexible nature of GDSLAB, a GDSTTS system may be setup using either Standard Controllers or Advanced Controllers or both Standard and Advanced Controllers using either an 8 channel data acquisition pad or RFM’s for data acquisition. The system can also be setup using either the Bishop & Wesley Triaxial Cell or the GDS Motorised Cell.

STDTTS (Standard Controllers)

The following diagram shows a STDTTS system configured with an 8 channel data acquisition pad and using the Bishop & Wesley Triaxial Cell.

The system can also be configured using RFM’s attached directly to each controller, as shown in the following diagram.

ADVTTS (Advanced Controllers)

The Advanced Triaxial Testing System is similar to the Standard Triaxial Testing System but uses Advanced Controllers for increased accuracy. The following diagram shows an ADVTTS system configured with an 8 channel data acquisition pad. The Advanced Controllers use an IEEE interface card in the PC for communication, therefore it is not necessary to use a Mux.

The system can also be used using an RFM connected to each controller for data acquisition.

The Triaxial Cell

Classic Bishop & Wesley hydraulic (bellofram) cell

At the centre of this system is the classic Bishop and Wesley hydraulic triaxial cell. This provides a test chamber in which the test conditions may be accurately controlled.  The parameters that can be controlled/measured are:

• Cell pressure

• Back Pressure

• Specimen volume change

• Pore pressure

• Axial Force

• Axial Displacement

            The axial force is provided by hydraulic pressure acting on a ram in the lower chamber of the triaxial cell.  By controlling the lower chamber pressure both axial displacement and force may be controlled as required.

Table of Specifications for the GDS Bishop and Wesley Style Cells:

Specifications marked * are optional

GDS Motorised Cell

At the centre of this system is the GDS motorised triaxial cell. This provides a test chamber in which the test conditions may be accurately controlled.  The controlled/measured parameters are identical as when using the classic Bishop and Wesley cell. 

            The axial force is provided by a direct screw drive to actuate the base pedestal through the bottom of the cell, enabling the axial strain and therefore the axial forces to be directly applied. 

            The drive of the cell is interchangeable with the drive of the Standard 3MPa/200cc pressure/volume controller.  By simply unscrewing the motor lead of the pressure controller  and screwing in the motor lead of the GDSMC the pressure controller becomes the axial force/displacement controller for the GDSMC. 

The Pressure Controllers

Three GDS Pressure Controllers, optionally fitted with a GDS Remote Feedback Module (RFM), are connected to the triaxial cell. 

            Each pressure controller senses pressure from a pressure transducer inside the controller pressure cylinder, converts this into digital form and then turns the stepper motor to increase or decrease the pressure as required.  The RFM unit allows the controller to read a second transducer in addition to the internal pressure transducer.  If the second transducer has an output which is proportional to the controller pressure then the parameter read by the transducer can be controlled directly by the controller. 

               Example

               The lower chamber controller senses axial force via its RFM.  By moving its stepper motor the controller can move water into or out of the lower chamber so increasing or decreasing the lower chamber pressure (if the test specimen is in contact with the top ram) and hence controlling the axial force. 

            Each transducer connected to an RFM must have a personality module (PM). The PM is specific to its transducer Each transducer must only ever be used with its associated PM.  The PM is a small brown box between the transducer and the controller.  The PMs transform the output of the transducer into a form compatible with the controller.

            The following connections must be made:

If these connections are incorrect the system will not function correctly and your transducers may be damaged. 

RS232 connectors for GDSTTS system with preferred controller arrangement.

GDSTTS using 8 channel Serial Pad

8 channels of 16 bit data acquisition.  Each channel can be user defined with gain and range (span) settings.  +/- 5 Volts supply voltage is available individually for each transducer. 

            A wide range of signals are accommodated by the inputs, which range from +/- 3mV to +/- 10V full scale. Sensor energising is provided as +/- 5V DC. 

            Power is derived from a standard IEC power cable.  The internal universal power supply accepts voltages in the range 85 Volts to 264 Volts AC.

Connecting Transducers for GDSTTS

The first three channels on the parallel or serial pad are by default associated with the software as follows:

            Channel 0: Load Cell

            Channel 1: Pore Pressure Transducer

            Channel 2: Displacement Transducer

            Any further channels may be used for other user defined transducers but the first 3 channels are normally connected as below (see figures below).

Channels for parallel/serial pad use in GDSTTS.  

Pin Number                  Connection

                                                        1                    + 5 volts excitation

                                                        2                    - 5 volts excitation

                                                        3                              0 volts

                                                        4                + input from transducer

                                                        5                 - input from transducer

Transducer connections (diagram shown looking directly into the socket on the acquisition device or ‘pad’).

Setting Up the System

Setting the Datum of Pressure Measurement

There are four identical pressure transducers in the Bishop & Wesley cell-based system.  

            There are one each in the three pressure controllers as follows:

• Lower chamber pressure controller

• Cell pressure controller

• Back pressure controller. 

In addition, there is a fourth transducer measuring pore pressure.  This measurement is routed through the 8 channel data acquisition pad or back pressure controller RFM.  When using an RFM the back pressure controller is used as a data logging host i.e. it does not control pore water pressure (pwp) - it merely measures and displays pwp and makes the reading available to the computer interface. 

            Naturally there will be small differences between measurements of the same pressure made by these transducers.  This is because they have slightly different accuracies (their specified deviation from a standard value) and calibrations (actual relationships between a standard value and the read value). This is quite normal and should be taken into account when interpreting your results because you do not have any control over these inherent discrepancies.

            You do, however, have control over setting the common zero or datum of pressure measurement.  This is so that all four pressure measuring systems (i.e. the transducers and their associated analogue-digital conversion) measure pressure from the same “base line”.  This is how you do it. 

            First you need to set up your datum of pressure measurement. Normally this will be an elevation equal to the mid-height of the triaxial test specimen.  Probably the best way of doing this is to connect a short length (say 300mm)of small bore nylon tubing to the back pressure connector of the cell.  This is the connection to the top cap drain.  Fill the cell with water.  You will not have a test specimen in place for this procedure.  Apply a small positive cell pressure using the cell pressure controller.  You can do this by setting a target pressure.  Open the valve to the back pressure line.  Water will flow out of the cell from your short tube. Stop pumping when the tube is full of water and water drips out of the open end. Fix the open end of the water-filled tube at an elevation corresponding to the mid-height of the test specimen (or the base of the test specimen if you prefer) 

            Now the water in the cell is at a pressure corresponding to this elevation head.  Connect your back pressure controller to the base pedestal pore water port and open the valve. Now the cell pressure controller, back pressure controller and pore pressure transducer all share the same pressure set by the external tube.  You can now zero the displays of these values.  The lower chamber pressure controller can also be zeroed at this time.  Now all four displays of pressure are zeroed to the same datum of pressure measurement!         

Common mistakes when setting datum of pressure measurement

Setting Up the Triaxial Cell

General reference may be made to Bishop and Wesley's paper "A hydraulic triaxial apparatus for controlled stress path testing" included with this Users Handbook.  It may be noted that any test, including conventional tests, may be referred to as a "stress path test". 

            Although Bishop and Wesley set out to design "a simple form of triaxial apparatus in which the stress paths encountered in engineering practice can be approximated to more readily than in conventional equipment", in a GDS computer controlled system their versatile cell is equally adept at carrying out classic "standard" tests as well as advanced tests. 

            Normally, you will set up your system with the computer to your left, the test cell to your right, with the bank of three linking controllers length ways in the middle.  In this way, the interface cables from the computer are conveniently "daisy-chained" to the controllers, while the pressure connectors from the controllers are immediately adjacent to the test cell. (Refer to figure 1) 

            One controller will be connected to the valve to the lower chamber of the cell (this becomes the "lower chamber controller"), one to the valve to the cell chamber itself (this becomes the "cell pressure controller"), while the third is connected to either the valve to the pore water duct to the base pedestal or to the valve to the drain to the top cap (this becomes the "back-pressure controller").  When making connections, remember to flush air out of the connectors using a syringe of deaerated water.

            Before applying cell pressure, set the cell pressure controller to zero and observe the lower chamber pressure displayed on the lower chamber digital controller.  The pressure in the lower chamber controller is caused by the self-weights of the triaxial cell piston and the test specimen and associated porous stones and top cap.  This pressure should be "nulled off" by pressing the PRESSURE ZERO function key on the lower chamber digital controller (i.e. pressing ‘Reset’ then ‘8’).  Cell pressure may now be applied.  If the triaxial extension device is being used, refer to section 2.5.4 below. 

            Before the load cell is brought into contact with the test specimen top-cap it is necessary to zero the load cell reading. This can be achieved by using the following key sequence on the lower chamber controller;

RESET, 0, •, 6, RESET

            This sequence will apply a ‘soft’ zero offset to the load-cell reading (if using RFM’s only).  If using an 8 channel serial acquisition device, select to zero the required transducer within the ‘pad form’ as described in section 4.1.1. 

            Following the test preliminaries, the top cap may be brought into contact sensitively, (docked) against the adjustable reaction head and load-cell by slowly operating the large knurled nut on the top of the triaxial cell. Docking is achieved when the pressure in the lower chamber just exceeds the cell pressure or when the load cell reading increases by an acceptable amount. Immediately after docking you should set the ‘soft’ zero for the displacement transducer connected to the cell pressure controller by using the key sequence;

RESET, 0, •, 6, RESET

            For triaxial compression tests, remember to lubricate with silicone grease the cup-and-cone seating between the top cap and the reaction head.  This avoids subsequent "stick-slip" and ensures smooth alignment.  For triaxial extension tests, refer to section 2.5.4.

            The Bellofram Rolling Diaphragm (BRD) fitted to the B&W 38/50mm cell has a maximum working pressure of 345psi and an effective area of 4.54 sq.inches.  This Bellofram Inc specification translates into SI units as 2400kPa giving a maximum deviator force of 7kN.  Assuming a maximum lower chamber pressure of 2400kPa, a useful table of approximate figures would be as follows:

• Note the cell pressure reduces the effective lower chamber pressure by the amount of cell pressure because the cell and lower chamber BRDs are matched in area.

Checking the Friction on a Triaxial Cell

In the GDS STDTTS system, axial total stress in the triaxial cell is calculated from the load-cell reading, the cell pressure, and the current average area of the test specimen. 

            The control of the axial stress via the load-cell reading is best if the BRD's in the lower chamber and cell and the Rotolin bearing guiding the piston are frictionless.  This assumption may be investigated and a correction found as follows:

·        Set up an empty triaxial cell with a digital controller connected to the lower chamber.  Ensure the valve to the lower chamber is open.

·        Switch on the digital controller or zero the volume if it is already switched on.

·        Put the lower chamber to EMPTY for 10 seconds, then press RESET.

·        When the pressure has settled, read the lower chamber pressure display.

·        Now put the lower chamber controller on FILL for 10 seconds and then press RESET and read the pressure display again.

·        Take half the difference of the two pressure readings. This is the friction correction.

            Normally, the friction correction will be less than a few kPa, say 3-4kPa.  If the friction exceeds say 5kPa this is abnormal and it must be suspected that the Bellofram Rolling Diaphragm (BRD) is becoming clogged with soil particles.  If this happens the cell must be stripped down for cleaning as soon as possible and before any particles puncture the BRD.

Area Correction for the Triaxial Test

The cross sectional area of the test specimen is continually corrected for the effects of change in volume (where back pressure is provided by a GDS Digital Controller) and axial deformation. Axial stress is therefore based on the average cross sectional area defined as the cross sectional area of the volumetrically equivalent right cylinder (Bishop and Henkel, 1962).

Using the Triaxial Extension Device

The GDS triaxial extension device enables triaxial extension to be carried out as routinely as triaxial compression.  The device prevents cell pressure from acting vertically on the top cap resting on the test specimen.  This allows axial stress to be reduced below cell pressure.

            The setting up, docking and undocking procedure is as follows:

·        Set up the test specimen in the usual way, using the top cap with plain ends

      (i.e.  without a metal hemispherical seating).

·        Fit the bell-mouthed flexible sleeve onto the top cap with the bell-mouth uppermost.

·        Lightly apply a thin coating of silicone grease to the inside of the bell-mouthed sleeve.

·        Fill and de-air the cell in the usual way.  When water runs out of the top vent tube, close it off with a straight connector and plug.

·        Carry out isotropic consolidations, B checks and saturation stages as required.

To dock the top cap and sleeve to the vented reaction head, the following procedures should be used:

·        Shut the back-pressure valve on the cell.  As cell pressure is released during docking,  Shutting the back-pressure valve "locks in" the current state of isotropic effective stress for fully saturated soils.

·        Shut the cell pressure valve and vent the cell to atmospheric pressure using the bleed valve on the top of the cell.

·        Close the bleed valve on the cell and vent the top vent tube to atmospheric pressure by removing the plugged connector.

·        Raise the open end of the water filled vent tube above the cell and secure in position.

·        By turning the large knurled nut on the reaction head, lower the vented reaction head, displacing some water from the cell through the vent tube, while observing the pressure in the lower chamber as indicated on the lower chamber pressure controller.

·        Continue to lower the reaction head until it locates in the flexible sleeve, and the lower chamber pressure registers a small permanent increase of pressure of, say, 1kPa.

·        Now lower the open end of the water filled vent tube to below the cell.  This applies a small negative pressure to the sleeve and seals the top cap to the vented reaction head.

·        Pressure to the required value in small increments of, say, 1kPa (or use the RAMP function).  During this procedure, continuously adjust the reaction head as required to maintain a pressure in the lower chamber equal to the cell pressure minus the friction correction plus, say, 1kPa. A seal between the top cap and the reaction head has been obtained if the cell pressure controller succeeds in restoring cell pressure. If the cell pressure cannot be achieved then the flexible sleeve has not sealed correctly. It is then necessary to repeat the procedure.

·        Tighten the lock nut on the reaction head.

·        Open the valve to back pressure.  Testing may now proceed.

            To undock at the end of the test, the following procedures should be used:

·        Turn off all valves to the cell and vent the cell pressure to atmospheric pressure using the bleed valve in the top of the cell.

·        Raise the open end of the water filled vent tube above the cell and secure in position.  This applies a small positive pressure to the interface between the top cap and the reaction head.

·        Release the lock nut on the reaction head and slowly raise the reaction head by turning the large knurled nut.

·        Set the cell pressure to zero pressure.  When zero pressure is reached, disconnect the cell pressure line from the cell and empty the cell of water.  Dismantle the cell in the usual way.

Triaxial Test Hints

CO2 helps de-airing 

            De-airing pore water ducts, porous stones, connections and cohesionless test specimens prepared dry may be greatly facilitated by first purging with carbon dioxide CO2.  This heavier-than-air gas does not mix with air or with water at room temperature and pressure.  Complete purging may be detected by holding a lighted match over the outlet.  The lighted match extinguishes in CO2. Remember to purge the triaxial cell and use de-aired water there too.

            Nold de-aerator 

            Good quality de-aerated water may be obtained with the Nold de-aerator which utilises cavitation caused by a high speed rotated vane to release air from tap water.  This air is then removed by a vacuum pump.  De-aeration should be carried out immediately before use unless the de-aerated water is stored under a good vacuum.

            De-airing connectors 

            When making connections with the hydraulic connectors, ensure that the valves and connectors are filled with de-aired water right up to the outside orifice.  This may be facilitated by using a hypodermic needle. Remember to grind off the point of the needle for safety.

            Side drains 

            When testing clays in the triaxial cell, filter paper side drains may be used to facilitate the axial equalisation of pore pressures.  This reduces test time, gives more uniform stresses and strains, and means that the pore pressure measured at the base pedestal is more typical of the pore pressure regime in the test specimen.  For very soft clays, however, conventional vertical strip side drains may make a significant contribution to the rigidity of the test specimen. 

            This problem may be overcome by using filter paper spiral drains cut to form a continuous helix. Of course, for triaxial extension, spiral side drains are essential. Remember to select a testing rate which allows equilibration of pore pressure.  If this is not done, you may end up testing a stiff outer shell of consolidated soil.

            Membrane penetration

            When testing granular soils down to fine sands, membrane penetration causes false volume change measurements (unless cell pressure and back pressure do not change) which may be corrected for.

            Bellofram care 

            The Bellofram Rolling Diaphragm (BRD) used in the Bishop & Wesley-type hydraulic triaxial cell seals the loading ram into the lower and upper chambers.  The loading ram should only be actuated in such a way as to ensure the convolute of the BRD is maintained i.e. by using pressure differential and never by externally pushing on pulling or the base pedestal. 

            When preparing test specimens of soil, particularly granular soils, on the base pedestal, it is absolutely essential that silt, sand and gravel grains do not drop down into the BRD. One way of preventing this is to pack the annular space around the pedestal with damp paper towels during preparation.

            Rotolin bearing care 

            Axial force is exerted on the test specimen by means of a piston fixed to the movable base pedestal.  The piston moves vertically up and down in a linear guide comprising a cage of ball bearings housed in a turret joining the cell to the base. 

            The ball bearings and piston are made of hardened steel and are open to the air.  Accordingly these moving parts should be protected from moisture and subsequent corrosion by periodically spraying with an aerosol of suitable water repellent and light lubrication such as "WD 40".  This is normally supplied with a small tube which enables spraying up into turret through the apertures from which the axial deformation yoke projects.

            Connecting tubing 

            Polythene and PVC tube should be avoided because of their relatively high permeability to air. Remember to site the test cell close to the end of the pressure controllers and cut the nylon connecting tubing to minimum lengths.

A simple guide to the fitting of swagelock pressure fittings.

            1       Ensure the end of the tube is cut straight using the supplied cutter.

            2       Place the Backing nut  with the screw towards the end to be attached.

            3       Place the Backing ring in front of the nut.

            4       Place the Olive  in front of the Backing ring.

            5      Push the tube square into the connection and slide the nut to the connector.

6       For the first operation tighten the nut by hand until "finger tight" and then apply three quarters (270 degrees ) of a turn using a spanner.

7       For subsequent use during normal operation tighten the nut by hand and then apply one quarter (90 degrees) turn using a spanner.     

            Temperature control 

            High quality triaxial and consolidation testing may only be carried out in a rigorously temperature controlled environment.  Temperature variation causes changes in the volume of water in the soil and test cell, and changes in dimensions in the test cell which for strain controlled tests means changes in loading.  Accordingly, measurements of pore pressure, axial loading and axial deformation may be affected.

            Back-pressure 

            In spite of our best efforts in de-airing our test systems, this process must be imperfect! Accordingly, it is good practice to test at high back pressures wherever appropriate and thereby ensure the compression of any residual air with consequent improvement in saturation.  For undrained tests on dilatant soils, elevating pore pressure before test also ensures pore water pressures do not become negative thus causing cavitation and invalidating the undrained condition. Remember to elevate cell pressure as well to keep effective stresses unchanged.

 Setting up the Standard pressure controllers

For each pressure controller you must set up the pressure limits and baud rates. To do this you must first get into the Systems Functions (SYSFUN) menu by pressing the keys RESET, 0.  The pressing key 4 enables you to set the pressure limits in MPa.  You should set pressure limits as follows:

            Lower chamber controller                                               3

            Cell pressure                                                                   2

            Back pressure                                                                 2

            To set the baud rate, you again enter the SYSFUN menu and press key 5.  The pressing key 1 returns 4800 which is the correct baud rate.  The whole key sequence is therefore: RESET, 0, 5, 1, RESET.

Setting up the Remote Feedback Modules (RFMs)

If you have one or more  RFMs in your set-up you should be very careful to protect the transducers when they are not in use.  For example you may have a one kN submersible load cell which you use when testing soft soils.  When you are testing stiff soils you will de-activate the RFM because the load could exceed  one kN, however you must also remember to physically remove the low-range load cell from the triaxial cell because even though the load cell is not activated from the RFM and may not even have power supplied it is still possible to damage the load cell physically by overranging it.

            The stages in using an RFM are simple:

• Install the transducer in you test apparatus and ensure that it is measuring a safe value (preferably zero).

• Connect the transducer to the correct personality module. If the wrong Personality Module is used then the calibration factors will be wrong and you could easily overrange and damage your transducer.

• Connector the Personality Module to the controller RFM socket using the trailing Lemo connector from the personality module.

• Power on the controller and set up the transducer details for the RFM.  These are range, decimal place, max value and min value. The method of doing this is described in the controller handbook and summarised below. You can now activate the RFM and the transducer reading will be displayed on the controller.

• When the controller (and therefore the transducer) has been powered on for at least two hours you should set the soft zero offset for the transducer. 

         You can now start the software.  The RFM details associated with the software should be checked and amended as necessary. As described above the ‘soft’ zero for the RFMs need to be set at different stages of the test setup. 

            In summary, you enter the RFM menus using the key sequence REST, 0, •.  In the menu you move on by using the -> key.  You should always follow these steps:

            Step 1: key 1 - RFM ACTIVE

            Step 2: key 2 - RANGE

            Step 3: key 3 - DECIMAL PLACE

            Step 4: key 4 - SET MAXIMUM

            Step 5: key 5 - SET MINIMUM

            Step 6: key 6 - REMOVE ZERO OFFSET

Typical values that you will use are summarised in the table below.

Checking the output of the load cell

The axial pressure controller displays the output of the load cell RFM in engineering units.  This will be in Newtons (N) or kilo Newtons (kN) depending on how your system is set up. 

            You can check the output of the load cell by using the mechanics of the hydraulic triaxial cell (Bishop and Wesley Cell only).  Firstly, empty the cell and install a rigid block as a dummy test specimen.  Then zero the display of pressure on the axial controller.  The valve to the lower chamber will be open at this time.  The cell will remain empty or you can fill it with water and set a cell pressure if you wish. 

            The area of the Bellofram Rolling Diaphragm is about 2931 sq.mm.  Using Bishop & Wesley’s equations, the relationship between net lower chamber pressure (i.e. less the cell pressure, if any) p and the axial force F is p(kPa)=0.3412*F(N).  You can set a lower chamber pressure of 100kPa (P TARGET=100), say, and the load will read approximately (100/0.3412)= 293N (or 0.293kN). 

            Alternatively, you can set the RFM in control (RESET, •, +) and set an axial force of 0.293kN (X TARGET =0.293).  You should then read approximately 100kPa (or 100kPa + cell pressure, if any) on the associated axial pressure display.  Remember to take the RFM out of control when you have finished (RESET, •, –).