On sample transducers -  Sample preparation

This document provides the most common practice for the installation of on-sample transducers to a Triaxial soil specimen. The following is a guide only; modifications to the procedure are up to the user.

The following instructions refer to a sample fitted with 1 radial and two axial transducers. Moreover, a mid-pore pressure transducer is fitted for the determination of either positive or negative pore water pressures within the specimen. This configuration works considerably better in 70 or 100 mm samples rather than 50 mm.

Sample must be prepared prior to the fitting of the on-sample transducers. Samples prepared either in a mould or trimmed from undisturbed samples which are stiff enough to support their weight without deformation work better on this occasion. This method is not appropriate for samples that are either too soft to stand upright without deformation or for granular soils without cohesion. In these last cases, other methods should be used which are not covered in this document. Even though it is not shown in this document, side drains can be installed to the specimen before fitting the rubber membrane around it.

1.     Ensure that the rubber membrane is in a good condition and has no holes prior to installation. The membrane must be completely dry. Draw two lines across the rubber membrane at the upper and lower limit of the sample and a line exactly in the mid-point (Figure 1).

2.     Fit the rubber membrane to the membrane stretcher and apply some suction with the suction ball through the suction port (Figure 2). Pass the membrane around the sample trying to bring the two drawn lines exactly at the specimen’s edges (Figure 3).

3.     Take the radial calliper (do not fit the LVDT yet) and pass it around the sample, holding it with a rubber band to support it (Figure 4 - Left). Place it in the middle of the specimen and mark the position of the supporting pads on the membrane (Figure 4 - Right).

4.     Place the axial callipers (symmetrically around the axis of the specimen and without the LVDT fitted) and mark the position of the supporting pads (Figure 5). At this stage, all three callipers must be in place and kept with the rubber band to make sure they don’t interfere and the LVDT pins will be able to move freely later on.

5.     Find the most suitable position for the PwP transducer making sure it is close to the middle of the specimen and is not interfering with the LVDTs and the callipers (Figure 6) and mark it on the membrane.

1.     Remove all the callipers from the sample.

2.     Pinch the PwP insertion point of the membrane and cut it with a scissors making a small hole (about 2-3 mm diameter). Place the PwP transducer grommet into the hole and under the membrane, paying attention not to disturb the sample (Figure 6). Pass the pwp transducer carefully inside the sleeve and slightly twist it to make a good contact with the soil. Put a generous amount of silicone on the grommet and carefully cover the contact points between the membrane and the pwp transducers to avoid leaks (Figure 6 (R)).

3.     Install the axial callipers by putting some silicone on the pads and sticking them onto the membrane (Figure 7). Use the two metal brackets that keep the upper and lower parts of the calliper in place and at the right distance. When both transducers are in place, put a rubber band around them (and the sample) to keep them in place.

9.     Push the four pins slightly into the sample to support the weight of the pads while the silicone is drying (Figure 8).

10.     When the silicone has dried and the axial transducers are firmly attached onto the sample, take the rubber off and remove the metal brackets carefully by removing the four screws that hold them (Figure 9). Place a small amount of silicone around the pins to seal the holes from any leaks.

11.     Put a small amount of silicon onto the radial transducer’s pad and place it into the marked position. Make sure the transducer is kept horizontal until the silicon dries out – use a rubber band to hold it in place (Figure 10).

12.     Ensure that all callipers are firmly in place and the silicone has dried out completely (usually, this can take 12-24 hours) and remove the rubber bands. Pass the LVDTs through the access holes of the Triaxial cell and then attach them to the correct calliper (Figure 11). The sample is now ready to be tested.

Note 1: Horizontal Bender Elements can be fitted on the same way the mid-PwP transducer was fitted (steps 5-7). The position of the elements should be chosen based on the following criteria:

·         The two elements must be positioned on a horizontal level (Figure 12);

·         The two elements must be positioned exactly opposite (i.e. symmetrically around the vertical axis);

·         The elements do not interfere with the callipers, LVDTs and mid-PwP transducer;

Note 2: Instead of silicon, super glue can be used to attach the calliper pads onto the membrane. Silicon should still be used around the rubber sleeve for the mid-PwP transducer to seal it properly. Silicon should also be used to seal the holes opened when the small pins are pushed in (Figure 8).

Appendix A – Selection of on-sample transducers in the csDYNA module of Clisp studio

When a dynamic Triaxial test is created in Clisp studio (csDYNA module), the user needs to select the on-sample transducers as feedback for the appropriate calculations.

1.     Create a test in csDYNA (Figure 13).

2.     On the Hardware Setup window, select the data logger which is used with the on-sample transducers. Then, select the channels that correspond to the appropriate Inputs and the type of Top Cap fitted (Figure 14). Then, press the Apply button.

3.     On-sample transducer inputs can be used by the software for the calculation of the sample diameter during the Ko consolidation stage. In the consolidation setup window (Figure 15), tick the Use the radial displacement transducer for Diameter values to use the on-sample transducer readings in the Ko consolidation stage. The software will control the load and pressure controllers appropriately in order to maintain the sample diameter constant based on these readings. If a mid-PwP transducer has been installed, the Use the mid-PwP transducer in calculations option needs to be ticked.

Appendix B – Selection of on-sample transducers in the  csStressPath module of Clisp studio

When a stress path test is created in Clisp studio (csStressPath module), the user needs to select the on-sample transducers as feedback for the appropriate calculations.

 1.     Create a test in csStressPath (Figure 16).

2.     On the Hardware Setup window, select the data logger which is used with the on-sample transducers. Then, select the channels that correspond to the appropriate Inputs and the type of Top Cap fitted (Figure 17). Then, press the Apply button.

3.     On-sample transducer inputs can be used by the software for the calculation of the sample diameter during the Ko consolidation stage. In the consolidation setup window (Figure 18), tick the Use the radial displacement transducer for Diameter values checkbox to use the on-sample transducer readings in the Ko consolidation stage. The software will control the load and pressure controllers appropriately in order to maintain the sample diameter constant based on these readings. If a mid-PwP transducer has been installed, the Use the mid-PwP transducer in calculations option needs to be ticked.

Appendix C – Selection of on-sample transducers in csTriaxialAdv module of Clisp studio

1.     Create a test in csTriaxialAdv module (Figure 19).

2.     On the Hardware Setup window, select the data logger which is used with the on-sample transducers. Then, select the channels that correspond to the appropriate Inputs & Top Cap fitted (Figure 20). Then, press the Apply button.

3.     Select the Use the mid-PwP transducer in calculations to calculate pore water pressure based on the mid-PwP transducer. Select the Use the radial displacement for Diameter values to determine the actual sample diameter based on the readings of the Radial transducer (Figure 21).

Appendix D – Selection of on-sample transducers in csUNSAT module of Clisp studio

When an unsaturated Triaxial test is created in Clisp studio (csUNSAT module), the user needs to select the on-sample transducers as feedback for the appropriate calculations.

1.     Create a test in csUNSAT (Figure 22).

1.     On the Hardware Setup window, select the data logger which is used with the on-sample transducers. Then, select the channels that correspond to the appropriate Inputs (Figure 23). Then, press the Apply button.

2.     Tick all 3 checkboxes to use the on-sample transducers.