Pictures showing fixing the axial LVDT at middle marked position

For the horizontal transducer sanity checks, target a small displacement (e.g. 2mm) on the horizontal displacement (encoder) and check that both horizontal transducers move by the same value. Conform the distance moved is the same as the target value (measure with a ruler).

Top Cap and Pedestal Alignment Check

The short range horizontal LVDT should be as close to zero as possible when the top cap and pedestal are exactly in line. This is important so that when the machine is loading the sample, the sample is loaded normal to the axis and not at an oblique angle. 

To do so, put the top cap and pedestal (without the porous discs) in the EMDCSS and tighten the bolts. From the GDSLab object display, use the CP command to bring the top cap down to the maximum positive position using the Axial Encoder option. Also, target zero position on the horizontal LVDT. Check that the top cap and pedestal are now align with each other.

When setting the transducer in place, it is near impossible to get it exactly to 0, therefore a slight offset is applied. A soft zero offset can be applied in the transducer window. This offset will remain fixed unless the user redefines it. It is also important to check this offset every so often to ensure that this offset hasn’t wavered, and redefine it if necessary.

Before shipping the EMDCSS machine to you, the horizontal LVDT is set close to 0 when the top cap and pedestal are aligned. If the offset value is high, then please follow the instructions in the ‘LVDT Transducer Setup’ section to bring the offset close to 0.

Picture showing top cap and pedestal aligned with each other at zero horizontal LVDT position

In addition to this, the vertical short range displacement transducer needs to be set up correctly, in terms of specimen height etc.  For example, when using a specimen height of 20mm, when the sampled is docked the transducer needs to be near the top of its range.  The vertical LVDT transducer has a range of +/- 2.5 mm, therefore the transducer should read approximately -1.5.  This is in order for the transducer to be able to read the full displacement of the sample, without going over its reading threshold. 

Checking Machine Functionality

The EMDCSS machine should stop automatically when it reaches the upper or lower limit. This can be checked by targeting values beyond the upper travel limit (eg -60mm). A warning will be displayed on the Object Display status bar when this occurs.  If the machine does not stop automatically or makes any loud noises when reaching the limits, push the Emergency Stop button immediately and contact GDS for further instructions.  Carry out the same procedure for the bottom limit switch (+60mm). Then repeat the same procedure for the shear limit switches.

Double check the load-cell calibration. If you have a (small) proving ring or known working load cell, put that between the topcap and pedestal, and compare its reading with the reading shown in GDSLab for the system load cells. If you do not have such a convenient load measuring system then adopt some ad hoc means. For example, you could use the "rubber bung" dummy test specimen described below. You know roughly your own weight. Press your whole weight down onto the dummy test specimen and observe the deformation or ask a colleague to observe it for you.  You will now have a rough but very convenient indicator of load. 

Machine functionality checks also involve running a few tests (explained below) on a dummy sample. Please refer to the GDSLab handbook for a step by step guide on setting up a test stage (Chapter 3) and selecting the relevant test modules (Chapter 6) for carrying out the tests.

Running the First Test

Install a dummy test specimen.  This could be a rubber sample for instance.  Place the rubber sample in the cell.  Perform a constant rate of strain and constant rate of shear test (using the Advanced Shear test module).  Choose parameters appropriate to the dummy test specimen. Use a strain rate of 50mm per hour and termination conditions of 2mm for each axis.  Plot the results and check for noise levels.  On the screen, plot load cell against displacement for each axis.  From the plot, calculate the stiffness of the dummy test specimen in kN/mm.  Please refer to the article below for the importance of stiffness estimate in dynamic load controlled tests.

Running the First Dynamic Test

Select the Dynamic Shear module and choose axial displacement control. Select 0.1Hz at +/- 0.2mm axially as well as in the shear plane (if the dummy test specimen will stand it).  Choose 100 data points per cycle, 5 cycles on, 0 cycles off, terminate after 5 cycles.

Running Other Tests

Going back to the GDSLAB object display, you can check that the load control is working by setting a small change in target load. Observe the deformation of the dummy test specimen. From your ad-hoc calibration of the stiffness of the dummy test specimen, check that the observed deformations are about right.On the screen, note the resolution (i.e. variation) of control of axial force (load).  Normally, you should expect better than 1% of full range at loads greater than approximately 1/20th of full range.

If all is OK you can then start testing with real soil.

User Adjustments

So long as the system hardware remains unchanged there will be no user adjustments required on a day to day basis. If transducers are changed it is very important to update the calibration details for the data acquisition system. The main user software adjustment between test is the selection of stiffness estimate of the test specimen (explained above).

Setting Up a Sample

Test Procedure

The typical testing procedure to be carried out on an EMDCSS is consolidation stage, followed by a period of dynamic shearing and then a monoclinal shearing, to failure.  This will essentially give the user the required data.

Please refer to helpsheet 216 ‘ADVDCSv2 User Interface’ for selecting the waveform shape for dynamic cyclic test, and to enable the realtime graphs for transducers to be displayed during the test. Refer to the GDSLab handbook for setting up the test parameters and the test stages.

LVDT Transducer Setup

It may be necessary at some point during the life of the EMDCSS device to realign both the horizontal and vertical short range transducers. 

Although they are quite difficult to access it should be possible to adjust the position without removing the EMDCSS front cover. If you find it too difficult to access, the lower front cover can be removed. Align the topcap and pedestal using the Object Display. This is best achieved by getting them as closely lined up as possible, then with the flat edge of a metal ruler progressively use small increments to align them perfectly.

As shown in the figure below, the horizontal LVDT transducer wire is fed through a hole in the base of the device and the LVDT body clamped in place.  As mentioned previously, it is vital that when the topcap and pedestal are exactly in line, the reading of the transducer is as close to 0 mm as possible.  This is so that any displacement fore or back of the pedestal can be measured accurately.  This horizontal transducer has a range of +/- 10 mm.  When aligning it there is bound to be a slight reading either side of zero which can be set as a zero offset.

Photo showing placement of horizontal displacement LVDT

When locking the LVDT into place, also ensure that the black lead is pushed as far down, toward the base of the device, as possible to avoid interaction with the plate that sits over the top of it.

As with the horizontal LVDT, the vertical LVDT is threaded through a hole in the lower side of the device and up to the bracket (shown in the picture below). The stainless steel armature has a magnet, clearly distinguishable, in one end of the armature.  This end needs to be fed through the LDVT and be in position through the LVDT bracket.  The other end of the armature is placed within the stainless steel block on the base of the device.  It is important to leave some lee-way for future adjustment - therefore the user should only position the armature a portion of the way into the block before tightening it in place.

Photo showing alignment of vertical LVDT

The armature should also be as straight as possible. The bracket holding the LDVT in place can be re-positioned by undoing the bracket screws attached to the device and manoeuvring the bracket so that the LVDT and rod line up perfectly.

The metal plate positioned below the bracket holding the LVDT in place is a resting plate for the armature when working with the sample (see picture below). By pulling the armature up and sliding the metal plate across, there is an indentation on the plate that the armature can rest in.  This is necessary for two reasons; for ease of access to the sample, as well as not to bend the armature while taking the pedestal/topcap in and out.

Photo showing metal plate to rest rod on while preparing the sample

User Maintenance

The EMDCSS requires only occasional user maintenance and should perform for many years, assuming it is cared of correctly during the use.  Care must be taken especially during sample preparation, insertion and removal not to drop particulate matter onto the linear guides or to allow water to drain onto the linear guides.

 It is recommended that the user undertakes a check for friction annually and applies a small amount of oil to the linear guides if required at the same time.  There should be no other user maintenance required.

Annual Test for Friction

The following tests check that the level of friction from the linear guides in the EMDCSS is within the expected range.  These should be carried out with no sample present so the axial loadcell and horizontal loadcell 2 can measure only the friction in the system.

To check for static friction on both axes use the object display to set position to 0mm, 2mm, 0mm, -2mm, 0mm, 2mm, 0mm.  Do this for both vertical and shear axis.  Note the relevant loadcell (axial loadcell or shear loadcell 2) reading at each position.  The friction value is: 

                   (max load – min load)/2

• Static axial friction should be <±0.025kN worst case ≤±0.03kN

• Static shear friction should be <±0.025kN worst case ≤±0.03kN

To check dynamic friction setup an advanced shearbox stage to ramp from 0mm to +10mm in 1 minute, logging every 3 seconds. Then setup a second ramp coming back to 0mm in 1 minute, again logging every 3 seconds.  Dynamic friction is (max load– min load)/2 where max load is max load observed during positive ramp and min load is minimum load observed during negative ramp.

• Dynamic axial friction should be <±0.025kN worst case ≤±0.03kN

• Dynamic shear friction should be <±0.025kN worst case ≤±0.03kN

 If you have difficulty performing these tests of are concerned your EMDCSS does not pass the tests please contact GDS technical support by sending an email including your results files to support@gdsinstruments.com.

Annual Lubrication

A small amount of lubrication may be required to maintain the linear guides of the EMDCSS.  Please note that excess lubrication will likely not reduce the friction in the machine, but may instead lead to dust/particle accumulation and thus additional friction in the guides.

Note: a water dispersing oil such as WD40 is NOT suitable for lubricating this machine.  A good quality machine grease such as Castrol EP2 is recommended.

If following the friction checks the apparent friction on the axis is less than ±0.01kN you should not have to apply any lubrication.  If the friction exceeds ±0.01kN continue as described below.

To lubricate the horizontal guides set the EMDCSS to its reverse horizontal travel limit.  This can be easily done using the object display and targeting beyond the travel range, eg horizontal displacement =-20mm.  Next place a small drop of grease onto each of the exposed guides as far back into the machine as possible (reaching underneath the back rubber skirt at the front of the base platen).  A small syringe is ideal for this.  Lastly cycle the machine between the forward and rear travel limits 5 times to distribute the grease across the guides.

The vertical guides are more difficult to reach but can be exposed by setting the machine to its upper travel limit.  They can then be accessed behind the topcap position.  Again, place a small drop of grease onto the bearing as high up as possible, then cycle the vertical axis between upper and lower travel limits 5 times to distribute the grease.

Note: Take care when inserting hands/fingers into the EMDCSS.  It is recommended to push the STOP button whenever there is a risk of trapping hands/fingers being trapped in the machine.