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146 GDS Helpsheet - DSS - Simple Shear Sample Preparation

Owned by Dan Zanet
Last updated: Feb 02, 2021 by Diogo Teles (Deactivated)
9 min read

HARDWARE

EMDCSS, VDDCSS, MDDCSS, GDSSSS & ADVDCSSv5

Sample preparation

This helpsheet is designed to help you prepare your sample.

Option 1 - Using support brackets

Items Required:

  • Base pedestal with porous disk

  • Top cap with porous disk

  • Membrane

  • Both lower lock rings & screws

  • O-rings

  • Teflon coated sample rings

  • 2 off pedestal support brackets

  • Soil sample

 

Place the sample membrane over the pedestal and then place and screw down the lower lock ring.

 

 

 

 

 

Place the o-ring on the pedestal (left side photo) then place and screw down the upper lock ring to engage the o-ring (middle photo). Place the sample rings allowing 3 rings above the desired sample height (right side photo).

 

Wrap the membrane down over the sample rings. Fit the support brackets. Place, compact and level the soil sample.

 

Place the top cap on the sample. Tighten the support bolts against the top cap and then slide membrane around the top cap. Roll the o-ring down over the membrane.

 

  • Place the prepared sample into the EMDCSS.

  • Extend using vertical displacement control parameter so that the loadcell is within a few mm of the top cap.

  • Target ≈0.01kN axial load in the object display.

  • As the loadcell contacts the top cap, make sure it’s aligned correctly, gently correct it by hand if required.

  • When they are correctly aligned and docked, set the EMDCSS to hold displacement and screw the top cap into the loadcell above.

  • Then zero both axial displacement transducers.

 

Losen and remove the top cap support brackets. Finally, screw in the draining tubes if required.

 

Option 2 - Using vacuum former

  • Base pedestal with porous disk

  • Top cap with porous disk

  • Membrane

  • Both lower lock rings for vacuum former

    • 3 spigot holes each

    • screws

  • O-rings for vacuum former

    • 99.5x3mm silicone ring

    • 94.5x3mm nitrile ring

  • Teflon coated sample rings

  • Vacuum split former

  • Spigots

  • Soil sample

 

 

 

 

  • Place the sample membrane over the pedestal

  • Put the lower o-ring followed by the lower lock ring

  • Screw them in place.

 

Place a second o-ring on the pedestal then place and screw down the upper lock ring to engage the o-ring. Place on the sample rings allowing 3 rings above the desired sample height.

 

Place the silicon (orange) ring along the bottom groove of the vacuum former, as shown in the picture above. Insert the 4 bolts that will hold the pedestal to the EMDCSS platen. These need to be in place at this stage as they will be covered by the split former (blocking access to the bolt holes) when the assembly is placed in the EMDCSS. Place the pedestal on a small block or other spacers so that is stands raised and with the bolt heads flush with the base of the pedestal. The pictures below show this being done by placing the pedestal on top of the top cap.

 

Place the two-part split ring of the vacuum former around the sample rings, with the bottom cut-outs positioned in line with the pedestal bolt sockets. The three spigot holes on the vacuum former also need to be aligned with the slots on the middle ring sample ring. Push down until the silicon ring seal slips in place against the middle sample ring.
Insert the 3 spigots in the spigot holes visible on top of the vacuum former. These will ensure the alignment of the sample rings. Next finger tighten the bolts on the side of the sample former to push the spigots against the sample rings.

Use only enough torque to prevent the spigots from moving. Overtightening these bolts may damage the sample rings.

Fold back the top of the membrane over the vacuum former and then use an o-ring to hold the membrane in place.
The membrane wall should be as flush as possible with the inner perimeter of the sample rings. Pull through slack where possible. If the membrane and o-ring start to slide up off the former folding the membrane back upwards and adding a second o-ring will prevent this.

 

 

Connect a vacuum line or pipette ball to the drainage pipe on the side of the vacuum former using a rubber hose and
apply vacuum. The membrane will then be pulled flush against the inside of the sample rings. When the sample is in place the vacuum former will later be moved to the pedestal stand on the EMDCSS under vacuum. This should be taken into account when setting up the vacuum pump, ensuring the hose has enough length or the pump has enough mobility to allow that.

 

 

 

Form the specimen inside the mould using the selected preparation method.

If tamping is used to achieve the target density then special care should be taken not to puncture the membrane when tamping along the edges.

Attach the top cap to the EMDCSS ensuring the porous disc and all four upper bolts to the loadcell are tightened in place. Place an o-ring on the top cap and roll it up until it is close to the top edge.

Using GDSLAB’s Object Display raise the axial actuator (negative direction) until there is enough clearance to install the specimen and pedestal set.

Keeping the vacuum former under vacuum move the pedestal to its stand on the EMDCSS and bolt it securely in place.

 

Use GDSLAB’s Object Display to move the actuator down until it is 2 to 5mm away from the top of the specimen.

 

Measure the gap (if any exists) between the top of the sample and the top of the membrane. Then measure the thickness of the porous disc on the top cap.

 

Set the horizontal and vertical load cells to zero then target a small vertical seating load (suggested between
0.001kN and 0.005kN). This will make the vertical actuator move down. Carefully observe the top cap’s approach to the specimen and cancel its movement if it seems out of alignment.



When the target sitting load is reached the top cap should be completely docked (in contact) with the specimen.
Measure the height of the portion of the porous disc still visible and calculate the difference from its total thickness.
The difference should be equal or very similar to the gap between the top of the sample and the top of the membrane. If it is not, check if the specimen if correctly aligned with the top cap.

 

 

 

 

Once docking is complete, turn off the vacuum pump and disconnect the hose from the side drain on the former. Pull down the o-ring holding the membrane and place it around the base of the pedestal, so it does not obstruct the disassembly of the split former. Slide the member up around the top cap.

 

 

 

 

Roll the o-ring on the top cap down over the membrane and then fold the membrane over the o-ring.

Next, loosen the 3 spigot bolts and then undo the bolts holding the two halves of the split former together. Remove the split former together with the spigots carefully to avoid deformation to the specimen.

The nitrile o-ring, previously placed around the base of the pedestal, can now be rolled up and placed around the lower sample ring, together with the silicone ring. This will be their position during the test. Connect drainage pipes as required.

 

Installation of top cap LVDT

Items required:

  • LVDT

  • LVDT clamp bracket

  • LVDT clamp insert (fitted inside the LVDT clamp bracket)

  • LVDT mounting plate

  • Screws

 

Insert the LVDT in the LVDT clamp insert, then insert the screws in the LVDT clamp bracket and mounting plate to connect these two parts together. Attach the assembly to the base pedestal and tighten the screws.

 

 

 

 

The LVDT transducer has 3 marking points. The LVDT transducer is linear in the range, symmetrically around a zero voltage point. This is shown by the middle marking point on the LVDT armature. The two outer markings shows the range of motion over which the data recorded is linear.

 

 

Adjust the position of LVDT such that when it is in contact with the top cap, the armature is on the middle marked position. This way the LVDT will be able to record the data on both sides of the zero voltage point. Tighten the upper screw on the clamp bracket with an allen key such that the LVDT is fixed firmly but be careful not to over tighten it.