June 02, 2020

Vibrating Wire Extensometer Installation Procedure

It consists of one or more anchors and a reference plate. Anchor or anchors are set in same borehole or in different boreholes drilled adjacent to the first borehole. The extensometer helps to accurately measure distance between various anchors with respect to reference plate and thus monitor with passage of time their relative displacement.

Encardio Rite Borehole Extensometer with Vibrating Wire is used to measure the extension (displacement) that takes place with time in a bored hole or in several bored holes in a rock mass.

We will be looking at the Vibrating Wire Extensometer Installation Procedure in this article.

It consists of one or more anchors and a reference plate. Anchor or anchors are set in same borehole or in different boreholes drilled adjacent to the first borehole. The extensometer helps to accurately measure distance between various anchors with respect to reference plate and thus monitor with passage of time their relative displacement.

Tools & Accessories required for installation

The following tools and accessories are necessary for the vibrating wire extensometer installation procedure:

  • Soldering iron 25 watt
  • Rosin 63/37 solder wire
  • Thread sealant (Loctite 577 or equivalent)
  • Cable jointing compound
  • Acetone (commercial)
  • Spanner 6, 8, 10, 18, 24, 30 and 38
  • Allen key 2, 4 3.4.8 Screw driver (tip size 6 x 0.8 and 3 x 0.5 mm)
  • Phillips head screw driver (tip size 2 mm)
  • Hacksaw with 150 mm blades
  • Cable cutter
  • Wire stripper
  • Insulated pliers 165 mm
  • 150 mm flat file
  • Toothbrush
  • wire brush
  • Cloth for cleaning (lint less)
  • Cello tape/Insulation tape
  • Micrometer depth gage 100 mm range
  • Digital indicator model EDI-54V
  • Digital multi-meter

Vibrating Wire Extensometer Installation Procedure

Three-point-reference-head-assembly
fibreglass-connecting-rod-assembly

  • Refer to the above image and remove the cover (19) to take out the polythene bag that contains requisite quantity of reference buttons (15), link plates (16), sensor tightening nut (17), locknuts (18), M6 x 100 cheese head screw (24) and locking nuts (25).
  • Remove guide plate (7) with the help of a 4mm Allen Key and hollow bolts marked 1, 2 and 3 from flanged housing (5). Place the housing on the clean ground, ensuring that it is at least 1m away from the borehole. Loosen the other two hollow bolts slightly from holes marked V and G.
  • The next step in the vibrating wire extensometer installation procedure is to cut grout and air vent tubes (12 mm od x 10 mm id supplied in coil form) to the length that is required. In most cases, the lower end of the grout tube is placed at a level higher than the anchor which is the closest to the face of the borehole.Make sure that the grout covers the anchor closest to the borehole mouth or is around 1 m from face of the borehole, whichever is closest. When you’re determining the length of the grout tube, remember than around 1.5m is of it is required to be outside the reference plate as it used to easily attach to the grout pump. Similarly, the air vent tube should project around 0.3 m is outside the reference plate.Like we did for the grout covers, the lower end of the air vent tube should always be at a higher level than the lower end of the grout tube. When it is time to insert the last anchor in the borehole, make sure the lowest end of the grout tube is tied to the connecting rod with a cable tie.


NOTE:
 When installing with upward to the horizontally sloping borehole, make sure the grout tube is 2m longer than the specified depth of deepest anchor and taped around 100 mm below far end of the anchor. To prevent is from swinging loose, use a PVC tape and tape the grout tube at every 2m to the plastic tube of connecting rod assembly.

NOTE: In an upward to horizontally sloping borehole, ensure that the air vent tube is 1.5 m longer than the specified depth of deepest anchor and taped around 100 mm above the far end of the anchor. Similarly to what we discussed above, tape the air vent tube with a PVC tape at every 2m to the nylon tube of the connecting rod assembly. Top-end of air vent tube should be at a level higher than the deepest anchor.


CAUTION:
 When installing the upward to the horizontally sloping borehole, attention should be paid to make sure that the air vent tube is taped around 100 mm beyond the rear of deepest anchor and does not get detached. This is very necessary for the grout to fill in up to end of the longest anchor.

  • Take the grout tube through the hollow both (12) of the hole marked G at bottom of flanged housing and corresponding holes on guide plate (7) and reference plate (14), taking care that when flanged housing is assembled to the adaptor (6), the grout tube is around 1.5 m outside the reference plate.In a similar way, guide the air vent tube by a hollow both of the hole marked V at the bottom of the flanged housing ensuring that when flanged housing is assembled to the adaptor (6), the vent tube is around 0.3 m outside the reference plate (14).
  • Use a spanner of size 18mm to tighten corresponding hollow bolt (12) which will securely hold the nylon grout and the air event tubes but not so tight that it squeezes them.
    CAUTION: In upward to the horizontally sloping borehole, be particularly careful in tightening the hollow bolts (12) properly as grout may leak through into the reference head assembly during the grouting process.
  • The next step is the vibrating wire extensometer installation procedure involved replacing the hollow bolts (12) in holes marked 1, 2 and 3 on flanged housing loosely tightening them with the hand.
  • Fix the male end of the connector of the longest fibreglass assembly (2) to the groutable anchor(1) by screwing it in. During the fixing, the nylon tubing will also get tightly grip to groutable anchor. You may need a little grease for ease in assembly.
  • The next step is to the place the deepest groutable anchor in the borehole until 1.5m of the near end of fibreglass rod is outside the face of the borehole. Trim nylon sleeve such that female connector and fibreglass rod is exposed by around 100 mm from connector end.


WARNING:
 Fibreglass rod is supplied in coil form using cable ties. Make sure you’re very careful while handling it as it can unwind and snapback.

    • Guide the end of the fibreglass rod through hollow bolt (12) of the hole marked ‘1’ at bottom of flanged housing (5) and corresponding holes on guide plate (7) with end protruding around 80 mm from guide plate (7) face. For this process, the fibreglass rod will have to bent. Tighten the hollow bolt-on connecting rod such that it does not slip into the borehole.
  • Repeat the above 2 steps of the vibrating wire extensometer installation procedure for the second deepest anchor and guide the connecting rod end through hole marked ‘2’ at bottom of flanged housing (5). Follow the same procedure for the near anchor.

extensometer-setting-arrangement
NOTE: When Installing the extensometer, always start by assembling and mounting the deepest anchor first. It is a good practice to avoid confusion at a later date in the identification of anchors. Holes on top of flanged housing are marked clockwise with identification numbers 1, 2, 3, G respectively and V in middle.

NOTE: While installing the extensometer on a sidewall of the roof, it is generally expected to use an extension. In such a case, it is recommended to place the connecting rod end as close as possible to the reference plate (14).
downward-borehole

Similarly, in the case of vertically down borehole settlement, it is advisable to set the connecting rod end as close as possible to guide plate (7). In other cases, it may be necessary to set connecting rod end in the middle such that either compression or tension can be monitored.

In other words, if only the compression is expected, set zero reading of sensor by extending shaft by around 10 mm. If you’re expecting only the extension, set zero reading of sensor by extending shaft by around 10 mm less than sensor range. You also set the sensor in mid-position if required but make sure never to set the sensor at the very end. A set of 100 mm long cheese head screws (24) and locknuts (25) is provided for this purpose.

  • Construct cheese head screw (24) and locking nuts (25) as shown in the figure below. Lightly loosen the hollow bolts (12) related to holes ‘1’, ‘2’ and ‘3’. Adjust the end of a connecting rod in the required position and tighten the locknuts (24).

NOTE: If EDI-54V shows negative displacement reading, move the structure away from the borehole face and vice-versa.

  • Re-assemble guide plate (7) to the adaptor (6) with Allen screws (9) taking care that holes marked ‘1’, ‘2’, ‘3’, ‘G’ and ‘V’ are aligned in reference plate (14), guide plate (7) and flange housing (5).
  • Use spanner size of 18mm to tighten the hollow bolts to secure the nylon tube but make sure you do not squeeze or restricts the free movement of female end connector inside the nylon tube.

NOTE: When dealing with upward to the horizontally sloping borehole, tighten the hollow bolts (12) with attention and care as the grout may leak through into the reference head assembly during the grouting process.

NOTE: You have to take extra precaution to make sure the extensometer assembly does not drop into borehole accidentally. It is advisable to secure it properly by attaching a safety rope to the anchor.

  • The next step in the vibrating wire extensometer installation procedure is to soak rags or jute cloth strips in quick-setting cement water mix and wrap it outside flanged housing (5) to build up approximately 5 mm thick layer. Lift and insert housing in the borehole using a to and fro screwing motion sealing the annular space between housing and borehole.
  • Secure the expandable anchors (21) in holes which were drilled before to bind the flanged housing with M16 nuts and washers which are provided. Allow the flanged housing set in the borehole for some time.
  • The next step in the vibrating wire extensometer installation procedure is to fix the grout pipe to the grouting machine and pump until the cavity is filled with grout and it overflows out of air vent pipe. You should pay extra care to ensure that the grout flowing out of air vent tube does not find its way into the flanged housing and disrupts the installation procedure.
  • Pump clean water through the grout tube till clear water comes out of air vent tube. This is done to keep the grout tube clean for topping up the grout later on. Allow the grout to settle in for some time.

NOTE: The cement water proportion depends upon the site conditions. Make sure to check with the engineer for the grout. The generally accepted and successful composition for hard rock is 50 kg of cement with 30 kg of water

NOTE: Make sure the grout covers the anchor closest to the mouth or is around 1 m from the face of the borehole, whichever distance is less. You can confirm this by inserting a  3 mm wire into the borehole through the vent pipe.

CAUTION: Take precautions to ensure that the grout does not overflow into the flanged housing as it may get into the protective nylon tubing and jam the setting rods.

  • During Vibrating Wire Extensometer installation procedure, ensure that all the anchors are securely and completely grouted, pump the grout into the system again after the setting period of the grout is over. It may be that the heavier grout settles at the bottom, leaving the anchors near the face of the borehole in water instead of the grout. To ensure that this is not the case, pump the grout in again so that the water at the top is pumped out through the air vent tube and replaced by grout.

CAUTION: It is important to keep all the anchors properly grouted to get the correct displacement readings.

NOTE: For very deep holes, staged grouting may be necessary. It is recommended to take the advice the grouting engineer at the site. If this is necessary, procure Borehole Extensometer with provision for one extra connecting rod hole and use this hole for an extra grout tube.

NOTE: You don’t have to fully grout the borehole when using packer anchors. Use a hand grout for better control. Remember, when you have to saturate packer geotextile sufficiently, you should pump water first. When the pressure builds up to around 400 to 500 kPa, stop pumping the grout and wait for the pressure to dissipate. Once a few minutes have passed, pump again till you feel a distinct pressure. The recommended grout mixture is 1 part cement and 1 part water by weight. King and tape the grout tube to maintain the pressure and disconnect the pump. Clean the pump thoroughly once the above steps are done.

  • The next step in the vibrating wire extensometer installation procedure involves removing a lot of parts. Remove screws (24), locknuts (25), reference plate (14), and spacers. To remove the grout and vent tubes, you will need to make a cut. Once they are extracted, round edges of grout and vent tube with a hand file. While cleaning the reference head assembly, make sure you do in a detailed manner. Reassemble reference plate with spacers to guide plate.
  • Remove guide plate assembly from the adaptor (6).
  • Keeping in mind that the cable has to ultimately connect to the junction box, embed the first sensor in the flanged housing. Using grub screws, fasten the sensor to the guide plate, guaranteeing that 10mm of the tube face is outside the face of the guide plate.
  • Pull shaft of sensor and assemble to connecting rod end with reference button (15), link plate (16), sensor tightening nut (17) and locknut (18).
  • Secure the sensors by screwing them to their respective connecting rods to keeping in mind the position of extensometer “1”.

CAUTION: Make sure to never rotate the shaft of the displacement sensor with respect to the outside body as it can permanently damage the sensor. Carefully pull or push the shaft only axially when installing the instrument. A pin has been provided at the end of displacement sensor that sits flush in a groove in the sensor body. Make sure you keep the pin seated inside the groove during assembly operation and tightening such that no force is exercised on its shaft and there is no rotational movement.

CAUTION: You may need to replace the thread used while mounting the sensor. Make sure you don’t use the thread sealant as it may be difficult to remove them later on.

  • Using Encardio Rite Readout unit, measure the displacement of the sensor. Reading at this stage should be around the initial reading. To set the reading precisely, lose the respective sensor with grub screws (8) and reposition the sensor in guide plate and tighten the grub screws again.
  • Set your reading to zero in the Model EDI-54V to set the position of the sensor. Subsequent readings will determine relative displacement between anchors.
  • Use a micrometre to note down the initial reading. Make sure you also record the date, time, and temperature while taking the initial reading.
  • Connect the nipple (22) to the socket of flanged housing and attach the junction box assembly (23) to the cables from sensors routed to the junction box.
  • Connect leads to respective connector pins in the junction box.
  • Replace cover (19) over guide plate (7).

This brings us to the end of the vibrating wire extensometer installation procedure. Hope this was helpful. If you have any questions or doubts, feel free to comment below.

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