June 02, 2020
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.
The following tools and accessories are necessary for the vibrating wire extensometer installation procedure:
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.
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.
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).
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.
NOTE: If EDI-54V shows negative displacement reading, move the structure away from the borehole face and vice-versa.
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.
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.
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.
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.
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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