VW2100 Vibrating Wire Piezometer
RST Instruments Ltd.
Page 21
cut short or extra bentonite may be added to attain the required pumping viscosity.
In the end, the low permeability cement bentonite grout will provide adequate
permeability for the vibrating wire piezometer diaphragm to react to any pressure
changes occurring at the location. A number of installation methods have been
identified using the fully grouted method:
•
Install piezometers one by one from the borehole bottom to the collar over
multiple days. Use a single PVC plastic tremie pipe, which is reduced in length,
as each successive installation is competed to the hole collar.
•
Attach the multiple piezometers to a PVC plastic tremie pipe and install to depth
in the borehole. Use the PVC plastic tremie pipe to grout the entire hole in one
stage and leave it in place. Note that you need to ensure that the piezometers
being grouted into the borehole will not be over ranged by the grout column being
placed. Vibrating wire piezometers can be over pressured to 200% of the full-
scale range. However, in practice, it is recommended that 150% of FS not be
exceeded to ensure an adequate safety buffer.
•
For deep holes with lower range piezometers, multiple grouted in PVC plastic
tremie pipes may be required. If multiple PVC tremie pipes are used, they should
have their annulus fully grouted to ensure that no internal to external pressure
communication can occur, if one or both PCV pipes should break.
•
Install piezometers attached to a PVC plastic tremie pipe inside a casing or
hollow stem auger. Leave in place while casing or auger stem is pulled out.
Downhole grouting may be carried out before the casing or auger stem is pulled
or following. This method is well suited to boreholes with wall stability issues.
•
Complete drilling and then grout the hole with casing or hollow stem auger still in
hole. Next, pull the casing or auger stem and top up the hole collar with grout.
Install piezometers in the borehole, from the bottom to the top. Add weights to
each piezometer as required, to overcome viscous resistance of the grout while
lowering the piezometer.
•
Attach piezometers directly to the outside of the inclinometer casing and grout in
place. Piezometers should be placed midway between the casing couplings.
•
Attach directly to the outside of corrugated polyethylene settlement pipe
(Sondex) or similarly attach to magnet/reed switch casing between the magnet
sensors so that pore water pressure and settlement can be measured along the
same borehole.
•
Install a series of vibrating wire piezometers inside a length of perforated 2-inch
PVC plastic pipe. The piezometer filter housings will be located in close proximity
to one or more of the perforation holes and will therefore be able to monitor the
external pressures when fully grouted in-place. This technique is useful in deep
installations inside of drill casing or hollow stem augers to prevent cable and/or
sensor damage when rotation is required during casing extraction. Later tremie
grouting outside the PVC pipe will result in the piezometers being fully grouted in-
place.