RST Instruments VW2100, Инструкция по эксплуатации, Страница: 19 / 34

VW2100 Vibrating Wire Piezometer
Instruction Manual
ELM0005P RST Instruments Ltd. Page 19
below and it will be obvious why longer drill rod or casing pulls will be more desirable
if possible.
In general, boreholes in bedrock are more stable than boreholes in soil. Boreholes in
cohesive soils are also more stable than boreholes in less cohesive, granular soils.
Bentonite Plug Method
Bentonite chips are recommended for downhole backfill work because they are a
made from solid bentonite which will not hydrate as quickly when exposed to water
compared to bentonite pellets, which are a manufactured product. Bentonite pellets
will become sticky very quickly when exposed to water and can easily clump
together, bridging inside the casing well above the target zone. Use of either
bentonite products for downhole seals should be limited to holes which are less that
20 meters, due to the difficulty involved with this method.
CAUTION: D
O NOT ROTATE THE DRILL CASING
. T
HE DRILL CASING CANNOT BE
ROTATED WHEN BEING PULLED
. R
OTATING THE DRILL CASING WILL LIKELY RESULT IN
DAMAGE TO THE INSTALLED PIEZOMETERS
.
1
Place a 60 cm bentonite seal at the bottom of the borehole to seal (if required).
2
Raise the drill casing 15 cm and start placing the bentonite chips until the
bentonite level is 30 cm below the required piezometer elevation.
3
Pull the drill casing as the bentonite is set in place. Be very careful not to bridge
or plug the drill casing with the bentonite.
◼
This is accomplished by ensuring the bentonite level is at all times below the
casing bottom and by slowly dropping the bentonite chips one at a time down
the hole. Feeding the bentonite chips in too rapidly will result in bridging of
the chips in the drill casing or borehole. Bridging will make completing
downhole installations extremely difficult.
◼
Tamping is not required because the natural swelling of the chips will provide
an adequate seal to the borehole walls once the bentonite chips are in place.
4
Lower a cylindrical weight down the drill casing to the top of the bentonite plug to
ensure the hole is clear of any obstructions prior to setting filter sand in place for
the piezometer zone.
5
Rinse the borehole with clean water to remove any obstructions or debris.
6
Place 30 cm of fine, clean sand in 15 cm increments by dropping from surface.
The drill casing will also have to be pulled as the sand back-filling proceeds.
7
Lower the piezometer into the hole and take the initial readings, as described in
Section 4.3.
8
Raise the drill casing 15 cm and backfill the hole around the piezometer with fine,
clean sand. Repeat until the sand is 30 cm above the top of the piezometer.