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2.2.2 Installing the Model 4850-2
The Model 4850-2 is designed to measure radial pressures on the tunnel lining.
To accommodate irregularities in the rock surface, it is necessary to fill the space between
the rock surface and the cell with quick setting mortar.
The rock surface is prepared by smoothing it off and flattening it as much as possible with
whatever hand tool will do the job. Nail, pins, pads, or pieces of rebar grouted into
boreholes adjacent to the cell location are now fixed in place. A quick setting mortar pad
is troweled onto the surface and the cell is then pressed down onto the pad causing the
mortar to extrude sideways thus eliminating any air bubbles or spaces between the cell
and the ground. When in place the cell must be gripped firmly using the previously
installed hardware. See Figure 5.
The cable is routed to the readout location and held firmly in place by tying it off to other
rebars, nails, etc. driven into the ground or to the reinforcing mesh, of one is used. At the
readout location the cable can be coiled inside a box, cast inside the shotcrete lining as
before.
The pinch tube should be bent so as to protrude from where the lining will be or can be
wrapped in foam, etc. so that it can be easily retrieved after shotcreting.
Shotcrete Lining
Pinch Tube
2.3 Initial Readings
Before shotcreting, take initial readings on all the cells and record in the field book. Take all
initial temperatures also using either a Readout Box or a digital ohmmeter.
Model 4850-2
Radial Pressure Cell
Instrument Cable
Junction Box
Figure 5 - Model 4850-2 Installation Detail
Mortar Pad
Nails
(driven into the ground
and bent over to
grip the cell)
5
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