Geokon 4850, Instruction Manual

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2.4 Re-Pressurizing the Cell
2.4.1 Standard Re-Pressurization Technique
After shotcreting, the cells temperature and initial reading can be read again. Once the
temperature has stabilized to ambient then the cells can be inflated using the pinch tube
and a special set of accessory pliers. The cell is first connected to the readout and then the
pliers are used to squeeze the pinch tube flat beginning at the capped end. (See Section 3
for readout instructions.)
CAUTION: Do not pinch the pinch tube closer than one inch from the end,
otherwise the seal screw in the end of the tube could be damaged.
As the tube is progressively squeezed flat, the hydraulic oil is forced out of the tube and
into the cell and the pressure will rise. It is necessary to make a chart showing the
relationship between the length of flattened pinch tube and the corresponding reading on
the readout box (which can be converted to a pressure if so desired, but this is not
necessary).
Length of Tube Pinched
Decrease
"Knee"
in
Reading
(or increase
in pressure)
Stop Here
Figure 6 - Cell Re-Pressurization Graph
As the cell expands inside any space that may exist, the pressure rise accompanying each
pinch will be small (only one or two digits). But as soon as the cell starts to fill the space
the pressure rise with each pinch will become larger.
A graph of the readings should show a pronounced “knee” where cell concrete contact is
made (as shown in Figure 6). As soon as this “knee” is passed the pinching can cease and
the pinch tube is bent out of the way, so that it lays flat on the tunnel lining surface.
However, it is also possible that the cell is already in good contact with the concrete, so
the pinching will immediately cause a pressure rise in the cell. If this is the case, then
cease pinching immediately.
Continued pinching after the cell has made good contact could cause the concrete around
the cell to split open which is not desirable and could lead to erroneous readings. Record
the new initial pressure after the cell has stabilized.