During such cases, cabling is done in the cable trench. A mm wide and up to mm deep cable trench should be dug at the left-hand side looking towards the sensors from the trench end side.
It is done to route cables from all the sensors at that elevation along the offset trenches as shown in the images below. Make sure to keep the ground clear of any sharp rock or objects which may accidentally damage the cable. Make sure to clean and level the trench with 50 mm of well-graded sand at the bottom of the trench along the path. Decide the depth and width depending upon the number of cables the trench has to carry.
The trench runs into abutment and then along abutment to the downstream side or directly to the downstream side towards the observation room, depending on the cable layout design. The work of these cable trenches is to carry the individual cables from piezometers to the observation room. Make sure to keep the ground clear of any sharp objects. The trench should also be thoroughly cleaned and leveled with 30mm of fine sand at the bottom of the trench along the path. The successive cables should be kept apart by mm and a distance of mm must be left free from the sides of the trench.
NOTE: During installation in embankments, the sensors and cables are embedded in a protective covering of sand or selected fine embankment material. Some installation may involve the positioning of a series of sensors and cables on a ready layer filled with not less than mm of compacted selected fine material. The prepared layers of compacted material may be placed in a trench or on an exposed ramp.
This is done to ascertain an acceptable grade without undue interference with construction operations. For example, working with rockfill dams and earth fill cores, it is handier to install sensors and cables in trenches in core and fine filter zones, and in ramps in a coarse filter, and compacted rockfill shell zones.
In rockfill dams with earth fill cores, for example, it is frequently convenient to install sensors and cables in trenches in core and fine filter zones and ramps in a coarse filter and compacted rockfill shell zones.
However, in an earth embankment, it is convenient to install in a trench. This allows sufficient degrees of compaction of backfill to be easily obtained without any damage to the sensor or cable trench. As sensors and cables are covered and compacted, repeated readings should be taken to ensure that they continue to function properly. NOTE: In case there is more than one layer of cable to be installed in a given array, they should be kept separated from each other at an interval of not less than mm of hand compacted sand or selected fine embankment material.
NOTE: Vertical routing of signal cables from sensors should be avoided while working in embankment dams. In case the vertical run of cables cannot happen, they should be routed vertically only through the filter. The image above shows the preferred method for vertical routing of instrumentation cables through the filter. As there is the availability of access galleries in concrete dams, the cable from the sensors is first routed to the gallery.
These cables may be terminated in junction boxes inside the gallery. Using a readout unit or a data logger, the data from from the various sensors can then be taken out. If need be, the signals from the junction boxes can be transferred through multi-core cables to any observation room outside the dam structure.
Installation in a concrete dam takes place with several piezometers that are installed at a chosen elevation at different cross-sections, along with other sensors. As shown in the figure below, there are three piezometers, five strain rosettes, five no stress-strain containers, five stress meters and two temperature meters are installed at elevation m.
The cables from the sensors are taken to junction boxes and mounted inside one of the cross galleries. It may be that the gallery is above or below the elevation where the sensors are installed. As a general practice, all the cables from sensors at any particular elevation are routed to a vertical shaft on the upstream side of the dam.
Through the vertical shaft, the cables are lowered or raised. During any cross-section, the filling of the dam is done till around 25 cm higher than where the sensors are to be mounted, leaving 0. In cases where the instrument has to be installed with other sensors, like the strain rosette and the no stress strain meter that require more space, a larger trench is left.
The route of the cable should be correctly marked, making sure it ends into the vertical shaft and runs parallel to the line of the sensors. Depending upon the number of cables the trench has to transfer, the depth and width of the channel are decided. In case all the cables at an elevation fit in one row, the depth of the channel can be around 10 cm. In case there is more than one row required for the cables, the depth should be increased by 10 cm per row.
Save my name, email, and website in this browser for the next time I comment. Once you have prepared the piezometer for installation, you can proceed further. The first step is to ascertain the installation depths of the multilevel piezometers. The cable connected to the piezometer should have a required length of the installation depth plus an extra length of 1 to 2 m. Using Guargum as a drilling method, drill a hole of mm diameter to a depth of around 1 m below the elevation at which the piezometer is to be installed.
If needed, use casing to prevent the sidewall of the borehole from collapsing. Make sure to wash the borehole clean, top to the bottom, by pumping in freshwater. The next step is to connect the bottom-most piezometer to sacrificial grout pipe about to mm from the end. For sacrificial grout pipe, a 20mm, bell-ended or threaded, PVC pipe can be used. Use a nylon cable tie to attach the piezometer, with its tip pointing upwards to the PVC pipe.
This is done to prevent the leakage of de-aired water confined between diaphragm and filter and also to reduce the possibility of de-saturation. After following the above steps, it is time to lower the assembled instrument into the borehole.
Using a nylon cable tie or BOPP tape, attach the cable from the piezometer to the grout pipe at nearly the same intervals. Use the same method to attach subsequent lengths of grout pipe. The cable should be taped above the grout pipe at regular intervals. The borehole should be held at the top in such a way that there is a gap between the grout pipe and the borehole bottom.
Once the complete assembly is lowered down the borehole, take readings of the piezometer to check the function. The reading should be in line with the height of the water column above the piezometer.
To full the annular space between the piezometer assembly and borehole, prepare a grout mix as instructed by project authorities. If no instruction is given, prepare the grout mix as mentioned below: Cement 50 kg Bentonite 15 kg Water liters. The first step is to mix cement in water. Slowly add bentonite in the cement-water mix such that the clumps are not formed. After mixing the grout for minutes, check the consistency.
Add more bentonite, if needed, to make the grout as consistent as cream. Make sure it is not very thick as that would make it difficult to pump into the borehole.
Using the sacrificial grout pipe, Pump grout in the annular space between borehole and piezometer until it reaches the ground surface. In case, a casing is used for preventing the borehole walls from collapsing, pull it out as the borehole is grouted. The next day, top up the settled grout to the ground surface. After the borehole is grouted, take readings of the piezometer.
The readings should ideally be more at this stage due to the grout column above the piezometers, which is denser than water. The grout takes at least 2 days to cure and the readings normalize as the grout cures.
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