Geokon 4855, Инструкция по эксплуатации, Страница: 9 / 20

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2. INSTALLATION
2.1 Preliminary Tests
It is always wise, before installation commences, to check the cells for proper functioning. Each
cell is supplied with a calibration sheet, which shows the relationship between readout digits and
pressure and shows the initial no load zero reading. The transducer’s electrical leads (usually the
red and black leads) are connected to a readout box and the zero readings given on the sheet is
now compared to the current zero readings. (See Section 3 for readout instructions.) The reading
should not differ by more than ≈ 50 digits, after due regard to corrections made for different
temperatures, barometric pressures and height above sea level and actual cell position (whether
standing up or laying down).
Take Initial No-Load Reading of pressure and temperature with the cell laying flat on the
ground. These are important readings and will be used in future calculations of pile-tip load.
By standing on the cell it should be possible to change the readout digits, causing them to fall as
the pressure is increased.
Checks of electrical continuity can also be made using an ohmmeter. Resistance between the
gauge leads should be approximately 180 ohms, ±10 ohms. Remember to add cable resistance
when checking (22 AWG stranded copper leads are approximately 14.7 Ω /1000' or 48.5 Ω /km,
multiply by two for both directions). Between the green and white should be approximately
3000Ω at 25 °C (see Table 4 in Appendix B), and between any conductor and the shield should
exceed 20 megohm.
2.2 Pressure Cell Installation
The short pieces of threaded re-bar are screwed into the lugs welded to the bottom plate of the
pressures cell. A depression is excavated in the ground and lined with sand. The shape of the
depression should match the profile of the bottom of the pile shaft, whether conical or
cylindrical. This depression is now filled with concrete of the same type as the rest of the pile
and is heaped in the middle so that when the pile tip pressure cell is lowered into it the concrete
extrudes out to the side carrying any voids with it. The concrete is left to harden.
The lugs protruding from the upper plate of the pressure cell are welded to the rebar of the rebar
cage as shown in Figure 1. (More details of this operation will be found in Appendix C) The
cables from the sensors and the hydraulic pressure line to the check valve are fixed firmly to the
rebar using nylon zip ties every meter or so. The cables and hydraulic line should be positioned
so that they are protected from the concrete tremie pipe and from being scraped as the rebar cage
is lowered into the hole.
Preparation of the bottom of the hole should ensure that the surface below the pressure cell is
clean of debris and is pre-filled with wet concrete to a depth of 30 cm. It is important that there
be no voids below the cell.
One technique to ensure that there are no voids below the pile tip load cell is shown in Figure 2.