How To Create An Inexpensive And Disposable Borehole Cable
In this "How To" we will look at how to make an appropriate and inexpensive borehole electrode cable suitable when you have a dry borehole (above the water table) or simply need to leave the cables installed for monitoring purposes.
- PVC pipe (whatever size will fit inside your dry borehole)
- Stainless steel hose clamps (local hardware store)
- Insulated multi-stranded cable (it needs to be rated to at least 400V and handle at least 2Amp.
- 2-part epoxy (local hardware store)
- wire ties (local hardware store)
- PVC cleaner (local hardware store)
- PVC primer (local hardware store)
- PVC glue (local hardware store)
- HOT air gun
- Layout the length of PVC pipe needed on the ground. If you require more than one length of PVC you can join several pieces together using a coupler at the field site.
- Mark the outside of the PVC pipe with a permanent marker at even intervals appropriate to the electrode spacing you want to use. I.e Write electrode # every 1 meter.
- Slide the stainless steel hose clamps over the PVC pipe.
- Remove enough insulation on the free end of the insulated cable to be able to wrap the bare copper wire around the hose clamp one time.
- Position hose clamp over the marked electrode position.
- Tighten hose clamp securily so that the bare copper is squeezed between the hose clamp and the PVC pipe tightly.
- Use the HOT air gun to heat the PVC while the hose clamp is under tension so that the hose clamp melts slightly into the PVC. This will keep it from sliding. Don't over heat either.
- Mix up 2-part epoxy (type that will bond both to plastic and metal). We are doing this to keep any water from the copper to stainless steel interface. Any moisture will degrade this connection and over time it will fail.
- Repeat for all electrode positions.
- Finish up by wire tying the loose electrode cables to the outside of the PVC pipe.
- Lastly terminate them into bannana plugs or directly into an appropriate connector. Contact Us for the connector specification.
note: Do not run the insulated 18awg cable inside the other hose clamps to secure the wires to the outside. You could potentially create an electrical short between two or more electrodes.
- Make sure the borehole is large enough to lower the PVC electrode cable string down the hole without collapsing the borehole or damaging the electrodes or cable AND enough room to install a small PVC pipe for pumping the grout to the bottom of the hole.
- Important: Grout the borehole from the bottom going upwards to minimize any pockets of air. Unless you specify to the driller in advance of the job he may figure on just pouring in the grout from the top which is inadequate.
- Try to match the grout resistivity to the formations average resistivity value using the AGI SoilTest Box and performing a simple 4-pin Wenner Soil Test Sounding.