Drilling quality boreholes with jetting method
In Timor Leste, we are starting to drill shallow boreholes using the jetting method. The tools and local skills of our partners are very basic, but we are achieving relatively decent construction quality results, at first glance. However, early water quality testing results are showing moderate microbiological contamination. We have no obvious direct sources of contamination and are assuming that a primary reason might be that we do not have a decent sanitary seal in place. Another issue of concern is that we are drilling with galvanized iron pipes (4") and the filter slots are being cut by hand. Although the water Ph is relatively neutral, we would prefer using a PVC lining to avoid future corrosion and iron contamination issues. We are drilling through sandy an gravel soils.
So, that said, a couple of questions: 1. Does anyone have experience on jetting with PVC pipes directly? 2. Are their easy or recommended ways to install a good sanitary seal?
- One query would be whether you could be getting contamination from the water you are using for jetting? Are the wells chlorinated after construction? There is a lot of pipe handling during construction, but I guess that would not lead to prolonged contamination. Is the contamination appearing immediately or only after several months?
- How deep is it to the water table?
- Do you have access to good dense local clay, because this would make an effective low cost seal with concrete above, combined with an apron, drainage and proper ground compaction
- PVC slotted pipe can be used and you can make very fine slots (from memory around 0.2mm is the minimum but needs good equipment) or make bigger slots and wrap with cheap geo-textile, with rope underneath to maximise the intake area. It would be good to know what you are using at the lower end of the pipe - A ball, a blank- end screwed or bonded on or nothing?, If it is blanked off it is more difficult to jet with small slots, but if it is not, and no ball, the sand will slowly fill up the screened section over time. You can of course buy both stainless steel and plastic jetting points but these are not such a cheap solution.
Best wishes Sally
My advice would be that you should talk to Richard Cansdale - his contact details are part way down this page: http://www.swsfilt.co.uk/weblog/
Richard has a lot of experience of well jetting and he makes jetting heads that doubles as roughing filters.
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