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What is a Borehole?: Text
What is a Borehole?: Image

In the Luangwa Valley, conventional big rigs which use air compression and are really designed to drill in rock usually fail to drill successful boreholes. They drill a hole and the spoil is blasted out with air. This works in a solid substrate but in our alluvial valley where the substrates are all sand, clay, mud, gravel, and unconsolidated sedimentary layers, it is not suitable and very often fails.

Instead of drilling downwards, the air compressors blast a cavity underground which inevitably

collapses. So - what often happens is that they drill to a reasonable depth of 30 to 40 meters but before they can get the casing in, the borehole walls collapse meaning they can only case to 10 or 12 meters. They will claim that they drilled to the required depth, and technically - they have, but closer inspection reveals that the hole has collapsed and is shallow.

In rocky conditions, this collapsing does not happen and the rock would form a natural casing but with unconsolidated substrates, the air compressed method of drilling is unsuitable.

Once the hole is drilled, the big rig teams would then case to the depth that the borehole has collapsed - usually about 12 meters or so, then quickly add the surround, soakaway, and pump. In their report, they might say that they drilled to 30 meters which may well be true technically - but the key is to look at the number, quality, and depth of casings that go down the hole and the number of pipes on the India Mark II handpump that go in.

Water at 12 meters is more likely to incorporate the water table level. It is more likely to be contaminated by environmental factors such as nearby pit latrines and run off from fields - possibly with cotton pesticides and likely to run dry in the dry season.

Our Little Beaver Lone Star LS200+ rig from Texas works differently. It is a rotary mud drill which uses water as the medium to remove the spoils instead of blasting spoil out with compressed air. The technology is new for Zambia and is ideal for the Valley - however, our rig cannot drill through rock so we are restricted to drilling in the Valley floor. Not only is the method suited to the Valley substrates but the rig is light and easy to manouevre. We can access remote parts of the valley that the big rigs could never reach.

What is a Borehole?: Text
What is a Borehole?: Image

As the drill bit drills in to the ground, the spoil is washed out of the hole with water. We achieve this by circulating water through the drill pipes and out in to sumps that we dig in to the ground at the

drill site making a closed water circulation. The walls of the hole are kept in place with a special inert

polymer called DD2000 which prevents them from collapsing during the drilling process.

Once the hole is drilled - to at least 35 - 50 metres, we put the high quality PVC casing down. Once in place, we put in a gravel pack around the casing and a sanitary seal to prevent any surface or water table accessing the borehole. The polymer is washed out using chlorine which breaks down the temporary seal thereby allowing aquifer water to flow in to the casing pipes. We perform a pump and yield test in order to calculate the capacity of the borehole, then lastly the concrete surround, soakaway and pump is installed.

We are accessing water at a depth of at least 24 metres plus. It is at this level and below that the plentiful aquifers of clean fresh, safe water 'flow'. There are two types of casing - solid and slotted. We put the slotted casing low down in the borehole which allows the fresh water to seep in through the gravel pack layer and fill the borehole casing. Casings at the top of the borehole are solid and there is a sanitary seal inserted above the aquifer level so no water from higher up can access the borehole.

This means that contamination from above ground cannot enter so it is not necessary to test the water after the rains because, unless the top of the borehole has somehow been breached, surface water cannot enter. However - there is no harm in testing from time to time and in giving the whole system a flush through with chlorine. This is meant to be undertaken by the local authority under the WASH program (Water and Sanitary Health). The borehole committee should

liaise with the council on this to ensure their borehole is included in the local authority stock.

The drilling process usually takes 5 days or so and the construction of the surround, run-off and ‘pond’ about 2 – 3 days.

What is a Borehole?: Text
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