Percussion drilling is a method of drilling boreholes that involves repeatedly dropping weighted tools into the ground to remove material from the bottom of the hole. We’re using percussion drilling in Sierra Leone because in many ways it is the simlest of all the motorised drilling techniques, requiring only the drill tools, a tripod and a powered winch. All of which can be built here with locally available materials.
- The percussion drill bits dropped by the drill rig to sink the wells. From L-R Small diameter bailer (for sand and suspensions), A cross cutter (for loose, soft rock), a larger diameter bailer and a clay cutter (for compacted clays).
The most common tool we use is a ‘clay cutter’ which consists of a weighted tube, open at the cutting end that fills with consolidated clay as the tool is dropped. Once full the tool is brought to the surface and emptied. With this tool in good ground conditions we can make progress of about 2m an hour., which means in theory at least we can hit the water table during the first day of drilling. However there are a number of problems we face which slow things down, or can halt progress altogether:
1) Hole not straight: The tools we use are weighted and so act as plumb-bobs to a certain extent, keeping the hole straight and vetical. However differences in soil density, small rocks or just the swinging of the dropping tool can mean that the dropping tool move off course. Small deviations are not really an issue as the pumps we install are pretty tolerant, however when the tool his a stone at the base of the hole it can deflect off at a sharp angle. This can lead to an abandoned hole or, at worst a tool stuck in the ground.
2) Rocky soil: A few different types of rock are common in the areas we drill, ranging from soft sedimentary rocks which can be crubled in the hand to impenetrable igneous (or ‘iron’) rocks. We can generally tell when we are approaching a rocky layer when the sound of the drill hitting the ground changes to a deep boom, followed by a metallic ‘ping’ in the case of igneous rocks. Hearing that metallic sound signals the end of drilling at that location, even far more expensive rotary drilling equipment struggles with hard rock. But getting through the softer rock also poses problems, anytime progress slows down the drill tool begins to cut away at the sides of the hole, slowly making the hole wider and more shallow. This can lead to the formation of a cavity below ground, which is then at risk of collapsing, possibly trapping the tool. Repeatedly striking even soft rocks also gradually damages the tips of the drilling tools and so we often have to abandon a hole eventually after hitting rocks anyway. Moving to a new location and trying again often resolves this problem as in most cases the rocks are not in a contiuous layer, however much time is wasted digging half-completed wells and the drilling team is only paid for successful wells.
3) Digging through a saturated zone: Hitting water when digging a well should generally be a time for celebration, although even this can pose problems. When we hit the primary aquifer (the ‘water table’) we continue to dig into the saturated material because the depth of the well in a saturated zone affects the maximum yield of the well, as well as ensuring that water is available year-round. At the end of the dry season, at the beginning of my trip, we only hit water at the primary aquifer, and we only had to dig a couple of metres into the aquifer to ensure a sustainable well with a yield good enough for a typical handpump. However now the rains are drenching the gound almost every night it is common to find saturated zones above the main aquifer, as the recharge water accumulates above zones of consolidated clay. The problem with drilling through saturated clay or sand is that this material is less structurally stable than dry material, and so the risk of a cave-in is far higher. Once the borehole has started to cave drilling has to be stopped, if we’re in the primary aquifer when this happens we may be deep enough to complete the well, otherwise the hole must be abandoned. We can typically only reach about 2m into saturated material before it starts to cave, which is adequate at the end of the dry season, when the water table is at its lowest. At other times, or if an electric pump is required however we need to be able to drill at least 5m into an aqifer to guarentee yield and sustainability.
I hope that a casing I am currently designing will be able to aleviate all these problems. Drilling through a series of joined pipes should keep the tool from cutting away at the walls of the hole and ensure that the tool cuts a straight and vertical hole, even when the ground conditions are rocky. The casing will also sure up the sides of the hole when drilling through instable material, preventing a collapse.
Typical casing used in commercial drilling operations is custom-built from 6″ steel pipe, threaded at each end to join many pipes together. The problem with using this however come down to cost and availability: it costs about £200 per 1.5m length and it would need to be imported from the UK, at further expense. So my challenge has been to design a solution, find suitable materials locally, build and test it. Now that the design is complete and the materials found we’re ready to begin building and testing. With a big drilling contract for a new landmark eco-project in the rainforest close to the Liberian border relying on our ability to dig wells in difficult conditions the pressure is on to make this work…