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Using the "Sludger Technique" for wells in Nepal


 

By John La Roche, a New Zealand engineer and author

The New Zealand Water and Wastes Association, along with the New Zealand government, have helped fund a water well project in Nepal that uses the "sludger technique."

The project, which also received support from the charity Water for Survival, is benefiting hundreds of Nepalese villagers and school children. 

"Sludging" is a traditional low-cost method of producing water wells. It is used extensively in many parts of Asia to bore holes of 25mm to 150mm in diameter to depths of up to 60 metres. The water table must lie within 6.5 metres of the surface and the strata must be soft alluvial deposits such as those found in the river flood plains of Bangladesh, Nepal, and northern India.

Sludging most often uses a 40-mm galvanized iron pipe which is moved up and down by the action of a bamboo lever pivoted about a simple frame. Drilling starts initially from a small excavated pit filled with a mixture of water and cow dung used as a drilling mud which helps to stabilize the bore during the sludging operation.

The pipe is raised and lowered by two assistants, usually villagers, using the bamboo lever. On each upstroke the mistri (technician) uses his hand to seal the top end of the open pipe. This creates a vacuum causing the column of water inside the pipe to be lifted as the pipe is raised. On the down stroke, the mistri removes his hand as the pipe drops faster than the column of water inside it. The cycle is repeated with the column of water moving upwards relative to the pipe until it begins to be pumped out at the top.

The weight of the pipe acting on a case-hardened cutting socket on the bottom of the pipe helps to 'fluidize' the soil in the bore on each down-stroke. The soil is then sucked up with the pipe and spills out of the top a few cycles later. Sinking rates of up to 20 metres per hour may be achieved.

At 1.5-metre intervals, the discharge is sampled and the strata layer identified and usually logged. An experienced mistri is able to gauge this not only visually but by the feel and note of the boring pipe impacting in the hole. Additional pipe lengths are added as the sinking continues to a depth of about 2 metres below the preferred screen depth.

The galvanized iron pipe is then withdrawn and replaced by a PVC rising main fitted with a sand trap at 1 metre and a well screen at 2 metres above the bottom. The PVC pipe is joined to a galvanized pipe within 3 metres of the ground surface where it is fitted with a mild steel cross and firmly concreted into the pump platform. A suction handpump is then fitted and a drained concrete apron slab constructed to keep the well area clean and hygienic.

A constraint to the sludging technique is the difficulty of penetrating through hard materials. A single small stone in the path of the boring pipe may cause problems, but a simple technique to avoid the abandonment of a borehole uses a modification of the well pointing technique known locally as "hammering." This involves a simple fabricated driver acting on the boring pipe. The boring pipe is fitted with an oversized cone at its lower end and a driving socket at its upper end. The outside diameter of the cone should be the same as the sludging socket.

If further stones are anticipated, then hammering is used to the desired depth. In these circumstances it is usually not possible to withdraw the pipes and they have to be left in place to form the rising main incorporating a well screen. A 32-mm PVC spiral-cut screen is inserted into a pre-drilled 40-mm galvanized pipe located by flanges top and bottom.

The upper flange butting the lowest joint in the rising main is secured using a standard socket, while the lower flange of slightly smaller diameter seals the bottom of the screen to allow a natural gravel pack to develop over time in the space between the PVC and galvanized iron pipes. This arrangement ensures the screen is protected during driving and enables considerable cost savings over commercially fabricated metal screens.

A drawback of the procedure is that there is little knowledge of the aquifer and the relative position of the screen during driving. Test pumping at regular intervals during hammering will help determine the presence of water and the quantity available.

The Nepal project has included the training of official caretakers for each well, three health motivators, and one sanitation worker who will encourage better health and hygiene including the construction of latrines and drying racks.

 


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