Black Cotton Soils and Latrine Construction
Has anyone any recommendations or suggestions on the best way to approach latrine construction in areas with black cotton soils?. To add to the problem it is also rainy season at the moment.
You could try a circular pit, with linings made from stone, brick or sandbags, depending on the mass of your slab. We recently used circular pits with wooden slabs supported by 3 beams, with a gobe plaster (clay, fibre & dung) for a sanitary seal. A heavier slab will require a ring beam. Depending on the target life time of the latrine wood or wicker could be used.
The slab needs to be raised at least 300 mm above ground level. Use compacted spoil from the pit to hipped against the edges of the slab to provide a sloping edge away from the pit to prevent run off entering. Also provide drainage channels or bunds (constructed from pit spoil) to prevent standing water around the edges of the pit. Where light weight slabs are used allow approx 200 mm bearing surface from the circular sides of the pit. If you must use a ring beam ensure that it is raised above ground level.
Depending on your programme parameters it is likely that a larger number of low volume pits (1 m usable diameter to 2 m depth) is preferable to larger volume pits to mitigate the risk of collapse. In Nepal we faced similar challenges to those described in you mail. As the context was humanitarian we focused on simple, low cost construction with smaller pits that were easily replicable at community level.
If you must use a rigid lining (e.g. For permanent institutional latrines) provide a sand or gravel pack between the pit lining and the excavated faces. This will accommodate differential contraction/expansion of the soil.
I agree with the comment on barrels by a previous poster. If long term latrines are required you could look at using concrete rings, again with a flexible lining in the annular space between the ring and excavated faces, to accommodate soil movements.
Hope this helps
It would be worthwhile knowing how permanent the construction is to be - for a season or permanently. Similarly to what scale and kind of infrastructure - is it a programme for 100s of schools or for a diplaced population camp? An idea of water table would help as well.
Initial thoughts would be that you would have to dig the latrine in the same way as a well in unconsolidated sediments with concrete rings or some other lining being lowered down as you dig.
I don't know how deep the water table is but if it is very high, you might need to consider an above ground construction with a concrete tank as the pit with the superstructure sitting on top of it accessed up steps.
Obviously the cost of either of these options is higher than for a regular latrine.
Hi Kate, We have had quite a few responses:
The main issue is 1) stability of the pit both during construction and once it is being used ie will the superstructure collapse into the pit as it gets wet a secondary issue is 2) the permeability of the soil - even if the pit is stable, will it fill up quickly and become unusable.
1 pit-linings which become expensive and do you have the materials?, even then they could be heaved out of the ground during the wet season
2 Above ground latrines - they are typically more structural and so are likely to crack if there is a lot of movement. If urine diversion is possible (and culturally acceptable - do people use water for anal cleansing, if so urine diversion is not a viable solution), this allows the solids to biodegrade much more quickly and the volume reduction is more marked than if the liquids are mixed with solids
3 very shallow pits with a light superstructure that can be moved when the pit is full. This requires a lot of monitoring and manpower to make it successful.
Questions you have to ask yourself:
1 How bad is the black cotton soil - some of the documents Charles metions will give you a simple tests and a better idea of this. Understand that if it is severe, either digging into the ground or building heavy structures on the ground could lead to major cracks (making containment tanks leak) or collapse of pit sides.
2 What is the situation interms of population numbers, density and the likely long term scenario - are they to be there for years or is it only short term, also funding - what is possible?
I hope this is of use - come back to us for more advice/clarification or further documents - we can probably give you designs on above ground latrines or Jeff's 'fosas migratorias' if necessary
This is an intractable problem that cannot be solved easily or cheaply in my experience. Where you find black cotton soils, there tends to be a lack of other building materials, including things like sand and suitable clay for brick making. In Zimbabwe where the standard 'Blair' latrine is built of brick, in some districts they resorted to transporting people long distances so that they could make their bricks and then they transported the people, the bricks and sand back to their homes at enormous expense!
I have tried lining pits with cement mortar reinforced with chicken wire, but I doubt they lasted long and I feel sure that there has been no monitoring The forces exerted by black cotton soils can be considerable, enough to gradually lift manhole rings out of the ground.
This is a circumstance where there is very good reason to avoid the problems by building something other than the standard VIP latrine, such as urine diversion toilets with above ground receptacles, or possibly 'fosas migratorias' that consist of a small relatively portable structure ... (more)
Something NOT to do (a lesson learned...) Do not line the pit with a barrel. The rain water fills underneath and it will pop out like a champagne cork.
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