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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.
Options are
1 pit-linings which become expensive and do you have the materials?, even then will they 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
Toby
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 that can be placed over a small hole that is used for a relatively short period. There may well be problems with the acceptability of either of these options, and disposing of the urine from a urine diversion toilet may be difficult if people do not have the water to dilute the pee for irrigation or are unwilling to do this.
I can provide more information on these alternatives, if required.
Regards,
Jeff
This is a very difficult problem to solve with no easy answers. Need to go back to the original assessment. What is the problem and is a pit latrine the solution? One thing to avoid is digging large pits that fill with rainwater runoff in no time at all and then become unusable as latrines. Keep rainwater runoff away from pits by using the dug black cotton soil to form a raised platform around the pit and divert runoff.
Key questions: How many people are being served/Density of population? Inidivudal families or large numbers of displaced people?
Jan
Another point for Toby:
-service connections for water or drain pipes can also be very tricky where they pass from ground to building if the building is on a slab and designed to ride out ground movement rather than prevent it.
Charles
Toby - a response from our Charles with some things to consider, but as Charles says maybe some one can give more specific advice.
As a comment from myself, it is not clear from the guys in the field what actually they want to know - as so often seems to be the case. So is this a problem of the practicality of construction, or how to assess the size needed for infiltration, or the building and foundation designs?
Regards
James
Tricky. I did a bit of work on expansive clays years ago and the BCSs were often mentioned. As a starter, I'd look for web advice/foundation details for expansive soils and not just BCSs (see below). I was on a project with highly plastic dry non bentonitic clay. Single story RC frame buildings could still develop 75mm cracks due to heave!
Main features: highly expansive on wetting and deep fissure cracks when dry give rise to higher bulk permeability but ultra low permeability when moist unless the fissures have been infilled with windblown material. Ie Don't rely on any soakaway capacity into these soils.
Building design: I as I recall, most design methods try to balance a few options such as: -taking the footings down below the zone of active moisture content change/ volume change and not having floors or walls separately founded on the shallow soil -dig it all out and put in a block of less expansive soil under the whole building, -keeping the point load on the foundation as high as possible to contain the swelling forces, - avoiding moisture content changes during the life of the building. Prewetting might be useful especially if you have some sort of surface possibly flexible membrane/cover around the building so that the soils don't dry out later by evaporation; - avoid moisture content changes due to changes in vegetation under the building and its immediate surround during site clearance and subsequent life. - keeping the building fabric material as flexible as possible with flexible connections so movement does not induce massive cracking - keeping the building dimensions as small as possible so each division of the overall 'block' can move as a unit on its own foundation independently of others. - note problems appear with large footprint structures like airport pavements and roads since sealing off evaporation at the surface tends to lead eventually to moisture build up underneath the areas that are paved
google "NAVDOCS Expansive Clay" gave this as a hit: UFC 3-220-10N Soil Mechanics
google "Foundations on Expansive Clay" gave this as a hit: TM 5-818-7 FOUNDATIONS IN EXPANSIVE SOILS
This has a design method and a few index tests like Free Swell and Atterberg limits which give you a clue as to how badly off you are.
If it is a very remote area, it may be worth looking to see how traditional buildings cope.
I am afraid I cant put any numbers to the above or give better sources of information to this off the cuff reply. This is about the limit of what I know so I hope you get a more useful reply from someone else
Charles
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slight change to wording
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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.
Options are
1 pit-linings which become expensive and do you have the materials?, even then
will
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
Toby
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 that can be placed over a small hole that is used for a relatively short period. There may well be problems with the acceptability of either of these options, and disposing of the urine from a urine diversion toilet may be difficult if people do not have the water to dilute the pee for irrigation or are unwilling to do this.
I can provide more information on these alternatives, if required.
Regards,
Jeff
This is a very difficult problem to solve with no easy answers. Need to go back to the original assessment. What is the problem and is a pit latrine the solution? One thing to avoid is digging large pits that fill with rainwater runoff in no time at all and then become unusable as latrines. Keep rainwater runoff away from pits by using the dug black cotton soil to form a raised platform around the pit and divert runoff.
Key questions: How many people are being served/Density of population? Inidivudal families or large numbers of displaced people?
Jan
Another point for Toby:
-service connections for water or drain pipes can also be very tricky where they pass from ground to building if the building is on a slab and designed to ride out ground movement rather than prevent it.
Charles
Toby - a response from our Charles with some things to consider, but as Charles says maybe some one can give more specific advice.
As a comment from myself, it is not clear from the guys in the field what actually they want to know - as so often seems to be the case. So is this a problem of the practicality of construction, or how to assess the size needed for infiltration, or the building and foundation designs?
Regards
James
Tricky. I did a bit of work on expansive clays years ago and the BCSs were often mentioned. As a starter, I'd look for web advice/foundation details for expansive soils and not just BCSs (see below). I was on a project with highly plastic dry non bentonitic clay. Single story RC frame buildings could still develop 75mm cracks due to heave!
Main features: highly expansive on wetting and deep fissure cracks when dry give rise to higher bulk permeability but ultra low permeability when moist unless the fissures have been infilled with windblown material. Ie Don't rely on any soakaway capacity into these soils.
Building design: I as I recall, most design methods try to balance a few options such as: -taking the footings down below the zone of active moisture content change/ volume change and not having floors or walls separately founded on the shallow soil -dig it all out and put in a block of less expansive soil under the whole building, -keeping the point load on the foundation as high as possible to contain the swelling forces, - avoiding moisture content changes during the life of the building. Prewetting might be useful especially if you have some sort of surface possibly flexible membrane/cover around the building so that the soils don't dry out later by evaporation; - avoid moisture content changes due to changes in vegetation under the building and its immediate surround during site clearance and subsequent life. - keeping the building fabric material as flexible as possible with flexible connections so movement does not induce massive cracking - keeping the building dimensions as small as possible so each division of the overall 'block' can move as a unit on its own foundation independently of others. - note problems appear with large footprint structures like airport pavements and roads since sealing off evaporation at the surface tends to lead eventually to moisture build up underneath the areas that are paved
google "NAVDOCS Expansive Clay" gave this as a hit: UFC 3-220-10N Soil Mechanics
google "Foundations on Expansive Clay" gave this as a hit: TM 5-818-7 FOUNDATIONS IN EXPANSIVE SOILS
This has a design method and a few index tests like Free Swell and Atterberg limits which give you a clue as to how badly off you are.
If it is a very remote area, it may be worth looking to see how traditional buildings cope.
I am afraid I cant put any numbers to the above or give better sources of information to this off the cuff reply. This is about the limit of what I know so I hope you get a more useful reply from someone else
Charles
