3.4 Sanitation in Areas with Unstable Soils

In general, the top 0.5m of a pit should always be lined to support the weak soils that are found close to the surface and to provide a foundation for the cover slab and superstructure above. Remember that the lining must extend by about 10cm above the ground to prevent surface water entering the pit.

Step 1. Decide whether the soil is instable, and thus if lining is required.

The decision whether to line the rest of the pit will depend on the type of soil. The simplest method is to seek out local knowledge and examine other excavations in the area, such as those for hand-dug wells. If existing excavations have not collapsed and are not lined, it is fairly safe to assume that pit-latrine excavations will not need lining. If in doubt, pits should always be lined.

Step 2. Decide what type of lining is most appropriate.  

The most important action in determining what is appropriate for the local context is to ensure local knowledge, skills and material is utilised. So look for examples of lined latrines of local (host) communities and ask around for skilled local labourers to help you design. Take the expected life span of the latrine into account and consider if the pits are going to be emptied or not. Pit-lining is most cost-effective where pits are to be emptied regulary.

The pit can be lined using any material that is strong enough to prevent the walls of the pit from collapse, can support the weight of the structure above, and is durable enough to ensure that it will last for as long as the pit will be in use. Common lining materials include:

• Bricks/stone – time consuming, but allows for infiltration as some bricks or stones can be left out
• Pre-cast concrete rings – the liquid can not escape the pit easily unless the ring is made with drainage holes: ring moulds required; expensive
• In-situ cast concrete – relatively time-consuming to construct mold; no infiltration (pit needs to be emptied); expensive
• Concrete blocks – can be build honeycomb style to allow infiltration; circular block moulds can be used for circular pids
• Wood – time consuming to position to provide proper retaining wall; prone to rotting (even when treated)
• Sandbags – easily available; cheap; cement can be used in bags for stability in areas of shallow groundwater
• Oil drums – holes must be made in sides for liquid to infiltrate; small diameter limits diameter of pit size and ease of excavation; corodes
• Ferrocement – time consuming and relatively expensive
• Corrugated-iron (CGI) sheets – holes are required to allow for infiltration, need to support bracing
• Tyres – high quantity of tires required, allows infiltration and provides stability
• bamboo/cane – rots faster than wood and less strong; may be easily available; can be suitable when pits are supposed to be closed when full.

Thin liners such as oils drums, CGI sheet, and bamboo/cane are not strong enough to take the weight of a floor slab or superstructure and therefore might require strengthening at the top by a ring beam.

Step 3. Pay extra attention to workers safety during excavation

Pits are usually dug by hand. Great care must be taken to protect the workers from pit collapse. When soils are loose or likely to collapse the pit walls must have temporary supports until a lining can be constructed. There must always be one person at the surface and at the top of the excavation to help in case of an accident.

Step 4. Consider alternative toilet or latrine designs

For areas where the cost involved and skills required to mitigate the loose or instable soil will make it preferable to avoid underground excavations, other alternatives that don’t require digging can be considered, like raised latrines, urine-diverting dry toilets or container based toilets.