Most pit latrines provide sanitation for a single household, usually necessitating a pit about 1 m across and 3 m or more in depth (although much larger pits are common in some areas), or two shallow pits of up to 1.5 m in depth. The pit may be circular, square or rectangular in plan. Circular pits are more stable because of the natural arching effect of the ground around the hole, with no sharp comers to concentrate the stresses (Fig. 7.1). However, people often find that square or rectangular pits are easier to dig. The depth of the pits often follows local traditions. It is usually advantageous to dig the pit as deep as possible, but this depends on soil conditions, cost of lining and the level of the groundwater.
Fig. 7.1. Strength of different pit shapes (A)
Fig. 7.1. Strength of different pit shapes (B)
The need for a pit lining depends upon the type of latrine under construction and the condition of the soil. In septic tanks and aqua-privies, for example, which require watertight compartments, the pit is always lined. However, in pit latrines it is only necessary to have a lining if the soil is likely to collapse during the life span of the latrine.
It is not easy to decide in advance whether a soil will be self-supporting. If other excavations in the locality (such as shallow wells) have proved to be self-supporting over a number of years, then it is probably safe to assume that a pit for a latrine can be dug without support. Granular soils such as sands and gravels normally require support. Cohesive soils, such as silts and clays, and soils with a high proportion of iron oxides, such as laterites, are often self-supporting. However, silts and clays may lose their self-supporting properties when wet, particularly where there is a varying water table.
If there is any doubt about the conditions it is better to assume that the soil is not self-supporting. Increasingly it is recommended that all pits should be lined, especially where the design life is over five years. Failure of an unlined deep pit can be extremely hazardous for the person excavating it. If the failure occurs some years later it can be expensive for the owner and disturbing for the users. In all cases the top 300-500 mm should be lined and sealed to support the slab (and where necessary the superstructure) and to prevent contamination of the surface and entry of vermin.
The lining may be of any material that supports the soil and that will last as long as the design life of the pit. Commonly, materials such as fired bricks, concrete blocks, concrete, ferrocement and local stone are used, but stabilized soil blocks, old oil drums (though with a limited life in corrosive groundwater) and unglazed fired clay pipes have also been successful.
Quarried stone, where available cheaply, makes a satisfactory lining. The more regular blocks should be used for the top 500 mm with mortar joints. Less-regular stone can be used for the remainder of the lining without mortar in the vertical joints. The builders or masons must be skilled and experienced if the lining is to last a reasonable length of time. Where local stone is used, its durability must be confirmed. Some stone will deteriorate when exposed to air or water or to frequent changes between wet and dry conditions.
The use of timber or bamboo is not generally recommended, since they are subject to insect and fungal attack and often have a limited life. Some hard woods can be satisfactory provided they are treated with tar, creosote or other preservative to lengthen their life. Care must be taken to ensure that none of the preservatives leach into the ground-water as even low levels of some preservatives can be toxic (WHO, 1984). Woven cane and bamboo have been used for the lower part of a lining with stronger materials used for the top 500 mm. However, unless the pits are designed to have an extremely short life, cane and bamboo should be avoided.
In almost all cases, pits of up to 1.5 m in depth can be excavated to their full depth and then lined from the bottom up. If the soil is very loose, the sides of the excavation may have to be sloped to prevent collapse. The space between the lining and the soil can then be backfilled, preferably with a granular material such as sand or gravel. Granular materials are used because they fill the space between the soil and the lining without leaving large voids. They also act as a filter to prevent soil particles being washed into the pit. Voids behind the lining produce locally increased loads on the lining which may cause collapse.
It is usual to provide a foundation for the lining similar to that provided for a domestic house. In most soils, a foundation width equal to twice the wall thickness is usually sufficient (Fig. 7.2). In very soft ground it may be necessary to construct wider foundations to prevent the weight of the lining itself forcing it into the soil (Fig. 7.3). Where the superstructure load is not directly applied to the lining, a widened foundation may not be required since the load applied to the ground at the base of the lining is small and considerable skin friction builds up between the sides of the lining and the ground.
Fig. 7.2. Lining for a shallow pit in firm ground
Fig. 7.3. Lining for a shallow pit in soft ground
Soakpits or leaching pits require a porous lining to allow the wastewater to escape into the ground. The method of achieving this depends upon the lining material used. With bricks, blocks or local stone, a proportion of the vertical joints are left unmortared. These unmortared joints may be confined to specific courses (e.g., every third or fourth course) rather than being spread throughout the lining. This enables the fully mortared courses to carry the load exerted by the soil on the lining. Where the ground is relatively strong, a more open, honeycomb technique is used, with only small dabs of mortar joining the masonry. Alternatively, specially manufactured bricks with angled ends to suit round pits and a central opening to allow for infiltration may be used (D. J. T. Webb, personal communication).
Concrete, ferrocement and fired clay ring linings are made porous by creating holes of 25-50 mm in diameter through the lining. Alternatively, the ring joints are held open by small stones or bricks. Additionally, concrete linings may be made of "no fines" concrete, that is, concrete without any fine aggregate (sand). A mix of one part of cement to four parts of clean gravel (with stones of 6-18 mm in diameter) is suitable. Where precast rings are used, the upper and lower 100 mm of the ring should be made of conventional concrete for extra strength.
The method of excavating deep pits depends upon the stability of the soil during the construction period. In soils that are self-supporting, the pit may be dug to its full depth and the lining installed afterwards. If the ground is not self-supporting, the lining must be constructed as the pit is dug.
Where a lining is not required for support during excavation, the pit is dug to the full depth, making allowance for the thickness of the lining to be installed subsequently. Accurate dimensions are maintained by using a plumb bob to ensure verticality and a template, either circular or rectangular to retain the horizontal dimensions. Ensuring correct dimensions minimizes the costs of lining and backfilling. Sometimes the soil near the surface is weathered and likely to collapse. In that case, the top metre of soil may be supported with a temporary lining (Fig. 7.4). If the finished lining is to be of precast concrete rings, then the top metre of soil will have to be excavated to a larger diameter so that the rings can pass inside the temporary lining.
Fig. 7.4. Excavation for a pit to be lined with precast concrete rings
When the hole has been excavated to the design depth, the bottom is levelled and cleaned. In firm ground, a foundation can be constructed by cutting a groove into the walls of the pit and building a ring beam. In exceptionally soft ground, where the lining is likely to sink into the floor of the pit, the ring beam foundation can be replaced by a floor slab of "no fines" concrete, 75-100 mm in thickness, covering the whole base of the pit. This will distribute the weight of the lining over a larger area of the pit base, thus reducing the load per unit area and preventing upwards heave of the soil (see Fig. 7.3).
Construction of linings
The use of precast concrete (Fig. 7.5) or fired clay rings for the lining of pits has the advantage that the lining can be prepared before excavation begins. This is particularly useful in weaker soils because it reduces the time the soil remains unsupported. The rings to be placed at the bottom of the hole may be porous, designed to allow the liquid wastes to seep into the surrounding soils or they may be sealed to create a wet tank, designed to increase the rate of sludge digestion. The ring nearest to the surface should be fully sealed to prevent entry of surface water and rodents and also contamination of the soil. As with shallow pits, any space between the back of the rings and the soil should be filled with sand or gravel.
Fig. 7.5. Pit bottom lined with precast concrete rings
Brick, blockwork and stone lining
These are built in a similar way to precast concrete linings, i.e., by building up from the foundations. With very deep pits it may be wise to allow time for the cement mortar to gain strength before filling any space behind the lining, to prevent the weight of the fill from deforming the lining. Except for the top 300-500 mm, the joints are left open as described above to ensure infiltration of liquid to the soil.
In situ concrete lining
In this method the hole is lined with concrete cast in the hole (Fig. 7.6). After excavation, shuttering is positioned to a convenient height allowing for compaction, and the space between is filled with concrete. Normally the concrete does not require steel reinforcement for structural strength. However, a small amount of steel may reduce shrinkage cracking. The lining can be made porous by leaving small holes in the concrete (short lengths of 25-50 mm of pipe fitted between the shuttering and the soil will be satisfactory). Alternatively "no fines" concrete can be used.
Fig. 7.6. Pit with concrete lining in situ
When mortar is plastered over layers of fine wire mesh (such as chicken wire) the resulting material is called ferrocement. It is strong, light, requires no shuttering and is easy to construct. It is now widely used for such structures as water tanks and latrine slabs and can be adapted for use as a pit lining.
In some countries the term ferrocement refers to any cement-based material reinforced with steel. Specifically it now describes a material consisting of several layers of small-diameter steel mesh (usually hexagonal chicken wire, with wire of 0.7-1.3 mm in diameter and openings of 12 mm). The layers are tied together with fine wire at 150-mm intervals and then plastered with a rich cement mortar (one volume of cement to two volumes of sand) to give a finished thickness of about 25 mm.
After excavating the hole, as much loose material as possible is removed from the pit walls. Cement mortar is applied directly to the walls of the pit to give a layer approximately 12 mm thick. This layer is then covered with two or three thicknesses of steel mesh, held in place with long staples driven through the mortar into the soil. A second coat of mortar is then applied and pushed firmly into the holes in the wire mesh. On completion, the mortar covering the mesh should be at least 10 mm thick. Where a porous lining is required, holes can be punched through the mortar while it is still weak.
Ferrocement rings may also be precast on the surface and used in the same way as concrete rings.
Excavation in loose ground
Where the ground is very loose and liable to collapse if left unsupported, or where the excavation enters the water table, the most common method of construction is to prefabricate the lining on the surface, place it in a starter excavation, dig out the soil below and allow the lining to sink as the hole is dug. This method is called "caissoning" (Fig. 7.7).
Fig. 7.7. "Caissoning" a pit
A hole is excavated as deep as possible (experience of the local ground conditions will determine the depth). A precast concrete ring fitted with a cutting edge is then placed in the hole. Additional rings are placed on top until ground level is reached. Excavation now begins inside the rings. As the ground is dug away from under the cutting edge, the rings start to sink under their own weight. Additional rings are then placed on top until the required depth is reached.
This method may also be used for linings of bricks or blocks. However, the lining must be constructed sufficiently far above the ground to ensure that the mortar has fully set before the lining enters the ground. The honeycomb method of construction cannot normally be expected to have sufficient strength to be sunk as a caisson.
Where caissoning is employed because of a high groundwater table, excavation should take place towards the end of the dry season when the water table is at its lowest. As the lower ring enters the water it is possible to continue excavation for up to one metre by scooping material in a bucket or with a specially shaped shovel.
Any space around the outside of the lining should be backfilled with compacted earth taken from the pit or, where available, with sand and gravel. If the ground is particularly weak, the top of the pit may be backfilled with weak concrete or a soil-cement mixture to give additional strength. Strengthening may be important if the top of the pit has become overly enlarged during excavation.