Chapter 7- Masonry dams
The ALDEV (African Land Development) design
Other types of rockcatchment dams
Roofs on rock catchment dams
Garlands of stone gutters
Masonry dams built in stages
Setting out dams walls
7.8 Calculating the cost of constructing dam walls
Bill of quantity and cost of dam walls
7.10 Construction of dam walls built of rubble stone masonry
7.11 Construction of draw-off piping
Ferro-cement roofs over dam reservoirs
Bill of quantity and cost of roofs
Manual on construction of a ferro-cement roof
A series of rock catchment dams were constructed of the ALDEV design during the late 1950s in Kitui, the two most famous being Ngomeni built in 1955 near Kyuso and Kaseva built in 1956 near Mutomo in Kitui. Kaseva rock catchment had a storage volume of some 3,000 cubic metres that was enlarged to about 5,000 cubic metres by a Danida project Mutomo Soil & Water Conservation in 1989. The dam reservoir has only dried up 4 times during the last 47 years, namely during the long droughts of 1975, 1985, 1995 and 2002.
More than 100 rock catchment dams of the ALDEV design have been constructed in Kitui, Makueni, Taita-Taveta and Zambia without any failure by the author during the last 20 years. The success being due to the simplicity of the design that does not require any reinforcement provided the width of the foundation for a dam wall is equal to 3/5 of the heig ht of the dam wall.
Rainwater falling on a rock is diverted to the dam reservoir
by two garlands of gutters built of flat stones mortared onto the rock.
The garlands are also used for enlarging the catchments so even a small
rain shower can provide huge volume of run-off water.
catchments so even a small rain shower can provide huge
volume of run-off water.
Rock catchment dams should preferably be build on rocks that gives some free storage volume such as, gorges between two rocks, rock pools and rock shelves.
7.3.1 Single-winged masonry dams in gorges.
Evaporation can consume up to half of the volume of water stored in open rock catchment dams without roofing. Roofs of galvanised iron sheets can be tied onto galvanised water pipes that are anchored onto pillars built of concrete blocks in the reservoir. The disadvantages are that: a) rodents can always find their way into a dam reservoir and drown there, b) water vapour corrodes the iron sheets and wires.
A more permanent option is to erect pillars of PVC pipes
filled with concrete onto which beams of reinforced concrete carrying
vaulted roof sections of ferro-cement is anchored.
Rock catchments require garlands of stone gutters to:
a) divert rainwater run-off from a catchment area to the water reservoirs of rock
catchment tanks and dams, and
b) increase catchment areas by winding their way around rocks, often to the opposite side.
Garlands of gutters must have an upward gradient (slope) of at least 3 cm for every 100 cm length to allow run-off water to flow towards the water reservoir by gravity.
If a dam wall is build in stages, the gutters should start at the points where the final height of the dam wall will reach.
It is advantageous to construct masonry dams in stages because:
1) A community can provide free labour to build one or two stages during a one dry
season without the work being too tiresome for them.
2) When the first stage is completed, the dam can provide water from the first rain
shower. This performance will encourage a community to build more stages.
3) Where funds are insufficient, people can sell some of the water from the first stages
and use the money for buying cement for he next stages.
4) Any leakage in one stage will be closed by the following stages.
Where a design and an estimate of the construction cost are required, the outline for the foundation of the dam wall must be marked on the rock. The site criteria are:
1) Dam walls may be build on rocks
having a down- and outward slope less than 15 cm depth for every 100 cm .
The gradient of a rock is measured by holding a spirit-level horizontal on a 100 cm length of timber while measuring the distance to the rock. In this case, the gradient is more than 15 cm thereby proving that the gradient is too steep for the foundation for a dam wall.
Foundation for dam walls must consist of solid rock without
It is advisable to construct dam walls
The procedure for marking the foundation for a dam wall
is as follows:
Water is poured into the pipe until the waterlevel has reached the 2 m mark on the stick. The waterlevel in the other end of the pipe is now marked onto the rock. The other end of the wall is marked in the same way.
Then mark with white paint both ends of the dam wall
and the place with the stick, which is the lowest point of the foundation.
Thereafter the width of the foundation can be marked
onto the rock using two design criteria:
Fig. 16. The outline of the foundation for a 200 cm high dam wall has been marked onto the rock with white paint.
cm, has been marked at the lowest point and the width of the crest, 30 cm, has been marked at each end of the dam wall, strings are drawn between these points to show the outlines of the foundation.
The outlines are marked with dots of white paint so that the builders can identify the correct place without problems.
The cost of constructing a rock catchment dam can be calculated when the volume of its dam wall is known because that determines requirements of materials and labour.
Volume of dam walls
The volume of dam walls can be calculated by drawing sketches with the length and height of the dam wall bearing in mind that the base of a dam wall must always be 3/5 of its height and that the crest should always be 30 cm wide.
The sketches are then divided into triangular (A) and
rectangular (B) units whose volume can be calculated using the formula
below. The volume of the units can then be added together to give the
total volume of the dam wall.
Fig. 21. An example of estimating the volume of a dam wall.
The measurements of the dam wall in the example are:
Height of middle wall 2.0 m
Volume of left wall
Volume of middle wall
Volume of right wall
Total volume of the three walls
When the total volume of a dam wall has
been found, the numbers of cubic meters is
75% rubble stones + 25% mortar with a mixture
of 1 part of cement to 4 parts of sand.
Materials and labour for 1 cubic metre of rubble stone masonry
Preferably the communities should provide free locally available materials, skilled and unskilled labour as their contribution for their water project. In the above example, that amounts to US$ 30 which is about 49% of the total construction cost.
Bill of quantity and cost for the 56 cu.m. dam wall shown on the former page
Preferably the communities should provide free locally available materials, skilled and unskilled labour as their contribution for their water project. In the above example, that would amount to US$ 1,680 which is about 49% of the total construction cost.
The width of the base for a wall must always be 3/5 of
its height and the width of the crest must be 30 cm as follows:
Remember that the foundation of walls
Where a dam wall has a bend, two templates are erected, both being at 90 degrees to the upstream side of the wall.
Where more than one template is required, their crest must be at the same level.
Templates are mortared onto the rock surface with their upstream sides being vertical.
When the template, or templates, are mortared into their vertical position the level of the crest is transferred from the template(s) onto the two places on the rock where the two ends of the dam wall will be mortared onto. The two places should be protruding parts of the rock that can provide good support for the dam wall.
The crest level of the dam wall can be transferred onto
the rock using a transparent hose-pipe. One end of the pipe it tied to
the upper part of the template while the other end of the pipe is laid
on the rock where the dam wall will end.
Water is filled into the pipe until the water-level reaches the height on the crest on the template. The water-level in the other end of the pipe laying on the rock is now horizontal with the crest and is marked onto the rock. Both ends of the dam wall are marked on the rock using the two water-levels in the pipe.
Strings are now drawn along the inner sides of the templates
to the two places on the rock where the 30 cm wide crest has been marked.
All loose sections on the rock are removed and the rock surface is roughen
with hammers within the strings.
Rubble stones, which have been brought to the construction site, are cleaned for all dirt and soil in a wheelbarrow with water.
The largest and flattest of the stones are laid out along the marked outline of the dam wall where they will be used for building the outer sides of the wall.
Smaller and rounder stones are also cleaned. They will be used for filling in the wall.
The rock surface between the strings is swept and cleaned with water thoroughly. If any dirt or loose part are left it might create leakage under the dam wall.
Dry cement is then dusted onto the moist rock surface within the strings until all parts have been covered in a thin layer of moist cement.
Simultaneously, mortar has been mixed of 1 part of cement
to 3 parts of coarse and clean sand, called 1:3.
Within the same hour, mortar of mixture 1 cement to 4 sand (1:4) is made and used for mortaring the flatter stones onto the foundation along the strings. Short sticks are used to support the stones.
A draw-off pipe is made from a length of 1 1/2" (38 mm) galvanized iron pipe being 3 meters long with thread at one end.
The surface of the pipe is roughen by a hammer to ensure a good bond with the stone masonry that will not create leakage.
The draw-off pipe is concreted into in the lowest part of the dam wall in an exact horizontal position to facilitate extracting water from the dam reservoir by gravity.
After about 12 hours the mortar in two lines of stones lining the sides of the dam wall have hardened so much that the construction of the wall can continue.
The space between the two lines of flat stones can now be filled with smaller and rounder stones compacted into mortar of mixture 1:4.
Ensure that no stone is touching another stone without mortar because that may cause leakage.
The surface of the filled-in stones and mortar is left with a rough surface to ensure a water-tight bond with the next course of stones as seen below.
The following day, the space between the two lines of
flat stones is filled with smaller and rounder stones in mortar 1:4 and
so on until the whole wall has been built up to the crest.
Water can be drawn manually from the water reservoirs of rock catchment dams it is a tiresome and dangerous to climb up to a dam situated high above the ground and climb down again with 20 kg of water in a jerry-can on the back.
Since it is fairly easy and cheap to gravitate water from rock catchment dams to tap stands at the ground level, a draw-off pipe should always be installed. Besides reducing labour and danger on drawing water, contamination of the water is also reduced because people do not enter the water reservoir.
There are two types of draw-off piping, namely
Fig. 22. Water is gravitated directly from a water reservoir situated on a rock shelf or in a gorge between two rocks by means of 18 mm galvanized piping. A perforated PVC pipe is pressed onto the upper end of the pipe that is placed in a filter box made of porous concrete blocks.
The lower end of the pipe is connected to a tap stand with watertaps. The whole length of piping between the intake and tap stand is mortared onto the rock with large stones for every 5 meters or so. Although this anchoring of the pipe prevents baboons from breaking the pipe, it cannot keep elephants from pulling the pipe apart when they are thirsty and cannot enter the water reservoir.
2) Gravity flow over a siphon is applied when the floor of a water reservoir is situated
Fig. 23. As with direct gravity flow, the intake/filter box is situated at the lowest point in the water reservoir. However, for a siphon flow a non-return valve must be installed in the pipe just outside of the filter box. The valve is mounted at that it allows water to flow out of the box and prevents water from flow into the box.
The second feature for a siphon system is that a vertical pipe with a removable G.I. cap is connected to the highest point of the draw-off pipe, which should, preferably, be next to the downstream side of the dam wall.
The draw-off pipe for a siphon system should therefore slope towards the reservoir, while the draw-off pipe for direct gravity flow should slope away from the reservoir.
The flow of water is started by closing the watertaps
at the tap stand and unscrewing the cap on the vertical pipe. Water is
then poured into the vertical pipe slowly until all air bubbles have left
the pipe. The cap is thereafter screwed airtight onto the pipe. Water
will now flow to the watertaps when they are opened.
Fig. 23. Plan of a vaulted ferro-cement roof anchored onto beams of reinforced concrete that are supported by pillars made PVC pipes filled with concrete.
Fig. 24. Sections of a ferro-cement roof on the reservoir of a rock catchment dam.
The above roof has a horizontal area of about 10 m x
6 m = 60 sq.m. but due to the
Materials and labour for 1 sq. m. of ferro-cement roof with beams and pillars.