WALLS, VAULTS, DOMES USING SOIL CEMENT, COMPRESSED MUD BLOCKS, NUBIAN TECHNIQUE OF ARCH ROOFS
Submitted by : Gayatri s Sneha j Janani t Ketan a Sagar c
Why Mud? One of the wonderful and endearing things about mud is that there is a wide range of muds and every single s ingle one has its own individual characteristics. All of it is god made and not machine made, so it is not standardized and it is almost limitless in quantity. Therefore to many, especially to the engineering world, it is maddening rather than endearing because you have to know kno w your own mud and how to handle it. All ladies have beautiful eyes and an d bewitching hair and shapely lips but ideally i deally you have to learn t o live with, love, cherish and understand your own particular woman. Treat your mud as you should treat your wife and you will have a wonderful house for life. - Laurie Baker Man has always built with natural materials; building with mud /earth, the most natural of all building materials, poses a special challenge. Architecture today, is dominated by cold hard, machine made building materials. While all previous attempts to replace these with a more plastic, environment- friendly material have resulted in a negation of the structural/technical dimension of Architecture, building with mud offers the unique possibility of a synthesis. As a an exploration into the possibilities of mud as a building material is especially relevant in a country like India where more than 80% of the population are sheltered in structures built with biomass. The advantages and possibilities of mud construction are endless. Mud is environmentally the most sustainable material. Besides its obvious eco friendly nature, its easy availability makes it almost a no cost material, abundantly present, generally on the site itself. This cost-effective material is also energy efficient and can be used to produce aesthetically very appealing structures.
COMPRESSED STABILISED EARTH BLOCK TECHNOLOGY The new development of earth construction really started in the nineteen fifties, with the technology of the Compressed Stabilised Earth Blocks (CSEB): a research programme for affordable houses in Colombia proposed the first manual press the Cinvaram. Since then, considerable scientific researches has been carried out by laboratories. The knowledge of soil laboratories concerning road building was adapted to earth construction. Since 1960 1970, Africa has seen the widest world development for CSEB. Social programmes and prestige demonstration projects are not computable anymore. Africa takes, these days, a further step with semi industrialization and standards. India developed CSEB technology only in the nineteen eightys, but sees today a wider dissemination and development of CSEB. The soil, raw or stabilized, for a compressed earth block is slightly moistened, poured into a steel press (with or without stabilizer) and then compressed either with a manual or motorized press. The input of soil stabilization has made it possible to build higher with thinner walls, which have a much better compressive strength and water resistance. With cement stabilization, the blocks must be cured for four weeks after manufacturing. After this period of time, they can dry freely and be used like common bricks with a soil cement stabilized mortar.
Soil stabilisation
Many stabilizers can be used. Cement and lime are the most common ones. Others, like chemicals, resins or natural products can be used as well. The selection of a stabilizer will depend upon the soil quality and the project requirements: Cement will be preferable for sandy soils and to achieve quickly a higher strength. Lime will be rather used for very clayey soil, but will take a longer time to harden and to give strong blocks.
MANUFACTURE of CSEB : Components: Mixture of sand, soil, cement in 4:4:1
Cement or lime and water can be used as stabilizers Soil having less clay content are used.
The processes include:
SIEVING & PULVERIZING MIXING COMPACTION CURING AND STACKING
SIEVING & PULVERIZING y y y
Sieving for removal of larger sized particles. Pulverisation to break up the lumps of soil particles. Storage of soil in dry condition.
MIXING Mixing is divided into two parts as dry and wet mixing y
y y
Soil and cement are mixed proportionately. uniform color. The dry mixture is layered thin and water should be sprinkled on it.
y
If the ball flattens - more water.
y
If it disintegrates - right amount.
y
If it crumples - less water.
Compaction y
y
y
Proper soil-cement mixture has to be used within two hours. should be placed in a mould and compressed. water and mixture coming out of mould implies mixture should be prepared with less water.
CURING & STACKING y
Blocks must be kept on a flat surface area protected against the sun and wind.
y
Should be rotated for uniform drying.
y
watering of bricks reduces the shrinkage.
y
Drying will take approximately 14 days.
SELECTION OF THE EQUIPMENT AND PRODUCTS
The development of CSEB means nowadays a wide range of equipment and products of different size and shape. To select the best adapted equipment and product to ones need, one should pay special attention to these factors:
ADVANTAGES OF CSEB A local material
Ideally, production is made on the site itself or in the nearby area. Thus, it will save transportation, fuel, time and money. A bio-degradable material
Well-designed CSEB houses can withstand, with a minimum of maintenance, heavy rains, snowfall or frost without being damaged. Their strength and durability have been proven since half a century. But lets imagine a building fallen down and that a jungle has grown on it: the bio-chemicals contained in the humus of the topsoil will destroy the soil cement mix in 10 or 20 years And CSEB will come back to our Mother Earth... No other building material can do that. Limiting deforestation
Firewood is not needed to produce CSEB. This will save forests, which are being depleted quickly in the world, due to short view developments and mismanagement of resources. Management of resources
Each quarry should be planned for various utilisations: water harvesting pond, wastewater treatment, reservoirs, landscaping, etc. It is crucial to be aware of this point: very profitable if well managed Disastrous if unplanned! Energy efficiency and eco friendliness
Requiring only a little stabilizer the embodied energy in a m 3 can be from 5 to 15 times less than a m³ of fired bricks. The pollution emission will also be 2.4 to 7.8 times less than fired bricks. Cost efficiency
Produced locally, with a natural resource and semi skilled labour, almost without transport, it will be definitely cost effective, more or less according to each context and to ones knowledge. An adapted material
Being produced locally it is easily adapted to various needs: technical, social, cultural habits.
A transferable technology
It is a simple technology requiring semi skills, easy to get. Simple villagers will be able to learn how to do it in a few weeks. An efficient training centre can transfer the technology in a weeks time. A job creation opportunity
CSEB allow unskilled and unemployed people to learn a skill, get a job and rise in the social scale. Market opportunity
According to the local context (materials, labour, equipment, etc.) the final price will vary, but in most cases it will be cheaper than fired bricks. Reducing imports
Produced locally by semi skilled people, no need to import from far away expensive materials or transport over long distances heavy and costly building materials. Flexible production scale
Equipment for CSEB is available from manual to motorized tools ranging from village to semi industry scale. The selection of the equipment is crucial, but once done properly, it will be easy to use the best adapted equipment for each case. Social acceptance
Demonstrated, since long, CSEB can adapt itself to various needs, from poor income groups to well off people or government needs. Its quality, regularity and style allow a wide range of final house products. To facilitate this acceptance, banish from your language stabilized mud blocks , when speaking of CSEB. Often people associate in their minds the name mud with poor building material.
DISADVANTAGES OF CSEB : y y y y y y y y y y
Proper soil identification is required or lack of soil. Unawareness of the need to manage resources. Ignorance of the basics for production & use. Wide spans, high & long buildings are difficult to do. Low technical performance compared to concrete. Untrained teams producing bad quality products. Over-stabilization through fear or ignorance, implying outrageous costs. Under-stabilization resulting in low quality products. Bad quality or un-adapted production equipment. Low social acceptance due to counter examples (by unskilled people, or bad soil & equipment).
Soil cement: o
A building material, containing natural soil, portland cement and water.
o
Soil - Sandy soils are the best
o
o
o
Portland cement - to react with water and form an interlocking bond, porosity of the material depends on cement Water acts as a binder Hard, semi-rigid durable material formed by hydration of the cement particles.
TYPES OF SOIL CEMENT:
1. A cement-modified soil has
Small proportion of Portland cement.
Similar to a soil, but with improved mechanical properties
lower plasticity
increased bearing ratio
shearing strength
decreased volume change.
The result is caked or slightly hardened material
2. A cement-treated base :
Mix of granular soil aggregates or aggregate material with Portland cement and water.
It is similar in use and performance to soil-cement base.
3. Soil cement base (scb):
contains higher proportion of cement than cement-modified soil.
used as a cheap pavement base for roads, streets, parking lots, airports, and material handling areas.
Distributes load over broader areas.
In-place or nearby located materials can be used for construction.
This conserves both material and energy.
Their strength actually increases with age, providing good long-term performance.
Characteristics:
o
Masonry strength is sensitive to block strength.
o
Works better with soil-cement mortars.
o
Strength depends on cement content of the mortar.
o
Used for the construction of load-bearing masonry.
Outstanding Features and Benefits of soil cement:
Creates a stabilized surface Increases load-bearing strength of all types of soils
Environmentally safe
Offers maximum weatherability to wind, rain ultraviolet light and other weather conditions
Prevents water from seeping into and destabilizing the surface
Dries clear, providing an aesthetically pleasing appearance
DOMES Application of Domes: y y y y
Plain masonry built with blocks or bricks Floors for multi-storey buildings, they can be leveled flat Roofs, they can be left like that and they will be waterproofed Earthquakes zones, they can be used with a reinforced ringbeam
They are built free spanning: y y
It means that they are built without form This way is also called the Nubian technique
Timber Saving: y
Domes are built with bricks and blocks (rarely with stones)
Variety of Plans and Shapes: y y
Domes can be built on round, square, rectangular rooms, etc. They allow a wider variety of shapes than vaults
Stability Study: y
The shape of a dome is crucial for stability, and a stability study is often needed. Be careful, a wrong shape will collapse.
Need of Skilled Masons: y
y
Building a dome requires trained masons. Never improvise when building domes, ask advice from skilled people. Be careful, a badly built dome may collapse
Need of Good Quality Materials: y
Domes built with compressed earth blocks should be made of blocks of very regular thickness
CASE STUDY ALMEDY MOSQUE RIYADH, SAUDI ARABIA Ar chitect: Auroville Earth Institute / Satprem Maïni
Client: Arriyadh Development Authority Built Ar ea: 547 m² Cost: US$ 375000
The aim of the project was to demonstrate that the ancient material of stabilised earth could be used to create a modern architecture. A mosque was considered an ideal prototype building, as it would be used daily by many people. The main elements of the mosque, from the ground level to the top of the minaret, are made of compressed stabilised earth blocks (CSEB), stabilised with 8.3 per cent cement. CSEBs are water-resistant compared to raw earth, and have a higher mechanical resistance. About 160000 blocks of
various kinds for walls, columns, arches, vaults and domes were laid in 49 days by 225 workers. Description Building data
The massing of the mosque was carefully considered to reflect and articulate the construction system, with orthogonal building blocks giving way to curved domes, arches and barrel-vaults. Furthermore, the strongly defined courtyard enclosure helps to extend the space of the building to the exterior. In short, the massing of the mosque could be divided into three components: prayer hall, courtyard and minaret, all reflecting the local Najdi building style. Sustainability issues were central to the design. Day lighting concerns were also critical, especially in the middle of the prayer hall (al harem). Control of direct light is carefully conceived and complemented through subtle openings in the barrel-vault ceiling. Overall, earth materials play the central role in internal climate control. Supplementary mechanical ventilation/air-conditioning is used during the peak summer months. The main prayer hall is air-conditioned using eight split-units carefully concealed within the Quran shelves. Quantitative data: 160,000 earth blocks were made in 32 shapes and sizes, both in a Belgian press and in a press designed in Auroville Ground floor area: 432 square metres Total floor area: 547 square metres Total site area: 800 square metres Structure, materials and technology
The foundation and slabs are of reinforced concrete, whereas the basement is of stone. The main body of the mosque is formed of 160,000 blocks of compressed stabilised earth reinforced with 8.3 per cent cement. Waterproofing was done using bituminous materials (exact details not available from Jazirah Gate Company). The structural system consists of load-bearing walls and columns that carry the whole weight of the elaborately designed barrel-vault roof. The blocks were over-stabilised to improve their capacity to bear the roof. The minaret is also a load-bearing structure, but its walls are slightly tapered (by about 1 degree) to evoke the local indigenous architecture. Its wall thickness varies from 59 centimetres at the base to 24 centimetres at the top. Composite techniques were adopted for lintels and beams: U-shaped compressed stabilised earth blocks with 8.3 per cent cement and reinforced cement concrete. The stringer of the arches contains precast reinforced cement concrete. Arches were built around a self supporting frame, using a technique developed by the Auroville Earth Institute. All vaults and domes were likewise built with free- spanning techniques developed by the AEI. The structural efficiency of the semi-circular barrel-vaults is optimised, with their thickness decreasing from 29 centimetres at the base to just 7 centimetres at the top. The domes are all constructed of 9 centimetre blocks, so as to minimise their weight. Origin of technology, materials, labour force, professionals
The compressed stabilised earth block techniques were developed by the Auroville Earth Institute in India. Technical Assessment Functional assessment
Technically speaking, the building satisfies the make it a masjid, a place for daily prayer only.) Surprisingly, the ablution/lavatory facilities are also missing from the plan of the mosque. No
explanation of this was given by the architect, but on our site visit we discovered that the ablution space was built separately, close to the main mass of the building, allowing it to be used by both the people praying in the mosque and visitors to the adjacent park.usual functional requirements of a mosque.It conforms to the traditional layout: the prayer hall forms a free, uninterrupted space punctuated only by the rhythm of the pillars, with the mihrab punched out of the main body of the building. However, there is no minbar, leading one to conclude that a Friday prayer is not performed in this mosque. Climatic performance
As with all structures built out of mud/earth blocks, the Al- Medy mosques climatic performance is exemplary. This can be clearly felt from the instant one enters the interior. The thickness and shape of the walls and ceilings, combined with the low thermal conduction of the earth-based material, provide ideal insulation from the harsh weather outside. In addition, the courtyard walls cast shadows that give a limited amount of protection from the sun; however, the entrances are not immediately sheltered with a canopy, lintel cover, or recessed porch. The arid, hot climate provides the ideal conditions for the use of earth blocks, with very little rain to trigger their disintegration. In addition, the compression applied to the blocks helps to reduce the hollow space in between them, and in the process reduces their water absorption. The architect has devised an efficient system of natural lighting that allows just enough natural light to enter the building without producing unwelcome glare from direct sun rays. This is achieved by inserting slender openings into the barrel-vault roof. Thick wooden window shutters further control the suns penetration into the building. The distribution of windows and doors, in tandem with the configuration of the ceilings, is designed to increase natural ventilation. All these measures work well when the weather is moderate. .
TOWARDS THE FUTURE Building with earth is definitely an appropriate, and cost and energy effective technology. Obviously one has to know the material and master its disadvantages, which normally are variations in the soil quality and hence the block quality, shrinkage cracks, lower strength than high quality fired bricks or concrete, production of the blocks on site, etc. Since half a century, research and development has proved the potential of earthen techniques. Earth can be used as a quality and modern building material almost everywhere in the world. One of the main key points for a general revival and dissemination of earthen techniques is respect for Nature and management of resources. Another important parameter is the training aspect. Quite a few training centres are blossoming all over the world, but it appears that government organisations have an essential role to play. They should bring awareness, from the schools to the masses, and give a political direction for the implementation of the research and development done by the best research and training centres.
Vault Vault ,a barrel form ,is simply deep three dimensional arch..It was first constructed in ancient Egypt and Middle East.In architecture, a vault is an arched structure of masonry, forming a ceiling or canopy. Vaulting makes it possible to roof over a comparatively large space using bricks, stone blocks or concrete. Until the development of metal girders and trusses in the 19th century, the only alternative was to use long wooden rafters or long stone lintels much simpler than vaulting; but cruder and more expensive as well, and ultimately limited by the length of wood or stone available. The Romans made an art form of the vault and it has remained popular ever since the large vault of the superstructure has also been designed using CSEBs as the primary material. Detailed funicular studies were carried out exploring the possibilities of several different arch/vault types. The profile considered most suitable for the wide 9.5 metre span was the shallow Segmental Arch - which rises 2.26 metres at the centre and extends 41.5 metres in length. The total area of the vault is 425 square metres. Through the optimisation method developed by the Auroville Earth Institute, the strongest and yet lightest possible arch has been designed with a thickness of 17.7 cm at the springer beam reducing to a mere 9 cm at the keystone. The tapering profile require CSEBs of 4 different sizes with 50 blocks needed per single arch, and 755 arches/courses required to complete the entire vault. In total 40,000 CSEBs have been custom manufactured for this alone. The vault is to be constructed course by course and more importantly, without the use of any supporting formwork. Instead, Sharanam will employ the Free Spanning Technique developed, again, by the Auroville Earth Institute which requires only end templates, string lines and a thin stabilised soil mortar.
Construction processs
Medieval vaulting was created over a period of 600 years between 900 and
1500AD (roughly). There were also different types of vault in different places . There must have been very many experiments in their construction. They all had one thing in common though. They were built of stone at a height without the benefit of precise predictable measurement. They would, probably, all be built under a roof to pro tect the work and the workers and all would require a scaffold to support the stones the mortar and the workmen. The scaffold would also need to support machines, usually treadmills, to lift large stones on occasions. Large bosses for example at the apex of vault ribs. Medieval mortar took a long time to achieve its final set and medieval vaulting
is noted for the large size of the mortar joints. This means that it must have been supported for some time before the support was removed. The large mortar joints were an economical solution to the problem of lack of precision in that stone would not have to be precisely cut and the mortar joint could be more or less wide depending on circumstances. It may be interesting to note that there are medieval paintings showing the construction of cathedrals and churches. They show details and quite mundane tasks but they do not show the construction of vaults, which suggests that they were out of sight, under a roof and above scaffolding. The obvious way to construct a vault is by constructing scaffolding to hold up all the pieces. This is extremely expensive, however. Most classic vaults are self -supporting in construction, or built with jointures. In a self-supporting vault, the starting arches have to be built with scaffolding, but after that, the bricks or stone blocks interlock, permitting additions until the vault is complete. One of the easy ways to start an arch or construct a vault uses a "jointure", a beam or clamp with faces to grip the top blocks. Each time a block is added, the jointure is walked up to the next block on that side. The construction of jointures used to be a trade secret of masons. Some classic vaults are made to be constructed with jointures. There are several inexpensive ways to construct concrete vaults. One of the simplest is to form or dig a mold in the earth, pour the concrete, and then remove the earth. This is extremely effective with unskilled labor, and produces beautiful, durable buildings. Another way is to construct hollow pillars and an inner vault surface of flimsy, good-looking surfacing materials (pretty wood paneling or gypsum board). These also include the internal rebar, anchor studs, ducting, and piping. Then, these are filled with low density concrete, usually made with pumice or zeolite in place of silica gravel, and possibly with the addition of glass bottles which are even lower density. The result is improv isable (the early stages are more like constructing a stage set than real construction), sturdy and attractive. Further, no wood, steel or other scarce materials are required for construction up to three stories tall.
Application of Vaults:
Plain masonry built with blocks or brick Floors for multi -storey buildings: they can be leveled flat Roofs: they can be left exposed and they will be wa terproofed in conventional way with a cement-lime-sand plaster Earthquakes zones: they can be used with a reinforced ring beam
Two Ways to Build Vaults:
Using a form to support the bricks during construction: this form is either made of wood or steel, and it can be re-used n Building free-spanning, that means without form : this way is also called the Nubian technique
Advantages
Time Saving: Vaults can be built with adobe, burnt bricks , CEB or stones Stability Study: The shape of a vault is crucial for the stability, and a stability study is often needed; be careful, a wrong shape will collapse Disadvantages
Need of Skilled Masons: Building a vault requires delicate hands of trained masons never improvise when building vaults, ask advice and specification from skilled people be careful, a badly built dome may collapse Need of Good Quality Materials: Vaults built with compressed earth blocks should be made of blocks of very regular thickness.
FREE SPANNING TECHNIQUE
The free spanning technique is an ongoing development of the Nubian technique that the Auroville Earth Institute is working on since a few years. It allows courses to be laid horizontally This technique combines also the use of vertical courses, like in th e withorizontal courses, vertical ones or a combination of both.the vault rises with horizontal ..
Limit of stability of the horizontal courses It is essential to study the location of the centers of gravity so that the weight of the masonry never goes beyond the Springer .The transfer of loads always takes the shape of catenary curves, and assumes the most direct way. The 6m span semicircular vault cannot be built horizontally anymore, as no force can balance the gravity forces. The c onstruction has to go on with vertical courses Load t ra nsfer in the shape of a catenary in an equilater al vault with a half dome.
Advantages of free spanning technique
The Free Spanning technique with horizontal courses presents an advantage compared to the Nubian technique: the glue is sandier and the quantity of glue is proportionally less, as the blocks are bigger. Therefore, the vault tends to crack less because there is less shrinkage due to the glue. y
Very flat segmental vaults and certain shapes of vaulted structures (i.e. groined domes) cannot be built with horizontal courses. The reason is that gravity forces cannot be balanced by the forces of the load transfer which assume the shape of catenary arches.
y
Therefore, the Free Spanning technique shows its full possibilities with a combination of horizontal and vertical courses.
y
This technique is the most efficient method to build vaulted structur es
Construction process
The Nubian technique needs a back wall to start sticking the vertical courses onto and the vault is built arch after arch. At the other end it is nearly impossible to lay the last course between the vault and the opposite wall. This technique was developed to start building the vault on both opposite walls at the same time. It presents the advantage of going faster, as more masons can work on the same structure. As both halves of the vault get closer to each other, there will finally be a gap between both, which has to be closed. The method presented hereafter allows bridging without support for this gap between both halves of the vault. 1. Start the vault on both sides 2. 2. Check the linearity of the last course 3. Grind a block to adjust its length 4. Apply 2-3 mm of glue on the block 5. Insert the block. Note the mortar on the sides 6. 6. Adjust the block by sliding it vertically 7. Grind the keystone to adjust its thickness 8. Apply glue on the 4 laying faces 9. Pour water on the keystone 10. Insert the key stone 11. Hit gently to get the keystone to wedge it 12. Wedge the keystone with stone chips
Basic principle of construction:
The basic principle is that The line of thrust should always remain in the middle third of the arch section and pier. Disregard of this rule will cause collapse. Compressed stabilised earth blocks must have been well cured for 1 month and left for drying for 3 more months. The reason is that earth blocks, even stabilised ones, always shrink because of clay in the soil. This time period is essential to allow the blocks to shrink fully. If this requirement is not followed and the blocks are used too early, they will shrink in the vaulted structures. Thus they will create tension in the masonry and t he latter will crack.
One should know that arches always tend to crack, even if they are well built. The reason is that the arch has a different behaviour than the masonry above it, and the line of thrust changes when the arch is loaded. Compressed stabili sed earth blocks used for building AVD should have a very accurate and regular thickness.
The Nubian technique of construction of vaults:
This technique came from Nubia , in the south of Egypt, hence the name Nubian. The Nubian technique was revived and disseminated by the Egyptian architect Hassan Fathy. The Nubian technique traditionally needs a back wall to stick the blocks onto. The vault was built arch after arch and therefore the courses were laid almost vertically. The binder, about 1-1.5 cm thick, was the silty-clayey soil from the Nile and the blocks used were adobes, the sun dried bricks. The unevenness of the adobes made it necessary to slightly incline the courses, so as to increase the adhesion by force of gravity. The basis of this technique is that the blocks adhere to each other with earth glue. The principle is that the dry block sucks by capillarity the water along with the clay of the glue which will bind the blocks. It is essential that the blocks are very thin, so as to have a high ratio sticking area/weight: the larger the area of the block is and the thinner the block is, the better will be the adhesion. When compressed stabilized earth blocks are used to build vaults using this technique, the course can be absolutely vertical as it is not needed any more to incline the courses for the adhesion. The even regularity of CSEB allows building with a cement -stabilized earth glue of only 1-2 mm in thickness.
Building a vault with the Nubian technique:
The back wall should be built first. It can have exactly the shape of the extrados of the vault or it can be quadrangular and the extrados of the vault will be drawn onto it. A template is needed to ensure the shape of the vault. It can advantageously be the future window frame on which are temporarily fixed some spacers to get the extrados shape of the vault. The template can also be made of welded Tor steel, which can be re-used afterwards for reinforced cement concrete. it is necessary to create a net of string lines between the back wall and the template. it is better to lay the net of string lines outside in the masonry. The reason is that any mistake in accuracy, with a block laid lower or slipping dow n, will not change the linearity of the string line. In certain cases, it is sometimes necessary to lay the string lines below the masonry. It is then indispensable to work with a very high accuracy and to leave always 1 mm gap between the blocks and the string line.
V ults and do
s built with th Nubian t
hniqu :
s and domes is like glue and should be more clayey than the one The binde for v for walls in order to stick the blocks prop erly against each other. Nevertheless this glue should not b e too clayey as it should not have an e cessive shrinkag e which will induce a lot of cracks in the structure later on. soil and sand should be sieved with 1 mm mesh. If th e mortar for walls (1cement: 6soil: 6sand) gives satisfa ctory results the following mix can successfully be used for vaults and domes: 1cement: 9soil: 3sand. If th e mortar for walls is 1cement: 5soil: 7sand, meaning that the soil is too clayey, the specification for vaults and domes could be 1cement: 8soil: 4sand or, if n eeded, less soil and more sand. If th e mortar for walls is 1cement: 7soil: 5sand, meaning that th e soil is too sandy, the specification for vaults and domes could be 1 cement: 10soil: 2sand or, if n eeded, more soil and less sand. If the soil is too sandy, no sand should be added and th e mix could be 1cement: 12soil.
If th e soil is really too sandy and the mix 1: 12 do es not give good results, the cement/soil ratio could be increased to 1cement: 11soil or 1cement: 10soil, or even more. Th e fluidity of the glue is essential for th e adh esion. The fluidity and thickness of th e glue varies according to the work:
THE EA TH IS SAC ED, AND AN SOIL FOR B ILDING IS A PRECIOUS MATERIAL: DONT WASTE IT. TREAT THE EARTH WELL. IT WAS NOT GIVEN TO YOU BY YOUR PARENTS. IT WAS LOANED TO YOU
BY YOUR CHILDREN. -KENYAN PROVERB
