|Compressed Earth Blocks - Volume II. Manual of design and construction (GTZ, 1995, 148 p.)|
Innovation in a rural area
Senegal is a vast country with a tradition of building with earth which is still visible today in most rural housing; but major changes are apparent occurring not only in building materials (with the sand-cement block and corrugated iron replacing earth and thatch) but also in the shape of the building, adopting models which are external in origin. These mutations are more and more influencing the rural built landscape which is swept up in the current of very rapid change emanating from urban models (Dakar). Nevertheless' in many situations far away from the capital, there remains a gap as far as access to modern building materials and technologies is concerned. These new «modern» solutions are still very often out of reach of the great majority of the population, physically (problems of transport) and economically (high cost). The use of local materials is once again being considered, but aiming at introducing significant improvements.
SOCIAL CENTRE AT OURO-SOGUI, SENEGAL
Social facilities in a village community
During the year 1987-88, the village community of Ouro-Sogui, a small town located in north-east Senegal, not far from the Mauritanian frontier and approximately 500 km from Dakar, designed a project for a social centre with guest house facilities. Anxious to use building and architectural solutions which would remain accessible whilst introducing significant improvements to traditional practices, the village decided to opt for using the compressed earth block which bridged the gap between traditional and modern building. This approach emerged through a link maintained between the village community and expatriate French residents who were able to obtain information about recent developments in building with earth. The Association for the development of Ouro-Sogui (ADO) contacted the municipality of the town of Valence (in Drome, France), which found the project attractive and rapidly responded to the appeal of Senegalese community by creating it own association «Drome Ouro-Sogui». It is in the context of these associations that this small bilateral cooperation project took place. The project requested the help of CRATerre-EAG and resulted in a combined construction, production and training site for the building of the social centre and guest house of Ouro-Sogui.
TECHNICAL FEATURES OF THE PROJECT
Social centre and guest house: total habitable surface area: 280 m², achieved in three work phases.
- First phase: pilot building and training phase whilst building the guest house: 44 m² of habitable surface area.
- Second phase: first workshop block: 83 m² of habitable surface area.
-Third phase: second, larger workshop block, meeting room and dry latrine facilities: 152 m² of habitable surface area.
Implementation: Association for the development of Ouro-Sogui (ADO)
With the help of the Drome Ouro-Sogui association (ADOS) and of CRATerre-EAG.
Construction: local masons and labour.
Foundations: Cyclopean concrete poured into trenches dug in previously compacted gullies: lateritic rubble stones and mortar dosed at 150 kg/m³.
Footings: three courses of compressed earth blocks stabilized at 8% and laid in an earth mortar stabilized at 10%.
- External walls: compressed earth blocks stabilized at 4%, laid in earth mortar stabilized at 6%. Block dimensions: 29.5 × 14 × 9 cm. Walls built 29.5 cm thick using a header and stretcher bonding pattern.
- Internal walls and partitions: in compressed earth blocks (as for external walls) but 14 cm thick using a stretcher bonding pattern.
Ring-beam: made from wood, using 27 mm local hard red-wood planks. Wood treated against insect and termite attack.
Roof structure: made from wood, using 22 cm wide local hard red-wood planks. Rafters made from planks previously sawn lengthways into 11 cm widths and nailed into place. 6 × 8 mm purling. Gable-end rafters fixed to the wooden ring-beam. 22 cm local hard red-wood edging planks, sawn down into 11 cm widths.
Roof covering: 23 mm galvanized corrugated iron sheeting 200 x80 cm.
False ceiling: woven organic material (e.g. reeds or palmyra branches).
For reasons linked to the introduction of a new building material and new building techniques, combined with the need for an overall cost which remains within a cheap and accessible range, the architectural design of the project choses a simple approach. In addition, on-site training as well as the time-tabling of the project in several phases make it vital to consider building approaches which would be easily assimilated and reproduceable by the local population. The building and architectural concept of the project was tried out during the building of the guest house. This consisted in a double bay system, with a covered verandah, the longest sides of which correspond to the direction in which the rafters of the roof structure are laid, suggesting the principle of future extensions lengthways by adding further bays. Only the masonry of the peripheral walls is loadbearing and supports the roof rafters, the interior walls serving only to divide off spaces. Buttresses on the outside elevations help to improve the stability of the walls and to take up the roof rafters using braced double-legged brackets. For later phases of the building, the same building and architectural solutions were adopted, which ensured good quality workmanship after the first phase of experimentation, of training and of acquisition of knowledge and skills on the part of the local population.
Fig. 284: Site plan for the project as a whole.
Fig. 285: Plan of first phase of work: guest house and first workshop block.
Fig. 286: Guest house: plan of masonry walls 29.5 and 14 cm thick.
Fig. 287: Guest house: plan of Cyclopean concrete foundations.
The north-east region of Senegal is marked by a rainy season from May to October with maximum temperatures reaching 40°C and relative humidity varying between 60% (min) and 100% (max). The prevailing winds, which bring rain, blow from the south-west. This wet season is succeeded by a hot, dry, very sunny season from November to April, with temperatures easily reaching 40° C. The «harmattan» wind which then blows from an easterly northerly direction, accentuates these dry conditions and raises a great deal of dust. The proximity of the desert regions of Mauritania explains the wide temperature range between day and night time.
These extreme and markedly seasonal climatic conditions demand that buildings be particularly well adapted from a climatic point of view.
Fig. 288: Elevation of elevations of guest house.
- There must be minimal exposure to bad weather and direct sun. An option meeting this requirement is to be closed to the east (exposure of blind gable walls), protection from the sun for south-facing walls (wide roof overhangs or a verandah), and protection from dust coming from the north.
- Natural ventilation must be used to the full with north-south breezes blowing through the building, shaded areas stimulating convection of the elevation, pierced openings but which still offer protection from dust, and high ventilation beneath the ridge to enable trapped heat to escape.
- Thermal inertia must be exploited notably to lessen the temperature differences between day and night time. The 29.5 cm thick masonry fulfils this role by retaining heat accumulated during the day.
Nailed wood roof structure
The principle of orienting the gable walls of the building on an east-west axis with a span between gutter walls of nearly 7 metres, prolonged by a roof overhang of 50 cm on either side suggest the need to design the roof structure using wooden planks nailed together. This approach can also be used, with the same attachment system, to build a ventilation ridge and bracing anchoring systems for attaching the rafters to the earth block walls.
The roof structure as a whole is made out of local hard red-wood planks 22 cm wide, sawn down lengthways into 11 cm wide planks. This is easier to assemble when laid out on a flat area using indicator wedges. To make them easier to transport and to put in place, the trusses are assembled in two halves. The end trusses are put into place first and then the intermediate ones. Rigidity is ensured by nailing on purlins using 6 × 8 cm battens. Each truss is attached to the masonry by bracing elements connected to a transverse bracket resting on the top of the elevation buttresses.
Fig. 289: Three transverse sections of the guest house, at different levels (see plan). Note the false ceiling of woven organic material.
Fig. 290: The north and south-facing elevations are designed in order to ensure maximum cross-ventilation. This is achieved by the openings (doors and windows) and by an upper ring of fixed open-work insets in the form of wooden frames to which are fixed organic material (e.g. reed or palmyra leaf) panels.
Walls-roof structure junction
The roof structure is held in place correctly notably by being placed on the gable walls which «brace», the thickness of the 29.5 cm walls. To this bracing system using trussed rafters is added the thickness of 22 cm local hard red-wood edging planks following the slope of the gable and acting as a formwork to pour a topping mortar between the trussed rafters, up to their uppermost edge. Anchoring within the gutter walls is achieved by using false slanting tension jambs, also bracing, which take up a vertical jamb linked to the trussed rafters and supported on the inner edge of the wall. The wooden pieces of the false brackets are screwed together and screwed into the masonry using rawlplugs.
Fig. 291: Section from gable wall to gable wall: Note the longitudinal rigidity through slanting pieces nailed into place between the tie-beam and the trussed rafters.
Fig. 292: Detail of support of false ceiling. The underface of the tie-beams of the different rafters of the roof structure enable small section supports for the false ceiling to be attached.
Fig. 293: bracing rafter of gable wall with false brackets for attaching to the wall.
Ring-beam - wooden lintel
A wooden ring-beam is placed on the walls; this also acts as a lintel for the external doors and windows at a height of 2.10 m above finished ground level.
The ring-beam is made out of 27 mm thick local hard red-wood planks which have been treated against insect and termite attack.
Two plank widths, 20 cm and 10 cm, are placed one above the other giving the ring-beam a total width of 30 cm. The planks are placed one above the other in such a way as to avoid joints occurring one above the other. They are then nailed together with 60 mm nails. At the corners and wall junctions, they are nailed together in the middle of the wood. To improve the connexion between the ring-beam and the masonry, with mortar, the surface of the planks is roughed down with an adze.
Fig. 296: Plan of laying of wooden ring-beam around the top of the 29.5 cm thick walls.
The ring-beam is prepared on the ground but attached to the walls and put into place on a bed of mortar.
The CEB, a vector of industrial cooperation
At the request of the Association of African Architects, the United Nations Industrial Development Organisation (UNIDO), and the Centre for Industrial Development (CID), pooled their efforts to launch a programme for the promotion of industrial investment projects in the building materials sector in Africa. A meeting was then organized in France between African building promoters (from Benin, Cameroon, Congo, Guinea, Togo, Zaire), the CFATerre-EAG team and manufacturers of production equipment, under the patronage of UNIDO and CID, on the theme of investment criteria and technical selection of equipment for the earth building industry. On this occasion, it was decided to launch an industrial cooperation initiative with the African countries invited. In December 1988, SICAD, and then in January 1988, the General States of AFRICA BAT, enabled this cooperation project to take concrete form, notably with Zaire.
A SCHOOL IN KINSHASA, ZAIRE
Compressed earth block construction at the service of small contractor promotion
In Zaire, there is clear evidence of a significant deterioration in the national built heritage, in both rural and urban contexts where living conditions are often very precarious. To this evidence can be added that of an increase in the costs of building materials which are increasingly inaccessible to the population. The lack of foreign currency to encourage the importation of building materials or local investment limits the possibility of industrial development in the building sector. Faced with this situation, the state of Zaire has launched a national policy for the promotion of small enterprises with good job-creation potential, notably in rural areas. This policy also aims to mobilize the wide-scale use of low-cost building materials and technologies requiring little capital investment. With this in mind, consideration has been given to transferring compressed earth block technology, at a decentralized level, to small contractors and local communities. Nevertheless, such a transfer could not be envisaged without a preliminary phase of information and technological training, in aspects of both production and construction. This then was the aim of this pilot project for a school in Kinshasa, in the context of a joint programme run by UNIDO/CID/Wallone region/CRATerre-EAG, together with 10 Zairian contractors, on the «Promotion of industrial cooperation in the building materials sector».
TECHNICAL FEATURES OF THE PROJECT
School project, combining production, training and site work on the production of earth blocks (training in a brickworks), with the design and construction of a demonstration building.
Project implemented with the support of: UNDP, the Department of Public Works, Urbanism and Housing of Zaire, the Mama Mobutu Foundation and Appro-Techno.
In collaboration with: ANEZA, OPEZ and SOFIDE.
With the participation of the following companies: EGEDEZA, GTAC, LOGEC, FINDATION, MONY, NZOLANTIMA, LA SIDELA, TRAGEMA-ETAZ, and the following NGOs: ECZ and the Salvation Army.
Building: neighbourhood school consisting in one 52 m² classroom.
Foundations: Reinforced concrete with peripheral ground beams, on a rubble infill, crushed and tamped.
Wall masonry: compressed earth blocks measuring 29.5 × 14 × 9 cm. Walls 29.5 cm thick, using a header and stretcher bonding pattern until the 7th course, then 14 cm thick until the top edge of the wall (see coursing plans).
Roof structure: central wooden truss with trussed rafter and bracing tie-beams. Exterior prolongation with overhang brackets for the roof overhang and small lateral porches. Roof covering: galvanized corrugated iron sheeting.
The pilot building undertaken in Kinshasa was the first phase of the building of a much larger number of schools. The site plan (fig. 301) shows the layout of a group of four classroom modules designed using the same building and architectural principles as module 1, which was built during the pilot phase. To this group of classrooms is added an administrative and service building which repeat the main features of the classrooms, whilst using a larger, tripped roof. The building principles used by the project are designed to be easy for the «trainee» enterprises to build them, whilst at the same time demonstrating a configuration which could be used to implement various project designs. The 29.5 and 14cm thick masonry walls, using vertical stiffening in the form of buttresses or pillars integrated into the walls prove well suited to larger sized buildings and provide a suitable solution to ensure the stability of the walls, notably to overcome problems of relative height to width. The roof structure, with a central truss and purlins resting on the gable-end walls, includes overhang brackets to support a roof overhang which provides protection from the sun and from bad weather and is well suited to the climatic context.
Fig. 301: Site plan of the whole school project with its four classrooms and its administrative end service building.
Fig. 302: Plan of classroom with two lateral access entrances with porches. Note the possibility of using the wall-space between the buttresses for storage (shelving).
Fig. 303: Plan of foundations using compacted infill and reinforced concrete peripheral ground beams.
Fig. 304: Coursing of bonding pattern of the first four odd number courses 29.5 cm thick (footing).
Fig. 305: Coursing of bonding pattern of the first three even number courses 29.5 cm thick (footing).
Fig. 306: Coursing of bonding pattern of the next eight even number courses 14 cm thick with interior buttresses and stiffening pillars for the gable or to support the roof structure.
Fig. 307: Coursing of bonding pattern of the next seven odd number courses 14 cm thick with interior buttresses and stiffening pillars for the gable or to support the roof structure.
Fig. 308: Coursing of bonding pattern of the last three odd number courses 14 cm thick with buttresses, stiffeners and pillars up to the top edge of the wall.
THE BUILDING CONCEPT OF THE PROJECT
The building of the masonry walls adopts the solution of a massive stabilized compressed earth block footing, built 29.5 cm thick up to the height of the window sills, i.e. up to the seventh course of earth blocks. The thickness of the window sills, built from fired brick, is included in the sixth course. From the eighth course onwards, the masonry is built up 14 cm thick, whilst at the same time taking care to ensure the stability of the walls, and their height to width ratio, by including in the thickness of the walls 29.5 cm thick buttresses located at the jamb opening angles. At the gable-end wall, an axial stiffening pillar 29.5 cm thick and 45 cm wide is bonded into the wall masonry.
Finally, two massive pillars, 29.5 cm thick and 91.5 cm wide, are also bonded into the gutter walls in the median transverse axis; these are designed to receive the bracing tie-beam of the roof structure.
This stability of the walls is also reinforced by using a peripheral ring-beam, poured at the height of the twenty-third course of blocks. The concrete for this ring-beam is poured into special compressed earth blocks, with longitudinal grooves in which lie a single layer of rods.
The classroom is extended in both directions from the gutter walls, on the classroom access side, by two small open verandahs, which are covered by a direct prolongation of the roof structure beyond the lower edges of the roof, thanks to a simple false console system, anchored into the masonry wall. These details will later be precisely defined.
Fig. 309: Elevation of gable-end elevation and main elevation of classroom.
Fig. 310: Transverse section showing the wall masonry and the roof structure.
Fig. 311: Longitudinal section of classroom. Note
the principle of a median roof truss end purlins resting on the gables.
Fig. 312: Plan of roof structure detailing the wood sections and the principle of horizontal wind-bracina by dridging the purlins.
Fig. 313: Elevation of roof structure and details of roof and verandah brackets.
Wooden roof structure
The roof structure is designed using the principle of a single median truss on which rest the purlins (assembled with nailed wooden gussets) which ran either side of it to join the gable-end walls. These purlins then support the rafters on which the roofing sheets are laid.
The roof structure is entirely built from local 7 × 15 wood, so that the truss have to be built using tie-beams and trussed rafters bracing the king post and the struts. All the parts are nailed together.
The truss is laid out, assembled and put together on the ground and then put into position, temporarily held in place by wooden props. The bracing tie-beam rests on the top of the masonry pillars intended for this purpose, but with wooden wedges in between. The 7/15 purlins are then laid and made rigid by nailing in the bracing elements on their upper edge, which hold them in position.
The overhang consoles of the roof along the gutter walls, as well as that of the two verandahs, are fixed to the bracing tie-beam of the truss by a vertical piece of wood on the outside of the wall. This vertical piece of wood is also strengthened bye horizontal piece of wood going through the wall and supporting the slanting part of the bracket. Two 7/15 exterior bracing posts keep the verandah roofs stable.
Fig. 315: Plan, elevation and vertical section of a «naco» frame window opening.
Fig. 316: Plan, elevation and vertical section of wooden door opening.
These use a classic design with independent masonry breasts for the windows and depressed arches for the lintels. Wooden frames fitted as the masonry went up and attached using barbed wire laid into the masonry mortar along the jambs receive the glass slatted «naco» frames or wooden doors.
Culture and architecture: a new birthright for earth
The cities of contemporary Saudi Arabia reflect the main features of the «international style». And yet, hidden amongst high-rise offices or international hotels reminiscent of «down-town» American cities, there still sometimes exist old neighbourhoods, nestling around ancient palaces and mosques, which quietly restore the image of what was, only a few decades ago, Saudi architecture. The most ancient buildings of Riyadh, such as the military citadel of Al Masmak, or the nearby old historic city of Diraiyah, fifteen kilometres to the north-east of the capital on the Wadi Hanifa, and the houses of Najd are all built with earth. Similarly, the architecture of the regions of Najran and Assir, to the south, bear witness to an ancient and perfectly mastered art of earth building. The birthright of earth architecture in a resolutely modern environment is today linked to the revaluation of the country's cultural heritage to which Saudi Arabia is turning a new and carefully attention.
EXHIBITION PAVILION IN SAUDI ARABIA
Pavilion for a national traditional festival
The national traditional festival of Janadriyah (near Riyadh) is generally inaugurated during the last week of Shaban, just before the start of Ramadan. The event serves to reaffirm traditional values which are reflected in the festival by the expression of a multitude of craft activities which have been passed down through generations, such as cabinet-making, weaving, leatherwork, pottery, wood engraving and painting, dancing, singing and theatre. A large number of people from nearly every corner of Saudi Arabia assemble at Janadriyah to celebrate these values and this craftsmanship in a festive atmosphere. On the occasion of the 1988 festival, the General Secretary of the Royal Commission of Jubail and Yanbu was invited to set up a permanent exhibition of the regional products of these two towns. It was decided to adopt the idea of turning once again to the tradition of building with earth whilst adapting it to the present-day requirements of workmanship offered by contemporary technologies. The stabilized compressed earth block met this criterion and a project agreement was reached in December 1987, in the context of a collaboration between the French Embassy at Riyadh and the Royal Commission of Jubail and Yanbu.
TECHNICAL FEATURES OF THE PROJECT
An exhibition pavilion, with a covered surface area of 200 m².
Owner: The Royal Commission of the towns of Jubail and Yanbu.
Design: Ibrahim Aba-Alkhail, architect from Riyadh, in collaboration with CRATerre-EAG.
Implementation: CRATerre-EAG with the help of Saudi enterprises and masons.
In collaboration with: the Joseph Fournier University of Grenoble, the King Abdulaziz University and the King Saud University of Petrols and Minerals (materials analysis).
With the support of the department of international affairs of the Ministry of Culture, of Communication and of Major Works; of the French Embassy at Riyadh (cultural service); and of the Georges Pompidou National Centre for Art and Culture, Paris.
- Foundations: reinforced concrete ground beams.
- Floor: reinforced concrete.
- Wall masonry: stabilized compressed earth blocks measuring 29.5 × 14 × 9 cm. Loadbearing walls 29.5 cm thick using a header and double stretcher bonding pattern. 45 cm thick pillars supporting interior arches or reinforced concrete lintel beams over the interior patio. 14 cm thick roof parapets.
- Roofs: mixed system of terrace roofs, using compressed earth block vaulting and reinforced concrete girders, and compressed earth block cupolas (at the four corners of the building) on pendentives. Water-proof render using bitumen and cement mortar over mesh.
The architectural demands of the project are modest - no more than 200 to 250 m² and not very complicated. The requirement is for a design for a suitably lit exhibition area. Ventilation has also to be provided, with «naturally», inspired solutions being preferred to mechanical machinery. The programme insists on the design of a building in keeping with the expression of an architectural tradition which can be celebrated in the context of the Janadriyah festival, by emphasising the use of local materials and traditionally-inspired decorations, whilst at the same time not merely imitating traditional building forms and techniques.
The architectural aspect finally exploits the principal of a general plan in the form of a square, giving access to a succession of exhibition spaces around a central open courtyard (or patio) which can be used by visitors crossing or for external exhibitions. This inner-facing part is in perfect harmony with Saudi architecture. In addition it enables natural ventilation to be exploited, by playing on hot air convection and the air movement created between the small external elevation openings and the open courtyard. The wall mass, terrace and domed roofs provide thermal resistance in keeping with natural cooling principles.
Fig. 319: Plan of square exhibition pavilion, with cupolas at each of the four comers connected by vaulted spaces around an interior courtyard.
Fig. 320: Elevation of pavilion elevations and sections of exhibition galleries (M) and courtyard (BB) showing the use of flat and domed roofs.
Fig. 321: Coursing of wall bonding patterns and arch and cupola pillars, for odd number courses.
Fig. 323 a: Bonding patterns of pillar and of corbel (courtyard elevation).
Fig. 323 b: Bonding pattern of corbel.
Fig. 323 b: Bonding pattern of corbel.
The massive pillars 45 cm thick and 1.07 cm wide which support the concrete lintel beams at the centre of each interior elevation of the courtyard consist of 24 courses of blocks, taking them to a height of 2.64 m. The bonding pattern uses headers and stretchers, laid at right angles to each other from one course to the next, with 3/4 blocks used to face the sides of the pillars. the bonding pattern for the corbels at the top of the pillars in four courses (25 to 28) uses the same principle with more 3/4 blocks for the last course, the widest, beneath the concrete beam.
Fig. 324: Coursing of wall bonding patterns and arch and cupola pillars, for even number courses.
The design of the vaulting roof system for the flat roof terraces and of the cupolas at the four corners of the building, all heavily loaded with stabilized compacted earth, exert strong forces on the walls and demand the use of a ring-beam. This overcomes any risk of structural cracking and directs the downward transmission of loads and forces vertically onto the walls. Special ring-beam blocks are made, with a longitudinal gully to incorporate a classic reinforced iron ring-beam with 2 sayers of 8 mm diameter rods and 6 mm stirrups every 30 cm.
Fig. 325: Detail of ring-beam design.
Fig. 326: Detail of water-proofing of flat roofs.
Fig. 327: Detail of working drawing for foundations.
Water-proofing the roofs
This is done using the classic way, i.e. tamped stabilized earth followed by a layer of cement mortar, of bitumen felt and of rolled gravel. The sides of the cupolas are infilled until the surface of the roof is levered with their summits so that they can receive the same water-proofing treatment.
Terrace roofs and vault and dome roofs pose a major problem for good drainage and rainwater runoff. In order to ensure good runoff, the spaces between the sides of the domes and the walls are filled with compacted earth, till they are flat and levered with the summit of the cupolas. Water-spouts are used for each roofing system separately.
The projection of the water away from the external wall is ensured by the large size of the water-spouts and their position on a corbel, on the elevation, using a special block shaped to take the slant of the spout, which both improves their stability and increases their length accordingly. Particular care is taken with the waterproof coating of the water-spouts, with interior facing, on the roof side, with multilayer protection and with a sandcement mortar.
Fig. 332: Vertical section of the external wall from the ring-beam to the merlon.
Fig. 333: «Exploded» view of the masonry structure of the pavilion.
The finishings of the building
Particular care is taken with the finishings of the exhibition pavilion as a whole. Lighting combines artificial lighting, with spotlights placed under the vaulting and at the springpoint corbels of the pendentives of each of the cupolas, with natural light from the interior patio. Painted bands of traditional motifs inspired by the decor of the dwellings of the ancient city of Diralyah (located 15 km north of Riyadh), as well as the decoration of the small triangular ventilation openings, lend a traditional touch, but not to excess. Finally, the great circular doorway of the main entrance to the pavilion is also richly decorated with multicoloured geometrical motifs, in the form of arabesques in the great tradition of Saudi painted decoration. This very high quality work is carried out by the Saudi artist, Ali Al Rezeza, who contributes the sculpture of the plaster mouldings all around the entrance doorway.
The exhibition and its impact
Traditional craft products from the region of Jubail and of Yanbu (weaving, pottery, leatherwork, tools, arms) from various periods were assembled during the construction of the pavilion and then exhibited, together with photographs showing the history of the two towns. A special section of the exhibition was devoted to the story of the construction of the pavilion and proved of particular interest to the public. A great many people visited the exhibition, confirming the favourable impact of this demonstration operation, which had both a cultural and a technological dimension.