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close this book Compressed Earth blocks: Manual of design and construction
close this folder The project's building dispositions
View the document Types of wall
View the document Types of structure
View the document Foundations and footings
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View the document Reinforcement
View the document Floors: structures
View the document Jack arches and vaulting
View the document Roof classification
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View the document Characteristic strength of CEBS
View the document Safety and height to width coefficients
View the document Permissible constraints
View the document Building economics

Foundations and footings

Two types of problem

Particular care should be taken with the foundations and footings of a compressed earth block building and the building should be protected from two main types of problem:

- structural problems,

- problems linked to humidity.

This is because buildings constructed from compressed earth blocks, by the very nature of the material, are vulnerable to inherent structural risks or to humidity which can cause very serious damage. One must therefore be particularly vigilant in respecting the rules and codes of good practice which are specific to building with earth. This does not mean, however, that problems stem only from the nature of the material; they can arise because of external factors - differential settling, landslides, and natural disasters such as earthquakes and floods - which will be even more damaging if the building has been badly designed or built.

Choosing a system of foundations and footings

This will depend on the nature of the ground on which the structure is to be built and the type of structure envisaged. There is a danger of structural weakness when building on unstable or weak sites. This danger can be increased by a poor design (underdimensionning or insufficient strength for example) or if the foundations are badly built (located excentrically to the downward loads for example). On poorly-drained sites, humidity can increase the risk of structural weakness as this can considerably weaken the cohesion of the material, its strength and therefore that of the wall.

The problems outlined here should not, however, lead one to overdimension the foundations and footings, nor to make too great a use of reinforced concrete. The choice of foundations and footings should above all be well-suited to the nature of the ground, the nature of the building (private or open to the public), the nature of the loads and permissible overloads, the climatic constraints of the environment (rain, snow, wind, etc.), the building principles of the structure (the type and thickness of wall, whether or not there is a cellar or a sanitary pit, etc.).

The table in fig. 81 suggests structural designs for foundations and footings given the nature of the wall systems and the site ground.


Fig. 80: Key to figure 81.


Fig. 81: Summary table of structural concepts depending on the type of wall and the nature of the ground for the foundation.

Water and humidity: a danger not to be underestimated

Earth buildings, whether built from compressed Barth blocks or from other earth building materials, remain particularly vulnerable to water. The designer of earth buildings must be well aware of this danger and must not underestimate its importance. He should take appropriate measures to eliminate it. It is vital to remove sources of humidity, particularly at the base of walls and at the level of foundations and footings.


Fig. 82: Weakness due to prolonged exposure to humidity

Problems with foundations

At the base of the walls, from the foundations upwards, the danger of capillary rise can stem from several sources: seasonal fluctuations in the water table, water retention by plants or shrubs growing too close to the walls, damage to the clean water supply or waste water system, absence of drainage, a damaged drainage system, or stagnation of water at the base of the walls. A lengthy period of humidity can weaken the base of earth walls, notably when the material loses its cohesion and passes from a solid to a plastic state. The base of the wall may then no longer be able to support the loads and will be in danger of collapsing. Humidity also encourages the emergence of saline efflorescences which attack the materials and hollow out cavities where small animals can nest (insects, rodents, etc.) and this can further aggravate the process of wearing away which has already started.


Fig. 83: Weakness due to humidity undermining the base

Problems with footings

Above the natural ground level, the base of the wall can be attacked by water. This can be due to water splashing back, waterspouts, badly designed or damaged gutters, puddles being splashed by passing vehicles, washing the floors inside, morning condensation (or dew), a roadway gutter flowing too close to the wall, surface waterproofing (cement pavement) which prevents evaporation from the soil, a water-proofing render which causes moisture to be trapped between the wall and the render or the growth of parasitical flora (such as moss) and saline efflorescences.

All these problems are well-known and completely solvable. The informed designer should not on the other hand adopt a "shielding" approach, which might not only be very expensive but could also provoke the very weaknesses it seeks to avoid by excessive water-proofing. Above all the building must be allowed to breathe. The correct attitude is to resolve the problems by attacking their causes, not their effects. Appropriate solutions can only emerge from a good understanding of the nature of the various risks which we detail below.


Fig. 84: Weakness due to humidity resulting from excesive waterproofing.


Fig. 85: Key to figs. 82 to 93.

HUMIDITY RISKS

Infiltration without accumulation

This humidity risk is very common where the foundations are built on a permeable site, the geotechnical composition of which is predominantly sand and/or gravel. This type of site ensures good drainage away from the building. When it rains, water infiltrates rapidly from the surface to underground. This infiltrated water does not therefore get the chance to accumulate and stay in contact with the foundations. There is therefore no risk of sufficient capillary rise to reach the wall and cause damage.


Fig. 86: Infiltration without accumulation.

Infiltration with temporary accumulation

This risk frequently occurs in cohesive clay or silty soils. If the way the foundation is built is combined with good surface drainage, such as the one shown in diagrammatic form in fig. 87, in the form of an incline draining water away from the building, then this humidity risk is less great. In a cohesive soil, water penetrates less quickly from the surface to underground and towards the infill material. The latter, when it consists of permeable material (sand and gravel, for example) will only accumulate water temporarily, but this water will have difficulty in disappearing from the adjacent cohesive soil. Nevertheless, this kind of temporary accumulation can result in water suction occuring in the foundations for a short time.


Fig. 87: Temopary accumulation.

Infiltration with prolonged accumulation

This risk can occur in all types of soil with poor surface drainage, even permeable, sandy and or gravelly soils when the ground slopes towards the building (a situation to be avoided at all costs). In this event, the slope acts as a captor and accumulator of water, which then stays in prolonged contact with the foundations. Capillary rise follows, and this can be significant during the rainy season. This capillary rise, depending on the design of the building, can even reach the footings and the base of the wall. Serious damage can occur.


Fig. 88: Prolonged accumulation.

Capillary rise with or without infiltration

The most serious humidity risk occurs when the structure is in contact with or in close proximity to the water table. When the foundations are directly in contact with this water table, capillary action is continuous. This phenomenon is all the more sensitive when the soil is cohesive as the latter, once saturated with water, remains in a permanents/ate of humidity. In a permeable soil when the foundations are always above the level of the ground water, a normal cycle of evaporation can take place and the danger is less, but still present. The permanent exposure of the foundations to the risk of capillary rise represents a great danger of damage to the base of the structure.


Fig. 89: Capillary rise.

Infiltration without accumulation

Since the water disappears very quickly underground, all that needs to be done is to evacuate as quickly as possible the same amount of remaining water which penetrates towards the foundations. In this case, the foundations and footings can be subjected to the weak capillary risk resulting from the infiltration, but they must without fail be able to withstand the risks of water flow and/or water splash-back occurring at the base of the structure, at the surface. The use of materials such as stone, fired brick or rendered sand-cement block can reduce this risk. Any rendering can be restricted to the interior surface of the footing in order to leave the way open for evaporation towards the outside to occur and to avoid any humidity traces on the inside. It is not necessary to use impermeable materials for the foundations nor to install a drainage system.


Fig. 90: Several examples of how to treat a humidity risk resulting from infiltration without accumulation.

Infiltration with temporary accumulation

Since in this case the cohesive soil absorbs water, good surface drainage is required in order to evacuate water from the vicinity of the building. A pavement or banking up may suffice but care must be taken not to make these impermeable to migrations of humidity or moisture. This is unfortunately what often occurs when, with the best of intentions, a pavement made of too high dosage cement is built. This prevents even the small amount of water which remains at the level of the foundations from escaping, since it is trapped by the impermeable surface and so naturally moves towards the footings and the base of the wall. There is no need to use an impermeable render, or even a bitumen one, on the vertical face of the foundations, nor to build impermeable foundations, nor even a deep drainage system, since the water accumulation is only temporary. The structure must be allowed to breathe.


Fig. 91: Several examples of how to treat a humidity risk resulting from infiltration with temporary accumulation.

EXAMPLES OF SOLUTIONS

Infiltration with prolonged accumulation

When there is a danger of prolonged water infiltration, the water must be intercepted before it penetrates underground and evacuated as quickly as possible. The principle of drainage is perfectly appropriate here. Drains can be built right against the foundations but then the external vertical surface of the foundations will have to be rendered or made impermeable. They can also be installed at a distance in the order of one metre from the foundations, but on condition that they are located deeper than the foundations. These more distant drains are more efficient if they are used in conjunction with an evacuation incline at the base of the wall and if the top layer of the drain layer is bowl-shaped to aid evacuation. It is also prudent to add a horizontal anti-capillary barrier (e.g. polythene, bitumen, or high dosage mortar) between the footing and the earth block wall.


Fig. 92: Several examples of how to treat a humidity risk resulting from infiltration with prolonged accumulation.

Permanent capillary rise

The source of humidity is permanently present and occurs on both sides of the foundations which are in contact with the water table. On the outside, this humidity occurs as a result of the accumulated effect of rain and capillary rise. On the inside, it occurs as a result of capillary rise. Drains must be built against the foundations (which should be water-resistant) and even under the floor covering of the ground-floor if this is directly on the ground. Distant drains are not recommended. Water-proof horizontal barriers are also needed between the footing and the earth block wall. If the floor covering is directly on the ground it can be laid on a water-proof film which is itself unrolled on a rough surface of stones and rolled gravel which acts as an anti-capillary barrier. It is preferable to previously dig up the ground supporting the building and make sure that some permeable materials (e.g. gravelly-sandy soil) are present. If the building is over a sanitary pit, this must be ventilated.


Fig. 93: Several examples of how to treat a humidity risk resulting from permanent capillary rise.


Fig. 94: The use of cyclopean concrete for foundations and footings is an attractive solution from the technical and economic point of view.

Choice of materials and specifications

When digging foundation trenches, the first thing is to dig them as regularly and cleanly as possible. This means both looking for good ground, as far as possible, without having to dig too deep (which costs more) and making sure the sides of the trenches are straight. Traditional principles of laying out a building using wooden stakes and strings are very useful for ensuring that the foundation trenches are correctly traced out.

The second thing is to avoid allowing the newly-dug trenches to be exposed to bad weather for too long. This is why 4 to 5 cm of blinding concrete, dosed at 150 kg/m³, is recommended at the bottom of the trench. This will also help to start off the masonry work of the foundations. On top of this blinding concrete, the body of the foundations can be built from stones, fired bricks, full sandcementblocks, cement or cyclopean concrete, and in exceptional cases from compressed earth blocks stabilized at 10% if the risk from humidity is not too great. The footings can also be built from stone, fired bricks, rendered sand-cement blocks, cyclopean concrete masonry or compressed earth blocks stabilized at 8% there is not too much risk of humidity occuring as result of splashback. Concrete foundations should be dosed at 200 kg/m³; if they contain reinforcement, at 250 kg/m³; and if they consist in a reinforced concrete footing plate or ground-beam, at 300 kg/m³. In the latter case, the quantity of steel can be estimated at between 50 and 70 kg/m³, including 25 to 40 kg for the transverse reinforcement which absorbs tensile stress.

Using cyclopean concrete

For cyclopean concrete foundations, rubble stones are incorporated in successive layers of cement mortar which coats each layer of stone with a covering at least 3 cm thick. This type of structure is perfectly suitable for. a low-cost construction on good ground, but must be well done. Notably, the rubble stones should not touch each other, nor be located only at the sides of the foundations, in which case the central part of the foundation would be filled only with mortar, giving a weak structure.

Stones which take up the whole width of the foundation should be laid at regular intervals, forming a kind of toothing.

The other aspect to be considered is how much cement to use in cyclopean concrete which should be dosed at 250 kg/m³ (250 kg of cement, 400 litres of sand and 800 litres of gravel). Once the rubble stones have been laid in layers of concrete, 1 m³ of cyclopean concrete ultimately contains less cement that solid concrete (approximately 125 kg) which is interesting from an economic point of view. All in cases, the total width of the foundations should be at least 40 cm, and at least 20 cm thicker than the wall thickness, divided between both sides of the wall faces starting from the longitudinal axis. The height of the body of the foundations should be at least equal to half the width. If the foundations require an anticapillary water-proof layer, this can be made using highly dosed cement mortar (500 kg/m³), bitumen-based paint or a bitumen or plastic film if these materials are available.

Cyclopean concrete can continue to be used for the footings ; above the foundations, in which case the cyclopean concrete must be shuttered and the stones placed right up against the shuttering. The principle of toothing stones (approximately every 60 cm and in alternate rows - one at each corner and one in the middle) to ensure the solidity of a cyclopean concrete footing should be carefully checked on site.

Ring-beam at foundation level

When building on poor soils which are unstable and which may cause differential settling, a foundation ring-beam is recommended. This will stabilize the sides against potential movement in the foundations. These movements are essentially vertical, and as a result the foundation ring-beam will be designed like a beam with vertical bending moment. Such a ring-beam therefore has to be a beam with reinforcement running from top to bottom. At the same time if the body of the foundations is mainly built from masonry, it is possible to reduce the amount of steel used. By locating the reinforced concrete ring-beam halfway up the body of the foundations, one can assume that there is an area of compression above and below this reinforced steel and the whole can therefore act in both directions. This means using masonry which has perfect compressive strength and hollow sand-cement blocks cannot be used.

MATERIALS AND SPECIFICATIONS

To take one example, 3 cm² steel rods or 2 cm² high adherence steel rods, can be sufficient. The concrete coating of these steel rods should be at least 4 cm thick. The height of the reinforced ring-beam can therefore be reduced to 10 cm using 212 rods or 310 rods. The cement dosage should be a minimum of 250 kg/m³.

The principle of using a ring-beam in the foundations cannot be applied to small, single-storey buildings founded on good to medium strength soils (rocky soils, compact sandy-gravelly soils, or cohesive soils) and if loads are evenly distributed. In other cases, it is preferable to use the solution of a reinforced concrete ringbeam which is integrated into the foundations.