Concrete foundations

Contents - Previous - Next

For light buildings a curtain wall may be poured directly into a carefully dug trench 15 to 25cm wide. To have the finished wall extend above the ground, forms built of 50 x 200mm timber can be anchored along the top of the trench.

Figure 5.9 Curtain wall poured in a trench.

A relatively lean mix of concrete, 1:4:8, can be used. The concrete must be placed carefully to keep the walls of the trench from sloughing off and mixing in, thereby causing weak spots. If the soil is not stable enough to allow digging a trench form, a wide excavation and the use of simple forms will be required.

Additional information on ratios, materials, forms, placing and curing concrete will be found in Chapter 3.

Concrete Block Foundation

It is desirable for all dimensions of a block wall to be divisable by 225mm. this will allow full or half blocks to be used at all corners and openings without the need to cut blocks to odd lengths. Blocks must be dry when used or the mortar joints will not develop full strength.

Concrete block foundations should be started in a full bed of mortar on a poured concrete footing. A 1: 1 :5 ratio of cement-lime-sand makes a good mortar. The corner blocks should be carefully located and checked for levelness and plumb. After several blocks have been laid adjacent to the corners, a line stretched between the corners can be used to align the top outside edge of each course of blocks as shown in Figure 5.10. After the first course, face-shell bedding is used. That is, mortar is placed along the vertical edges of one end and the side edges of the top of the block. This will save up to 50% of the mortar and is about three-quarters as strong as full bedding.

Figure 5.10 Face-shell bonding in blockwall.

Masonry units must be overlapped so that the vertical joints are staggered in order to obtain adequate strength. Where small units such as bricks are used, the bonding must be both along and across the wall. However, blocks are only bonded longitudinally. Cross bonding is required only at points of reinforcement such as pilasters. A halflap bond is normal, but where necessary to permit bonding at returns and intersecting walls, this may be reduced to one-quarter of the block length, though not less than 65mm.

The strength of blocks of either dense or lightweight aggregate is sufficient for normal small-scale work, but where loading is heavy only dense concrete blocks are suitable. Hollow blocks may be used for load-bearing walls, but the courses directly supporting floor and roof structures should be built of solid construction in order to distribute the loading over the length of the wall and thus avoid the concentration of stresses.

The thickness, length and height of the wall determine its structural stability. Table 5.7 indicates suitable relationships for free-standing, single-thickness, unreinforced, concrete block walls not externally supported and not tied or fixed at the top and designed to resist wind pressure. Longer and higher walls and, for example, walls retaining bulk grain may need the extra strength of being tied to a pier or crosswall.

Figure 5.11 Reinforcing block walls.

Table 5.7 Stabilizing Hollow Block Walls

Thickness of wall Height of wall Maximum length of wall panel between piers, cross walls, etc.
100mm 1.8m 3.6m
150mm 3.0m 3.0m
215mm 3.6m 4.0m
215mm 4.5m 3.0m
305mm 4.5m 4.0m

Floating Slab or Raft Foundation

A slab foundation is a large concrete floor covering the entire building area through which all the loads from the building are transmitted to the soil. It is both building floor and foundation and is well suited to garages, shops, small stores, and homes without basements. The concrete floor and the foundation are cast in one piece. The slab is cast about 100mm thick and lightly reinforced at the top to prevent shrinkage cracks. Steel bars are placed at the bottom under walls or columns to resist tensile stress in these zones. Light surface slabs can also be used to carry lightly loaded structures on soils subject to general earth movement.

As with all foundations the centre of gravity of the loads should coincide with the centre of the slab. This is facilitated when the building has a simple regular plan with load-bearing elements such as walls, columns or chimneys, located symmetrically about the axis of the building.

Pier Foundation

Isolated piers or columns are normally carried on independent concrete footings sometimes called pad foundations with the pier or column bearing on the centre point of the footing. The area of footing is determined by dividing the column load by the safe bearing capacity of the soil. Its shape is usually square and its thickness is governed by the same considerations as for foundation footings. They are made not less than 1 1/2 times the projection of the slab beyond the face of the pier or column or the edge of the baseplate of a steel column. It should in no case be less than 150mm thick. As in the case of strip footings, when a column base is very wide, a reduction in thickness may be effected by reinforcing the concrete.

When piers are used to support prefabricated building frames of steel or laminated wood, the bolts for anchoring the frame to the piers must be grouted into the concrete and very accurately positioned. this requires skilled labour and supervision.

Figure 5.12 Design of floating slab foundation.

Figure 5.13 Simple rigid frame structure.

Post or Pole Foundation

For lightweight buildings without suspended floors, post or pressure-treated pole frames are suitable and inexpensive. The posts are placed in holes dug into the soil and a footing provided at each post. This is important since otherwise either gravity loads or wind uplift can lead to building failure.

The concrete pad under the pole provides the necessary support for gravity loads. The concrete collar around the base of the pole offers resistance to uplift. The pole is secured to the collar by several spikes driven near the base prior to placing the pole on the pad and pouring the concrete for the collar. While earth backfill should be well tamped to provide the greatest resistance to uplift a concrete collar, that extends to ground level, offers better protection against ground moisture and termites.

Bracing of the poles to the roof and other building frame members offers adequate lateral stability. Figure 5.14 illustrates the pad and collar design.

Figure 5.14 Pole foundation.

Pier and Ground Level Beam Foundation

As mentioned previously, this design may be chosen for application where safe bearing layers are so deep as to make a curtain wall very expensive. The ground level beam must be designed to safely carry the expected load. Ordinarily the beam is made 150 to 200mm wide and 400 to apart. First the piers are formed and poured on footings of suitable size. The soil is then backfilled to 150mm below the top of the piers. After placing 150mm of gravel in the trench to bring the level even with the top of the piers, forms are constructed and the beam is poured. The reinforcing shown in Figure 5.15 is necessary. The size and spacing must be carefully calculated.

Protective Elements for Foundations


Figure 5.15 Pier and ground level beam foundation.

Several steps can be taken to prevent ground or surface water from penetrating a foundation wall. If the building is located on sloping land where a footing drain can be terminated at ground level within a reasonable distance, the installation of a continuous drain around the outside of the foundation will reduce both the possibility of leaks and the lateral force of saturated soil bearing against the wall. The recommended drain design consists of 100mm drain tile placed slightly above the level of the bottom of the footing. The tile should be installed with little or no gradient so that the ground water level will remain equal at all points along the footing. Gravel is used to start the backfilling for the first 500mm and then the excavated soil is returned and tamped in layers sloping away from the wall.

The water resistance of poured concrete basement foundation walls may be improved by applying a heavy coat of bituminous paint. Block walls should be given two coats of cement plaster from the footing to above ground level and then covered with a finish coat of bituminous paint.

Moisture creeping up the foundation wall by capillary action can cause considerable damage to the lower parts of a wall made of soil or wood. While a mortar cap on top of the foundation wall usually provides a sufficient barrier, the extra protection of a ship of bituminous felt sometimes is required. To be effective such as damp-proof cause must be set at least 150mm above the ground and be of the same width as the wall above.

Pitch-roof buildings that are not equipped with eave gutters can be further protected from excessive moisture around the foundation by the installation of a splash apron made of concrete. The apron should extend at least 150mm beyond the drip edge of the eaves and be sloped away from the wall approximately 1:20. A thickness of 50mm of 1:3:6 concrete should be adequate.

Foundations for Arch or Rigid Frames

Additional resistance to lateral forces is needed for foundation walls supporting arch or rigid frame buildings. This can be accomplished with buttresses, pilasters or by tying the wall into the floor. Figure 5.16 shows each of these methods.

Termite Protection

Figure 5.16 Methods of strengthening foundations.

Subterranean termites occur throughout East Africa and cause considerable damage to buildings by eating the cellulose in wood. They must have access to the soil or some other constant source of water. They can severely damage timber in contact with the ground and may extend their attack to the roof timbers of high buildings. Entrance to unprotected structures is gained through cracks in concrete or masonry walls, through the wood portion of the house or by building shelter tubes over foundation posts and walls.

The main objective in termite control is to break the contact between the termite colony in the ground and the wood in the building. This can be done by blocking the passage of the termites from soil to wood, by constructing a slab floor under the entire building, and/or installing termite shields, treating the soil near the foundation and under concrete slabs with suitable chemicals, or by a combination of these methods.

Creepers, climbers and other vegetation likely to provide means of access for termites should not be permitted to grow on or near a building.

Chemical protection is useful if termite shields are not available, but is also recommended in combination with mechanical protection. Creosote oil, sodium arsenite, pentachlorophenol, pentachlorophenol, pentachlorphenate, copper napthenate, benzene hexachloride and dieldrin are the products predominantly used. The protective duration is 4 to 9 years depending on soil and weather conditions. Timber elements are impregnated before use. The timber surface is protected only if sprayed with insecticide prior to painting. Cracks, joints and cut surfaces must be protected with special care as termite attacks always start in such locations.

Slab on the ground construction: Firstly, the construction site must be carefully cleaned and all termite colonies be traced down, broken and poisoned with 50 to 2001 chemical emulsion. Secondly, after the top soil has been removed and any excavation is completed, poison should be applied at a rage of 5I/mē over the entire area to be covered by the building. The soil used as backfill along the inside and outside of the foundation, around plumbing and in the wall voids is treated at a rate of 61/m run and before casting the floor slab any hardcore fill and blinding sand should also be treated. Existing buildings can be given some protection by digging a 30cm wide and 15 to 30cm deep trench around the outside of the foundation. After having sprayed the trench with poison, the excavated soil is treated and replaced.

It is advisable to do the soil poisoning when the soil is fairly dry and when rain is not pending, otherwise there is risk of the chemical being washed away instead of being absorbed by the soil.

It is also advisable to cover the poisoned band of soil with concrete or with a substantial layer of gravel. This protects the poison barrier and helps to keep the wall clean and free of mud splashes. If the wall is rendered, it is preferable to poison any rendering that is applied within 30cm or so of the ground. To poison concrete or sand:cement mortar, simply use a 0.5 to 1.0% dieldrin emulsion instead of the usual mixing water. There is no effect on the amount of water required or the binding strength of the cement.

All preservatives are toxic and should be handled with care. Some are extremely toxic if swallowed or allowed to remain in contact with the skin. A recommendation for first-aid from the supplier of the preservative should be insisted upon. When using dieldrin, aldrin or chlorodane, children and animals should be kept away from the area where treatment is to be carried out.

Termite shields: The termite shield should be continuous around the foundation irrespective of changes in level and should be made of 24 gauge galvanized steel. The edge of the shield should extend horizontally outwards for 5cm beyond the top of the foundation wall and should then bend at an angle of 45° downwards for another 5cm. There should be a clearance of at least 20cm between the shield and the ground. All joints in the shield should be double locked and properly sealed by soldering or brazing or with bituminous sealer. Holes through the shield for anchor bolts should be coated with bituminous sealer and a washer fitted over the bolt to ensure a tight fit.

Protection of existing buildings: A building should be regularly inspected inside and out and especially at potential hiding places. The outside should be checked for such things as staining on walls below possibly blocked gutters, accretion of soil, debris or added-on items like steps which might bridge the termite shield. Ground-floor window and door frames and timber cladding should be probed to discover decay or termite damage. All timber, whether structural or not, should be inspected, special attention being paid to places which are infrequently observed such as roof spaces, under-sides of stairs, builtin cupboards and flooring under sinks where there may be plumbing leaks.

Extensively damaged wood should be cut out and replaced with sound timber pre-treated with preservative. In the case of decay the source of moisture must be found and corrected and where subterranean termites are found, their source of entry must be traced and eliminated. Termites within the building must first be destroyed. The treatments to be applied include some measure of soil poisoning, the provision of barriers and the surface treatment of timber and wood-based materials.

In the case of drywood termites fumigation is the only reliable method of extermination and this should be carried out by trained men under proper supervision.

Figure 5.17 Termite protection.

Contents - Previous - Next