Examples of foundation materials

 

Natural stone foundations

KEYWORDS:

Special properties	Suitable where concrete is expansive
Economical aspects	Low cost
Stability	Good
Skills required	Skilled labour
Equipment required	Masonry equipment
Resistance to earthquake	Medium to good; depends on overall design
Resistance to hurricane	Good
Resistance to rain	Good
Resistance to insects	Very good
Climatic suitability	All climates
Stage of experience	Widely used

SHORT DESCRIPTION:

• Stone foundations are made of rubble (undressed stone) or squared stone; similar construction is possible with broken brick and concrete from demolished buildings.

• The quality of mortar is of importance to achieve good strength. An example of a good mix is:

- 4 parts cement

- 1 part lime

- 12 parts clean sand

- sufficient water to make a workable mix.

• Construction should start on firm, uniform strong subsoil. It should not be started on grass, black fertile soil, filled up materials or mud.

• Under the foundation there should be a layer of lean concrete (min. 5 cm) or tamped sand; minimum depth 40 cm.

• In earthquake areas, reinforcement with wire mesh or steel rods is required, but professional advice should be sought. Further information: Bibl. 01.01, 01.05, 01.06, 20.05.

Stone in Earth Mortar (from Vorhauer, Bibl. 20.05); Stone in Cement Mortar (Bibl. 20.05)

RAMMED EARTH FOUNDATIONS

KEYWORDS:

Special properties	Only used for earth constructions on dry sites
Economical aspects	Low cost
Stability	Poor to medium
Skills required	Semi-skilled labour
Equipment required	Excavation and tamping equipment
Resistance to earthquake	Low
Resistance to hurricane	Low
Resistance to rain	Low
Resistance to insects	Low
Climatic suitability	Only very dry climates
Stage of experience	Traditional method

SHORT DESCRIPTION:

• Rammed earth foundations are made of well graded soil, preferably wig a stabiliser for water resistance and higher strength.

• The site must be well drained and great care is needed to protect the foundation from ground moisture, especially with a plastic foil or bitumen felt. Bitumen paint, or a facing of rubble stone or burnt bricks are alternatives.

• When in doubt about suitability of rammed earth foundations, they should not be used. Stabilized soil blocks can be used instead, but similar protective measures are necessary.

• Wherever possible, the earth foundation should be placed on a concrete footing.

• The foundation is made in formwork, in the same way as the walls: layers of 10 cm soil are tamped down to 6 - 7 cm, before the next layer is filled up.

Further information: Bibl. 02.06, 02.08, 02.19, 02.32, 20.05.

Procedure of Constructing a Rammed Earth Foundation (Bibl. 20.05)

Burnt brick foundations

KEYWORDS:

Special properties	Good alternative to concrete foundation
Economical aspects	Medium costs
Stability	Medium to good
Skills required	Masonry skills
Equipment required	Masonry equipment
Resistance to earthquake	Medium to good
Resistance to hurricane	Medium to good
Resistance to rain	Good
Resistance to insects	Good
Climatic suitability	Most climates, except consistently wet areas
Stage of experience	Widely used

SHORT DESCRIPTION:

• Burnt brick foundations are principally the same as masonry wall constructions, but begun under the ground, either directly on a bed of tamped sand or lean concrete, or on a concrete footing.

• A widened base is preferable to distribute the weight of the walls.

• Care must be taken to lay the bricks in perfectly level courses, and measures for waterproofing are important.

• A good mortar for masonry foundations is:

- 4 parts cement

- 1 part lime

- 12 parts clean sand

- sufficient water to make a workable mix.

• In earthquake areas, masonry foundations should be reinforced with wire mesh or thin rods. Professional advice should be sought. Further information: Bibl. 20.04, 20.05.

Burnt Bricks in Cement Mortar (Bibl. 20.05)

Concrete foundations

KEYWORDS:

Special properties	Strongest foundation
Economical aspects	Expensive
Stability	Very good
Skills required	Skilled labour
Equipment required	Form work, cement mixer
Resistance to earthquake	Very good
Resistance to hurricane	Very good
Resistance to rain	Very good
Resistance to insects	Very good
Climatic suitability	All climates
Stage of experience	Commonly used worldwide

SHORT DESCRIPTION:

• Concrete foundations on hard, uniform ground can be made without steel reinforcement, if not in an earthquake or hurricane prone area.

• All non-uniform andproblem soils require reinforced concrete foundations, especially in areas of medium to high rainfall and natural hazard regions.

• Depending on the strengths required, concrete mixes can vary from 1: 3: 4 (cement: sand : gravel) to 1: 4: 7, the higher proportion of cement being required for reinforced concrete.

• Water contents of fresh mixes should make them just easily workable. Excessive water leaves pores in the concrete, making it weak and water absorbent. Foundation trenches should also be properly wetted to avoid excessive absorption of the water from the mix.

• The concrete should be wet-cured for 3 to 7 days before building the walls. A damp-proof course should be laid between foundation and wall.

Further information: Bibl. 20.03, 20.04, 20.05.

Cast Concrete Foundations (Bibl. 20.05)

Reinforced Foundations

Placing concrete footing without shuttering: the reinforcement is laid after the lowest course of lean concrete is hardened. The richer second layer holds the reinforcement.

Foundation strip poured into shuttering of wood or plywood. These should be oiled before pouring concrete, to facilitate removal after hardening.

The finished foundation, with the trench filled up with the previously excavated soil and well compacted.

Foundations on Expansive Clay (Bibl. 20.03)

• Certain clayey soils respond to moisture movements (in rainy and dry seasons, moisture extraction by trees, etc.) with excessive swelling and shrinkage, which can severely damage foundations and consequently entire buildings.

• Damage can be avoided by either installing foundations which penetrate through the zone of ground movement, or by constructing foundations and superstructures which are tolerant of ground movement.

• Pile-and-beam-foundation: Small diameter piles are installed below the zone of clay movement; RC ground beams, which span between pile heads are constructed on compressible material (eg expanded polystyrene), which absorbs ground movement without affecting the beams and superstructure.

• Pad-and-beam-foundation: Pads are installed on stable ground below the movement zone; RC columns support ground beams, which are constructed in the same way as in the pile-and-beam-foundation.

Pile-and-beam-foundation; Pad-and-beam-foundation

Split-bamboo piles

KEYWORDS:

Special properties	Used for subsoil stabilization
Economical aspects	Low cost
Stability	Good
Skills required	Special training
Equipment required	Drop hammer
Resistance to earthquake	Good
Resistance to hurricane	Good
Resistance to rain	Good, helps to drain water
Resistance to insects	Low
Climatic suitability	All tropical areas
Stage of experience	Experimental

SHORT DESCRIPTION:

• Split-bamboo piles have been developed to improve the bearing capacity of soft compressible soils and to reduce settlements for various types of construction works, such as buildings, roads, etc.

• The hollow bamboo culms are filled up with loosely wound coconut coir and jute thread wrapped in jute fabric; holes in the culm permit the water in the soil to tackle in, thus drying out the soil and improving its load-bearing capacity.

Further information: Dr. M.A. Aziz or Dr. S.D. Ramaswamy, Department of Civil Engineering, National University of Singapore, 10 Kent Ridge Cresent, Singapore 0511; Bibl. 20.01.

Split-Bamboo Pile

Split-bamboo piles filled up with loosely wound coconut coir strands of about 6 mm diameter each tied up with spirally wound jute thread along its length and wrapped with a layer of thickly knit jute burlap have been successfully used. Treated split-bamboo steps were holed at random points and tied up together at regular intervals with galvanized iron wire after putting the coconut coir wicks inside along its entire length (Fig. 1).

FIGURE 1; FIGURE 2

Stabilized Area

These specially made split-bamboo piles were used in stabilizing the soft compressible subsoil of an actual construction site (Fig. 2) which consisted of a top layer of about 2 m thick soft to medium stiff sandy clayey silt underlain by a layer of about 6 m thick very soft silty clay which was again underlain by a layer of medium dense silty clayey sand. The split-bamboo piles, each about 8 m long, 80 to 90 mm diameter, were driven by a drop hammer at 2 m spacing in a square grid. After installation of the piles the entire area was covered with about 2 m surcharge of sandy materials (Bibl. 20.01).

Wooden post foundation

KEYWORDS:

Special properties	Used for spot and pile foundations
Economical aspects	Low cost, if sufficient timber is available
Stability	Low to good
Skills required	Carpentry and construction skills
Equipment required	Carpentry and masonry equipment
Resistance to earthquake	Low to good
Resistance to hurricane	Low to good
Resistance to rain	Low to good
Resistance to insects	Low
Climatic suitability	All, except consistently wet climates
Stage of experience	Traditional methods

SHORT DESCRIPTION:

• Wooden post foundations can only be used for lightweight structures, that is buildings made of timber, bamboo and/or other vegetable material.

• The main drawback of using timber for foundations is the risk of weakening due to attack by insects (mainly termites and beetles), fungus and rodents. Hence, protective measures are necessary. (See sections on Timber and PROTECTIVE MEASURES.)

• Timber posts can be driven into the ground, if the climate is predominantly dry, the site is well drained and destructive biological agents (mainly termites) are not common in the area. Further information: Bibl. 14.18, 14.22, 20.04, 20.05.

Simple Wooden Post Foundations (Bibl. 20.05)

Only for dry areas without termites.

Wooden Posts on Concrete Footings (Bibl. 20.05)

Only for dry areas without termites.

Wooden Posts without Ground Contact (Bibl. 20.05)