Topic 2 : Foundation

3.1 Introduction

Loading in buildings consist of the combined dead and imposed load which exert a downward pressure upon the soil on which the structure is founded. This in turn prompts an active force in the form of an upward pressure from the soil. The structure is in effect sandwiched between these opposite pressure thus the design of the building must be able to resist the resultant stresses set up within the structural members and the general building fabric. Hence supporting subsoil must be able to develop sufficient reactive force to provide stability to the structure to prevent failure due to unequal settlement and to prevent failure of the subsoil due to shear.


3.1.1 Definition of Foundation
A foundation is the base on which a building rests and its purpose is to safely transfer the load of the building to a suitable subsoil.

3.2 Choice of foundation type

The choice and design of foundation for domestic and small types of buildings depends mainly on two factors. The total loads of the building are taken per metre run and calculated for the worst case.



The nature and bearing capacity of the subsoil can be determined by:
a. Trial holes and subsequent investigation.
b. Bore holes and core analysis.
c. Local knowledge.

3.3 Types Of Foundation
Foundation is usually made of either mass or reinforced concrete and can be classified under two headings:

a) Shallow foundation
Those which transfer the loads to subsoil at a point near to the ground floor of the building such as strips and raft.

b. Deep Foundation
Those which transfer the loads to a subsoil some distance below the ground floor of the building such as piles.

3.3.1 Raft Foundation

Raft foundation is often used on poor soils of lightly loaded buildings and is capable of accommodating small settlement of soil. In poor soil the upper crust of soil (450-600mm)is often stiffer than the lower subsoil and to build a light raft on this crust is usually better then penetrating it with a strip foundation.

Figure 3.1: Raft Foundation

Figure 3.2: Raft Foundation for Wall
(Souce:Fig. 3.1 and fig 3.2:Chudley,R. (1999), Consctruction Technology, Addision Wesley; Longmans)


3.3.2 Pad Foundation

This type of foundation is used to support and transmit the loads from piers and column.The most economiced plan shape is a square but if the columns are closer to the site boundry, it may be necessary to use a rectangular plan shape of equivalent area. The reaction of the foundation on the load and ground pressures is to cup, similar to a saucer and therefore main steel is required in both directions. The depth of the base will be governed by the anticipated moment and shear force, the calculation involved is beyond the scope of this volume.




Figure 3.3: Type of Pad Foundation
(Souce: Fig 3.3: IKRAM Note)


3.3.3 Strip Foundation

The oldest and the most common form of foundation is a strip foundation where a trench is excavated, concrete placed in the bottom and the wall built upon it. The depth is determined by the need to place the strip below the level where expansion due to frost will affect its stability (usually 1m) and the nature of the sub-soil. The width is governed by the relationship between the imposed load and the bearing capacity of the ground and also by the practical necessity of making it wide enough for a man to work it.




Figure 3.4: Strip Foundation
(Souce:Fig 3.4 Holmes, R.(1995), Introduction To Civil Engineering Construction, University of the West of England, Bristol.)




3.3.4 Buoyancy foundations

If a building is required to have a basement storey, the formation of that basement will involve the removal of a large quantity of sub-soil. The mass of this sub-soil, in many cases, can equal or exceed the total mass of the finished building and its designed imposed loads. If this occurs then the completed structure will impose no greater stress on the sub-soil through its basement floor than did the soil it replaced. Therefore the capacity of the soil to carry the load is not in doubt and the building 'floats' in the ground precisely the same way that an ocean liner floats in the sea.

As the sub-soil at the levels to which this form of substructure is carried is almost always water-bearing the basement would need to be lined with a waterproof membrane which would then cause the upward hydrostatic pressure to counter-balance a proportion of the downward thrust of the building.

Figure 3.5: Buoyancy foundations

INPUT 3B

3.4 Pile Foundation

A pile can be loosely defined as a column inserted in the ground to transmit the structural loads to a lower subsoil. Piles have been used in contact two hundred years ago and until the twentieth century were invariably of driven timber.



Figure 3.6: Piling


3.5 Classification of Piles

Piles may be classified by the way in which they transmit their loads to the subsoil or by the way they are formed. Piles may transmit their loads to a lower level by:

a) End Bearing
The shafts of the piles act as column carrying the loads through the overlaying weak subsoil to firm strata into which pile toe has penetrated. This can be a rock strata or a layer of the firm sand or gravel which has been compacted by the displacement and vibration encountered during the drive.



Figure 3.7: Friction Pile

b) Friction
Any type of foundation imposes on the ground a pressure which spreads out to form a bulb of pressure. If a suitable load bearing strata cannot be found at an acceptable level, particularly in stiff clay soils, it is possible to use a pile to carry this bulb of pressure to a lower level where a higher bearing capacity is found. The friction of floating pile is mainly supported by the adhesion or the friction action of the soil around the perimeter of the pile shaft.


Figure 3.7: End Bearing Pile


3.5.1 Types of bearing piles

The classification of bearing piles is related to the effect on the soil. There are two main types: displacement piles and replacement piles.

a.Diplacement Piles

A displacement pile is either driven, jacket , vibrated or screwed into the ground. This section displaces the soil outwards and downwards but the material is not actually removed. There are two types of displacement pile: large displacement piles which includes all solid driven piles and small displacement pile, in which very little soil is displaced. This would include the screwed piles and H piles.


b.Replacement Piles

Replacement piles may be classified as Supported or Unsupported.In both cases a hole is formed in the ground by some form of cutting or boring tool and is then filled with reinforced concrete. The unsupported hole will normally require a short tube at the top to prevent debris from falling into the concrete during placing. Support to holes may be provided by means of medium or heavy sectional casing, sscrewed together as boring proceeds,or by means by a head of drilling mud (usually bentonite suspension).

3.6 Types Of Pile

3.6.1 Driven Piles

a) Timber piles
Timber piles are usually square sawn hardwood or softwood in lengths up to 12.000m in sections,with sizes ranging from 225 x 225 mm to 600mm x 600 mm .Most timber piles are fitted with an iron or steel driving shoe and have an iron ring around the head to prevent splitting due to driving. Although not particularly common they are used in sea defences such as groynes and sometimes as guide piles for large in conjunction with steel sheet piling.



Load bearing capacities can be up to 350 kn per pile depending upon section size and or species.There are two types of timber piles: Natural logs named as Bakau Piles, and treated timber piles which are chemically treated against the decay.


b) Bakau Piles

The bakau pile is generally tapered and has a diameter of 75 to 125mm.. The piles are generally used as friction piles at poor ground condition which have a high ground water table.The bakau piles are generally used for light buildings (column load of approximately 30 tonnes).Suitable in soft clay areas.

c) Treated Timber Pile

The piles are made from kempas, a kind of broadleaf tree. The cross sectional area of the pile is 5 inches by 5 inches and six inches by 6 inches,and the pile is 20 to 24 feet long. The permissible degree of bow or wrap of the pile within 20 feet long is 1½ inches from a straight axis through the pile.

The permissible degree of wrap of a pile more than 20 feet long is 2 inches. Design working ads of 5 inches by 5 inches piles an d6 inches by 6 inches piles are the 15 ton/pile and the 20 tonnes respectively.


d) Composite Piles
It is composite wood and concrete pile. The timber is kept below groundwater and a greater over-all length is achieved. A closed-end pipe pile may be used in place of the timber section.

Combination of two or more of a preceding type or combination of different materials in the same type of pile. Composite piles are used in ground conditions where conventional piles are unsuitable or uneconomical concrete and timber are the type used because it is cheap and easy to handle of the timber piles with the durability concrete. The timber is terminated below ground water level and the an upper portion formed in concrete.

e) Steel Piles

Steel piles, like timber, are driven by percussion means and have a variety of suitable cross-sections. In addition to the common sheet piles, the three main types are H sections, Box piles and tube piles. The main use of steel piles is for temporary works, retaining walls and marine structures. The problem of corrosion of the steel can be overcome by suitable protection.

Sheet piles have the advantages of being robust, light to handle capable of carrying high compressive loads when driven on to a hard stratum, and capable of being driven hard to a deep penetration to reach a bearing stratum or to develope a high skin frictional resistance, although their cost per metre run is high compared with precast concrete piles.

f) Precast Concrete Piles

Precast concrete piles used on medium to large contracts where soft soils overlaying a firm strata are uncountered and at least 100 piles will be required. The precast concrete driven pile has a little frictional bearing strength since the driving operation moulds the cohesive soils around the shaft which reduces the positive frictional resistance.

i) Steel H- Section Piles

H section piles are in the form of wide- flanged steel section and rolled in accordance with standard. The displacement piles, and the H section piles may be driven by any type of hammer, but the head of the pile should be protected by a helmet.

3.6.2 Driven Cast-In-Place Pile

• Driven cast-in-place pile installed by driving,to the desired penetration,a close ended steel tube or concrete shell and the void created is filled with concrete. Steel tube or concrete shell can be withdrawn or left in place.
• Readily adjustable in length to suit the desired depth of penetration.
• Economic if no casing required.

3.6.3 Bored Cast-In-Place Pile

• A borehole is formed in the ground by sugar etc and the void concrete to form bored pile.
• Usual sizes varies from 400mm diameter to 1000mm diameter.
• Allowable load varies from 800kN to 1500kN.
• Length of bored piles easily adjustable to suit the penetration depth.
• Suitable in redidual soil
• Uses high slump self compacting concrete.
• Trem concreting if water in borehole.