Pile foundation is a substructure that carries and transfers the load of a structure to an allowable bearing soil located at some depth below the surface. It is a load carrying and transferring system. It is a formation that strengthens the bearing capacity of unstable and weak soil for construction purposes. The main components
of the pile foundation are the pile cap and the piles. Another essential component is the beam that links all the piles together.
FUNCTIONS OF THE PILE FOUNDATIONS
I. To transmit load from a structure to a solid ground.
II. To resist vertical, lateral and uplift load.
This is a long slender formation which transfers load to deeper soil or rock of high bearing capacity displacing shallow soil of low bearing capacity. It is a convenient method of foundation for works over water, swampy and areas with low bearing capacity on the surface.
MATERIALS FOR CONCRETE PILING
The main types of materials used for piling are;
III. Reinforcement rods
IV. Tremi- pipes
CLASSIFICATION OF PILES
Piles are classified with respect to:
(i) Load transmission and functional behaviour.
(ii) Type of material.
(iii) Effect on the soil.
CLASSIFICATION OF PILE WITH RESPECT TO LOAD TRANSMISSION AND FUNCTIONAL BEHAVIOUR
End Bearing Piles: These piles transfer their load directly to soil of good bearing capacity. They derive most of their carrying capacity from the penetration resistance of the soil at the pile shoe (cast steel shoe).
Friction/Cohesion Piles: These are used where there is no reasonable bearing stratum. They rely on resistance from skin of pile against the soil (i.e. their carrying capacity is derived mainly from the frictions of the soil in contact with the shaft of the pile). BORED PILES (Replacement Piles): Are generally considered to be non-displacement piles; a void is formed by boring or excavation before pile is produced. Piles can be produced by casting concrete in the void. Some soils such as stiff clays are particularly amendable to the formation of pile in this way, since the borehole walls do not required temporary support except clotting to the ground surface. In unstable ground, such as gravel, the ground requires temporary support from casing or bentonite slurry.
There are three non-displacement methods: bored cast-in-situ piles, pre-cast piles and grout or concrete intruded piles.
ADVANTAGES AND DISADVANTAGES OF DIFFERENT PILE MATERIAL
+ The piles are easy to handle.
+ Are relatively inexpensive where timber is plentiful.
+ Sections can be joined together and excess length easily removed.
- The piles will rot above the ground water level. Have a limited bearing capacity.
- Can easily damage during driving by stone and boulders.
- The piles are difficult to splice and are attacked by marine borers in salt water.
Prefabricated concrete pile (reinforced) and pre-stressed concrete piles.
+ Do not corrode or rot.
+ Are easy to splice. Relatively in expensive.
+ The quality of the concrete can be checked before driving.
+ Stable in squeezing ground, for example, soft clays, silts, and peat pile material can be inspected before piling.
+ Can be re-driven if affected by ground heave.
Construction procedure is not affected by ground water.
+ Can be driven in long lengths carried above ground level, for example, through water for marine structures.
- Relatively difficult to cut.
- Displacement, heave and disturbance of the soil during driving.
- Can be damaged during driving. Replacement piles may be required.
- Sometimes problems with noise and vibration.
- Cannot be driven with very large diameters or in condition of limited headroom.
Steel piles (rolled steel section).
+ The piles are easy to handle and can easily be cut to desire length.
+ Can be driven through dense layers. The lateral displacement of the soil during driving is low.
+ Can be driven hard and in very long lengths.
+ Can carry heavy loads.
+ Can be successfully anchored in sloping rock.
+ Small displacement piles particularly useful if ground displacement and disturbance is critical.
- The piles will corrode.
- Will deviate relatively easy during driving.
- Are relatively expensive.
PILE FOUNDATION DESIGN
During pile design, the following factors are taken into consideration:
- Pile material compression and tension capacity.
- Deformation area of pile at the top and the end of the pile.
- Eccentricity of the load applied on the pile.
- Soil characteristics.
- Ground water level.
- Pile spacing and arrangement.
- Calculation and predication of design bearing capacity of both single pile and group in different soil types using pile design formulae.
Piles are designed that calculations and prediction of carrying capacity is based on the application of ultimate axial load in the particular soil conditions at the site at relatively short time after installation.
PILE DESIGN FORMULAE
Two widely used simplified methods that can satisfy all of the above conditions are referred as geotechnical and dynamic methods.
This method takes into account the soil conditions (the physical characteristics of the soil); the behavioural responses of soil applied pile load; soil resistance to pile load. Soil are classified as either granular/ non-cohesive or clays/ cohesive. The load settlement response is composed of two separate components, the linear elastic shaft friction Rs and non linear base resistance Rb. The concept of the separate evaluation of shaft friction and base resistance forms the bases of “static or soil mechanics” calculation of pile carry capacity. The basic equations to be used for this are written as:
Q= Qb + Qs – Wp or
Rc= Rb + Rs – Wp
Rt= Rs + Wp
Where: Q=Rc equals to the Ultimate compression resistance of the pile.
Qb=Rb= base resistance.
Qs=Rs= shaft resistance.
Wp= weight of the pile.
Rt= tensile resistance of pile.
Dynamic design methods
This method is otherwise referred to as a pile driving formula; it represents conditions at the time of pile driving. It is the most frequently used method of estimating the load capacity of driven piles. It relates ultimate load capacity to pile set (the vertical movement per blow of the driving hammer) and assumes that the driving resistance is equal to the load capacity to the pile, under static loading, they are based on an idealized representation of the action of the hammer on the pile in the last stage of its embedment.
Usually, pile driven formulae are used either to establish a safe working load or to determine the driving requirements for a required working load. The use of dynamic formulae is highly criticized in some pile design literatures. Dynamic method does not take into account the physical characteristics of the soil. This can lead to dangerous mis-interpretation of the results of dynamic formula calculation since they represent conditions at the time of driving. It does not take into account the soil conditions which affect the long term carry capacity, reconsolidation, negative skin friction and ground effects.
At the Site in Beckley Avenue, Gbagada, Lagos State there are twenty- four (24) pile points, surveyed for piling. Each of the pile point had single piles except for pile point eight (8) and two (2), which had three (3) and two (2) piles respectively. This additional pile was due to the difference in the loading and also following the Structural Engineer’s report. The total numbers of vertical piles are twenty-nine (29) piles.