A deep foundation is a type of foundation distinguished from shallow foundations by the depth they are embedded into the ground. There are many reasons a geotechnical engineer would recommend a deep foundation over a shallow foundation, but some of the common reasons are very large design loads, a poor soil at shallow depth, or site constraints (like property lines). There are different terms used to describe different types of deep foundations including the pile (which is analogous to a pole), the pier (which is analogous to a column), drilled shafts, and caissons. Piles are generally driven into the ground in situ; other deep foundations are typically put in place using excavation and drilling. The naming conventions may vary between engineering disciplines and firms. Deep foundations can be made out of timber, steel, reinforced concrete and prestressed concrete.
I-When do we need pile foundations:
·Top layers of soil are highly compressible for it to support structural loads through shallow foundations.
·Lateral forces are relatively prominent.
·In presence of expansive and collapsible soils at the site. ·Offshore structures
·Strong uplift forces on shallow foundations due to shallow water table can be partly transmitted to Piles. ·For structures near flowing water (Bridge abutments, etc.) to avoid the problems due to erosion.
II-Why to use pile foundations:
·Inadequate Bearing Capacity of Shallow
·To Prevent Uplift Forces
·To Reduce Excessive Settlement
III-How do they work:
·By Friction between the piles and the soil such as in Fig.1 :
·End bearing piles such as in Fig.2 were the bed rock reaction force is important.
IV-Types of piles:
1. Steel Piles
·Pipe piles, pipe piles are a type of steel driven pile foundation and are a good candidate for battered piles.Pipe piles can be driven either open end or closed end. When driven open end, soil is allowed to enter the bottom of the pipe or tube. If an empty pipe is required, a jet of water or an auger can be used to remove the soil inside following driving. Closed end pipe piles are constructed by covering the bottom of the pile with a steel plate or cast steel shoe. ·Rolled steel H-section piles, H-Piles are structural beams that are driven in the ground for deep foundation application. They can be easily cut off or joined by welding or mechanical drive-fit splicers. If the pile is driven into a soil with low pH value, then there is a risk of corrosion, coal-tar epoxy or cathodic protection can be applied to slow or eliminate the corrosion process. It is common to allow for an amount of corrosion in design by simply over dimensioning the cross-sectional area of the steel pile. In this way the corrosion process can be prolonged up to 50 years.
2. Concrete Piles
Concrete piles are typically made with steel reinforcing and prestressing tendons to obtain the tensile strength required, to survive handling and driving, and to provide sufficient bending resistance. Long piles can be difficult to handle and transport. Pile joints can be used to join two or more short piles to form one long pile. Pile joints can be used with both precast and prestressed concrete piles.
·Pre-cast Piles, These piles are molded and prepared to be transferred to the site, either to be immediately used or transferred as parts to be collected. ·Cast-in-situ Piles, These piles are molded in working site, molds are prepared then concrete is put in and waited for to mature. ·Bored-in-situ piles, These piles are drilled- bored- in situ, this process requires a “male” and a “female pile were one is drilled through the other. 3. Timber Piles , are simple designed piles. Main consideration regarding timber piles is that they should be protected from rotting...