DEEP FOUNDATION, deep foundation notes,deep foundation depth, Advantage of deep foundation, Deep foundation example,When deep foundations are necessary,Pile foundation minimum depth

 DEEP FOUNDATION 

When the ratio of depth of foundation to the width of foundation is greater than 1. Then foundation is called deep foundation 

i.e.       D /B> 1 

Deep foundation are those that transmit the load to deeper soil.Deep foundations are required when hard strata is available at larger depth. 

Types of Deep foundations 

(1)- Pile foundation

(2)- Pier foundation

(3)- Well or cassion foundation 

(1)- Pile foundation 

It is a deep foundation, is actually a slender column or long cylinder made of materials such as concrete or steel which are used to support the structure and transfer the load at desired depth either by end bearing or skin friction.

Classification of piles 

(1)- Classification based on material and composition 

i)- Timber piles 

Timber piles are made from tree trunks and are well seasoned, straight and free from all defects. In india timber piles mostly made up of sal tree trunks.these piles are available in length between 4 to 6 meter .

ii)- Concrete piles 

Concrete piles are either precast or cast in situ .precast piles are cast and cured at the casting site and then transported to the site for installation .precast piles are normally suitable for short length. Cast in situ piles are constructed by drilling hole in the ground and then filling the hole by concrete after placing the reinforcement.

iii) Steel piles 

Steel piles are usually of rolled H- section or thick pipe sections.these piles are used to withstand large impact stresses and where less distribution from driving is desired. Steel sheet pile and H pile are generally used to support the open excavation and to provide seepage barrier.

iv) Composite piles 

A pile which is made up of two material like concrete and timber or concrete and steel is called composite pile.

 If the required length of steel piles is less than the depth of pile, many times upper portions are built with concrete. Thus steel and concrete composite piles are sometimes used.

(2)- Classification based on method of installation 

i)- Bored piles 

Bored pile, also called drilled shaft, is a type of reinforced-concrete foundation that supports structures with heavy vertical loads. A bored pile is a cast-in-place concrete pile, meaning the pile is cast on the construction site. Bored piles are constructed in prebored holes either using a casing or by circulating stabilizing agent like bentonite slurry.

ii)-Driven piles 

These piles are driven into the soil by impact of hammer.these piles may be of concrete,steel or timber .Boring are not required in this type of piles.when a pile is driven into granular soil it densifies the soil and increases strength of soil .But when a pile is driven in a saturated caly ,the soil instead of being compacted gets remoulded with reduction in strength.

iii)Driven and cast- in- situ piles 

It is type of driven pile.They are constructed by driving a steel casting in to the ground. The hole is then filled with concrete by placing the reinforcement and casing is gradually lifted.

 (3)- Classification based on the  function

i)- End bearing piles 

End bearing piles, which is also known as point-bearing piles, are used to transfer superstructure load through low bearing capacity soil to a strong stratum such as rock or very dense sand and gravel.

ii)- Friction piles 

Friction pile is a kind of pile foundation. This type of pile utilizes the frictional resistance force between the pile surface and adjacent soil to transfer the superstructure load. Depending on the subsoil strata condition, resistance force due to friction can develop in a definite pile length of on the full length.

iii)- Tension piles 

Tension piles are called uplift piles.these piles are used to anchor down the structures subjected to uplift due to hydrostatic pressure.

(2)-Pier foundation .

A pier foundation is a collection of large diameter cylindrical columns which distribute the load of the structure to the firm strata below. It stood several feet above the ground. It is also known as “post foundation”

(3)- Well or cassion foundation  

Well or cassion is a large size hollow box or well which is sunk deep into the ground to support heavy loads.The load carrying capacity of such foundation is very large .Such foundation are generally used in bridges