Foundation is the lowest part of a structure which provides a base for the super-structure and transmits the loads (live load, wing load) on the structure including the dead weight of the structure itself to the soil below.
Types of Foundation:
Foundation can be broadly classified into two types–
·Deep Foundations
·Shallow Foundations
The most common forms of construction pertaining to deep foundation are:
·Piles
·Cofferdams
·Caissons.
The various types of shallow foundation are:
·Spread footing
·Grillage footing
·Eccentrically loaded footing
·Combined footing
·Mat or Raft foundation.
Importance of Pile foundation:
—Enables a structure to be supported by a layer of soil
—Provide safe foundation
—Used for -
ØBuildings
ØTrestles
ØBridges
Classification of Piles:
Depending upon materials used in their manufacture,
Piles can be classified as:
a)Concrete Piles
oCast-In-Place or cast-in-situ Piles
oPre-cast Piles
oPre-stressed concrete Piles
b)Steel Piles
c)Timber Piles
d)Composite Piles
Classification of pile based on function
·Bearing pile
·Friction pile
·Sheet pile
·Anchor pile
·Batter pile
·Fender pile
·Compaction pile
Precast Pile
Pre means before & cast means made. So precast pile refers to a pile which is made before, it is being used.
ØMost common pile
ØFrancois Hennebique first use it (1897)
ØA.A.Raymond built his company(1901)
ØIt can be constructed either in the factory or on site
ØIt is reinforced concrete pile
PRECAST PILES SHAPE& SIZE
ØCommonly manufactured in square
ranging from about250 mm to about 450 mm
maximum section length of up to about 12 m
Hexagonal
Circular
Octagonal
Triangular
H shapes
Soil suitability
vSuitable in a wide range of soil condition
vworking loads in excess of 1000kN
vUseful for very deep piling up to 40 m
vin soft ground or in aggressive or contaminated soils
Bearing capacity
•High bearing capacity
•Compact soil & increase the bearing capacity
•Concrete formations around the contracted sections increase the capacity.
Design of Precast Pile
·By the soil test, we would be find the depth of penetrate.
·By which length of a pile can be determined.
·After determine the total weight of the building, we would find the number of piles.
·The diameter of the pile normally varies from 35 cm to 65 cm and their length varies from 20 ft to 30 ft.
·Besides bearing load for easy transportation & movement big weir or reinforcement is used in the pre cast pile.
· For easy driven rectangular pile is often made.
·Longitudinal reinforcement usually consists of one bar 20 mm to 50mm in diameter at each angle of the section of the pile.
·The vertical rods are tied horizontally by bars 6mm to 10mm in diameter.
·The horizontal bars may be provided in the form of stirrups wound around the verticals.
·Pile shoe & pile stirrup is used in the lower part of the pile to driven the pile easily.
Process of precast pile
—Steel form is used for the precast pile manufacture.
—Before pore the concrete in to the form, Mobil or other kinds of oil have been used.
—Cement, sand, aggregate ratio is normally 1:2:4 in pre cast pile.
—But to make the foundation stronger mix ratio should be 1:1.5:3.
—When the concrete pore in the steel form it would be ramming by the vibrator.
—After 3 days, piles have been covering by the sheet.
—After 3 days of casting, steel form would be removed.
—Then the piles would be prepared for 4 weeks curing.
—Then the piles are transported to the site for driving.
Pile Driving
It is the process of driving piles into the ground.
It requires few piling equipment such as-
Cranes
Hammers
Cushions
Leads
Piles
Templates
Piles are driven in the ground by the following methods:
By drop hammer
By steam hammer
By water jets
By boring.
By Drop Hammer:
It is a pile driver in which the hammer is raised by a rope or cable then allows the drop on the pile cap.
Pile Installation Using Drop Hammer
Normally, 1.5 to 3 meter from above, the drop hammer would be put down in to the pile cap.
The weight of drop hammer varies from 230 to 1800 kg.
—By using the pile wipes machine, the drop hammer would be controlled.
—The exact weight of the drop hammer to be used is governed by the shape and length of pile as well as nature of ground to be penetrated.
By Steam Hammer
In this type of pile driver the hammer is automatically raised and then dropped a short distance by means of a steam cylinder and piston. There are two types of steam hammer pile drivers:
§Single acting type.
§Double acting type.
The hammers usually weight 2000 to 4000 kg and have a stroke length up to 15m. In case of single-acting steam hammer or drop-hammer the stroke length of fall should not exceed 1.35m.
By Water Jets
This method of driving piles involves displacing the soil below the shoe of the pile by means of one or more jets.
The jet consists of a 5 to 6cm diameter pipe with its lower end decreasing in diameter and forming a nozzle of about half the diameter of the pipe at top.
Water is forced through the pipe under a pressure of 7 to 17.5 kg/ sq.cm.
The nozzle of the jet of the jet pipe is usually kept about 15 to 30 cm below the pile shoe.
The water jet is stopped a few meters before the pile reaches its required depth and it is driven by hammering for that distance.
By Boring
If the pile is required to penetrate beds of hard soil, or soft rock to reach its required depth, driving of the pile by boring is an economical solution. In dry soils, boring may be done by auger, while in soft rocks, rotary well drills may be used.
The bore is made to the required depth of the pile. Precast piles are then put in the bore hole.
Advantages of pre-cast piles:
—They can be cast well before the commencement of the work resulting in rapid excavation of work.
—Their construction can be well supervised and any defect detected can be rectified before use.
—The reinforcement remains in their proper position and do not get displaced.
—They can be driven under water
—The driving of adjacent pile does not produce adverse effect upon the already driven pile.
—They can be loaded soon after they have been driven to the desired depth.
Disadvantages of pre-cast piles:
—They are heavy and great difficult is experienced for their handing and transportation.
—The exact length of a pile can rarely be pre-determined and as such it has to be lengthened sometimes, rendering the pile weak at the joint.
—If a pile is found to be too long after driving, it has got to be cut down which involves extra labor, time and expense.
—They are subjected to the driving shocks after the concrete has fully set. This may result in unsound construction.
—Precast pile cannot be long.
—Precast pile creates shake that can harm other structure.
—Precast pile cannot be done in busy areas and highly populated areas
—The piles are difficult to manufacture
—It is not economical
—Subject to longitudinal and transverse cracking
—Not aesthetic
—Not appropriate for curved or flared structures
—Complicated for skews
Solution
If only quality of pile is considered then precast pile is much better than cast in situ pile. But due to the difficulties & expense in transport & hammering the use of precast pile is rare in Bangladesh. If the depth of deep foundation is 20 to 30 feet, then precast pile is much better than cast in situ pile with the consideration of all circumstances.
Difference of cast in situ & precast pile
Precast pile
vReinforcement remains in their proper position.
vCan be loaded soon after they have been driven
vCan be driven under water
vDefect can be rectified before use
vThey are costly
vExact length of pile can rarely be predetermined
Cast in situ
qPossibility of the reinforcement getting displaced
Posts
