Tuesday, 6 March 2012

foundation

in construction, the most vital elements that we should get started with is foundation. the well-handled of this topic will execute our perspective and perception towards this field.below are some of the information in foundation..


                                           FOUNDATION

Foundation is a lower part of a structure which provides a base for the super-structure proper. The function of any foundation is to safely transfer the load of a building such as dead, imposed and wind loads to suitable subsoil. The Building Regulations require all foundation of building to:

a.       Safely sustain and transmit to the ground the combined dead and imposed loads so as not to cause any settlement or other movement in any part of the building or of any adjoining building or works.

b.      Be of such a depth, or be so constructed, as to avoid damage by swelling shrinkage or freezing of the subsoil.

c.       Be capable of resisting attack by deleterious material, such as sulphates in the subsoil.

The choice of foundation type depends on:

  1. The strength of the natural foundation and bearing capacity of the soil.
  2. The weight and total loads of the building.

Foundations are usually made of either mass or reinforced concrete and can be considered fewer than two headings:

       a.   Hallow foundation: Those which transmit the loads to subsoil at the point near the

                                 ground floor of the building such as strips and rafts foundation.

       b.   Deep foundation:  Those which transmit the loads to subsoil some distance below the 

                               ground the ground floor of the building such as pile foundation. 

The main purpose of foundation:

  1. To distribute the weight of the structure over large area so as to avoid over loading of the soil beneath.
  2. To load the sub-stratum evenly and this prevent unequal settlement.
  3. To provide a level surface for building operations.
  4. To take the structure deep into the ground and this increase its stability, preventing overturning.

in here, i ll post for strip and pile foundation..whereelse i ll try upload other types of foundations later.



Basically, pile and strip foundation are used piling to support and transmit the load.

 Basic principles of piling

The unsuitability of upper regions of subsoil may be caused by:

a.       Low bearing capacity of the subsoil.

b.      Heavy point loads of the structure exceeding the soil bearing capacity.

c.       Presence of highly compressible soils near the surface such as filled ground and underlying peat strata.

d.      Subsoil such as clay which may be capable of moisture movement or plastic failure.

e.       High water table

 

  Piles:

Basically piles can be defined as a column being inserted in the ground to transmit structural loads to a lower level of subsoil. Piles are relative long and slender members of the timber, concrete, steel or combination of these materials. Piles also used in normal soil to resist heavy up lift forces or in poor soil conditions to resist horizontal loads. The choice of pile depends on site and soil conditions, economic considerations and structural requirements. They are many types of piles such as timber pile, precast concrete pile, steel performed pile, composite pile and bored pile.

Types of piles:  Precast Concrete Piles

Precast concrete piles defined as reinforced concrete pile which is moulded in square form. The pile constructed of concrete, both reinforced and pretensions are used extensively.  The piles are cast and charred in a casting yard and then transported to the site for driving. The precast concrete pile can avoid cracks that could lead to corrosion of the reinforcement by ground water. The function of reinforced in a precast pile is to resist the stress produced which the pile is finally expected to receive. The reinforced concrete pile supply length up to 18.00m, section sizes such as 200x200mm, 250x250mm and 300x300mm carrying loading up to 1000KN.    
 
Advantages of piles:
1.      They can be cast well before the commencement of the work resulting in repaid execution of work.
2.      Their construction can be well supervised and any defect detected can be rectified before use.
3.      The reinforcement remains in their proper position and do not get displaced
4.      They can be driven underwater.
5.      The driving of adjacent pile does not produce adverse effect upon the already pile.
6.      The can be loaded soon after they have been driven to the desired depth.

Disadvantages of piles:
1.      They have heavy and hence great difficulty is experienced for their handling and transportation.
2.      the exact length of a pile can rarely be pre-determine and as such it has to be lengthened sometimes, rending the pile weak at the joint.
3.      If a pile found to be too long after driving, it has got to cut down which involves extra labour, time and expense.
Piles Driving:
A hammer with approximately the weight of the pile is raised a suitable height in a guide and released to strike the pile head. This is a simple form of hammer used in conjunction with light frames and test piling, where it may be uneconomical to bring a steam boiler or compressor on to a site to drive very limited number of piles.
It rests on the head of the reciprocating punts of the hammer itself, strikes below in such rapid succession that the pile is kept in almost continuous motion.
There are two main types of stem hammers pile drivers:
·         Single-acting steam or compressed-air hammers
·         Double-acting pile hammers
Single-acting steam or compressed-air hammers is use for driven pile in the site of block je.  
Single acting hammer is the hammer operated by steam pressure conveyed from the boiler of the pile driver. The force of each stroke depends upon the steam pressure and the number of blows of the hammer varies from 50 to 60 per minutes.

A.  Pile Foundation

These are frequently used with multi-storey building in cases where it is necessary to transmit the building load though weak and unstable soil condition lower stratum of sufficient bearing capacity. Pile foundations which transmit loads first to the piles which, in turn transmit the load to the soil.
The upper end of the pile called the head or butt. The lower end of a pile called the tip or nose. The members into whom the butt end of the piles is embedded called the pile cap.  The pile cap distributes the applied load to the individual piles which, in turn, transfer the load to the bearing ground.
The individual piles are spaced and connected to the pile cap or tie beams and trimmed in order to connect the pile to the structure at cut-off level, and depending on the type of structure and eccentricity of the load, they can be arranged in different patterns.          


Process of pile foundation:

  

1.      After site preparation, it necessary to driving the piles into the subsoil for the propose area.

2.      Single-acting steam hammers is use for driven pile in the site. The 200x200mm of reinforced concrete pile is use to pilling.

3.      After finish pilling on the propose area, it is necessary to excavate pit to receive pile cap. The minimum depth of excavate pit for pile cap with one pile is 750mm and it length is 500mm. The minimum depth of excavate for pile cap with two piles is 1050mm and  it length is 1100mm. The width for excavate pile cap is 500m

4.      After excavation, the need to break the piles of RC piles and cut of piles at a level by machines. The bars of piles embedment into pile caps shall be 600mm.     

5.      A concrete blinding layer 50mm thick as Grade 15 of weak concrete placed under the pile cap. The function of the concrete blinding are to fill any weak pockets encountered during excavation and to provide a true level surface from which the reinforcement can be positioned.

6.      Formwork is required for the pile cap before positioning the reinforcement and placing the concrete. Formwork is used to retain concrete in a pile cap until the concrete has developed sufficient strength to stay in position without support. The period which should elapse before formwork is struck depends on the type of cement, nature of component and the temperature.

7.      The high tensile reinforcement such as 10mm, 12mm, 16mm diameter are use to support the pile caps. In positioning the reinforcement in the formwork, they tie or join together with the bar of piles and the bar of column stump.

8.      The concrete Grade 30 is normally used for common pile caps.



B. Strip Foundation

Strip foundation normally provide for loads bearing walls, and for rows of column which are space so close that pad foundation would nearly touch each other. The majority of buildings up to four storeys in height have strip foundation, in which a continuous strip of concrete provides a continuous ground bearing under the load bearing walls. 
The strip foundations are built with external wall is placed centrally on the concrete strip which carries loads from floors and roofs and the weight on the foundation is spread fairly uniformly along the length of the walls.
These are suitable for most subsoil and light structural loadings such as those encountered in low to medium rise domestic dwelling where mass concrete is usually required for all other situation.
They consists of continuous mass concrete strips poured in the bottom of trenches and must be wide and deep enough to avoid soil movement that could cause instability.

Advantages
It is more economic to excavate and concrete the strip foundation that to work in a large number of individual pits. In fact, it is often thought to be more economical to provide a strip foundation whenever the distance between the adjacent square pads is less than the dimension of the pads, and for ease in construction closed spaced pad foundation can be formed by inserting vertical joints in a continuous strip of concrete.

Disadvantage
This foundation won't apply for low bearing capacity soil. The working procedure is too much such as digging, framing and other is the weakness. More concrete (footings are deeper), longer period before walls and roof can be constructed.
 an example of strip foundation:

No comments:

Post a Comment