Foundation
Foundation
Part of the structure below the ground level I called foundation. Foundation supports the weight of structure and the load carried by it. The size and shape of the structure caries the foundations. The type of foundation also depends on the bearing capacity of the soil.
Purpose of foundation
>The main purpose of foundation is to spread the load over a large area.
>To ensure safety of the super structure against soil erosion and soil movements.
>To increase the stability of the structure various environmental forces like rain and wind.
>To provide a bas platform for masonry and concreting.
Conditions for site selection
>Low lying area should be avoided.
>Site should be as even as possible, as leveling increase the cost of construction.
>The ground should be away from the site to enable easy drainage.
>The site should not have any filled up soil.
>the ground water table level should be low.
>the site should possess good foundation soil at a reasonable depth.
>Site for residential buildings should be away from Industries and near to schools and hospitals.
>Site should be near to places where facilities like water and power are already available.
>Site should be connected with good system of roads and railways.
>Friendly neighborhood is also essential for site selection.
>”Tests Pits” are made up to foundation level to find the resistance of the soil during site selection.
Bearing capacity of the soil
>It is the maximum load per unit area which the soil will resist safely.
Safe Bearing capacity = Ultimate bearing Capacity/ Factor of Safety.
>factor of safety normally adopted is 2-3.
>The bearing capacity of reclaimed or shrinkage soil is taken as 50 kN/m^2.
Various methods to improve the bearing capacity of the soil:
>reducing the air voids of the soil by ramming or driving piles.
>Drawing the sub soil water by laying open jointed drains in trenches.
>Restricting the soil movements by providing sheet pile enclosure.
>Increasing the depth of the foundation.
>Compacting the soil by injecting silicates in the soil mass.
>The last 2 methods highly increase the cost of construction.
Various loads on foundation
(a)Dead load
It refers to the weight of the floors, walls and roofs.
(b) Live Load
Refer to the temporary loads imposed.
(c) Wind load
It refers to the action of the effect of the wind on tall structure, generally above 30 fee height.
Wind load = P = 0.66(k1k2k3v)^2
Where v = velocity of wind
K1 = importance factor (=1)
K2 = shape, size or height
K3= topography factor.
Types of foundations
Two main classification foundations are shallow foundation and deep. Shallow foundation is opted for light structure and for the soil having high bearing capacity. Deep foundations are opted for multi stored buildings, bridges, light house and docks etc. Further they are classified as spread or pad foundations, raft or mat foundation. Grillage and pile foundations.
(a) Spread foundations or Pad foundation
In this type of foundation, the load of the structure is spread over a large area. It is adopted, if the bearing capacity of the soil is less. It is further classified as “Strip footings” and “column footings”.
Strip footings
Strip footings are used where soil of good bearing capacity is available at a depth of less than 3 metres from the ground level. It is also called wall footings, Provides continuous and longitudinal bearings for loads. It is further classified in to ‘simple footings’ and ‘stepped footings’.
Simple footings
In the simple footings a simple single footings is provided for the walls of ligh structures. Concrete is foundation may be of cement or lime. A minimum offset of the edge of the concrete must be equal to 15 cm.
Stepped footings
It consists of 2 or more footings of stone masonry or bricks masonry. Light structures are provided with stepped footings. As the depths increase, the widths of stepped footing increase. Plain cement concrete of 1:4:8 mixes is used for foundation bed. Foundation trench should be rammed properly before laying the concrete.
Isolated column footings
Used for single column. It may be square, circular or rectangular. The footing is of R.C.C with reinforcement at bottom. Column footing on piles should have a minimum thickness of 30 cm.
Combined column footings
Used when isolated footings over lap or if they are very close to each other. Used if they are more than one column located on the boundary.
(b) Raft or Mat foundation
Adopted for medium heavy structures where the soil has low bearing capacity. Used where unequal settlement is suspected. Supports the arrangement of column’s or walls in a row. It may also consists of a system of beam and slabs. In these types the load is transmitted to the soil below through a continuous slab.
(c) Grillage foundation
It is used to transfer the heavy load from a column, stanchion or piers to a large area of soil of low bearing capacity. This avoids deep excavation. It may be of steel or timber, having one or two tiers. In two tiers grillage, the top tier is placed perpendicular to the bottom tier. The entire arrangement is embedded in rich cement concrete. The grillage beams are placed, So that the entire foundation functions as a single unit.
Deep Foundation: If the soil available is not having the required bearing capacity, deep foundation is used. Pile foundation is the most popular deep foundation.
(d) Pile foundation
Pile foundation is used if the soil is loose. When the structure is tall and heavy, Piles are classified in to bearings piles and friction piles. End bearing piles support the load mainly by resistance developed at the pile point or base. Friction piles support the load mainly by resistance of the piles and the available at the limited depth. For retaining the earth, sheet piles are used. Under reamed piles are used when the soil happens to be highly swelling and shrinking.
Foundation for machine
Foundation for machine is designed to make entire arrangement safe during operation. Foundations for reciprocating and impact type of machines are described below.
(a) Foundation for reciprocating type of machine
The machine should be separated from other foundations (like main building) as far as possible. The amplitude of the vibration should not exceed the safe limits. Cantilever should not be over hanging. The cantilever should be designed for rigidity against vibrations. The foundation blocks should rest on firm foundation. The combined centre of gravity of machine should be at least equal to the distance of the centre of gravity of the foundation as possible. Design criteria (IS: 2974 Part 1:1964)
(b) Foundation for Impact type of machines
The stress produced by impact should be 0.8 times the allowable static stresses. The centre of gravity of the center foundation, anvil should coincide with the time of fall of the hammer. The maximum vertical irrational amplitude should be less than 1.2 mm. The area of foundation block should be such that the safe loading intensity of the soil never exceeds during the hammer operation. The mass of the foundation should not be less than 3 times that of the anvil.
Causes for failure of foundation
>Unequal settlement or shrinkage of sub-soil.
>Heavy rains and Earth quakes.
>Thrust of pitched roof or wind action on super structure.
>>Variation in the height of water table.
>Scouring and removing of sub-soil due to the animals and growing trees etc.
Remedial measures
>Unequal settlement can be prevented by resting the foundation on rock or hard moorum.
>Driving of sheet piles prevents the failure due to sliding and variation in failure height of water table.
>Increasing the width of the foundation prevents failure due to wind action.
>Foundation should have a depth where no ground movement occurs.
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