Foundation is that part of the structure, which is in direct contact with the loads and transmits them to the ground below. It is a very important part of every structure. Especially for massive structures. Its design is very important and difficult. Hence it should be designed with great care. Here we discuss different types of foundations with their suitability.
FUNCTIONS OF FOUNDATION
Following are the functions of the foundation:
1. To keep the intensity of pressure within safe limits of the bearing capacity of the soil, the used to distribute the load of the structure over a larger area of the sub-soil.
2. To provide a leveled base for the construction of piers and abutments.
3. To prevent the tilt and overturning of the piers and abutments
4. To prevent the lateral escape of the supporting material of the river bed. This will prevent piers from sinking. Thus the damage caused due to the failure of the bridge will be averted.
5. To avoid unequal settlement of the sub-soil and supper structure etc.
TYPES OF FOUNDATION (footings)
Generally, the following types of foundations are adopted for structure:
A. Shallow foundation
1. Spread foundation
2. Continuous foundation
3. Combined footing
4. Strap footing
5. Mat foundation
6. Grillage foundation
7. Stepped footing
8. Machine foundation
B. Deep foundation
1. Pile foundation
2. Raft foundation
3. Caisson foundation
C) Floating foundation
A) Shallow foundation (D≤ w, or D/W ≤2):-
If the depth of foundation is less than breadth then it is called the shallow foundation. It is of the following types:
- Spread foundation (l/w is between 1 and 2)
A footing provided to support an individual column is known as spread footing.
This type of foundation in shape is similar to provided for walls. It is best suited in such situations where the scouring of the river bed is minimum and good and hard soil is available within 2 to 3 meters below the river bed level. This type of foundation can also be provided even if the bed contains erodable material as sand, but the scouring is prevented by driving sheet-piles on up-stream and down-stream side and floor pitching. The minimum depth of this foundation is determined by the following formula.
D = p/d ((1 – sin ϕ)/ (1+sinϕ))
D= depth of the foundation
P= bearing capacity of subsoil in kg/m²
D= density of sub soil in kg/m3
Φ= angle of repose of the soil.
- b. Continuous foundation (l/w>2):-
If the length of the footing is more than the width of the footing and the chances of differential settlement are less, it is called continuous footing
- c. Strip foundation (l>>w):-
If the length of the footing is much more than the width of the footing, it is called strip footing.
(d) Combined footing
A footing that supports two or more columns is termed as combined footing The shape of rectangular footing may be rectangular, square, trapezoidal, or triangular. The shape of the footing is so proportioned that the CG of the supporting area is in line with the CG of the two links.
c) Strap footing:-
When two or more isolated footings are connected by a beam, is called strap footing and the beam connecting the footings is called strip footing.
f) Mat foundation
Mat is a combined footing that covers the entire area beneath a structure and supports all the walls & columns. The mat foundation with an extra beam connecting the column is called a raft foundation. Normally it is provided under the following circumstances:
1. Bearing capacity is low
2. Foundation covers more than 50% area
3. Heavy load to take care etc.
g) Grillage foundation:
It is a special type of isolated footing, generally provided for heavily load steel stanchions. It consists of one or two tiers of wooden or rolled steel sections with space filled with concrete.
h) Stepped footing:
It is provided only when the existing ground is in the form of slope in case sloping of ground 1st lower footing shall be constructed.
D) Machine foundation:-
This foundation is used for the support of machines. The weight of the foundation should be 2.5 to 3.5 times the weight of the machine.
B) Deep foundation (Dew & D/w>2)
If the depth, D of the foundation is greater than breadth then it is called the deep foundation. It is provided when a load of the superstructure is heavy, has a poor bearing capacity, the subsoil water level is high, fluctuation in subsoil water level, and the structure is situated seashore or river bed.
Deep foundation is of the following types:
- Pile foundation
- Mat foundation
- Pier foundation
a. Pile foundation (Load is taken by bearing & friction)
Under the following circumstances pile foundations are used:
(a) When the hard soil is encountered at great depths and the provision of spread foundation economical.
(b) When the provision of raft and grillage foundation is expensive.
(c) When concentrated heavy loads are to be taken by the foundation.
(d) When the scouring depth in the river bed is very much deep.
(e) When the top soil is compressible.
Followings are the types of the pile poundation:
1. End bearing pile (Used to transfer load through water or soft soil to a suitable bearing stratum)
2. Friction pile (Used to transfer load through the friction ground perimeter
3. Compaction pile (Used to compact loose granular soil and thus increasing the bearing capacity of the pile)
4. Tension pile (To protect the structure through uplift pressure)
5. Anchor pile (To provide an anchor against horizontal pool)
6. Feeder or Dolphin pile (Used to protect the structure in the water against impact caused by ships or other floating objects)
7. Better pile (This pile is used to resist inclined force)
Ex:- Simplex, Franki, Vibro, Mc-arthurs, Precast, Sheet pile.
Suitability of pile
- Grillage & Raft foundation not possible
- Transfer heavy Live Load & Dead Load
- Seasonal variation of Ground Water Table
- In marine structure
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b) Mat foundation
This type of foundation is best suited for situations where the bearing capacity of soil is very poor and the river bed contains soft or mal clay and good soil is not available within a reasonable depth. This foundation is also known as the floating foundation. In this case, a thick slab of reinforced cement concrete is laid over a layer of well-compacted lime concrete. Over this reinforced cement concrete piers may be constructed at specified intervals. From economical considerations, the provision of inverted tee (T) bears has been found useful. The inverted T-beam raft foundation is shown in fig.1.
c. Pier foundation:
In the pier foundation, the load is taken only by bearing.
Types of Pier:
1. Open caisson:- It is open at the top and as well as the bottom.
2. Closed cassion:- Where bearing stratum is available at shallow and no chances of erosion below the foundation. It is closed at the top and open at the bottom. It is also known as floating caission.
3. Pneumatic caission
d. Well foundation
In situations where due to scouring or bearing capacity considerations, foundations are to be taken to greater depths than 5 to 7 m, open excavation becomes costly and uneconomical due to the following reasons:
1. To retain the sides, heavy timbering is required.
2. Due to greater earthwork involved due to side slopes, progress in open excavation will be very slow.
3. The excavated material refilled in the open excavated foundation leads to loose soil which can be easily scoured
C) Floating foundation:-
When the pressure is exerted by the overburdened soil and structure. (including foundation) are equal then the foundation is called floating foundation.
Foundation on Black Cotton Soil
Black cotton soil has characteristics of shrinkage and swelling due to the moisture movement through them. During the rainy season, moisture penetrates into the soil and swelling occurs in an unexpected way. Similarly, during the summer season, moisture moves out of the soil, and consequently, the soil shrinks. These shrinkage cracks, sometimes also known as tension acts. The width of cracks may be up to 15 cm and maybe 2 m deep. The bearing capacity of this soil varies from 5 to 15 t/m2.
In the case of black cotton sail following precaution is taken:-
- Remove all black cotton soil
- Increase the concrete grade
- Increase the depth of the foundation
- Increase depth & breadth then fill with and, mooram etc. around and beneath the footing.
SELECTION OF FOUNDATION (footings)
The selection of the foundation depends upon the following factors:
- Nature of foundation
- Type of the structure
- Superimposed load etc.
- Velocity of flowing water
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