Thursday, May 19, 2016
The following are the different types of foundations, which are generally used for different structures :
1. Spread Footing Foundation.
2. Benching or Stepped Foundation.
3. Pile Foundation.
4. Raft Foundation.
5. Well Foundation.
6. Caisson's Foundation.
7. Cantilever Foundation.
8. Combined Footing Foundation.
9. Inverted Arch Foundation.
10. Grillage Foundation.
1. Spread Footing Foundation. This is the simplest type of foundation and is generally used for ordinary buildings on alluvial soils. This type of foundation can normally be used for three to four-storied buildings on common type of alluvial soils.
The spread footing foundation consists of a concrete base, generally lime concrete and a series of footings below the ground level. The depth and width of foundation depends on the bearing capacity of the soil and the intensity of load. The depth of foundation can be calculated by the Rankine's formula which is
d = P/W K²
K = 1-sinø
where P=safe bearing capacity of soil
w = wt. of soil per cu. metre
ø= angle of repose of soil.
The width of foundation is given by
W = P / L
where W =Width of foundation.
L = Load of structure per running metre.
P = Safe Bearing Capacity of soil.
The width of foundation should in no case be less than 2T+2J, where T is the thickness of wall and J is the concrete offset to be provided. If the width of foundation is taken as 2T + 2J, then the number of footings in this foundation will be equal to number of half bricks in the thickness of wall, excluding the concrete offsets. For example, for the foundation of 1½ brick thick wall, three offsets excluding concrete offset, will be provided.
2. Benching on Stepped Foundation. This type of foundation is provided on hilly places or in those situations where the ground is slopy. In this foundation the excavation trenches are made in the form of steps. All the steps should be preferable of equal length and depth. The function of providing steps is to avoid unnecessary cutting and filling. The plinth of the structure should start after the highest point of the ground. Sometimes R.C.C. pile is driven along the lowest base of the footing to avoid any slipping of the structure along with the foundation.
3. Pile Foundation. It is one of the important types of foundation which is used in the following situations :
1. When it is not economical to provide spread foundation and hard soil is at a greater depth.
2. When it is very expensive to provide raft or grillage foundation.
3. When heavy concentrated loads are to be taken up by the foundation.
4. When the top soil is of made up type and of compressible nature.
5. When there are chances construction of irrigation canals in the nearby area.
6. In case of bridges when the scouring is more in the river bed.
7. In marshy places.
Piles are vertical columns driven into the ground on which wooden or concrete platforms are supported. The piles are driven at regular distances. The size and distance apart, of the piles depends upon the bearing capacity and type of soil and the load of the structure.
Classification of Piles. The piles can be classified according to
(i) Material and (ii) Working.
1. Material classification. The piles are classified as
(a) Wooden Piles.
(b) Concrete Piles.
(c) R.C.C. Piles
(d) Sheet Piles.
(a) Wooden Piles. These are made from trunks of trees, such as Teak, Sal, Babul, Deodar etc. The wooden or timber piles are generally circular in shape, the diameter varying from 20 cm to 50 cm. The length of the pile is generally 20 times the diameter. The top of the pile is provided with an iron ring or cap and the bottom is sharpened and provided with iron shoe. Jf the soil is soft, blunt piles may be used, but if the ground Contains boulders, metal point should be Used. Timber piles should be driven below the permanent water table, otherwise they decay to fungi and insects. These piles are economical and can be driven rapidly without heavy machinery and much technical supervision.
(b) Concrete Pile. Concrete piles are made cast-in-situ. Holes of the specified diameter are made into the ground and filled with cement cencrete. Sometimes, the shell driven for making the hole is left inside and the concrete is filled. The advantage of this is that there is the shell to protect the cement concrete of the pile from getting disturbed or eroded by the action of acidic water encountered in the sub-stratum. These piles are used when they are to be driven to a hard stratum passing through plastic soils. These are sound in construction as they have not to bear hammer blows. These are cast in exact lengths and there is no wastage like in precast piles. The main drawback of these piles is that they cannot constructed under water.
(c) Reinforced Cement Concrete Piles. R.C.C. piles are generally precast and their feet are bevelled like wooden piles. The R.C.C. piles can be octagonal, square or circular in shape with steel helmets on their top. After the piles are cured and seasoned, they are driven into the ground. These piles are 15 cm to 60 cm in diameter and can be 3 m to 30 m in length R.CC. piles should not contain more than 4% steel. These piles can be cast early before starting the foundation work and the execution of the work can be done very quickly. Unlike timber piles these can be used above the ground water table. But these piles are very heavy and cause difficulty in transpiration and there are changes of their being damaged m transit.
(d) Sheet Piles. This class of pile is essentially used during the construction of foundation and not as foundation member of £ structure. Their main function is to enclose a certain area of the ground within which the foundation work can be carried and also to confine loose soil and prevent it from spreading. Sheet piles can be wooden, steel, concrete or R.C.C.
2. Working classification. This classification is based on the mode of working of the piles. According to this classification piles are divided into two groups, (i) Bearing pills, and (ft) Friction piles.
(i) Bearing Piles. These piles are used to bear vertical loads on their ends. Bearing piles are used in those places where the depth of hard stratum is not much.
When piles are driven upto the hard stratum, they transfer the load of the structure to the hard stratum below, those piles virtually act as columns.
(ii) Friction Piles. When the soil is very loose or soft to a considerable depth, friction piles are used. These piles balance the load of the structure by the friction offered by the surrounding soil on the sides of the piles. They are generally short in length and are not driven to the hard bed. The surface of the friction piles is made rough so as to increase skin friction.
The problem of friction piles is controversial. In some of the soils, the so is become loose due to some reason or the other and reduce the friction, which may result in the failure of the structure.
4. Raft Foundation. Raft or mat foundation is used in those places where spread footing or pile foundation cannot be used advantageously. This type of foundation is also recommended in such situations where the bearing capacity of the soil is very poor, the load of the structure is distributed over the whole floor area, or where a structure is subjected to constant shocks or jerks.
The raft foundation consists of a reinforced cement concrete Slab or R.C.C. T-beam slab placed over the entire area. The T-beam slab may consist of primary and secondary beams.
The T-beam may be inverted also. The inverted T-beam raft foundation is most suited to columned structures, such as in factories •or work-shops. The beams and the slab should be constructed all at a time so as to act as monolithic. The R.C.C. work is laid at the required depth of foundation and then upto the plinth, the inside spaces are filled with dry sand and gravel. The R.C.C. slab and beams can be laid directly over the rammed ground surface or over a bed of lime concrete.
5. Wall Foundation. Wells are a convenient method of securing a trustworthy foundation in deep sandy and soft solid Well foundation is generally provided .for in the construction of bridge piers, ghats etc., where the depth of water is moderate and the foundations are to be carried out in deep sandy soils of soft soils.
For the construction of well foundation in running water as for the construction of a bridge pier, a temporary dam is constructed to exclude the water from the place of construction. This temporary structure is known as cofferdam. The water from the inside of the cofferdam is pumped out. Now a well curb made of steel, concrete or wood with steel cutting edges, is placed over the desired position where the well is to be sunk. A masonry or concrete steining wall is constructed upto a height of 1 m. It is then allowed to dry. 1 he earth from the inside of the well is scooped out either by manual labour or by draggers, and then the well is allowed to sink. Another height of steining is constructed and the material from the inside is dragged out. The well sinks due to its own weight. The process is repeated till the well sinks to the acquired depth or reaches some hard stratum as the case may be. Before descending the outer surfaces of the steining is plastered smooth so as to minimize the frictional resistances. The sinking is tested by putting the desired loads on the top of the well.
When the sinking in all respects is completed, the lower portion upto a depth of nearly 3 m is plugged with cement concrete, the middle portion with sand and gravel and the top portion with cement concrete.
Now an R.C.C. well cup is constructed over the well. The top of the well cup should be below the bed level of the river. Over the well cap is now constructed the super structure of the bridge pier.
6. Caisson's Foundation. When the depth of water is considerable and the flow of water is such that cofferdam cannot be constructed easily and economically, then another method of well foundation is used which is called Caisson Foundation.
A caisson is a box made of steel, double walled and water-tight, Laving a well curb with cutting edges attached to its bottom. The drum is carried to the site, i.e. the position where it is to be placed. The drum is made to sink with the help of steel rails or sand bags-and is kept in position upright by means of steel ropes. The double walled steel caisson is filled with cement concrete, and the water from the inside in purred out. Now the soil from the inside is scooped out with the help of draggers and the caisson is allowed to sink slowly. The length of the caisson is increased by attaching another length of the caisson, and filled with cement concrete (with some reinforcement it required), soil is dragged out and it is allowed So sink to the required depth or when it reaches the hard stratum.The sinking is tested by putting the designed loads over it.
After the sinking is completed the bottom portion is plugged with cement concrete, middle portion with sand and gravel and •again the top portion with cement concrete. The steel caisson above She bed of the river is removed if possible and the remaining is allowed with the steining. In this case also R.C.C. well cup is constructed over the top of the caisson which the masonry pier is constructed.
7. Cantilever Foundation. This is a typical type of foundation, which is provided in such pleases where eccentric footings are to be provided for the external walls or columns due to restrictions of space or some other reasons. In this type of foundation separate footings are provided for the external and internal walls and they are simply connected with each other by a cantilever beam. The tendency of the exterior load to overturn, is balanced by whole or part of the downward pressure, acting at the other end of it.
8. Combined Footing Foundation. When two or more columns are supported by a single base area, the foundations to be provided in such cases are called combined footing foundations. The combined footings are also provided to establish the exterior columns along the boundry line, for white symmetrical footings are not possible. The exterior and the interior columns are constructed on the same base in such a way that the base area of the combined footing should be equal to the total load of the two columns, divided by the safe bearing capacity of the soil. The base area should be so shaped as to be symmetrical along the centre line of the columns.
9. Inverted Arch Foundation. This is not a common type of foundation. This type of foundation is used in such places where the bearing capacity of the soil is very poor and load of the structure is concentrated over the pillars. The other conditions of the soil are such that deep excavation are also not possible. For this foundation an inverted arch is constructed below the foot of pires etc. Generally segmental arches with a rise of l/5th to l/10th of the span are used. The span of arches will of course depend upon the arrangement of the pillars. The thickness of the arch ring, should not be less than 30 cm.
10. Grillage Foundation. This is also a very important type of foundation and is suitable for those situations where the load of the structure is pretty heavy and the hearing capacity of the soil is very poor. This foundation is specially suited where deep excavations are not possible. Grillage foundations are usually provided for the construction of stanchions.
It consists of a concrete base over which are placed one or two tiers of I-sections at right angle to each other. The area of the concrete base is calculated by dividing the total load of the structure by the bearing capacity of soil. A trench of the required dimensions, is excavated. Over this a cement concrete block generally 30 cm to 45 cm in thickness is spread and properly consolidated. When the contrite is partially dry, I-sections, i.e. Rolled Mild steel joists are placed at regular distances. (The size and the distance apart of the I-sections depends upon the load of the structure and the bearing; capacity of the soil). The lower flanges of the I-sections are connected to the concrete block by rich cement mortar. The I-sections are themselves connected to each other by pipes and bolts so as to form a rigid mass. Another tier of I-sections is placed at right angles to the previous one and connected by means of nuts and bolts. The whole unit is now embedded cement concrete so as to protect the steel from corrosion. Over this the structure is, constructed.
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The bearing capacity of a soil mainly depends on the closeness of its particles. The bearing capacity of a soil can be increased by the following methods:
1. By increasing the depth of foundation. The compactness of the foil increases as we go below the ground level. As the bearing capacity directly depends on the compactness of the soil, it will go on increasing as the depth of foundation is increased.
2. By draining of the sub-soil under. Water reduces the cohesive properties and hence reduces the bearing capacity of the soil. By draining off water from the sub-soil the bearing capacity of the soil is certainly increased.
3. By compacting the soil. If the soil is compacted thoroughly, the voids are decreased and bearing capacity is increased.
4. By confining the soil and preventing it form spreading and lateral movement. Spreading soils, if confined by sheet piling will resist more leads, that is, their bearing capacity will increase.
5. By increasing the width of foundation. By increasing the width of foundations, the intensity of load is decreased and on the same soil more loads can be placed. (Virtually speaking the bearing capacity of that particular area of the soil is increased).
6. By hardening the soil by grouting, i.e. pumping in the cement -grout into the ground. By grouting, the cohesive properties are increased and the soil will be able to take up more loads.
7. By solidifying the ground by chemical processes. In this case also the soil is compacted by mixing certain chemicals such as ' calcium chloride etc.
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The soils on which a structure rests may be classified into three categories:
(a) Hard soils. These soils are generally rocky in nature, incompressible and can bear fairly good loads. Solid rock, moorum and stony soils are examples.
(b) Soft soils. These are alluvial soils and are compressible when loaded. They can not take much load. Ordinary clay, loam, and common soils are examples of this.
(c) Spreading soils. These soils are compressible when they are confined and prevented from spreading. These soils when loaded spread out laterally. Sand and gravel are examples of this type of soil.
Bearing capacity of soils. Bearing power or Bearing capacity of a soil is defined as the maximum load that a soil can bear per unit area (usually Tonnes per sq. m) without yielding or causing cracks, displacement or rupture. This is the ultimate capacity of a soil. But as far as designing is concerned, we are less concerned with the ultimate load or the ultimate capacity of soil.
Methods of determining the Bearing Capacity of Soils
There are many methods of finding out the bearing capacity of soils. A very simple methods of determining the bearing capacity is as follows :
Dig a pit of size 2 mx2 m and of the required depth. The bottom of the pit is levelled by simply spreading the soil by hand. It should never be compacted. At the centre of this pit put a steel plate of 60 cm X 60 cm X 5 cm. Drive to pegs equidistant from the steel plate. Over the steel plate erect a wall 40 cm X40 cm either with bricks or stone or concrete blocks. Nearly 50 cm above the adjoining ground level. The difference: of levels between the top of the pegs and the wall is noted by a dumpy level. Now gently place the loads on the top of the wall by constructing a wooden platform. The load may consist of sand bags, girders or R.S.J. The loads are increased by a suitable amount. usually 0.5 tonne at an interval of 20 to 30 minutes. Before each increment of load the difference of levels between the pegs and the,top of the wall is noted. Note that the difference of level will remain constant till the soil yields. The moment the difference is increased, the increment of load must be stopped.
The bearing capacity of the soil will be the total load divided by the area of steel plate, that is
Bearing capacity = wt. of steel plate + wt. of wall + wt. of sand bags etc.
Area of plate (60 x 60 cm)
Safe Bearing Capacity. The safe load to be taken on a soil for the purpose of design is called Safe Bearing Capacity of the soil. The Safe Bearing Capacity of a soil may be defined as the bearing capacity of a soil divided by a number usually by constant and called factors of safety, i.e.
Safe Bearing Capacity = Ultimate Bearing capacity
Factor of Safety
The factor of safety depends on the type of building and the nature of the soil. Generally a factor of safety of 2 to 4 is taken for different purposes. Thus if the ultimate load of a soil is 6 tonnes/m2
and its factors of safety is 3 the working or design loads to be given to that soil will be 6/3 = 2 tonnes/m2. This is the safe bearing capacity of the soil.
Safe Bearing Capacity of Different Soils
Soft clay 2-3.75 tonnes/m2
Black cotton soil 5-7.5 tonnes/m2
Alluvial learn 7-5.16 tonnes/m2
Alluvial soil 5-7.5 tonnes/m2
Moist clay 11-18 tonnes/m2
Made-up ground 5 tonnes/m2
Ordinary clay 22 tonnes/m2
Clay mixed with and 22 tonnes/m2
Compact clay (dry) 33-55 tonnes/m2
Loose sand 22 tonnes/m2
Compact sand 22-32 tonnes/m2
Compact confind sand 44 tonnes/m2
Kankar or sandy gravel 22-32 tonnes/m2
Compact gravel 44-65 tonnes/m2
Moorum 22-44 tonnes/m2
Soft rock 25-85 tonnes/m2
Ordinary rock 85-110 tonnes/m2
Hard rock above 250 tonnes/m2
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