Foundations

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Foundations

Definition

Foundation is the lowest part of a structure which provides a base for the super- structure proper. super-

Purpose of Foundation

To distribute the weight of the structure over large area son as to avoid over-loading of the soil beneath. overTo load the sub-stratum evenly and thus prevent subunequal settlement. To take the structure deep into the ground and thus increase its stability ,preventing overturning.

Types Of FoundationsFoundations can be broadly classified into two types (i). Deep Foundations (ii). Shallow Foundations Again Deep Foundations can be classified as Pile, Cofferdams and Caisson Fall Foundations.

Shallow FoundationsWhen the foundation is placed immediately beneath the lowest part of the super structure, it is termed as the Shallow Foundations. The object of this type of foundation is to distribute the structural loads over a wide horizontal area at shallow depth below the ground level. Various types of Shallow Foundations are as follows 1.Spread Footings 2.Grillage Foundations 3. Eccentrically Loaded footings 4. Combined Footings 5. Mat or Raft Foundations.

Spread Footings

As the name suggests, in case of Spread Footings ,the base of the member transmitting load to the soil is made wider so as to distribute the load over wider area. The various Footings described under this category are: (a). Wall Footings (b). Reinforced Concrete footings (c ). Inverted Arch Footings (d). Column Footings

Wall FootingsIt consists of several courses of bricks ,the lowest course being usually twice the breadth of the wall above The depth of each course is usually 10 cm. A bed of lean concrete of uniform thickness is first spread over the entire length of the wall. The concrete bed provides a plain surface upon which the masonry work for wall footing can be started directly.

Reinforced Concrete Footings

In places where the wall are subjected to relatively heavy loading and the bearing capacity of the soil on which the wall footing is to rest is low. In such cases it is desirable to provide reinforced concrete footing below the wall. This appreciably reduces the volume of masonry work in footing and depth of bed concrete and as such proves to be economical. A 7 to 8 cm. thick bed of lean concrete is provided below the reinforced concrete footing to perform the function of concrete bed block.

Inverted Arch FootingsThis type of foundation used to be provided for multistoried buildings in olden times. However with the advent of reinforced cement concrete construction practice ,inverted ach construction is rarely done these days. The advantage of inverted arch construction is that in soft soils the depth of foundation is greatly reduced.

Column Footings (Independent)

An Independent footing is one which is provided under the column or other similar member for distributing the concentrated loads in the form of uniformly distributed load on the soil below. The footings may be square, rectangular or circular in plan. Depending upon the load to be carried and the bearing capacity of the soil, independent footing may be of brick masonry, stonemasonry, R.C.C., steel Grillage Etc.

(i). Footing for brick pillar: Square footing is the simplest and most economical to be provided under pillar ,columns. (ii). R.C.C Column Footings: On account of their low bending strength the footings constructed with brick ,stone or plain concrete require considerable depth to be safe to carry loads. R.C.C column footings may be circular, rectangular, or square in plan. (iii). Stone Pillar Footings: This type of footing is similar in construction to the footing for brick pillar. In this case, the regular offsets on all the four sides of pillar are slightly bugger in width and depth.

Grillage Foundations

When heavy structural loads from columns , piers or stanchions are required to be transfer to a soil of low bearing capacity ,grillage foundation is often found to be lighter and economical. This avoids deep excavation and provides necessary area at the base to reduce the intensity of pressure within safe bearing capacity of soil. Grillage Foundations can be broadly divided in the following two categories. 1.Steel Grillage 2.Timber Grillage

Ecentrically Loaded Footings

As far as practically , the foundation, should be so shaped and proportions that the centre of gravity of the imposed loads coincide with the CG of the supporting area of base. However, when walls or columns are to be placed close to property lines, the required supporting areas of the base cannot be placed concentrically with the imposed loads without overlapping the property lines. Hence , different methods are adopted to ensure the stability of wall or column without encroaching the area outside the property line of building.

Combined Footings

A combined footing is so proportioned that the centre of gravity of the supporting area is in line with the CG of the two column loads. A Combined Footing may be rectangular or trapezoidal in shape. Rectangular shape is only possible where loading condition is such that either the two columns are equally loaded or the interior column carries greater load. On the other hand, in case of trapezoidal footing , no such condition is applicable.

Raft FoundationIn made-up ground , soft clay or marshy site madehaving low value of bearing capacity, heavy concentrated structural loads are generally supported by providing raft foundation. It provides an economical solution to difficult site conditions. Raft foundation consists of thick reinforced concrete slab covering the entire area of the bottom of the structure like a floor.

Deep Foundations

In case, the strata of good bearing capacity is not available near the ground, the foundation of the structure has to be taken deep with the purpose of attaining a bearing stratum which is suitable in all respects. The most common forms of construction pertaining to Deep foundation are (a). Piles (b). Cofferdams (c ). Caissons

Pile Foundations

Pile Foundation is generally used when simple spread foundation at a suitable depth is not possible either because the stratum of required bearing capacity is at a greater depth. Depending upon their function or use of Piles may be classified into the following types. (i). Bearing Piles (ii). Friction Piles (iii).Sheet Piles (iv).Anchor Piles (v). Batter Piles (vi). Fender Piles (vii). Compaction Piles.

Bearing Piles: Bearing Piles are those which are driven into the ground until a hard stratum is reached. Such piles act as pillars supporting the super structure and transmitting the load down to the level at which it can be safely borne by the ground. Friction Piles: When piles are required to be driven at a side where the soil is weak or soft to considerable depth, the load carried by a pile is borne by the friction developed between the sides of the pile and the surrounding ground. Sheet Piles: Sheet Piles differ from bearing or friction piles in that they are rarely used to furnish vertical support but are used to function as retaining wall.

Anchor Piles: When Piles are used to provide anchorage against horizontal pull from sheet piling walls are pulling forces, they are termed as Anchor Piles. Batter Piles: When Piles are driven at an inclination to resist large horizontal or inclined forces ,the piles are termed as Batter Piles. Fender Piles: When the Piles are used to protect concrete deck or other water front structures from the abrasion or impact that may be caused from the ships they are called Fender Piles. Compaction Piles: When Piles are driven in grannular soil with the aim of increasing the bearing capacity of the soil. These are Compaction Piles.

Depending upon the materials used in their manufacture, piles can be broadly classified as 1. Timber Piles 2. Concrete Piles 3. Composite Piles 4.Steel Piles.

Cofferdams

A Cofferdam May be defined as a temporary structure constructed in a river or lake or any other water bearing surface for excluding water from a given site to enable the building operation to be performed on dry surface. Considering the materials used in construction , cofferdams may be divided into the following categories 1. Earthen Cofferdam 2. Rockfill Cofferdam 3.Single Walled Cofferdam 4.Double Walled Cofferdam 5. Crib Cofferdam 6.Cellular Cofferdam

Earthen Cofferdam: It essentially consists of an earthen embankment built around the area to be enclosed. It is constructed in places where the depth of water is not much, and the velocity of the current is very low. Rock fill Cofferdams: If the depth of water to be retained by the embankment of cofferdam is of order 1.8 to 3 m. Stone or rubble is used for the embankment . Single Walled Cofferdam: This type of Cofferdam is used in places where the area to be enclosed is very small and the depth of water is more. Double Walled Cofferdam: For Cofferdams required to enclose larger areas in deep water, single walled type becomes un economical as larger sections trussed struts would be necessary to resist the water pressure.

Crib Cofferdams: In deep waters where it is difficult to penetrate the guide piles or sheet piles into the hard bed below crib cofferdam is used. Cellular Coffer dam: This type of cofferdam is mostly used for dewatering large areas in places where the depth of water may be of the order of 181821m. The two common shapes of cellular cofferdam are 1. Circular type cellular Cofferdam 2. Diaphragm type cellular Cofferdam

Caissons

A Caisson may be defined as a water light structure made up of wood, steel or reinforced concrete constructed in connection with excavation for foundation of bridges ,abuttments in rivers and lakes. Types of Caissons 1.Open Caisson 2.Bo