What is Foundations

TYPES OF FOUNDATIONS

What is Foundations?

Foundation of a structure is like the roots of a tree without which the tree cannot stand. The construction of any structure, be it a residence or a skyscraper; starts with the laying of foundations.it is the lowest part of the structure that transfer the building lod safily to the soil or ground.

Before designing the foundation, the type of soil is determined. Depending on whether the soil is hard soil or soft soil, a specific type of foundation is adopted.

Foundations are made in various materials… They could be reinforced cement concrete foundations or brick foundations or stone rubble masonry foundations etc. The choice of material to be used in the construction of foundations also depends on the weight of the structure on the ground.

The bearing capacity of soil plays a major role in deciding the type of foundation. The safe bearing capacity of soil should be 180N/mm2 to 200N/mm2.

Foundations are broadly classified into shallow foundations and deep foundations. The depth of the foundation means the difference of level between the ground surface and the base of the foundation. If the depth of the foundation is greater than its width the foundation is classified as a deep foundation.

Shallow foundations can be made in depths of as little as 3ft (1m), while deep foundations can be made at depths of 60 - 200ft (20 - 65m). Shallow foundations are used for small, light buildings, while deep ones are for large, heavy buildings.

Shallow foundations are commonly used in smaller structures such as residences and small buildings whose floor height is limited to 10m whereas Deep Foundations are used in Skyscrapers…. Piles are the most commonly used Deep Foundations used in skyscrapers…

Types of foundations along with brief overview of each of them which include

a) Shallow Foundation which is further classified into

Spread footing or open trench foundations Grillage foundations Raft foundations Stepped foundations Inverted arch foundations Compensated Foundation

b) Deep Foundation which is further classified into

Pile foundation Well foundation Caisson foundation

3) Design Procedure of foundations

TYPES OF FOUNDATIONSStep-1: Given DataFactored load on column Pu in ‘KN’ Size of column, l x b in ‘mm’SBC of Soil in KN/sq.m Self weight of footing = 10% of Pu

Step-2:Area of footing = Total Load/S.B.C Provide a suitable size of footing L x BStep-3:Net upward pressure intensity,

Pu/area of footing.Step-4:

Design for flexureCritical section occurs at the face of column l = (L – l)/2Mutr =wl2/2

Step-5:Depth of footingd = [(Mu/(2.76×1000)) ]1/2

Step-6:Calculation of area of steel,Mu = 0.87fyAst(d-0.42xu)

Step-7:

Check for One-way shearCritical shear force occurs at a distance of‘d’ from face of the column

Nominal shear stress, Tu = v/bd for 0.15% of steel from IS456:2000, Table-19 Permissible shear stress

If Tc>Tv footing is safe in shear


Shallow Foundation

1) Spread Footing Foundations :

Foundations constructed by increasing the are at the base of the structure by means of offsets, are called spread footing foundations. In such foundations, spread is given under the base of a wall or a column by providing offsets. This spread is known as footing and the foundation itself is called spread footing.

(Spread Footing)

Broadly speaking, all types of shallow foundations can be referred to as spread footing foundations. However ,from design and construction point of view, they have been designated separately. The various types of spread footing foundations are ;

a) Wall footings:

Wall Footing

This is a common and the simplest type of spread footing foundation; It consists of a number of courses of bricks, the lowest being usually twice the thickness of wall above. In this type of foundation, the base width of the wall is increased by providing 5cm (one-fourth of brick length) offsets on either side of the wall. The depth of each course is usually 10 cm

In some cases, however, the bottom course is made 20cm deep. In the case of footing for store walls, the size of offset is slightly more than that of the brick wall footing.

Method of Construction of Wall Footing:

For constructing such a footing, a bed of lean cement concrete (1:8:16) is first laid over the entire length of the wall. The thickness of this lean concrete bed is usually kept 15cm and its width is kept 20 to 30 cm more than that of the bottom course.

In no case, the depth of the concrete bed should be less than its projection beyond the wall base. For foundations over firm soil or compacted ground, the concrete bed below the brick wall footing may be dispensed with. After laying the concrete for constructing the concrete bed, it should be properly compacted and cured before laying the base course of the wall.


Suitability:

This is the cheapest type of spread footing foundations and is largely used for walls of ordinary buildings.

b) Masonry Pillar Footing:

Isolated footings are used to support the individual pillars and columns constructed in brick or stone masonry. They are stepped down in the wall footing after providing 15cm offset of foundation concrete on all the sides.

(Masonry Pillar Footing)

c) Concrete Column Footing:

These are either stepped type, slate type or slope type, having projections in the vae concrete. To support heavy loads, reinforcement is also provided at the base. The reinforcement provided is in the form of steel bars and is placed in both directions. Concrete column footings may be either isolated footings or combined footings. Isolated footings are used to support the individual columns, whereas combined footings are used where projections of different columns are not possible on all the sides, due to limited space.

(concrete column footing)

The combined footings are generally rectangular in shape when both columns are of same sections and trapezoidal in shape when the columns are of different sections.

2) Grillage Foundation:

The foundation which consists of one or two tiers of wooden or rolled steel section with space filled up with concrete is known as Grillage foundation. This is so called because the bed constructed in this type of foundation is

TYPES OF FOUNDATIONScalled grillage. This type of construction avoids deep excavation and provides the necessary area at the base of the structure to reduce the intensity of pressure within the safe bearing capacity of the soil.

OBJECT :

The object of this type of foundation is to spread the load over a large horizontal area at the base of a structure.

TYPES :

Depending upon the material used in construction, grillage foundations are further classified into two types.

Steel Grillage Foundation Timber Grillage Foundation

(Steel Grillage foundation)

Steel Grillage Foundation

Method of construction of steel grillage foundation:

For constructing steel grillage foundation, a trench is excavated to the calculated width and about 0.90 m - 1.50 m deep. Bottom of the trench is rammed and leveled

Then a layer of lean cement concrete ( 1: 8 : 16 ), about 30cm thick, is laid and compacted.

After this, a layer of rich cement concrete (1: 2 :4 ), about 15cm thick is spread and compacted to form a concrbed.

Over the concrete bed thus prepared, the bottom tier consisting of a number of steel I - beams of designed dimensions are placed at specified distance apart, using spacer bars. The space in between and around the steel beams is then filled with cement concrete.

On this bottom tier, a second layer of steel I - beam is placed, if required.

The entire space is then filled with cement concrete 1: 2 : 4. On the grillage bed thus prepared, the structure in the form of a steel stanchion, column, pier is built.

TYPES OF FOUNDATIONSFor construction of the timber grillage foundation,

Wooden planks, usually 5 to 8 cm in thickness, are laid side by side longitudinally on the prepared bed of the trench excavated for this purpose.

The planks should be arranged in a width 45 to 60 cm more on either side than the designed width of the base course of the structure.

On this bottom layer of planks, rectangular timber sections of suitable size are laid at about 35 cm center to center.

Then a top layer of planks, usually 8 to 10 cm in thickness, arranged side by side, in width equal to that of the base course of the structure is to be built over it

On the timber platform thus prepared, the structure in the form of a timber column or even a masonry wall is built.

Suitability of steel grillage foundation:

Steel grillage foundations are useful for structures like columns, piers, stanchions subjected to heavy concentrated loads and hence are employed for foundations of the buildings such as theaters, factories, town, halls etc. Timber grillage foundations are usually provided for timber columns subjected to heavy concentrated loads.

This type of grillage foundation can also be safely used for light buildings where the soil encountered is soft and is permanently water-logged.

RAFT FOUNDATION :

The foundation consisting of a thick R.C.C slab covering the whole area of a mat is known as raft foundation .Method of construction of Raft Foundation:

( Raft Footing)

The whole area is dug out to the specified depth and 30 cm more wide than the area to be covered. The bed is compacted and sprinkled over with water. Then a layer of lime concrete or lean concrete ( 1: 8 : 16 ) is laid to a suitable thickness to act as a bottom

cover. After this, the reinforcement is laid. The reinforcement consists of closely spaced bars placed at right

angles to one another.

TYPES OF FOUNDATIONS Then the cement concrete (1 : 2 : 4 ) is laid and compacted to the required thickness. The concrete slab so laid is then properly cured When loads are excessive, thick concrete beams running under the columns can also be constructed.

SUITABILITY :

Raft Foundation Construction

This type of foundation is useful for public buildings, office buildings, school buildings, residential quarters etc, where the ground conditions are very poor and bearing power of the soil is so low that individual spread footing cannot be provided.

STEPPED FOUNDATION :

For constructing this type of foundation, excavation is done into steps having short length and uniform thickness and the masonry work is done on the horizontal bed of concrete thus prepared.

If there is any possibility of slipping of the structure bodily, R.C.C piles can be driven along its base concrete on the sloping side.

(STEPPED Footing)

INVERTED ARCH FOUNDATION :

The foundation consisting of inverted arches between the piers is known as inverted arch foundation. In this type of foundation, the load from the piers is transferred to the soil by constructing arches in inverted position at their at their base. The rise of the inverted arches is about one - fifth to one - tenth of the span and they are usually build in half-brick rings. The position of arches may be either along the row of piers or across the row of piers in both the directions depending upon the nature of soil and the type of land to be taken by them


.

(Invert Foundation)

METHOD OF CONSTRUCTION :

For constructing this type this type of foundation;

Excavation is done to the required depth. The bottom of the excavation is leveled and compacted Then the foundation concrete is laid to the required thickness and finished according to the layout of the

arches to be constructed under the piers. The arch rings, usually half brick thick, are then built in cement mortar and piers are constructed. The end piers should be designed and constructed to take the outward thrust caused by arch action.

SUITABILITY :

This of foundation is not commonly used for buildings, but it is quite suitable for other structures like bridges, reservoirs, tanks, supports for drainage lines etc.

DEEP FOUNDATION :

The foundation constructed sufficiently below ground level with some artificial arrangements such as piles, wells etc, at their base are called deep foundations. Deep foundation are further classified into the following types ;

Pile foundation Well foundation Caisson foundation

PILE FOUNDATION:

Piles are used if the bearing capacity of soil at shallow depth is not enough to withstand the structural load. So the piles are driven to transfer the loads from the structure to the high bearing capacity soil/rock stratum.

(pile steel) (Pile foundation)

TYPES OF FOUNDATIONSThe main functions of using pile foundations are listed below in short

To transfer the load from the structure to the foundation at a greater depth where the bearing capacity of soil is efficient to withhold the structure loads.

Pile controls the soil settlement which can be accompanied by surface foundations. Piles are used to increase the safety factor for high loaded structures. Piles enable the scope of construction in very low bearing capacity soils.

A foundation ( spread footing or grillage ) supported on piles is called a pile foundation. A pile foundation usually consists of a base of spread footing or grillage supported by piles at their bottom. Piles distribute the load of structure to the soil in contact either by friction alone or by friction combined with bearing at their ends.

SUITABILITY :

This type of foundation is suitable under the following situations ; When the soil is very soft and solid base is not available at a reasonable depth to keep the bearing power

within safe limits. When the grillage and raft foundation are very expansive. When the building is very high carrying heavy concentrated loads. When it is necessary to construct a building along the sea shore or river bed

Casson Foundation

In geotechnical engineering, a caisson (/ˈkeɪsən/ or /ˈkeɪsɒn/) is a watertight retaining structure used, for example, to work on the foundations of a bridge pier, for the construction of a concrete dam,or for the repair of ships. These are constructed such that the water can be pumped out, keeping the working environment dry. When piers are to be built using an open caisson and it is not practical to reach suitable soil, friction pilings may be driven to form a suitable sub-foundation. These piles are connected by a foundation pad upon which the column pier is erected.

How caissons work

Shallow caissons may be open to the air, whereas pneumatic caissons, which penetrate soft mud, are sealed at the top and filled with compressed air to keep water and mud out at depth. An airlock allows access to the chamber. Workers move mud and rock debris (called muck) from the edge of the workspace to a water-filled pit, connected by a tube (called the muck tube) to the surface. A crane at the surface removes the soil with a clamshell bucket. The water pressure in the tube balances the air pressure, with excess air escaping up the muck tube. The pressurized air flow must be constant to ensure regular air changes for the workers and prevent excessive inflow of mud or water at the base of the caisson.

Installation

To install a caisson in place, it is brought down through soft mud until a suitable foundation material is encountered. While bedrock is preferred, a stable, hard mud is sometimes used when bedrock is too deep.


(Casson Foundation)

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What is Foundations