SHREE SWAMI ATMANAND SARASWATI INSTITUTE OF TECHNOLOGY,SURAT-6 SHALLOW FOUNDATION Group Members : 130760106026 – Khokhani Nikunj 130760106027 – Koshiya Krushal 130760106028 – Koshiya Pandav 130760106029 – Lukhi Zalak 130760106030 – Makwana Yash 130760106031 – Makwana Nisarg 130760106032 – Makwana Parth 130760106033 – Mevawala Kajol Faculty Name : Prof. Avinash Satashia Prof. Khushbu Raval

Index Shallow Foundation Types of shallow foundation Spread footing Combine footing Strap footing Mat or raft foundation

Shallow foundation When the depth of foundation is equal or less than the width of foundation , then it is termed as shallow foundation. It is also known as open foundation. A shallow foundation is placed immediately below the lowest part of the superstructure. A footing is a foundation unit constructed in brick work, stone masonry or concrete below the base of the wall or column for the purpose of distributing the structural-load over a wide area.

Types of shallow foundation The shallow foundation can be classified into the following types: Spread footing Combine footing Strap footing Mat or raft foundation

Spread footing : The spread footing are those which spread the superimposed load of wall or column over a larger area. The spread footing support either a column or a wall. The spread footing may be of the following kinds : Single footing for a column Stepped footing for a column Slopped footing for a column Wall footing without step Stepped footing for wall Grillage footing

Single footing for a column : A spread footing for a single column is either known as the isolated footing or pad footing. The footing mat consist of simple concrete block projecting out from the column face on all sides. The thickness of concrete block should atleast be equal to side offset from the column face. Stepped footing for a column : If the column load is more or if the safe bearing pressure of the soil is less, the base area will be large. In such a case, it is necessary to provide masonry offsets, to achieve larger spread, before the load is transferred to the concrete base.

Slopped footing for a column : These are also knows as isolated or individual column footing. Due to the bending strength, the footing constructed with brick, stone or plain concrete require considerable depth to the safe to carry heavy loads. RCC column footing may be circular, rectangular or square in plan. Wall footing without step : It is also called strip footing, which provides a continuous longitudinal bearing. Thus, a spread footing for a continuous wall is called strip footing. When the wall carries light loads and the safe bearing pressure is very high, width of the footing may be very small

Spread footing for wall

Stepped footing for wall : When the wall carries heavy load and the safe bearing pressure of the soil is not very high, the base width required may be much greater. The masonry offsets are provide to achieve larger spread. The depth of concrete bed is normally not less than 15 cm. Thus, in the case off typical wall footing, it the wall having thickness ‘t’ cm., the foundation width is equal to ‘(2t+30)’ cm. and the depth of foundation is equal to ‘3t’ cm. Grillage foundations : A grillage foundation is an isolated footing generally provided, when heavy structural loads from columns, piers or steel stanchions are required to be transferred to a soil having poor or low bearing capacity.

Steel grillage foundation : this type of foundation is lighter and more economical, for which deep excavation is not required and provides more area at the base, so that the intensity of pressure can be reduced within safe bearing capacity of soil. It can be broadly divided into two categories, depending apon the material used : (a) steel grillage foundation (b) timber grillage foundation Steel grillage foundation : Figure shows steel grillage foundation for steel stanchion. steel grillage foundation consists of steel joints or beams which are provide in single or double tiers. In case of double tier grillage, the top tier is laid at right angles to the bottom tires.

Method of construction : A minimum clearance of 8 cm. is kept between the beams, so that concrete can be poured easily and compacted properly. A minimum cover of 10 cm. is kept on the outer sides of the external beams. The depth of concrete below the lower tier should be atleast 15 cm. Method of construction : the excavation for foundation is done to the desire depth and the bed is well levelled. The foundation bed is then covered with a 15 cm. thick rich mix of concrete, which is well compacted to make it impervious. The grillage beams are then placed on this bed at specified distance using separators. The top surface of grilage beams is kept in a horizontal plane and rich cement grout is filled all around the beams to secure them to concrete bed.

The concrete is then placed between and around the beams. The second tier of beam is then placed at the right angles to the first tier and the entire space is filled with concrete. The steel stanchion is then erected on the base plate fixed on the second tiers beam; with the side angles and gusset plates.

(b) Timber grillage foundation : This type of foundation is provide for heavily loaded masonry walls or timber columns. This foundation is specially useful in waterlogged areas, where the bearing capacity of soil is very low and where the steel beams may get corroded due to subsoil water. The loading on the soil is limited to 50 to 60 kN/ m2 The foundation uses timber planks and timber beams in the place of steel joists. No concrete is embedded between the timber joists. However, the bottom concrete provided in steel grillage foundation is replaced by timber platform constructed of timber planks.

Combined footings : A combine footing is a single footing, which supports two columns. A combined footing is provided under the following circumstances: When the columns are very near to each other, so that their footing overlap. When the bearing capacity of soil is less, requiring more area under individual footing. When the end column is near a property line so that its footing cannot spread in that direction. A combine footing may be rectangular or trapezoidal in plan. The aim of combine footing is to get uniform pressure distribution under footing.

For this, center of gravity of the footing area should coincide with the C.G. of the combine loads of the two column. If the outer column, near the property line, carries heavier load, provision of trapezoidal column becomes essential to bring the C.G. of footing in line with the C.G. of two column loads.

Strap footing : A strap footing consists of two or more footing on individual columns, connected by a beam, called a strap. When a column is near or right next to adjacent property limit,a square or rectangular footing concentrically located under the column would extend into the adjoining property, which may not be permissible. In that case. The strap footing may be provide. The strap beam, connecting the spread footing of the two columns, does not remain in contact with soil and thus does not transfer any pressure to the soil. The function of the strap footing beam is transfer the load of heavily outer column to the inner one. In doing so, the strap beam is subjected to bending moment and shere-force.

Raft foundation : A raft or mat is a combine footing, which covers the entire area below the whole building or structural and supports all the walls and columns. A raft foundation is preferred in the following situation : When the allowable soil pressure is low or the building loads are heavy, the use of spread footing would cover more than one-half of the area and it may prove more economical to use mat or raft foundation. They are also used, where the soil mass contains compressible that the differential settlement would be difficult to control. A raft foundation is also needed to reduce settlement on highly compressible soils.