SHALLOW FOUNDATION BRANCH:-CIVIL ENGINEERING SUBJECT:-BUILDING CONSTRUCTION SUBJECT CODE:-2130607 GUIDED BY:- KAMLESH DALAL BHAGWAN MAHAVIR COLLEGE OF ENGINEERING AND TECHNOLOGY GONDALIYA NAYAN 130060106512 ITALIYA DARSHAN 130060106517 SUVAGIYA SNEHAL 130060106562 SACHANI SANKET 130060106552 PONKIYA ZALAK 130060106561 INTRODUCTION The Foundation Can be classified broadly into two types: 1. Shallow foundations 2. Deep foundations SHALLOW FOUNDATIONS: When the depth of foundation is equal to or less than the width of foundation, then it is termed as shallow foundation. It is also know 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: 1. Spread footing 2. Combined footing 3. Strap footing 4. Mat or Raft foundation 1. 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: i. Single footing for a column ii. Stepped footing for a column iii. Slopped footing (Trapezoidal) for a column iv. Wall footing without step v. Stepped footing for wall vi. Grillage foundation (i) Single footing for a column: A spread footing for a single column is either known as the isolated or pad footing. In this case, the footing may consist of simple concrete block projecting out from the column face on all sides. The base dimensions of the concrete base should not be less than twice the appropriate lateral dimension of the column in that direction. The thickness of concrete block should atleast be equal to side offset from the column face. (ii) 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. The height and width of each should be so proportioned that the rate of spread does not exceed the permissible value for the masonry. (iii) Slopped footing for a column: These are also known as Isolated or Individual column footings, They have the projection in the concrete base. Due to the low bending strength, the footing constructed with brick, stone or plain concrete required considererable depth to be safe to carry heavy loads. The depth of plain concrete footing can be reduced much, by providing reinforcement at its base to take up tensile stresses. RCC column footing may be circular, rectangular or square in plan. Figure : Spread footing (vi) 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. It can be broadly divided into two categories, depending upon the material used: a) Steel grillage foundation b) Timber grillage foundation (a) Steel grillage foundation: Fig. 3.4 shows steel grillage foundation for steel stanchion. Steel grillage foundation consists of steel joints or beams (Rolled Steel Joints – RSJ) which are provided in single or double tiers. A Minimum cover of 10cm. is kept on the outer sides of the external beams as well as above the upper flanges of the top tier. The depth of concrete below the lower tier should be atleast 15cm. Figure : steel grillage foundation (b) Timber grillage foundation: Times type of foundation is provided 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 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 joints. However, the bottom concrete provided in steel grillage foundation is replaced by timber platform constructed of timber planks. Fig(a) Timber Grillage foundation for wooden post Fig. 3.5(a) shows a typical timber grillage foundation for a timber column. The excavation for the base is levelled. The bottom layer of timber planks of size 20 to 30cm. Wide and 5.0 to 7.5 cm. thick is laid, side by side; without any gap between them. Over the top of this layer, a timber beam of same section as that of the wooden post is placed at right angles. Fig. 3.5(b) Timber Grillage Foundation For Masonry wall Fig. shows the timber grillage foundation for wall. The foundation consists of bottom layer of planks, over which the wooden beams are placed at right angles to the direction of the planks. 3. Strap footing: A strap footing consists of two or more footing of individual column by a beam, called a strap. 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 beam is to transfer the load of heavily loaded outer column to the inner one. In doing so, the strap beam is subjected to bending moment and shear-force and it should be designed to withstand these. Fig. 3.7 Strap Footing FOUNDATIONS IN BLACK COTTON SOILS: Construction of building on black cotton or expansive soils is very much dangerous due to its volumetric changes with the change of atmospheric conditions. The differential settlement of the buildings, caused by the movement of the ground due to the alternate swelling and shrinkage, results in formation of cracks thus formed are sometimes 15 to 20 cm. wide and 2.5 to 4.0 m. deep. 1.Precaution for Foundation in Black Cotton soils: - The foundation should be taken to such depths, where the cracks cease to extend. The minimum depth of foundation should be atleast 1.50m. - The construction in black cotton soil should be carried out during dry season. - The external walls should be provided with plinth protection at ground level, so that moisture does not enter in foundation during monsoon. 2.Types of foundation in Black Cotton soils: i. Strip or pad foundation ii. Pier foundation iii.Under-reamed pile foundation (i) Strip or pad foundation: The strip foundation for walls and the pad foundation for columns may be provided for medium loads. When the soil is soft and having poor bearing capacity, a 30cm thick layer of ballast and moorum should be provided and rammed and then 30 cm. thick layer of coarse sand may be placed. Fig 3.9 (ii) Pier foundation: For the walls carrying heavy loads, the pier foundation with arches may be provided. The piers are dug at regular interval and filled with concrete, which are connected by concrete or masonry arch and the wall may be constructed over it. The arches are constructed with a gap above the ground level, which may permit the movement of soil during swelling and shrinkage operatings. Causes of failure of foundation • Unequal settlement of sub-soil. • Unequal settlement of masonry. • Horizontal movement of adjoining the structures. • Shrinkage due to with drawl of moisture from soil below the foundation. • Lateral pressure on walls. • Action of atmosphere. • Lateral movement of soil below foundation.