FOUNDATION Engineering Design of shallow foundation Prepared by : Prof. Alka M. Shah Civil Engineering Department Institute of Technology Nirma University

What is Foundation ? PURPOSE Lowest part of a structure directly in contact with the ground and transmits all the loads to the sub-soil below it Majority of structure fails due to failure of their foundation Major purpose is to transmission of load in such a way soil is not over-stressed and does not undergo deformation PURPOSE To support structure Distributes the loads over a larger area Minimizes the differential settlements Increases stability & prevents overturning Distribute non-uniform load uniformly to the soil

DEEP FOUNDATIONS D > B Types of Foundation FOUNDATIONS SHALLOW FOUNDATION D ≤ B DEEP FOUNDATIONS D > B

Suitability of Foundation SHALLOW FOUNDATION Bearing capacity of soil is more. ground water table(W.T) is low. dewatering of foundation is not required. top layers of soil are uniform and stable. Load on the structure is less. Deep FOUNDATION Bearing capacity of soil is low. ground water table(W.T) is high. dewatering of foundation is costly and difficult. top layers of soil are non uniform and unstable. Load on the structure is more.

SHALLOW FOUNDATION Advantages: Cost (affordable) Construction Procedure (simple) Material (mostly concrete) Labour (doesn’t need expertise) Disadvantages: Settlement Foundation gets subjected to pullout, torsion etc Irregular ground surface(slope, retaining wall)

TYPES OF SHALLOW FOUNDATION 1) SPREAD FOOTING i) Continuous footing (strip or wall) ii) Isolated column footing a) Square b) Circular c) Rectangular iii) Reinforced concrete footings 2) STRAP FOOTING 3) COMBINED FOOTING i) Rectangular ii) Trapezoidal 4) RAFT FOUNDATION

1) SPREAD FOOTING Carries a single column Used to spread out loads from walls of columns over a wider area Consists of concrete slabs located under each structural column and a continuous slab under load-bearing walls Commonly used due to their low cost & ease of construction Used in small to medium size structures with moderate to good soil condition For high rise buildings it is not efficient

Simple wall footing without masonry offsets (SBC is high) Wall footing with masonry offsets (Stepped wall footing) (SBC is low)

Wall footing A spread footing for a continuous wall is called strip footing. (Used to support load bearing walls)

Reinforced Concrete footing Used where the walls are subjected to heavy loads & soil bearing capacity is low

ISOLATED COLUMN FOOTING Used to support individual columns.

ISOLATED COLUMN FOOTING

2) COMBINED FOOTING Supports two columns the two columns are so close to each other that their individual footings would overlap one column is placed right at the property line C.G. of column load and centroid of the footing should coincide

COMBINED FOOTING Rectangular footing Trapezoidal footing

3) STRAP FOOTING Two isolated footings connected with a strap or beam Connects in such a way that they behave as one unit The strap acts as a connecting beam and does not take any soil reaction. The strap is designed as rigid beam. It does not provide bearing

to connect an eccentrically loaded column footing to an interior column Used to transmit the moment caused from an eccentricity to the interior Can be used instead of combined footing if the distance between two columns is large More economical than combined footings

4) RAFT FOUNDATION Large reinforced concrete slab supporting no. of columns & walls area of spread footings or combined footings exceeds about 50 percent of the gross area of the building Greater weight & continuity of a mat provides sufficient resistance

RAFT / MAT FOUNDATION

Raft/Mat Foundation is suitable under following conditions Structural loads are heavy or the soil condition is poor Soft or loose soils having less bearing capacity Foundation soil is non-homogeneous and prone to excessive differential settlements Lateral loads are non-uniform Columns are so close that their individual footings would overlap To resist water pressures when groundwater extends above the floor level of the lowest basement of a building There is a large variation in the loads on individual columns

Factors Affecting Foundation Selection FACTORS AFFECTING FOUNDATION CHOICE: Sub-surface soil Ground water table conditions Type of structure Magnitude of load Tolerances of structure i.e. permissible values of settlement and tilt etc.

PERMISSIBLE DIFPERENTUL SRTTLEMENTS AND TILT IS:1904

Basic requirement of foundation Foundation must be properly located considering any future influence which could adversely affect its performance Soil supporting must be safe against shear failure Foundation must not settle or deflect Should safe against sliding and overturning

Design of foundation include Soil Design Structural Design

Geotechnical Shallow Foundation Design Soil Design of foundation Include: Loads coming on foundation W (DL, LL, etc.) Determination of Allowable Bearing pressure of soil, qa Determination of area of foundation Area = [W + Self weight of foundation(10% of W)] / qa Layout of foundation: such a way that C.G. of foundation coincides with C.G. of load Upward soil pressure on footing p = W/A < A. B. P

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