Foundation Design PDF

FOUNDATIONS Pavan G S Assistant Professor Department of Civil Engineering National Institute of Technology, Surathkal, Karnataka 575025  INTRODUCTION Two basic components :. Superstructure. Substructure FUNCTIONS OF FOUNDATIONS....

FOUNDATIONS Pavan G S Assistant Professor Department of Civil Engineering National Institute of Technology, Surathkal, Karnataka 575025  INTRODUCTION Two basic components :. Superstructure. Substructure FUNCTIONS OF FOUNDATIONS. Reduction of load intensity. Even distribution of load. Provision of level surface. Lateral stability. Safety against undermining. Protection against soil movement  TYPES OF FOUNDATIONS. SHALLOW FOUNDATIONS Spread footings Combined footings Strap footing Mat foundation Loading…. DEEP FOUNDATIONS Pile foundations Pier foundation Caisson foundation Well foundation  SPREAD FOOTING Footings that spread the superimposed load of wall or column over a large area. Single / Isolated footing Stepped footing Sloped footing Wall footing without step Stepped footing for wall Grillage foundation Loading… Simple Strip footing for Stepped Strip footing for walls walls  GRILLAGE Grillage FOUNDATION foundations consist of a number of layers of beams usually laid at right angles to each other and used to disperse heavy point loads from the superstructure to soil  COMBINED FOOTINGS A spread footing which supports two or more columns is called as combined footing. Rectangular combined footing Trapezoidal combined footing Combined column wall footings  STRAP FOOTING When two isolated footings of columns are connected by a beam, it is called as strap footing.  RAFT / MAT FOUNDATION Raft foundation covers the entire area beneath a structure and supports all the walls and columns. Used when - use of spread footing would cover more then 50% of the area - allowable soil pressure is low Eliminates differential settlement Types : Solid slab system, Beam slab system, Cellular system, Piled raft system. Solid slab  system Beam slab system Piled raft foundation Loading… Solid slab Cellular raft system system  DEEP FOUNDATIONS PILE FOUNDATION Foundation that transfers building loads to the earth farther down from the top surface by means of vertical members. It may be constructed using timber, concrete or steel. May be used instead of raft foundation , when there is no firm bearing strata at a reasonable depth. Types: - End bearing pile - Friction pile - Combined end bearing and friction pile - Compaction piles  Steel piles Concrete piles Driven Bored piles piles COMPACTION PILES Used to compact loose granular soil Increases bearing capacity Compaction piles themselves do not carry load Can be made of weaker material – sand, timber, bamboo  PIER FOUNDATION Cylindrical column of large diameter to support and transfer superimposed loads to the firm strata below. Transfers load only through bearing Shallower in depth than pile foundation Can be conveniently constructed in case of stiff clays Types: Masonry / Concrete pier Drilled caissons  WELL FOUNDATIONS / Box like structures sunkCAISSONS from the surface to the desired depth Much larger in diameter than pier foundations Used for major foundation works such as bridge piers, break waters. Hollow inside, which is filled later with concrete or sand. Not used for buildings Caisson  Question ? Given a building (superimposed loads) and a construction site, how do we decide on the type of foundation and depth of foundation? What are the concerns that we need to be aware of or that need to be addressed?  What is the order of occurrence and extent of soil and rock strata? What is the engineering properties of the soil and rock strata? For e.g., load bearing capacity, elastic modulus Location of ground water table, and its variation.  These questions are answered by performing a SITE EXPLORATION exercise. Test is performed to sample soil along the depth direction. Soil is sampled along the depth, up to the, significant depth.  Methods of site exploration Open excavation pit Boring methods – Augur boring – Augur and shell boring – Wash boring – Percussion boring – Rotary boring Subsurface sounding Geophysical methods Open excavation method (Open Trial Pits) Simplest form of site exploration Pits are excavated at the site exposing sub-soil in natural condition Ideally performed for depths upto 3m The method becomes expensive for larger depths.  Boring methods Augur boring: used in cohesive and soft soils above water table. Hand augers for up to a depth of 6m. Mechanical augurs for larger depths. Disadvantages: samples are highly disturbed. Augur and shell boring cylindrical shells with cutting edge at the lower end. Manual for stiff clays and, mechanical for stiff to hard clays. Wash boring  Rotary boring A drill bit is fixed to the lower end of the drill rod Rotated mechanically to drive into the soil layer Suitable for both rocks and soils Solution of water and bentonite is forced down the hollow drill rods  Sub surface soundings Cone penetration tests: A penetrometer is used. Penetrometer is usually a cone. Resistance to penetration is empirically corelated withLoading… properties like bearing capacity, density index etc. Two types are there: – SPT (standard penetration test) – CPT (cone penetration test) Geophysical method Shock waves are created at the ground level, say be striking a plate on the soil with a hammer The radiating waves are picked up by receivers located in the soil. Receivers are termed as geophones. Based on the time taken to receive the shock waves, nature of soil profile is gauged Should be satisfied, along with boring test  Bearing capacity of soil Gross pressure intensity of soil (q): It is the total pressure at the base of the footing due to the weight of the superstructure, weight of the footing and weight of the earth fill if any    Safe bearing capacity (qs)  Depth of footings? What should be the minimum depth of footing from the ground level? D  Single/ Isolated footing Shape of the footing is kept as same as that of the column: Square column : Square footing Rectangular column: Rectangular footing Circular column: Circular footing Provide equal overhang ‘a’ in both directions. Value of ‘a’ is generally around 10 to 20 cms. Base dimension of the concrete base should be at least twice the lateral dimension of the column in that direction.s Thickness of cement concrete base should be at least equal to the overhang distance, ‘a’. Stepped pad footing STRIP FOOTING FOR WALLS Width B = W/qs, where, If the width B is greater than t+2a, then it is essential to provide masonry W = Gross pressure intensity offsets at base of the footing qs = Safe bearing capacity of the soil STEPPED FOOTING FOR WALLS Minimum depth required for stepped footing: Ratio to be maintained is n:1 for masonry and n1:1 for concrete base. This implies, (B-T)/2 = n1*d+n*(Dmin-d) B-T = 2[n(Dmin-d)+n1*d]