FOUNDATION

NAME: SNEHA COUTINHOSUBJECT: A.B.C.M.

YEAR: T.Y.B.ARCH ‘A’BHARATI VIDYAPEETH C.O.A.

FOUNDATION• A STRUCTURE ESSENTIALLY

CONSISTS OF TWO PARTS, NAMELY THE SUPER STRUCTURE WHICH IS ABOVE THE PLINTH LEVEL AND THE SUBSTRUCTURE WHICH IS BELOW THE PLINTH LEVEL.

• SUBSTRUCTURE IS OTHERWISE KNOWN AS THE FOUNDATION AND THIS FORMS THE BASE FOR ANY STRUCTURE. GENERALLY ABOUT 30% OF THE TOTAL CONSTRUCTION COST IS SPENT ON THE FOUNDATION.

• THE SOIL ON WHICH THE FOUNDATION RESTS IS CALLED THE “FOUNDATION SOIL”.

OBJECTIVES OF A FOUNDATION :• A FOUNDATION IS PROVIDED FOR THE FOLLOWING PURPOSES:• TO DISTRIBUTE THE TOTAL LOAD COMING ON THE STRUCTURE ON A

LARGER AREA.• TO SUPPORT THE STRUCTURES.• TO GIVE ENOUGH STABILITY TO THE STRUCTURES AGAINST VARIOUS

DISTURBING FORCES, SUCH AS WIND AND RAIN.• TO PREPARE A LEVEL SURFACE FOR CONCRETING AND MASONRY WORK

ESSENTIAL REQUIREMENTS OF A GOOD FOUNDATION:• THE FOUNDATION SHOULD BE SO LOCATED THAT IT IS ABLE TO RESIST

ANY UNEXPECTED • FUTURE INFLUENCE WHICH MAY ADVERSELY AFFECT ITS PERFORMANCE.• THE FOUNDATION SHOULD BE STABLE OR SAFE AGAINST ANY POSSIBLE

FAILURE.• THE FOUNDATION SHOULD NOT SETTLE OR DEFLECT TO SUCH AN

EXTENT THAT WILL IMPAIR ITS USEFULNESS

DISTRIBUTION OF SOIL PRESSURE

Soil pressure distribution in non-cohesive soil.

Soil pressure distribution in cohesive soil.

TYPES OF FOUNDATION

FOUNDATION

SHALLOW FOUNDATION

ISOLATED FOOTING

PAD FOUNDATION

COMBINED FOOTING

STRIP FOUNDATION

RAFT FOUNDATION

DEEP FOUNDATION

PILE FOUNDATION

FRICTION PILE

LOAD BEARING PILE

PIER FOUNDATION

ISOLATED FOUNDATION• IN FRAMED STRUCTURES

WHERE SEVERAL COLUMNS ARE TO BE CONSTRUCTED, ISOLATED FOOTINGS CAN BE ADOPTED.

• THE COLUMNS INVOLVED CAN BE PROVIDED WITH MASONRY OR CONCRETE FOOTING.

• IF MASONRY FOOTING IS PROVIDED, STEPS ARE GIVEN AND THE FOUNDATION AREA IS THUS INCREASED SO THAT THE STRESSES DEVELOPED AT THE BASE IS WITHIN THE LIMIT.

• CONCRETE CAN BE MOULDED TO ANY SHAPE AND HENCE A CONCRETE FOOTING MAY BE A SLOPING ONE TO PROVIDE SUFFICIENT SPREAD.

ISOLATED FOUNDATION

DEEP FOUNDATIONS• LOADS ARE SUPPORTED IN TWO WAYS.• IF THE LOAD IS SUPPORTED BY THE

EFFECT OF FRICTION BETWEEN THE SOIL AND THE PILE SKIN, IT IS CALLED FRICTION PILE.

• FRICTION PILES MAY BE MADE OF CAST IRON, CEMENT CONCRETE, TIMBER, STEEL, WROUGHT IRON AND COMPOSITE MATERIALS.

• IF THE LOAD IS SUPPORTED BY RESTING THE PILE ON A VERY HARD STRATUM, IT IS CALLED LOAD BEARING PILE.

• LOAD BEARING PILES ARE STEEL SHEET PILES, CONCRETE PILES AND TIMBER PILES.

• PILES MAY BE CAST-IN-SITU OR PRECAST.

• THEY MAY BE CASED OR UNCASED.

LOAD BEARING PILE FRICTION PILE

BEARING CAPACITY OF SOIL :• IT IS DEFINED AS THE MAXIMUM LOAD PER UNIT AREA WHICH THE SOIL WILL

RESIST SAFELY WITHOUT DISPLACEMENT• THE BEARING CAPACITY OF THE SOIL CAN BE FOUND BY LOADING THE SOIL,

NOTING THE SETTLEMENT AND BY DIVIDING THE MAXIMUM LOAD BY THE AREA ON WHICH THE LOAD IS APPLIED. THE MAXIMUM LOAD IS OBTAINED FROM THE GRAPH BETWEEN THE SETTLEMENT AND LOAD.

• SAFE BEARING CAPACITY OF SOIL = [ULTIMATE BEARING CAPACITY OF SOIL] / [FACTOR OF SAFETY]

• ON COMPLETION OF A STRUCTURE THERE MAY BE SOME DISPLACEMENT IN THE POSITION OF THE FOUNDATION.

• FOR ORDINARY FRAMED STRUCTURES OF CONCRETE, THE PERMISSIBLE ANGULAR DISTORTION IS 1/500 AND THE DESIRABLE VALUE IS 1/1000.

• THE MAXIMUM DIFFERENTIAL SETTLEMENT SHOULD NOT EXCEED 25 MM IN CASE OF FOUNDATIONS ON SANDY SOIL AND 40 MM IN CASE OF FOUNDATIONS ON CLAYEY SOIL.

• IN CASE OF NON-COHESIVE SOILS, SUCH AS SAND AND GRAVEL, THE ALLOWABLE BEARING CAPACITY SHOULD BE REDUCED BY 50%, PROVIDED THAT THE WATER TABLE IS ABOVE OR NEAR THE BEARING SURFACE OF THE SOIL.

• THE BEARING CAPACITY OF RECLAIMED SOILS OR SHRINKABLE SOILS CAN BE TAKEN AS 50KN/M2 IN THE ABSENCE OF THE SITE DATA.

SHALLOW FOUNDATION VS DEEP FOUNDATION

SHALLOW FOUNDATION• IN SHALLOW FOUNDATION, GENERALLY, THE DEPTH AT WHICH THE

FOUNDATION IS PLACED IS LESS THAN ITS WIDTH I.E. THE RATIO OF DEPTH OF FOUNDATION TO ITS WIDTH IS LESS THAN UNITY.

• THIS TYPE OF FOUNDATION SPREADS THE LOAD OF SUPERSTRUCTURE INTO THE GROUND LATERALLY. HENCE SHALLOW FOUNDATIONS ARE GENERALLY KNOWN AS SPREAD FOOTING.

DEEP FOUNDATIONS• IN DEEP FOUNDATION, THE DEPTH AT WHICH FOUNDATION IS PLACED IS

GREATER THAN ITS WIDTH.• THE DEPTH TO WIDTH RATIO OF THE FOUNDATION IS USUALLY GREATER THAN

4 TO 5.• UNLIKE SHALLOW FOUNDATIONS, DEEP FOUNDATIONS DISTRIBUTE THE LOAD

OF SUPERSTRUCTURE INTO THE GROUND VERTICALLY RATHER THAN LATERALLY.

RAFT FOUNDATION VS COMBINED FOUNDATION

RAFT FOUNDATION• RAFT FOUNDATION IS A THICK

CONCRETE SLAB REINFORCED WITH STEEL WHICH COVERS THE ENTIRE CONTACT AREA OF THE STRUCTURE LIKE A THICK FLOOR.

• SOMETIMES AREA COVERED BY RAFT MAY BE GREATER THAN THE CONTACT AREA DEPENDING ON THE BEARING CAPACITY OF THE SOIL UNDERNEATH.

• THE REINFORCING BARS RUNS NORMAL TO EACH OTHER IN BOTH TOP AND BOTTOM LAYERS OF STEEL REINFORCEMENT

RAFT FOUNDATIONS ARE USED:

• WHEN THE ALLOWABLE BEARING PRESSURE OF SOIL IS SO SMALL AND BUILDING STRUCTURES EXERTS TOO HEAVY LOADS TO BE ACCOMMODATED BY AREA OF INDIVIDUAL FOOTING WITHIN THE FLOOR AREA OF STRUCTURE, RAFT FOUNDATION IS SUGGESTED.

• WHEN SOIL IS VERY ERRATIC OR HAVE COMPRESSIBLE LENSES AND NOT EASY TO ASSES OR DEFINE EXTENT AND LOCATION OF CAVITIES OR OTHER WEAK POCKETS AND SUBSEQUENT UNCERTAINTY IN COMPUTING TOTAL AND AS WELL DIFFERENTIAL SETTLEMENT.

• WHEN EQUIPMENTS, IN CASE OF INDUSTRIAL OR OTHER COMMERCIAL FLOORS AND STRUCTURES TO BE PLACED ON SOIL HAVE VERY SENSITIVITY TO UNEQUAL SETTLEMENT.

• IN ONE WORDS, WHERE DEEP FOUNDATION LIKE PILE FOUNDATION ARE NOT ECONOMICAL AND FEASIBLE AND ISOLATED COLUMN FOOTING IS IMPRACTICABLE DUE TO LARGE FOOTING SIZE OR OVER-LAPPING OF NEIGHBOR FOOTING , RAFT FOUNDATION IS THE ECONOMICAL SOLUTION.

• WHERE STRUCTURAL SYSTEM SEEKS TO USE RAFT FOUNDATION LIKE CHIMNEYS, SILOS, WATER TOWER ETC.

• TO ACHIEVE FLOATING FOUNDATION WHERE SOIL HAS VERY LOW BEARING CAPACITY; THE PURPOSE IS TO BALANCE WEIGHT OF SUPER-STRUCTURE BY SOIL MASS REMOVED TO PLACE FOUNDATION.

• IN SOME CONDITIONS LIKE PITS TO BE SUPPORTED ON FOUNDATIONS SUCH THAT GROUND WATER TABLE IS ABOVE THEM OR WHERE BASEMENTS ARE REQUIRED ACCORDING TO ARCHITECTURAL REQUIREMENTS.

• STRUCTURES WHERE ISOLATED FOUNDATION MAY BE SUBJECTED TO BENDING MOMENT OF VARYING MAGNITUDE WHICH MAY CAUSE DIFFERENTIAL SETTLEMENT AND DIFFERENTIAL ROTATION OF ISOLATED FOOTING AND SUBSEQUENT DISTRESS IN STRUCTURE ABOVE; RAFT FOUNDATION IS SUGGESTED.

COMBINED FOUNDATION• WHENEVER TWO OR MORE COLUMNS IN A STRAIGHT LINE ARE

CARRIED ON A SINGLE SPREAD FOOTING, IT IS CALLED A COMBINED FOOTING. ISOLATED FOOTINGS FOR EACH COLUMN ARE GENERALLY NOT ECONOMICAL.

• COMBINED FOOTINGS ARE PROPORTIONED IN SUCH A WAY THAT THE CENTRE OF GRAVITY OF THE LOADS COINCIDES WITH THE CENTRE OF GRAVITY OF THE FOUNDATION. HENCE THESE FOOTINGS HAVE EITHER A TRAPEZOIDAL OR A RECTANGULAR SHAPE

• COMBINED FOOTINGS ARE PROVIDED ONLY WHEN IT IS ABSOLUTELY NECESSARY, AS

• WHEN TWO COLUMNS ARE CLOSE TOGETHER, CAUSING OVERLAP OF ADJACENT ISOLATED FOOTINGS

• WHERE SOIL BEARING CAPACITY IS LOW, CAUSING OVERLAP OF ADJACENT ISOLATED FOOTINGS

• PROXIMITY OF BUILDING LINE OR EXISTING BUILDING OR SEWER, ADJACENT TO A BUILDING COLUMN.

MACHINES USED

MACHINES

EARTH MOVING EQUIPMENT

EXCAVATORS

CONSTRUCTION VEHICLES

DUMPERS

TANKERS

TIPPERS

TRAILERS

MATERIAL HANDLING EQUIPMENT

CRANES

CONVEYORS

FORKLIFTS

HOISTS

CONSTRUCTION EQUIPMENT

CROSS SECTION OF SOIL

• O HORIZON: ORGANIC LAYER, HUMUS

• A HORIZON: ELUVIAL LAYER

• B HORIZON: ILLUVIAL LAYER

• C HORIZON:UNCONSOLIDATED MATERIAL

• R HORIZON: BEDROCK

ANGLE OF REPOSE• The angle of repose or the critical angle of repose, of

a granular material is the steepest angle of descent or dip relative to the horizontal plane to which a material can be piled without slumping. The angle of repose can range from 0° to 90°

• At this angle, the material on the slope face is on the verge of sliding.