Foundations PDF

1 Foundations 2 Functions of foundations  Reduction of load intensity  Even distribution of load  Provision of level surface.  Lateral stability  Safety against undermining or scouring  Protection against soil movements. 3 Essential Requirements of a Good...

1 Foundations 2 Functions of foundations  Reduction of load intensity  Even distribution of load  Provision of level surface.  Lateral stability  Safety against undermining or scouring  Protection against soil movements. 3 Essential Requirements of a Good Foundation.  Constructed to sustain the dead and imposed loads and to transmit these to the sub-soil in such a way that pressure on it will not cause settlement which would impair the stability of the building or adjoining structures.  Base should be rigid so that differential settlements are minimised  Should be taken sufficiently deep to guard the building against damage or distress caused by swelling or shrinkage of the sub-soil.  Should be so located that its performance may not be affected due to any unexpected future influence. 4 Types of foundations  Shallow foundations  Deep foundations  According to Terzaghi, a foundation is shallow if its depth is equal to or less than its width.  In case of deep foundations, the depth is much greater than its width. 5 Shallow foundations  Spread footings  Combined footings  Strap footings  Raft or Mat foundation 6 Spread footings  Spread the super-imposed load of wall or column over a larger area.  Spread footings support either a column or wall.  May be of the following kinds:  Single footing for a column  Stepped footing for a column Isolated footing  Sloped footing for a column  Wall footing without step Strip footing  Stepped footing for wall  Grillage foundation 7 Isolated Footing 8 9 Grillage foundation 10 Combined Footings  A spread footing which supports two or more columns is termed as combined footing.  Rectangular combined footing  Trapezoidal combined footing  Combined column-wall footing 11 12 Combined Footings  The combined footing for columns will be rectangular in shape if they carry equal loads.  The design of rigid rectangular combined footing should be done in such a way that centre of gravity of column loads coincide with the centroid of the footing area.  If the columns carry unequal loads, the footing is of trapezoidal shape.  Sometimes, it may be required to provide a combined footings for columns and a wall. 13 Mat Foundation  A raft or mat is a combined footing that covers the entire area beneath a structure and supports all the walls and columns.  It is provided When:  The allowable soil pressure is low.  The building loads are heavy.  The use of spread footings would cover more than one half the area. 14 Mat Foundation 15 Strap footing  Footings of two columns are connected by a beam  Used where the distance between the columns is so great that a combined trapezoidal footing becomes quite narrow.  Each column is provided with its independent footings and a beam is used to connect the two footings. 16 Strap footing 17 DEEP FOUNDATIONS  Types:  Deep strip, rectangular or square footings.  Pile foundation.  Pier foundation.  Well foundation or caissons. 18 Pile Foundation  Loads are taken to a low level by means of vertical members which may be of timber, concrete or steel.  May be of four types (based on load transfer)  End bearing pile  Friction pile  Combined end bearing and friction pile  Compaction piles 19 Pile Foundation 20 End bearing piles  To transfer load through water or soft soil to a suitable bearing stratum.  To carry heavy loads safely to hard strata.  Used for Multi-storeyed buildings so that the settlements are minimised. 21 Friction piles  Used to transfer loads to a depth of a friction-load-carrying material by means of skin friction along the length of the pile.  Such piles are generally used in granular soil where the depth of hard stratum is very great. 22 Combined end bearing and friction pile  Transfers the super-imposed load both through side friction as well as end bearing.  Such piles are more common, specially when the end bearing piles pass through granular soils. 23 24 Compaction piles  Used to compact loose granular soils, thus increasing their bearing capacity.  The compaction piles themselves do not carry a load.  Hence they may be of weaker material (such as timber, bamboo sticks etc.)-sometimes of sand only. 25 26 Types of piles – construction process Bored Piles Driven Piles 27 Types of piles – materials  Wooden piles  Concrete piles  Precast  Cast-in situ  Steel piles  Composite Piles 28 Pier Foundation  Consists of a cylindrical column of large diameter to support and transfer large super-imposed loads to the firm strata below.  Pier foundations transfer the load only through bearing.  Pier foundation is shallower in depth than the pile foundation.  Preferred in a location where the top strata consists of decomposed rock overlying a strata of sound rock.  In such a condition, it becomes difficult to drive the bearing piles through decomposed rock.  In the case of stiff clays, which offer large resistance to the driving of a bearing pile, pier foundation can be conveniently constructed. 29 Pier Foundation  Pier foundations may be of the following types  Masonry or concrete pier  When a good bearing stratum exists upto 5 m below ground level, brick, masonry or concrete foundation piers in excavated pits may be used.  Drilled caissons.  Concrete caisson with enlarged bottom  Caisson of steel pipe with concrete filled in the pipe  Caisson with concrete and steel core in steel pipe 30 31 Well Foundations  Box like structure-circular or rectangular-which are sunk from the surface of either land or water to the desired depth.  Much large in diameter than the pier foundations or drilled caissons.  Hollow from inside, which may be filled with sand, and are plugged at the bottom.  Used for bridge piers and abutments; wharfs, docks, breakwater; pump houses 32 33 Sub-soil Exploration  For New Structures  Selecting type and depth of foundation  Determining bearing capacity  Predicting settlement  Determining ground water level  For Existing Structures  Investigating the safety of structure  Predicting settlement  Determining remedial measures 34 Site Exploration  Objective  To provide reliable, specific and detailed information about the soil and ground water conditions of the site which may be required for safe and economic design of foundations  Information gathered include  Order of occurrence and extent of soil and rock strata  Nature and engineering properties of soil and rock formations  Location of ground water 35 Depth of Exploration  Exploration should be taken up to a depth known as significant depth (depth up to which an increase in pressure due to structural loading is likely to cause perceptible settlement or shear failure of foundation.  This may be assumed equal to 1.5 to 2 times the width of the loaded area  Isolated footing and raft = 1.5 x width  Adjacent footing with clear spacing less than twice the width = 1.5 x length  Plie foundation = 10 to 30 m or more  National Building Code - depth of exploration = 1.5 x width of footing, with minimum of 1.5 m 36 Methods of soil exploration  Open Excavation  Trial pits  Cheapest method  Soil sample collected at various levels  Soil could be inspected in their natural condition.  Suitable of depths up to 3m  Not suitable for greater depths and excavation below water table 37 Methods of soil exploration  Boring  Small holes are dug rather than open excavation  Soil is collected at the top from various locations beneath the ground  Collected samples are preserved for further lab testing 38 Methods of soil exploration  Sub-Surface Sounding  Measuring the resistance of soil to penetration at different depths  Penetrometers are used under static or dynamic loading  Resistance to penetration is empirically correlated with the engineering properties of the soil 39 Methods of soil exploration  Geo-physical methods  Used for larger depths of excavations  Involves the detection of difference in the physical properties of geological formations  Seismic refraction method and electrical resistivity method are most commonly used 40 Bearing Capacity of Soil  Supporting power of soil/rock; used in design of foundation  Gross Pressure Intensity (q)  Gross pressure intensity at the base of the foundation due to weight of super-structure, self-weight of footing and weight of earthfill  Net Pressure Intensity (qn)  Excess pressure after the construction of the structure  𝑞 = 𝑞 − 𝛾𝐷, D – depth of footing; 𝛾- unit weight of soil  Ultimate Bearing Capacity(qf)  Minimum gross pressure intensity at the base of foundation at which the soil fails in shear 41 Bearing Capacity of Soil  Net Ultimate Bearing Capacity (qnf)  Minimum net pressure intensity causing shear failure of soil 𝑞 = 𝑞 − 𝛾𝐷,  Net Safe Bearing Capacity (qns)  Net ultimate bearing capacity divided by a factor of safety 𝑞 =  Safe Bearing Capacity (qs)  Maximum pressure which the soil can carry safely without risk of shear failures 𝑞 =𝑞 + 𝛾𝐷 = + 𝛾𝐷 42 Bearing Capacity of Soil  Allowable Bearing Pressure (qa)  Net loading intensity at which neither the soil fails in shear nor there is excessive settlement detrimental to the structure in question  This depends on the type of sub-soil and the type of foundation the building has  Should never exceed the safe bearing capacity 43 Methods for Estimating Bearing Capacity  Analytical Methods involving the use of soil parameters  Plate Load Test  Penetration Test  From Building Codes 44 45 Depth of Footing  Minimum depth of foundation is given by Rankine’s Formula 𝐷 =  Where,  𝜑 – angle of repose  𝛾 – unit weight of soil in kN/m3  q – intensity of loading at the base of footing in kN/m2  Design of Shallow 46 foundation 47 Strip Footing  Width of footing is found on the basis of bearing capacity of the soil. 𝑊 𝐵= 𝑞  Where,  W – total super-imposed load on the base of the foundation (kN/m)  𝑞 - Safe bearing pressure (kN/m2) 48 1. Simple Strip Footing  When wall carries light load and safe bearing capacity (SBC) of soil is very high  Wall directly rests on top of concrete base, with no masonry offsets  Concrete base should project out by a value ”a” on either sides of the wall  “a” may vary from 10 to 20 cm  Width of concrete base should not be less than twice the width of the wall. 49 1. Simple Strip Footing  NBC recommends that the angle of spread of the load from wall base to the outer edge of ground bearing shall not exceed n1:1 (n1 horizontal to 1 vertical) where  n1 = 2/3 for lime concrete  n1 = 1 for cement concrete  B = T + 2a  Thickness of concrete base should at least be equal to the offset a for cement concrete and 1.5a for lime concrete base 50 2. Stepped Strip Footing  Wall carries heavy load and when SBC of soil is not very high, the base width required will be much greater than “T + 2a”  It is essential to provide masonry offsets to achieve larger spread, before the load is transferred to the concrete base  The height and offset of spread should be so proportioned that rate of spread does not exceed n:1 for brick masonry and n1 :1 for concrete base 51 2. Stepped Footing  n = ½ for brick and stone masonry  n1 = 2/3 for lime concrete  n1 = 1 for cement concrete  Minimum Depth of footing needed (assuming uniform rate of spread) n:1= : Dmin Dmin = (𝐵 − 𝑇) 52 2. Stepped Footing  Minimum Depth of footing needed (by different rate of spread) Dmin = 𝐵 − 𝑇 − 2𝑑 𝑛1 − 𝑛  Where, d = thickness of concrete block 54 Soln 1 55 Isolated or Pad Footing  Area of footing is found on the basis of bearing capacity of the soil. 𝑃 𝐵= 𝑞  Where,  P – total load transmitted by column, including footing (kN)  𝑞 - Safe bearing pressure (kN/m2) 56 Isolated Footing 57 Stepped Pad Footing 58 Soln 2