SHALLOW FOUNDATION Niraj Patel 9428891414 niraj pateldarshan ac

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SHALLOW FOUNDATION Niraj Patel 9428891414 niraj. patel@darshan. ac. in Building Constrcution (3130607) Darshan Institute

SHALLOW FOUNDATION Niraj Patel 9428891414 niraj. patel@darshan. ac. in Building Constrcution (3130607) Darshan Institute of Engineering & Technology

Topics to be covered § Necessity § Types § Setting out and excavation §

Topics to be covered § Necessity § Types § Setting out and excavation § Construction § Failures of foundation § Remedial measures Building Construction (3130607) 2 Darshan Institute of Engineering & Technology

What is Shallow Foundation ? § A foundation is shallow if its depth is

What is Shallow Foundation ? § A foundation is shallow if its depth is equal to or less than its width. § Also known as “Open Foundation”. § Footing is lowermost portion of it which is in direct contact with the soil and distributes the load to soil. Building Construction (3130607) 3 Darshan Institute of Engineering & Technology

Types of Footing § Spread Footing 1. Strip 2. Isolated § Combined Footing 1.

Types of Footing § Spread Footing 1. Strip 2. Isolated § Combined Footing 1. Rectangular 2. Trapezoidal § Strap Footing § Raft/Mat Footing Building Construction (3130607) 4 Darshan Institute of Engineering & Technology

Spread Footing § The spread footing are those which spread the superimposed load of

Spread Footing § The spread footing are those which spread the superimposed load of wall or column over a larger area. § They are generally classified as strip footing for wall and isolated footing for columns. Building Construction (3130607) 5 Darshan Institute of Engineering & Technology

Strip Footing § These are generally adopted for load bearing wall, as it provides

Strip Footing § These are generally adopted for load bearing wall, as it provides continuous longitudinal bearing. § As so it is called Strip Footing. These may be simple or stepped. § Whenever wall carries light load, small width of footing required is very small, in which is simple footing can be provided. § No offsets are provided and wall directly rest on footing. § Whereas, load on wall are heavy and SBC of soil is low, large base area is required to transfer the load safely. § Here, stepped footing is needed. Building Construction (3130607) 6 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 7 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 7 Darshan Institute of Engineering & Technology

Contd. § Generally, thickness of footing(in case of simple) is around 150 mm and

Contd. § Generally, thickness of footing(in case of simple) is around 150 mm and outward project of bed from wall is kept nearly equal to thickness of wall on either side. § While in case of Stepped footing width of footing is calculated based on thumb rule T=(2 t + 30) mm where t is thickness of wall. § Also each step from wall is projected 50 mm on both side progressively in case of brick wall and 75 mm in case of stone wall. Building Construction (3130607) 8 Darshan Institute of Engineering & Technology

Isolated footing for Column § These are provided to transfer the load of individual

Isolated footing for Column § These are provided to transfer the load of individual column to soil safely. § These are generally made from Reinforced Cement Concrete. § The footing may be simple, sloped or stepped, depending on the requirement of load to be transferred and bearing capacity of soil. § The shape of footing may be square, rectangular or circular in plan. § When heavy load are to transferred and spacing of column is less, isolated footing is replaced by combined footing. Building Construction (3130607) 9 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 10 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 10 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 11 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 11 Darshan Institute of Engineering & Technology

Contd. Simple Footing Stepped Footing Building Construction (3130607) 12 Darshan Institute of Engineering &

Contd. Simple Footing Stepped Footing Building Construction (3130607) 12 Darshan Institute of Engineering & Technology

Contd. Sloped Footing Building Construction (3130607) 13 Darshan Institute of Engineering & Technology

Contd. Sloped Footing Building Construction (3130607) 13 Darshan Institute of Engineering & Technology

Combined Footing § When a common footing is provided for two or more columns,

Combined Footing § When a common footing is provided for two or more columns, it is known as combined footing. § These are generally provided under following circumstances: 1. When are columns are very near and their individual footings overlap each other. 2. When SBC of soil is less and requiring more area, if isolated footing is provided. 3. When end columns are located near property line and its isolated footing projects beyond the property line. § The footing maybe rectangular or trapezoidal in plan. Building Construction (3130607) 14 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 15 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 15 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 16 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 16 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 17 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 17 Darshan Institute of Engineering & Technology

Contd. § The aim of providing combined footing is to get uniform pressure distribution

Contd. § The aim of providing combined footing is to get uniform pressure distribution under the footing. § But, while using this type of footing, one should keep in mind that the Centre of Gravity of footing area should coincide with the Centre of Gravity of Combined loads from columns. § Also when columns located at property line carries heavier load, then trapezoidal footing must provided to achieve above mentioned mechanism. § For rest of ordinary situations, rectangular footing is satisfactory. Building Construction (3130607) 18 Darshan Institute of Engineering & Technology

Strap footing § Here, the isolated footing of two or more columns are connected

Strap footing § Here, the isolated footing of two or more columns are connected by a beam called STRAP. § Whenever column with heavy loads are located near or right next to property line, large area of spread would not be available. § Here, general convention is to design it as combined footing. § But is column spacing is large, combined footing would be ineffective as due to generation of large bending moments and may result in failure of footing. § Here, Strap Footing is provided to overcome this limitation. Building Construction (3130607) 19 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 20 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 20 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 21 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 21 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 22 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 22 Darshan Institute of Engineering & Technology

Building Construction (3130607) 23 Darshan Institute of Engineering & Technology

Building Construction (3130607) 23 Darshan Institute of Engineering & Technology

Raft/Mat Footing § It is generally a combined footing that covers entire area below

Raft/Mat Footing § It is generally a combined footing that covers entire area below the structure to support the loads of columns and walls. § When SBC of soil is low then and spread footing covers large area of strata, then it is economical to provide Raft or Mat footing. § Also when soil mass contains compressible lenses at some portion of area, differential settlement is difficult to control. § In all of above situations, it is convenient to provide mat/raft. Building Construction (3130607) 24 Darshan Institute of Engineering & Technology

Flat Plate Type Raft Building Construction (3130607) 25 Darshan Institute of Engineering & Technology

Flat Plate Type Raft Building Construction (3130607) 25 Darshan Institute of Engineering & Technology

Raft with tie/two way Beam Building Construction (3130607) 26 Darshan Institute of Engineering &

Raft with tie/two way Beam Building Construction (3130607) 26 Darshan Institute of Engineering & Technology

Raft with Tie/Two way Beam Building Construction (3130607) 27 Darshan Institute of Engineering &

Raft with Tie/Two way Beam Building Construction (3130607) 27 Darshan Institute of Engineering & Technology

Grillage Foundations § It is special type of isolated footing which is provided for

Grillage Foundations § It is special type of isolated footing which is provided for steel stanchion especially where is soil bearing capacity is poor. § Depth is generally 1 -1. 5 m. § Load is transferred to very large area by joist (generally, I – section ISHB) or tier of joists, each tier being placed at right angles to above it. § It is of two types 1. Steel Grillage Foundation 2. Timber Grillage Foundation Building Construction (3130607) 28 Darshan Institute of Engineering & Technology

Timber Grillage Foundation Building Construction (3130607) 29 Darshan Institute of Engineering & Technology

Timber Grillage Foundation Building Construction (3130607) 29 Darshan Institute of Engineering & Technology

Contd. § Here, the no concrete is embedded in between the timber joists. §

Contd. § Here, the no concrete is embedded in between the timber joists. § Useful sometimes in the water logged areas. § Bottom layer of planks of thickness 5 -7. 5 cm is used as bottom layer over which the primary beam is placed of about size 10 cm x 15 cm right angle to direction of planks. § Over this layer timber beam of same section of that of timber post whose load is to be resisted is to be placed. Building Construction (3130607) 30 Darshan Institute of Engineering & Technology

Steel Grillage Foundation Building Construction (3130607) 31 Darshan Institute of Engineering & Technology

Steel Grillage Foundation Building Construction (3130607) 31 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 32 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 32 Darshan Institute of Engineering & Technology

Contd. § Here, the both tiers are generally spaced by spacer bar of 20

Contd. § Here, the both tiers are generally spaced by spacer bar of 20 mm and 25 mm pipe separators. § Minimum clearance of 80 mm is kept in order to facilitate placing of concrete. § Here concreting is done to keep the beam in position and protect it form moisture and water attack. § Min. concrete cover on outer beam is kept 100 mm. § Also depth of concrete bed is kept min 150 mm. Building Construction (3130607) 33 Darshan Institute of Engineering & Technology

Eccentrically Loaded Column • Special case of isolated footing where the footing is subjected

Eccentrically Loaded Column • Special case of isolated footing where the footing is subjected to eccentric loading. • Here the total pressure intensity is equal to sum of central point load and bending moment generated due to central point load. Building Construction (3130607) 34 Darshan Institute of Engineering & Technology

Contd. • Building Construction (3130607) 35 Darshan Institute of Engineering & Technology

Contd. • Building Construction (3130607) 35 Darshan Institute of Engineering & Technology

Setting out of Foundation Trenches § It is process of laying and marking of

Setting out of Foundation Trenches § It is process of laying and marking of excavation lines and centre line on the ground. § Once foundation is designed, layout plan is prepared. § For setting out the foundation of small buildings, centre line of longest outer wall is first marked on the ground. Building Construction (3130607) 36 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 37 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 37 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 38 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 38 Darshan Institute of Engineering & Technology

Example Building Construction (3130607) 39 Darshan Institute of Engineering & Technology

Example Building Construction (3130607) 39 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 40 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 40 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 41 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 41 Darshan Institute of Engineering & Technology

Excavation of trenches § Trenches marked by as per layout plan can be excavated

Excavation of trenches § Trenches marked by as per layout plan can be excavated manually by hand operated instruments or by mechanized equipment. § Hand operated instruments includes, spade, pick axe, crow bar, sledge hammer, boning rod, etc. § On the other hand, mechanized equipment like power shovel and trenchers can be used for speedier excavation. § Power shovel can be used dig trenches up to width of 1. 7 m. § Trenchers can be used for width up to 1. 5 m and depth 5 m. Building Construction (3130607) 42 Darshan Institute of Engineering & Technology

Hand Instruments Building Construction (3130607) 43 Darshan Institute of Engineering & Technology

Hand Instruments Building Construction (3130607) 43 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 44 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 44 Darshan Institute of Engineering & Technology

Mechanised Equipment Building Construction (3130607) 45 Darshan Institute of Engineering & Technology

Mechanised Equipment Building Construction (3130607) 45 Darshan Institute of Engineering & Technology

Trencher Building Construction (3130607) 46 Darshan Institute of Engineering & Technology

Trencher Building Construction (3130607) 46 Darshan Institute of Engineering & Technology

Trenching Process Building Construction (3130607) 47 Darshan Institute of Engineering & Technology

Trenching Process Building Construction (3130607) 47 Darshan Institute of Engineering & Technology

Timbering of Trenches § § § 1. 2. 3. 4. 5. Required when either

Timbering of Trenches § § § 1. 2. 3. 4. 5. Required when either depth is large or sub soil is loose. In both above cases, soil may cave in to trenches. Also known as shoring. Here timber planks or boards supported by struts are provided to give temporary support to sides of trench. These are generally categorized into Stay Bracing Box Sheeting Vertical Sheeting Runner System Sheet Piling Building Construction (3130607) 48 Darshan Institute of Engineering & Technology

Stay Bracing § Simplest of all, supporting the sides of relatively firm soil. §

Stay Bracing § Simplest of all, supporting the sides of relatively firm soil. § Useful when depth not exceed 2 m. § Trench is supported by vertical sheets of wooden log or polling boards against opposite wall and kept in position by one or two row of struts. Building Construction (3130607) 49 Darshan Institute of Engineering & Technology

Box Sheeting § Adopted for loose soils, up to depth of 4 -5 m.

Box Sheeting § Adopted for loose soils, up to depth of 4 -5 m. § Here, vertical sheets are placed very near sometimes with touching each other to support soils and kept in position by Struts. § If soil is very loose, then, sheeting is done horizontally supported by whale or vertical board with help of struts. Building Construction (3130607) 50 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 51 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 51 Darshan Institute of Engineering & Technology

Other methods Vertical Sheeting Runner System • Adopted for deep trenches (up to 10

Other methods Vertical Sheeting Runner System • Adopted for deep trenches (up to 10 m depth). • Method is similar to box sheeting except excavation is carried out in stages and at end of each stage, an offset is provided which vary generally from 2550 cm and each stage is generally 3 m in depth. • Used in case of extremely loose and soft ground which needs immediate support after excavation. • Here runners made of wooden plank with iron shoe are driven in advance of excavation to provide prior support. Building Construction (3130607) 52 Darshan Institute of Engineering & Technology

Vertical sheeting Building Construction (3130607) Runner system 53 Darshan Institute of Engineering & Technology

Vertical sheeting Building Construction (3130607) Runner system 53 Darshan Institute of Engineering & Technology

Sheet Piling § These are useful where 1. Soil is loose 2. Size and

Sheet Piling § These are useful where 1. Soil is loose 2. Size and depth of trench is very large. 3. Sub soil water. § These are designed to resist lateral earth pressure. § Mostly used for bridge foundation, mat footing, pier foundation. Building Construction (3130607) 54 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 55 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 55 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 56 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 56 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 57 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 57 Darshan Institute of Engineering & Technology

Cellular Cofferdam N U O RF PIE Building Construction (3130607) 58 N O I

Cellular Cofferdam N U O RF PIE Building Construction (3130607) 58 N O I T DA Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 59 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 59 Darshan Institute of Engineering & Technology

Failure of Foundation § Foundation may fail due to one or combination of any

Failure of Foundation § Foundation may fail due to one or combination of any of following reasons. 1. Unequal settlement of soil 2. Unequal settlement of masonry 3. Sub soil moisture 4. Lateral pressure on walls 5. Lateral movement of sub soil 6. Weathering of sub soil 7. Atmospheric conditions Building Construction (3130607) 60 Darshan Institute of Engineering & Technology

Unequal settlement of Sub Soil § It may lead to cracks and rotation. §

Unequal settlement of Sub Soil § It may lead to cracks and rotation. § Settlement occur due to 1. Non uniform nature of sub soil 2. Unequal load distribution on footing 3. Eccentric loading. Measure to avoid such situation 1. Resting foundation on rigid strata. 2. Proper design of base of footing 3. Avoid eccentric loading upto maximum extent. Building Construction (3130607) 61 Darshan Institute of Engineering & Technology

Types of settlement Building Construction (3130607) 62 Darshan Institute of Engineering & Technology

Types of settlement Building Construction (3130607) 62 Darshan Institute of Engineering & Technology

Settlement of sub soil Building Construction (3130607) 63 Darshan Institute of Engineering & Technology

Settlement of sub soil Building Construction (3130607) 63 Darshan Institute of Engineering & Technology

Unequal settlement of masonry § This implies to masonry in between the footing which

Unequal settlement of masonry § This implies to masonry in between the footing which is rises up to the ground level especially in case of load bearing wall masonry. § This masonry as usual have mortar joints which is liable to shrink or compress which leads to settlement. § This can be prevented by 1. Using thin mortar joints with proper strength and workmanship should be checked for same on site. 2. Restricting the rate of progress to 1 m per day if lime mortar is used and 1. 5 m if cement mortar is used. Building Construction (3130607) 64 Darshan Institute of Engineering & Technology

Sub soil moisture movement § This specially occurs in case of cohesive soils. §

Sub soil moisture movement § This specially occurs in case of cohesive soils. § Due rising/falling of sub soil water, swelling/shrinkage occurs in sub soil leading to differential settlement of foundation resting on it. § It may be dangerous in expansive soil as if swelling pressure exceeds the shear strength of soil, heaving of soil starts which results in failure of foundation. § For this special measurement are taken to control ground water. Building Construction (3130607) 65 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 66 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 66 Darshan Institute of Engineering & Technology

Lateral movement of wall § If the wall transmitting load is subjected to lateral

Lateral movement of wall § If the wall transmitting load is subjected to lateral pressure or thrust from a pitched roof or arch or due to wind action, then § It may subjected to moment which ultimately transferred to foundation. § If foundation is not designed for such situation, then it may fail in overturning or due to generation of tensile stress. Building Construction (3130607) 67 Darshan Institute of Engineering & Technology

Lateral movement of Sub soil § This is applicable to very soil which is

Lateral movement of Sub soil § This is applicable to very soil which is liable to move out under vertical loads especially where ground is sloping. § It also occurs where large pits are excavated near foundation and soil is granular. § This may also lead to collapse of structure. Building Construction (3130607) 68 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 69 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 69 Darshan Institute of Engineering & Technology

Weathering of sub soil § This happens when the small trees, shrubs are grown

Weathering of sub soil § This happens when the small trees, shrubs are grown near to foundation. § Roots of these plants can absorb moisture from soil and resulting in shrinkage and compression of soil due which there is settlement of soil. Building Construction (3130607) 70 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 71 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 71 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 72 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 72 Darshan Institute of Engineering & Technology

Atmospheric actions § Behaviour of foundation is largely affected by weathering agents like wind,

Atmospheric actions § Behaviour of foundation is largely affected by weathering agents like wind, rains, etc. § If porosity of soil is higher then, it will considerably reduce the strength of soil as effective pressure is decreased. § Also if water remains stagnant near foundation, it will create dampness which can spread over wall. § As a measure of this, backfilling should be done with good soil and well compacted. § Provide DPC to protect wall against moisture. Building Construction (3130607) 73 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 74 Darshan Institute of Engineering & Technology

Contd. Building Construction (3130607) 74 Darshan Institute of Engineering & Technology