FOUNDATION Dr. Sumit Khandelwal, Malaviya National Institute of Technology Jaipur, Jaipur, INDIA...

FOUNDATION Dr. Sumit Khandelwal, Malaviya National Institute of Technology Jaipur, Jaipur, INDIA Shallow and Deep Foundations, Building Technology, III Semester

Superstructure Plinth Plinth Level Level Substructure Substructure Foundation Foundation Major Building Parts

Foundation Shallow and Deep Foundations, Building Technology, III Semester Part of structure in direct contact with ground to which the loads are transmitted is foundation

Foundation The foundation system for a building is the critical link in the transmission of building loads down to the ground. Bearing directly on the soil, the foundation system must: Distribute vertical loads so the settling of a building is either negligible or uniform under all parts of the building Anchor the building's superstructure to prevent uplifting due to wind and earthquake forces. Shallow and Deep Foundations, Building Technology, III Semester

Purpose of Foundation Transfer building loads to soil and distributes it to larger area to reduce intensity at base below SBC Load distribution to soil is such that differential settlement can be avoided Provide a level, stable surface to safely support a building Anchor the building from wind, seismic and other lateral loads Shallow and Deep Foundations, Building Technology, III Semester

SOIL REACTIONS Loads From the Structure Foundation must resist Dead Load Weight of building Live Load Weight of Occupants Weight of Furniture Weight of Equipments Lateral Load Wind Seismic Any other such as uplift etc.

Requirements of A Safe Foundation Foundation system transfers all loads to soil such that structure is safe against settlements that may lead to collapse Foundation settlement shall be uniform as far as possible and the settlement should not damage the structure Foundation must be technically and economically feasible Shallow and Deep Foundations, Building Technology, III Semester

Types of Sub-soil & Characteristics Shallow and Deep Foundations, Building Technology, III Semester Rocks: Broken into regular and irregular sizes by joints Soils: Particulate earth material Boulder - Too large to be lifted by hands; Cobble - Particle that can be lifted by a single hand; Gravel - Course grained particle (size larger than 6.4mm); Sand - Frictional (size varies from 6.4 to 0.06mm;) Silts - Frictional, low surface-area to volume ratio,(size varies from 0.06 mm to 0.002mm); Clays - Cohesive - fine grained - high surface-area to volume ratio (size smaller than 0.002 mm)

ClaysPorous (sandy)

Site Inspection & Sub-soil Exploration Shallow and Deep Foundations, Building Technology, III Semester Inspection of site of work is done to estimate behaviour of ground; to estimate general level of ground and drainage pattern to estimate nature of soil by visual examination Soil-investigation is done to find the order of occurrence of the sub-soil strata; to collect disturbed/undisturbed soil samples to ascertain the ground water table and its variations to test soil samples in laboratory for: Particle size distribution, Liquid/Plastic limit, Shear/compressive strength, Water content, Shrinkage/swelling, Permeability, Consolidation (creep & settlement) Foundation shall be designed such that The soil below does not fail in shear Settlement is within the safe limits

Site Exploration Methods Test Pits Square or circular pit is excavated exposing sub-soil surface; Natural condition can be inspected; Possible to take disturbed and undisturbed samples; Probing Hollow tube or solid rod with pointed end is driven; Frequently taken out to examine arrested material; Boring Special techniques/instruments used to force/withdraw soil at ground; Only disturbed samples are available; Can be used only for identification purposes Shallow and Deep Foundations, Building Technology, III Semester

Site Exploration Methods Boring Auger boring Possible only for soft soils; Manual or mechanical Auger and shell boring Used for still soils; Casing is also used Wash boring Not suitable for rock and boulders; Speedy method Percussion boring Suitable for any type of soils/rock; Cutting tool used for rock; Core boring Hollow tube with rotary motion; Disturb soil samples; Rock core; Shallow and Deep Foundations, Building Technology, III Semester

Site Exploration Methods Sub-surface soundings Measurement of resistance of soil with depth under static or dynamic loading; Penetrometer is driven into ground by blows from standard weight; Number of blows required for standard penetration is Standard Penetration Resistance (SPR) of soil; Resistance empirically correlated with engineering properties of soil; Used mainly for cohensionless soils especially sand; Geo-physical methods Useful for large exploration depths; Normally associated with mineral/oil exploration; Shallow and Deep Foundations, Building Technology, III Semester

Site Exploration Methods Shallow and Deep Foundations, Building Technology, III Semester Choice of site exploration method depends on Number of sites Cost Nature of ground/sub-soil Topography As mentioned earlier, the most important objective of site exploration/soil-investigation is to determine bearing capacity of soil

Bearing Capacity of Soil Shallow and Deep Foundations, Building Technology, III Semester Important terms Gross Pressure Intensity: Total Pressure Net Pressure Intensity: Gross Overburden pressure Ultimate Bearing Capacity: Minimum GPI for shear failure Net Ultimate Bearing Capacity: UBC Overburden Safe Bearing Capacity: Maximum Pressure soil can carry safely without risk of shear failure, Allowable bearing pressure: Load intensity that can be exerted on soil considering both shear failure & settlement criteria

Foundation Settlement The cause of settlement is typically due to a reduction in the volume of air voids in the soil. As a building bears down on the supporting soil and transfer various types of loads, some settlement is expected. A properly designed and constructed foundation system should minimize settlement. Shallow and Deep Foundations, Building Technology, III Semester

Foundation Settlement Uneven or "differential" settlement can cause a building to shift out of plumb causing cracks in the foundation, structure, or finish. Extreme differential settlement can lead to failure of a building's structural integrity. Permissible settlement limits Total Settlement: 100 mm Differential settlement: 25 mm or 40 mm Shallow and Deep Foundations, Building Technology, III Semester

Uniform settlement is usually of little consequence in a building, but differential settlement can cause severe structural damage Foundation Settlement No SettlementTotal SettlementDifferential Settlement Shallow and Deep Foundations, Building Technology, III Semester

Bearing Capacity of Soil Shallow and Deep Foundations, Building Technology, III Semester Important terms Gross Pressure Intensity: Total Pressure Net Pressure Intensity: Gross Overburden pressure Ultimate Bearing Capacity: Minimum GPI for shear failure Net Ultimate Bearing Capacity: UBC Overburden Safe Bearing Capacity: Maximum Pressure soil can carry safely without risk of shear failure, Allowable bearing pressure: Load intensity that can be exerted on soil considering both shear failure & settlement criteria

Methods of SBC determination Shallow and Deep Foundations, Building Technology, III Semester Plate Load Test Used for UBC determination A rigid plate is loaded at foundation level Size of pit is five times the width of plate Settlement corresponding to different load UBC is taken as the load at which the plate starts sinking at rapid rate

Methods of SBC determination Plate Load Test This UBC is corresponding to shear failure Load-settlement curve is also prepared Shallow and Deep Foundations, Building Technology, III Semester Bearing pressure corresponding to permissible settlement can be obtained Safe bearing pressure (SBC) is the lesser of bearing pressure from shear failure and settlement criteria

Methods of SBC determination Shallow and Deep Foundations, Building Technology, III Semester Standard Penetration Test (SPT) Penetration resistance or number of blows or N value is determined Actual N value or corrected N value is used for calculation of SBC

Methods of SBC determination Shallow and Deep Foundations, Building Technology, III Semester Cone Penetration Test lculation of SBC

Methods of SBC determination Shallow and Deep Foundations, Building Technology, III Semester Analytical Methods SBC is calculated from Rankine or Terzaghi formula using the engineering properties of soil, determined in laboratory from soil samples collected by boring/test pits. Presumptive SBC values Taken from codes Can be used for lightly loaded structures Can be used for preliminary design of heavily loaded or lightly loaded important structures

Methods of SBC determination Shallow and Deep Foundations, Building Technology, III Semester Presumptive SBC values (as per IS1904-1961) S. No.Description of Sub-soilSBC (kN/m 2 ) 1Hard rocks without lamination and defects3300 2Laminated rocks1650 3Residual deposit of shattered and broken bed rock900 4Soft Rock450 5Gravel, sand and gravel, compact450 6Coarse sand, compact and dry (water table low)450 7Fine sand, loose and dry100 8Hard or stiff clay in deep bed, dry450 9Soft clay indented with moderate thumb pressure100 10Very soft clay that can be penetrated with the thumb50

Type of Foundation Primary factors affecting foundation choice Subsurface soil Ground water conditions Structural requirements: Secondary factors affecting foundation choice Construction access, methods & site conditions Environmental factors Building Codes & Regulations Impact on surrounding structures Construction schedule Construction risks Shallow and Deep Foundations, Building Technology, III Semester

Types of Foundation Shallow Foundation Spread Foundation: Can be used for both Masonary and Concrete members; Square, Rectangular or Circular Footings; Single or Combined; Mat/Raft Foundation: Used only for Concrete members Solid slab, Beam-slab, Cellular Deep Foundation Piles Pile Walls Caissons Diaphragm Wall Shallow and Deep Foundations, Building Technology, III Semester

Shallow Foundation Depth is not more than width (Terzaghi) Also called open foundation Transfers loads to the soil very near the surface Shallow and Deep Foundations, Building Technology, III Semester

Advantage of Shallow Foundation Affordable Cost Simple Construction Procedure Convenient Materials Skilled labour not required Shallow and Deep Foundations, Building Technology, III Semester

Spread Footing Also known as Footer The foundation consists of concrete slabs located under each structural column and a continuous slab under load- bearing walls It is an enlargement at the bottom of a column/wall that spreads the applied structural loads over a sufficiently large soil area For the spread foundation system the structural load is literally spread out over a broad area under the building Used in small to medium size structure with moderate to good soil condition Each column & each wall has its own spread footing, so each structure may include dozens of individual footings Shallow and Deep Foundations, Building Technology, III Semester

Continuous (Strip) Foundation A wide strip of reinforced concrete that supports loads from a bearing wall USES Under foundation walls FOUNDATION WALL (Concrete or Masonary) STRIP FOOTING (Concrete or Masonary) LOAD

Continuous (Strip) Foundation Width of foundation is to be calculated from SBC consideration Minimum depth of foundation is calculated from Rankine formula Depth of foundation may be increased for securing adequate bearing capacity

Continuous (Strip) Foundation Width of footing is calculated from total load at base of footing and SBC of sub-soil. Angle of spread of the load from wall base to the outer edge of bearing ground H/V=1/1 for Concrete H/V=1/2 for Masonary

PIER (Concrete or Masonry) SPREAD FOOTING (Concrete) COLUMN LOAD Isolated (Pad) Footing A footing that spreads the load over a broad area which supports one (or a few) load(s) USES Under piers or columns

Isolated (Pad) Footing

Combined Foundation It supports two columns or more columns Used under condition Closely placed columns Low SBC End Columns

Combined Foundation

Continuous Footing

Strap Footing

Footing Plan for a building

Raft Foundation Used to spread the load from a structure over a large area, normally the entire area of the structure Almost the entire building is placed on large continuous footing Normally consists of a concrete slab which extends over the entire loaded area. The slab, heavily reinforced with steel, carries the downward loads of the individual columns or walls. Slab may be stiffened by ribs or beams Raft foundations have the advantage of reducing differential settlements as the concrete slab resists differential movements between loading positions. Mostly used on soft or loose soils with low bearing capacity as they can spread the loads over a larger area Shallow and Deep Foundations, Building Technology, III Semester

Raft Foundation Is Required If The structural loads are so high or the soil condition so poor that spread footings would be exceptionally large. As a general rule of thumb, if spread footings would cover more than 50% of the building footprint area, a mat or some type of deep foundation will usually be more economical The soil is very erratic & prone to excessive differential settlements. The structure continuity and flexural strength of a mat will bridge over these irregularities. This is also true for highly expansive soils prone to differential heaves Shallow and Deep Foundations, Building Technology, III Semester

Raft Foundation Is Required If The structural loads are erratic and thus increase the likelihood of excessive differential settlements. Again, the structural continuity and flexural strength of the mat will absorb these irregularities The lateral loads are not uniformly distributed through the structure and thus may cause differential horizontal movements in spread footings and pile caps. The continuity of a mat will resist such movement Shallow and Deep Foundations, Building Technology, III Semester

Raft Foundation Is Required If The uplift loads are larger than spread footings can accommodate. The greater weight and continuity of a mat may provide sufficient resistance The bottom of the structure is located below the groundwater table, so waterproofing is an important concern. Because mats are monolithic, they are much easier to waterproof. The weight of the mat also helps resist hydrostatic uplift forces from the groundwater Shallow and Deep Foundations, Building Technology, III Semester

Raft Foundation Shallow and Deep Foundations, Building Technology, III Semester

Deep Foundations: Pile Foundation Shallow and Deep Foundations, Building Technology, III Semester Pile foundations Used to carry and transfer the load of the structure to the bearing ground located at some depth below ground surface. The main components of the foundation are the pile cap and the piles. Piles are long and slender members which transfer the load to deeper soil or rock of high bearing capacity avoiding shallow soil of low bearing capacity.

Pile Foundation Shallow and Deep Foundations, Building Technology, III Semester Pile foundation is required when Very heavy loads Uneven load SBC is very low at and near ground Problematic top soil Stability of soil High water table Fluctuating water table

Pile Foundation Shallow and Deep Foundations, Building Technology, III Semester The selection of a pile foundation type for a structure should be based on the specific soil conditions as well as the foundation loading requirements and final performance criteria. There are numerous types of foundation piles. A pile classification system may be based on type of material, installation technique and equipment used for installation. Foundation piles can also be classified on the basis of their method of load transfer from the pile to the surrounding soil mass.

Pile Foundation Shallow and Deep Foundations, Building Technology, III Semester Classification of piles with respect to load transmission/function Piles are used to transmit the foundation load to a deeper soil stratum which has a higher load carrying capability Piles that transmit their load to a particular soil stratum at the end of the pile are called end bearing piles Piles that transmit their load to the soil by friction between the pile surface and the soil are called friction piles Piles that transmit the load to the soil by a combination of both actions (friction and end bearing piles )

Pile Foundation Shallow and Deep Foundations, Building Technology, III Semester Classification of piles with respect to load transmission/function Piles that compact loose soil to improve SBC are called compaction piles Piles that provide anchorage are called anchor piles. If the anchorage is against uplift than there are called uplift piles

Deep Foundations - Purpose transfer building loads deep into the earth Basic types Drilled (& poured) Driven

Pile Foundation Shallow and Deep Foundations, Building Technology, III Semester Classification of piles with respect to material Concrete Piles Precast piles Cast-in-situ piles Timber Piles Steel Piles Composite Piles

Pile Foundation Shallow and Deep Foundations, Building Technology, III Semester Classification of piles based on installation technique Displacement Piles: Piles which are driven are termed Displacement Piles because their installation methods displace laterally the soils through which they are introduced. Replacement Piles: Piles that are formed by creating a borehole into which the pile is then cast or placed, are referred to as Replacement Piles (also called Bored piles) because existing material, usually soil, is removed as part of the process.

Pile Foundation Shallow and Deep Foundations, Building Technology, III Semester What is a Driven Pile? A Driven Pile is a deep foundation that is constructed by driving a concrete, steel or timber pile to support the anticipated loads in competent subsurface material. Prefabricated concrete piles are driven using a pile driver equipped with a hydraulic free fall hammer. Prefabricated concrete piles are primarily used in loose soils

Pile Foundation Shallow and Deep Foundations, Building Technology, III Semester What is a Bored Pile? A Bored Pile is a deep foundation that is constructed by removing the soil in the pile location by an excavating tool (bucket- auger core barrel- etc..) to correct depth. When the drill has arrived at the correct depth, the pile is concreted using a Tremmie pipe or pumped through the end of the centre pipe (CFA).

Pile Foundation Shallow and Deep Foundations, Building Technology, III Semester

Deep Foundation Shallow and Deep Foundations, Building Technology, III Semester The key design issues in relation to pile foundations include: 1. Selection of the type of pile and installation method; 2. Estimation of the pile size in order to satisfy the requirements of an adequate margin of safety against failure of both the supporting soil and the pile itself, both in compression and tension; 3. Estimation of the settlement of the foundation, and the differential settlement between adjacent foundations; 4. Consideration of the effects of any lateral loading, and the design of the piles to produce an adequate margin of safety against failure of the soil and the pile, and an acceptable lateral deflection; 5. Consideration of the effects of ground movements which may occur due to external causes (such as soil settlement and swelling), and the estimation of the movements and forces induced in the pile by such movements; 6. Evaluation of the performance of the pile from appropriate pile loading tests, and the interpretation of these tests to evaluate parameters which may be used to predict more accurately the performance of the pile foundation.

Deep Foundation Shallow and Deep Foundations, Building Technology, III Semester Classification of piles with respect to load transmission : Piles are used to transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock having a higher bearing capacity. The method by which this occurs is the basis of the simplest pile type classification. Three main pile types: 1. End bearing piles (point bearing piles) 2. Friction piles (cohesion piles ) 3. Combination of friction and end bearing piles

Shallow and Deep Foundations, Building Technology, III Semester

FOUNDATION Dr. Sumit Khandelwal, Malaviya National Institute of Technology Jaipur, Jaipur, INDIA...

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