Foundation

FOUNDATION

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

Shallow and Deep Foundations, Building Technology, III Semester

Major Building Parts

Superstructure Plinth Level Substructure Foundation

Foundation

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

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

Loads From the Structure Foundation must resist  Dead Load ◦ Weight of building 

Live Load ◦ Weight of Occupants ◦ Weight of Furniture ◦ Weight of Equipments

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Lateral Load ◦ Wind ◦ Seismic

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Any other such as uplift etc.

SOIL REACTIONS

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 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 Shallow0.002 and Deep Foundations, to volume ratio (size smaller than mm) Building Technology, III Semester

Porous (sandy)

Clays

Site Inspection & Sub-soil Exploration 

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

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Soil-investigation is done ◦ ◦ ◦ ◦

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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 Shallow and Deep Foundations,

Building Technology, III Semester

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;

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Probing  Hollow tube or solid rod with pointed end is driven;  Frequently taken out to examine arrested material;

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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;

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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 

Choice of site exploration method depends on ◦ ◦ ◦ ◦

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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

Shallow and Deep Foundations, Building Technology, III Semester

Bearing Capacity of Soil 

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 Shallow and Deep Foundations, Building Technology, III Semester

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

Foundation Settlement

No Settlement

Total Settlement

Differential Settlement

Uniform settlement is usually of little consequence in a building, but differential settlement can cause severe structural damage Shallow and Deep Foundations, Building Technology, III Semester

Bearing Capacity of Soil 

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 Shallow and Deep Foundations, Building Technology, III Semester

Methods of SBC determination 

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 Sand Bags Platform for loading Dial Gauge Testing Plate Foundation Level Foundation Shallow and DeepSoil Foundations, Building Technology, III Semester

Methods of SBC determination 

Plate Load Test ◦ This UBC is corresponding to shear failure ◦ Load-settlement curve is also prepared ◦ 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 Shallow and Deep Foundations, Building Technology, III Semester

Methods of SBC determination 

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 65 kg Hammer 750 mm

Tripod

Bore Hole Split Spoon Sampler

Shallow and Deep Foundations, Building Technology, III Semester

Methods of SBC determination 

Cone Penetration Test ◦ lculation of SBC

Shallow and Deep Foundations, Building Technology, III Semester

Methods of SBC determination 

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.

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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 Shallow and Deep Foundations, Building Technology, III Semester

Methods of SBC determination 

Presumptive SBC values (as per IS19041961) 2

S. No.

Description of Sub-soil

SBC (kN/m )

1

Hard rocks without lamination and defects

3300

2

Laminated rocks

1650

3

Residual deposit of shattered and broken bed rock

900

4

Soft Rock

450

5

Gravel, sand and gravel, compact

450

6

Coarse sand, compact and dry (water table low)

450

7

Fine sand, loose and dry

100

8

Hard or stiff clay in deep bed, dry

450

9

Soft clay indented with moderate thumb pressure

100

10

Very soft clay that can be penetrated with the Shallow and Deep Foundations, Building Technology, III Semester thumb

50

Type of Foundation 

Primary factors affecting foundation choice ◦ Subsurface soil ◦ Ground water conditions ◦ Structural requirements:

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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

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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 Shallow dozens of individual and Deep Foundations, Building Technology, III Semester footings 

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)

Continuous (Strip) Foundation Width of foundation is to be calculated from SBC consideration  Minimum depth of foundation is calculated from Rankine formula 

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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

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

COLUMN

PIER (Concrete or Masonry)

SPREAD FOOTING (Concrete)

LOAD

Isolated (Pad) Footing

Isolated (Pad) Footing

Isolated (Pad) Footing

Isolated (Pad) Footing

Combined Foundation 

It supports two columns or more columns

Used under condition Closely placed columns Low SBC End Columns

Combined Foundation

Combined Foundation

Continuous Footing

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 and Deep Foundations, true for highly expansiveShallow soils prone to 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

Raft Foundation

Shallow and Deep Foundations, Building Technology, III Semester

Raft Foundation

Shallow and Deep Foundations, Building Technology, III Semester

Deep Foundations: Pile Foundation 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. 

Shallow and Deep Foundations, Building Technology, III Semester

Pile Foundation 

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

Shallow and Deep Foundations, Building Technology, III Semester

Pile Foundation 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. 

Shallow and Deep Foundations, Building Technology, III Semester

Pile Foundation 

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 and Deep Foundations, combination of both actions Shallow (friction and end Building Technology, III Semester

Pile Foundation 

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

Shallow and Deep Foundations, Building Technology, III Semester

Deep Foundations - Purpose transfer building loads deep into the earth

Basic types – Drilled (& poured)

– Driven

Pile Foundation 

Classification of piles with respect to material ◦ Concrete Piles  Precast piles  Cast-in-situ piles

◦ Timber Piles ◦ Steel Piles ◦ Composite Piles

Shallow and Deep Foundations, Building Technology, III Semester

Pile Foundation 

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. Shallow and Deep Foundations, Building Technology, III Semester

Pile Foundation 

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

Shallow and Deep Foundations, Building Technology, III Semester

Pile Foundation 

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).

Shallow and Deep Foundations, Building Technology, III Semester

Pile Foundation

Shallow and Deep Foundations, Building Technology, III Semester

Pile Foundation

Shallow and Deep Foundations, Building Technology, III Semester

Deep Foundation  

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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 Shallow and accurately Deep Foundations,the parameters which may be used to predict more Building Technology, III Semester performance of the pile foundation.

Deep Foundation  

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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

Shallow and Deep Foundations, Building Technology, III Semester