Foundation

(1)

FOUNDATION

Dr. Sumit Khandelwal,

Malaviya National Institute of Technology

Jaipur, Jaipur, INDIA

Shallow and Deep Foundations, Building Technology, III Semester

(2)

Superstructure

Plinth

Level

Substructure

Foundation

(3)

Foundation

Part of structure in direct contact with ground to

which the loads are transmitted is foundation

(4)

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.

(5)

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

(6)

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

(7)

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

(8)

Types of Sub-soil &

Characteristics

Shallow and Deep Foundations,



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)

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Clays Porous

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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

 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

(11)

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

(12)

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;

(13)

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;

(14)

Site Exploration Methods



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

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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

(16)

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.

(17)

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

(18)

Uniform settlement is usually of little consequence in a building, but differential settlement can cause severe structural damage

Foundation Settlement

No Settlement Total Settlement Differential Settlement

(19)

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

(20)

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

Foundation Soil Sand Bags Platform for loading Foundation Level Testing Plate Dial Gauge

(21)

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

(22)

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

Bore Hole

Split Spoon Sampler Tripod

65 kg Hammer 750

(23)

Methods of SBC determination



Cone Penetration Test

(24)

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.



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

(25)

Methods of SBC determination



Presumptive SBC values (as per

IS1904-1961)

S. No. Description of Sub-soil SBC (kN/m2₎

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

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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

(27)

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

(28)

Shallow Foundation



Depth is not more

than width (Terzaghi)



Also

called

open

foundation



Transfers loads to the

soil very near the

surface

(29)

Advantage of Shallow Foundation



Affordable Cost



_{Simple Construction Procedure}



Convenient Materials



Skilled labour not required

(30)

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

(31)

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)

(32)

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

(33)

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

(34)

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

(35)

(36)

(37)

(38)

(39)

Combined Foundation



It

supports

two

columns

or

more

columns

Used under condition

Closely placed

columns

Low SBC

(40)

(41)

(42)

(43)

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(45)

(46)

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

(47)

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

(48)

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

(49)

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

(50)

Raft Foundation

(51)

Raft Foundation

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Raft Foundation

(53)

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.

(54)

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

(55)

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.

(56)

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 combination of both actions (friction and end

(57)

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

(58)

Deep Foundations - Purpose

transfer building loads deep into the earth

Basic types

– Drilled (& poured)

(59)

Pile Foundation



Classification of piles with respect to material

◦ Concrete Piles  Precast piles  Cast-in-situ piles ◦ Timber Piles ◦ Steel Piles ◦ Composite Piles

(60)

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.

(61)

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

(62)

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).

(63)

Pile Foundation

(64)

Pile Foundation

(65)

Deep Foundation

 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.