Transcript of FOUNDATION Dr. Sumit Khandelwal, Malaviya National Institute of Technology Jaipur, Jaipur, INDIA...
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- FOUNDATION Dr. Sumit Khandelwal, Malaviya National Institute of
Technology Jaipur, Jaipur, INDIA Shallow and Deep Foundations,
Building Technology, III Semester
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- Superstructure Plinth Plinth Level Level Substructure
Substructure Foundation Foundation Major Building Parts
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- 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
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- 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
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- 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
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- 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.
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- 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
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- 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)
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- ClaysPorous (sandy)
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- Methods of SBC determination Shallow and Deep Foundations,
Building Technology, III Semester Cone Penetration Test lculation
of SBC
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- 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
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- 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
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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 Shallow and Deep Foundations, Building
Technology, III Semester
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- 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
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- 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
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- Advantage of Shallow Foundation Affordable Cost Simple
Construction Procedure Convenient Materials Skilled labour not
required Shallow and Deep Foundations, Building Technology, III
Semester
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- 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
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- 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
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- 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
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- 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
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- 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
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- Isolated (Pad) Footing
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- Combined Foundation It supports two columns or more columns
Used under condition Closely placed columns Low SBC End
Columns
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- Combined Foundation
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- Continuous Footing
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- Strap Footing
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- Footing Plan for a building
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- 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
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- 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
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- 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
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- 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
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- Raft Foundation Shallow and Deep Foundations, Building
Technology, III Semester
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- Raft Foundation Shallow and Deep Foundations, Building
Technology, III Semester
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- Raft Foundation Shallow and Deep Foundations, Building
Technology, III Semester
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- 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.
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- 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
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- 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.
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- 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 )
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- 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
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- Deep Foundations - Purpose transfer building loads deep into
the earth Basic types Drilled (& poured) Driven
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- 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
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- 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.
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- 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
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- 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).
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- Pile Foundation Shallow and Deep Foundations, Building
Technology, III Semester
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- Pile Foundation Shallow and Deep Foundations, Building
Technology, III Semester
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- 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.
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- 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
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- Shallow and Deep Foundations, Building Technology, III
Semester