Download - Lecture Foundations
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Groundwater Contamination EU Water Framework Directive (2002)
Maintain quality of drinking water
In terms of groundwater:
1. Prevent input of pollutants
2. Recharge-discharge
balance
3. Reverse current pollutant
concentration trends
4. Do all the above within 15
years
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Groundwater Contamination Common contaminants:
1. Hydrocarbons (industry)
2. Pesticides
3. Human waste (sewerage)
4. Fertilisers: Nitrates
Contaminant sources:
1. Storage tanks (point source)
2. Septic systems
3. Fly-tipping of waste
4. Contaminated water courses
5. Landfills
6. Roads and railways (line
source)
7. Salt water intrusion
8. Farming (diffuse source)
9. Acid mine drainage (pyrite)
Contaminant Transport
Advection; with
groundwater
Diffusion (dilution) & dispersion
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Foundations
1. Foundation types
2. Foundation design
3. Considerations
4. Examples
5. Ground improvement
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Pad/strip footing Raft footing Piles
Foundation types Shallow Foundations Deep Foundations
Which foundation to use?
Geology
Geotechnical properties
Structure to be built
= suitable for task
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Foundation types
Pad footing Pad & strip footings
Shallow foundations
Single pad or continuous
strip
Distribute foundation
pressure to ground over a
sufficient area
Suit the pressures that
soil/rock can withstand
Footing size dependent
on strength of foundation
materials
Pad footing for single columns
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Strip footings Usually for supporting walls
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Raft footing
Shallow foundations
Where ground too weak for
pads or strips
Spread the pressure over
a much wide area
Reduce the load applied to
ground
Settlement can be
managed
Prevent lateral movement
between foundations
Improve rigidity & integrity
of building superstructure
High buildings in clay NB: referred to as a slab foundation also
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Raft footing Concrete & metal reinforcement
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Piled Foundations Pile group
Individual Pile Types
Where bearing material
exists at depth
Driven or drilled to required
depth
a) Precast concrete
b) Steel H pile
c) Steel shell pile (hollow)
d) Continuous flight auger pile
e) In-situ cast bored pile
f) Under-reamed bored pile
(greater end bearing pressure)
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Installing Piled Foundations
Driven Piles Drilled Piles
Driven in to ground by pile driver
Accommodated by lateral displacement
Diesel driver (hammer weight +
explosion)
Bachysoletanche.com
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Hole hollowed out by auger
Filled with concrete (+/- steel rebar)
Sometimes steel cased
Continuous flight augering
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Base of an offshore wind turbine
Turbine stem sits on three piles
Offshore Pile Foundations
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Piled Foundations Piles support loading in two ways:
1) End bearing
Loads transmitted to layer that
pile is resting on
Terminating a pile in gravels,
dense sands or bedrock
2) Skin friction
Friction between material and
sides of pile contributes to
carrying capacity
Reality: a contribution from
both
Cohesive materials: Suction piles
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Ground Settlement
Load applied during construction -> subsidence occurs; ground consolidates
As porosity decreases and grain packing increases
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Foundation failures
Weight of construction
> rock strength
Compaction or
consolidation of
permeable rocks
Failure into
cavity; shear or
flexural failure
Slope profiles too
steep; increase
driving forces
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Foundation Design Foundations are the part of the building structure that
transmit loads to the ground
The load that a soil/rock can support is its bearing
capacity
Soils and rocks have a range of bearing capacities due
to their shear strengths, groundwater level and
consolidation; permeabilities and grain packing
Foundation depth: confining effects, passive pressures
Clay consistency: loss of structure and consolidation
If foundation pressure for a given foundation is too
One pile to a group of piles
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Foundation Design Pressures exerted by a foundation into ground
become less significant with depth; 0.2q at 3B.
Depth to which foundation bearing pressure
significant depth.
Sig
nific
ant
Depth
Foundations are designed with
high margins of safety (FoS:2-3)
Designed to ensure that:
Applied foundation pressure is less
than that which would cause soil to
shear failure
ABP: foundation type & wide range of
soil properties
3 X
Fo
un
dati
on
Wid
th
100 kPa
20 kPa
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Foundation Bearing Pressures UBP ultimate bearing pressure, i.e. load at failure
Load which causes settlement greater than 10% pile
diameter
Do not want ground to fail therefore FoS applied:
SBP safe bearing pressure
UBP+ arbitrary Factor of Safety (usually 2-3)
Still potential for settlement to take place:
ABP allowable bearing pressure; SBP further
reduced to take in to account all possible failure
mechanisms.
The reduction factor applied to SBP may be
significant in soils but usually close to 1 in rocks
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Allowable bearing values Category Type Value
kN/m2 Remarks
Rocks
Strong sound igneous or gneissic rocks 10,000 Assumes
foundation to
grade 1/2
rock
Strong limestone and sandstone 4,000
Schists and slates 3,000
Strong shales & mudstones 2,000
Coarse
and very
coarse
soils
Dense gravel or sand and gravel >600 Width of
foundation
>1m. Water
table below
base of
foundation
Medium dense gravel or sand and gravel
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Safe Bearing Pressure
Rock Type Unweathered and
massive
Heavily fractured
or thinly bedded
Strong igneous, gneiss 10 MPa 6 MPa
Strong limestones and
sandstones
4 MPa 3 MPa
Schists and slates 3 MPa 2 MPa
Strong mudstones, weak
sandstones
2 MPa 1 Mpa
Shale, sound chalk, weak
mudstone
750 kPa 400 kPa
Can be defined in terms of rock type & weathering:
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Safe Bearing Pressure Or based on rock strength and degree of fracturing:
Unconfined
Compressive
Strength
(MPa)
RQD % / Fracture spacing in millimetres
25 / 60 70 / 200 90 / 600
100 4 8 12
25 1 3 5
10 0.2 1 2
SBP for a rock of given UCS,
RQD and fracture spacing
Fewer fractures
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Foundations in an area of old
mine workings To avoid subsidence:
1. Pile through to below mine workings
2. Fill mine chambers with grout
Normal footings where
subsidence at great depth
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Foundations in limestone