Transcript REINFORCED EARTH STRUCTURES

REINFORCED EARTH
STRUCTURES
• CONCEPT
• BASICS OF DESIGN
• CASE STUDIES
REINFORCED EARTH (R.E.)
Reinforced earth is a composite material
formed by the friction between the earth
and the reinforcement. By means of friction
the soil transfers to the reinforcement the
forces built up in the earth mass. The
reinforcement thus develops tension and
the earth behaves as if it has cohesion.
COMPONENTS OF R.E.
• SOIL
• SKIN
• REINFORCEMENT
SOIL FOR R.E.
• Development of sufficient friction between
earth and reinforcement.
• No interstitial pore water pressure develops
within the reinforced earth structures
• The placing and compaction of the earth fill
layers can be accomplished easily
• The soil must conform to certain electrochemical conditions to avoid corrosion
REINFORCEMENT FOR R.E.
• Reinforced members are composed of thin
wide strips also called ties.
• Should be flexible to ease placement.
• Should have adequate tensile strengths.
• Should have adequate service life taking in
to account corrosion and weathering.
FACING ELEMENTS
• Should retain the back fill between the
layers of reinforcements.
• Made of either metal units or precast
concrete panels.
• Should be able to deform without distortion.
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DESIGN COMPONENTS
• SKIN
• TIES
• FOUNDATION – Bearing capacity &
sliding
• SETTLEMENT
• OVERTURNING
• Drainage
DESIGN : SKIN
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Calculate the lateral earth pressure.
Based on the the spacing of ties.
Designed as thin shells.
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DESIGN : TIES
Two boundary conditions –
• The wall is perfectly flexible & frictionless.
• The wall will move laterally during
construction by sufficient amount to
mobilize a state of active earth pressure.
DSIGN : TIES contd
• The tensile force increases from zero at the
free end to maximum at the face of the wall.
• The tensile force also increases linearly
with depth.
• Tension mode of failure.
• Bond mode of failure.
• Factor of safety – 1.5 to 2.5.
DESIGN : FOUNDATION
• Resistance to Bearing Capacity Failure:
Bearing capacity of foundation soil must
withstand the substantial settlement without
damage to the structure.
• Sliding at the base of the structure:
Active earth pressure acts on the face wall
and the friction at the bottom. This can be
improved by help of passive earth pressure.
DESIGN : FOUNDATION contd
• Overturning : Tilting of the upper portion
of the wall may occur, if the reinforcing
strips are not long enough.
DESIGN : SETTLEMENT
• Not much design issues involved.
• Limiting values depend upon usage.
• Calculations based on normal settlement of
any structures.
• Settlements within the reinforced earth mass
itself and settlement of the foundation soil.
DESIGN : DRAINAGE
• Aim to avoid interstitial pore pressure.
• Choose backfill soil with good drainage
characteristics.
• Provide boulder/sand column drain in case
the R.E. is supporting embankment slope.
MATERIAL SELECTION: SOIL
• Must be able to develop friction.
• The backfill should be from non-organic
soils such as sand and Gravel which are not
affected by biological activity.
• Have good drainage.
• Primarily cohesion less soils are used.
MATERIAL SELECTION:
FACING ELEMENTS
• Can be chosen from metal and concrete.
• Based on durability and stability criteria.
• Aesthetics can be equally important when
used in urban areas.
MATERIAL SELECTION:
REINFORCING STRIPS
• Based on electro-chemical properties of
back fill.
• Can be chosen from metal, geo-synthetics
etc based on durability criteria.
• Select fixtures accordingly used to connect
strips to the facing elements.
COST COMPARISON
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Total
Clearing the jungle
:Rs 0.01
Earthwork in excavation
:Rs 0.90
CC 1:2:4 for foundation and coping
:Rs 10.24
CC 1:3:6 in the retaining wall
:Rs 77.42
Surface reinforcement for the concrete retaining wall :Rs 4.20
Ordinary backfill including compaction
:Rs 5.93
RCC parapet wall
:Rs 1.32
:Rs100.00