Lean Construction

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By Dr. Attaullah ShahSwedish College of
Engineering and Technology Wah Cantt.

• Reinforced Concrete Design-II
• Lec-3
• Retaining walls

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Purposes of Retaining walls

• Retaining walls are structures designed to
  restrain soil to unnatural slopes.
• They are used to bound soils between two
  different elevations often in areas of terrain
  possessing undesirable slopes or in areas where
  the landscape needs to be shaped severely and
  engineered for more specific purposes like
  hillside farming or roadway overpasses.

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Types of Retaining walls
Gravity walls depend on their mass (stone,
concrete or other heavy material) to resist
pressure from behind and may have a 'batter'
setback to improve stability by leaning back
toward the retained soil. For short landscaping
walls, they are often made from mortarless stone
or segmental concrete units (masonry units).
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• Cantilevered retaining wall
• Cantilevered retaining walls are made from an
  internal stem of steel-reinforced, cast-in-place
  concrete or mortared masonry (often in the shape
  of an inverted T).
• These walls cantilever loads (like a beam) to a
  large, structural footing, converting horizontal
  pressures from behind the wall to vertical
  pressures on the ground below.
• Sometimes cantilevered walls are buttressed on
  the front, or include a counter fort on the back,
  to improve their strength resisting high loads.
  Buttresses are short wing walls at right angles
  to the main trend of the wall. These walls
  require rigid concrete footings below seasonal
  frost depth. This type of wall uses much less
  material than a traditional gravity wall.

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Drainage of Retaining walls
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Sheet piling

• Sheet pile retaining walls are usually used in
  soft soils and tight spaces. Sheet pile walls are
  made out of steel, vinyl or wood planks which are
  driven into the ground.
• For a quick estimate the material is usually
  driven 1/3 above ground, 2/3 below ground, but
  this may be altered depending on the environment.
  
• Taller sheet pile walls will need a tie-back
  anchor, or "dead-man" placed in the soil a
  distance behind the face of the wall, that is
  tied to the wall, usually by a cable or a rod.
  Anchors are then placed behind the potential
  failure plane in the soil.

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Failure of Retaining walls

• Failure of retaining walls is more frequent as
  compared to other RC structures due to
• Poor design assumptions
• Changing and unpredictable subsoil and backfilled
  conditions
• Poor masonry work/material strength and improper
  bonding
• Lack of drainage facilities and provisions
• Can you think of some more reasons?

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Lateral pressure on Retaining walls

• Active lateral pressure ( Can you define)
• Passive lateral pressure
• The general equation ( Rankine Eq) for the
  active and passive coeff. for a surcharge angle
  of ? and internal angle of friction F is given
  as
• For horizontal backfill, the expression is given
  as
• The total lateral active and passive pressure are
  given as

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Earth pressure for various conditions
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Modes of failure of retaining walls

• The individual components of the wall may fail
• The wall as a whole may be displaced due to
  sliding
• The wall may get overturned
• The factor of safety against the sliding must be
  at least 1.5
• The horizontal force acting on the retaining wall
  is Ph which is resisted by the vertical component
  of lateral pressure and resisting weight of the
  load on the toe
• To have min factor of safety of 1.5
• If the required sliding resistance is not
  provided by the weight, then key may be provided
  beneath the stem
• The pressure under the footing must not exceed
  the allowable bearing capacity of soil

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Overturning of Retaining Wall
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The top width of the stem0.08h 0.0815
1.20ft 14.4 in Assumed 18inDepth of the base
0.12h to 0.16h14.4in to 28in Assume 24in Width
of the base 0.5h to 1.25 h 7.5 ft to 11.5 ft
Assume 10ft Width of heel 0.5d to d 12 to 24
in Assumed 9in
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Design of semi gravity retaining walls

• The retaining walls which resist the lateral
  pressure by means of its weight and developed
  soils.
• The wall thickness is selected such that no part
  of the wall is in tension
• The resultant force must pass through the middle
  third to fulfill this condition.

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Effect of Surcharge

• If the surcharge is above the backfill it will
  induce a uniform lateral pressure on the
  retaining wall in addition to the back fill
  pressure.
• In case the surcharge is away from the face of
  wall. The lateral pressure effect of the
  surcharge will be effective at 45 degree from the
  surcharge as shown in the figure ( Partial
  pressure due to partial surcharge)

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Estimating the sizes of Cantilever retaining
walls

• Highest of the wall
• Depends on the height of the back fill.
• The foundation of retaining wall is usually 3-6
  feet below the ground level depending the soil
  condition and weather effects. In colder regions,
  the depth has to be sufficiently below the ground
  level to avoid the damage due to frost action
• The various min dimensions are shown in the
  Figure.
• The reinforcement is main or stem reinforcement
  and temperature reinforcement along the face of
  retaining wall.

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Assigments