Retaining Walls

1
Retaining Walls

• An earth retaining structure can be considered as
  one of four types
• Gravity Walls
• Embedded Walls
• Reinforced Soil Walls
• Hybrid Systems

2
Gravity Walls

• Gravity walls rely on their significant mass and
  geometrical dimensions for stability against, for
  example, sliding, overturning. Little or no
  contribution to stability is made from the
  passive resistance of any soil acting on the face
  of the wall.
• Mass Construction
• Semi-Mass Construction
• Reinforced Construction

3
Gravity Walls

• Mass construction walls can be broken down into 5
  types
• Concrete
• Concrete with Masonry Facing
• Unreinforced Masonry
• Gabions
• Crib

4
Concrete Gravity Walls

• Mass concrete walls are suitable for retained
  heights up to 3 m. The cross section shape of the
  wall is affected by stability, the use of space
  in front of the wall, the required wall
  appearance and method of construction.

5
Gabions

• Gabions are free-draining walls constructed by
  filling large baskets with broken stone.
• Retention is achieved from a combination of the
  stones weight, and its interlocking and
  frictional strength. The wall face is battered at
  approximately 6 degrees from the vertical. The
  maximum height is approximately 10 m. They are
  constructed with either a stepped face or a
  stepped back.

6
Bin Wall or Crib Wall

• Crib walls are constructed by interlocking
  individual boxes made from timber,(temporary
  works), pre-cast concrete or metal members.
• The boxes are filled with crushed stone or other
  coarse granular materials to create a
  free-draining structure.

7
Semi-Mass Construction

• Semi-mass construction is a compromise between
  simplicity of mass concrete and low material
  content of reinforced concrete. Can be cost
  effective if reinforcement details are kept
  simple.

8
Reinforced Construction

• Reinforced concrete and reinforced masonry walls
  on spread foundations are gravity structures in
  which the stability against overturning is
  provided by the weight of the wall together with
  the weight of the retained material which rests
  on the slab The following are the main types of
  wall
• Concrete Cantilever
• Counterfort / Buttressed
• Precast
• Masonry
• Prestressed

9
Concrete Cantilever

• Cantilever walls or stem walls of reinforced
  concrete are the commonest type of gravity wall.
  They are composed of a vertical or inclined slab
  monolithic with a slab base. Simple forms of
  cantilever wall utilize the weight of the earth
  or backfill on the heel. This weight is added to
  the concrete weight to provide resistance against
  active thrust.

10
Counterfort / Buttressed

• Counterfort walls are cantilever walls
  strengthened with counterforts monolithic with
  the back of the wall slab and base slab. The
  counterforts act as tension stiffeners and
  connect the wall slab and the base to reduce the
  bending and shearing stresses. Counterforts are
  used for high walls with heights greater than 8
  to 12 m.

11
Reinforced Soil Walls

• Reinforced soils are used
• as an integral part of the design
• as an alternative to the use of reinforced
  concrete or other solutions on the grounds of
  economy or as a result of the ground conditions
• to act as temporary works
• as remedial or improvement works to an existing
  configuration.
• This category covers walls which use soil,
  reinforced with reinforcing elements, to provide
  a stable earth retaining system and includes
  reinforced soil and soil nailing.

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Soil Nailing

• Constructing a soil nailed wall involves
  reinforcing the soil as work progresses in the
  area being excavated by the introduction of
  passive bars, which essentially work in tension.
  These are usually parallel to one another and
  slightly inclined downward. These bars can also
  work partially in bending and by shear. The skin
  friction between the soil and the nails puts the
  latter in tension.

13
Soil Nailing

• The Hickman Bluff has a history of instability
  dating back to the New Madrid seismic activity of
  1811-1812 the instability has resulted in crest
  of the bluff retreating 3 to 5 feet per year.
  This has caused an approximately 400 ft. deep by
  600 ft. deep wide semi-circular recess in the
  bluff and resulted in the complete loss of
  Magnolia St. in 1990.

Hickman Bluff, Hickman, KY
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Soil Nailing
15
Soil nailing

• Working from the top downward, a mass of
  reinforced soil is gradually built up. In order
  to keep the soil from caving in between the bars,
  some sort of facing needs to be installed. This
  is generally made with some shotcrete reinforced
  by a welded wire mesh. This facing can be
  vertical, battered to a wide variety of angles,
  or made up of a series of benches.
• The passive bars are often referred to as "nails"
  and the soil reinforcing technique is known as
  "soil nailing."

16
Soil Nailing

• Over 100,000 S.F. of permanent soil nailed wall
  consisting of approximately 25 rows of soil
  nails, for a total of over 1700 each 45 ft. long
  on roughly a 6 ft. by 6 ft. grid, to stabilize
  the upper and lower bluff slopes. Approximately
  60 horizontal drains at 70 ft. long to lower the
  ground water table within the Continental
  Deposits.

Hickman Bluff, Hickman, KY
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Soil Nailing
Soil Nailed Retaining Wall Pittsburgh, PA
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Reinforced Earth

• Mechanically stabilized earth walls are
  structures which are made using steel or
  geosynthetic soil reinforcements which are placed
  in layers within a controlled granular fill.

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Mechanically stabilized earth walls

• The combination of reinforcements and earth
  create a composite structure which is internally
  stable as long as sufficient reinforcements are
  placed to within the earth to counteract the
  shear forces that result when soil is placed at a
  90 degree angle of repose.
• Originally invented in the late 1960's by Henri
  Vidal a French architect and engineer, Reinforced
  Earth which consists of soil, steel strip soil
  reinforcements and precast concrete facing panels
  was the first MSE system.
• Since that time other systems utilizing different
  facing systems (wire and concrete masonry blocks)
  and different soil reinforcement types (welded
  wire mesh, geogrids, geotextiles) have been
  used.

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What is the difference between soil nailing and
reinforced earth?

• A soil nailed wall will have been built downward
  with the soil being reinforced in situ, while a
  Reinforced Earth wall is constructed by building
  an embankment that is then strengthened as the
  work progresses, constitutes an essential
  difference.

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Reinforced Earth
22
Reinforced Earth
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Hybrid Walls

• These are walls which combine elements of both
  externally stabilized walls (e.g. gravity walls)
  and internally stabilized walls (e.g. reinforced
  soil).
• Anchored Earth
• Tailed Gabion
• Tailed Concrete Block
• Miscellaneous

24
Hybrid Walls

• Any wall which uses facing units (of any type)
  tied to rods or strips (of any material) which
  have their ends anchored into the ground is an
  anchored earth wall. To aid anchorage, the ends
  of the strips are formed into a shape designed to
  bind the strip at the point into the soil.

25
Tieback walls

• TIEBACKS and TIEDOWNS can be installed in
  virtually any ground condition using rotary,
  percussion or down-the-hole hammer tooling. 
• The critical design elements are the capacity
  (Kips), length of unbonded zone, corrosion
  protection (Class I or II or none), grout cover
  (usually 1/2"),  and testing specs. 
• In the construction of tiebacks, bars or cables
  are placed in predrilled holes with concrete
  grout.
• Cables are commonly high strength, prestressed
  steel tendons.
• To protect the tie rod from corrosion, it is
  coated with paint or asphaltic materials.

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Tieback Anchor Wall
Columbia Tower, Seattle, Washington
27
Tiedback Wall

• The modest 25 foot deep exposed wall shown in the
  above picture was actually excavated to over 50
  feet deep prior to construction of the subway
  tunnel box built for the Metropolitan Atlanta
  Rapid Transit Authority.

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Tailed Gabion

• Gabion elements fitted to geogrid 'tails'
  extending into supported soil.
• The first structure on record to use a
  combination of gabions and mechanically
  reinforced soil was built in Sabah, Malaysia in
  1979. A vertical skin of gabions was anchored to
  the backfill using metal strips.

29
Tailed Concrete Blocks

• Concrete block facing units fitted with geogrid
  'tails' extending into supported soil.

30
Sheet Pile Wall

• Steel sheet pile walls are constructed by driving
  steel sheets into a slope or excavation. Their
  most common use is within temporary deep
  excavations. They are considered to be most
  economical where retention of higher earth
  pressures of soft soils is required.

31
Failure - Earthquake
Pavement deformation and damage to rock
retaining wall caused by seismic activity
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Failure - Earthquake
Typical failure of a stone retaining wall