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Retaining Walls

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The skin friction between the soil and the nails puts the latter in tension. Soil Nailing ... which is internally stable as long as sufficient reinforcements ... – PowerPoint PPT presentation

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Title: 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.

12
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
14
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
17
Soil Nailing
Soil Nailed Retaining Wall Pittsburgh, PA
18
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.

19
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.

20
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.

21
Reinforced Earth
22
Reinforced Earth
23
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.

26
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.

28
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
32
Failure - Earthquake
Typical failure of a stone retaining wall
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