Transcript Lean Construction

1
Reinforced Concrete Design-II
Lec-3
Retaining walls
By
Dr. Attaullah Shah
Swedish College of Engineering and Technology
Wah Cantt.
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.
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).
− 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.
Drainage of Retaining walls
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.
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?
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 ϐ and internal angle of friction Φ is given as:
−
For horizontal backfill, the expression is given as
− The total lateral active and passive pressure are given as:
Earth pressure for various conditions
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
Overturning of Retaining Wall:
The top width of the stem=0.08h = 0.08*15 = 1.20ft= 14.4 in Assumed 18in
Depth of the base= 0.12h to 0.16h=14.4in 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
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.
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)
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.
Assigments