Hey guys, this is structures explained, and
in this video we will be learning about Retaining Walls. This video is divided into 4 parts. First we will learn about general types of
retaining walls and its components. After that we will see types of failures in
a retaining wall, then we will see general forces and behavior of retaining walls, and
lastly typical reinforcement in a cantilever retaining wall. So stick around till the end to know everything
about Retaining Walls. Let’s know what a retaining wall is. Retaining walls are used to hold earth or
any other material. It prevents soil from taking its natural position
and makes area above and below it usable. The wall has basically 3 parts. Stem, Toe Slab and Heel Slab. Toe and Heel slab make up for the foundation
of the wall. Some walls have a key provided in footing
to prevent it from sliding. The stem may be provided with drain holes
with slope for the water drainage. Similar sloping perforated pipe may be provided
below the same for water drainage. The soil behind the stem can be coarse aggregates
so that water percolates and exits via drains. The portion of wall in contact with the soil
is usually provided with some kind of waterproofing. Now let us know about various types of retaining
walls depending on their use. First type is a Gravity Wall which retains
soil by its own weight. Gravity wall is of a bigger size and usually
built in stone masonry and rarely in plain concrete. Second type is a Cantilever Wall which is
the most common type of retaining structure. It is usually used for retention height up
to 8 meters or 25 feet. The 3 components, which are stem, toe and
heel act as one-way cantilever slabs. The stem acts as a vertical cantilever under
lateral earth pressure. The heel acts as a vertical cantilever under
the action of net weight of the retained earth and the toe acts as a cantilever under the
action of net soil pressure. Hence the resulting deformed shape would look
something like this. The main reinforcement resisting the tension
forces will be provided in these regions as the concrete is weak in tension. Third type of retaining wall is a Counterfort
Wall. Such types of walls have supports called ‘counterforts’
connecting stem and heel slabs. The counterforts are concealed in the retained
earth. These walls are provided where retention height
is more than around 7 meters or 23 feet. The counterforts subdivide the Stem and heel
into rectangular panels. These panels are now supported on 3 sides
and free at one edge. Fourth type of retaining wall is a Buttress
Wall which is similar to the Counterfort wall but the supports are now on the toe side and
not buried in earth. Between the two, counterfort and buttress,
Counterfort wall is preferred as it provides usable space in front of the wall and looks
clean. In terms of efficiency and economy Buttress
wall is preferred. Fifth type of retaining wall is restrained
on the top and can be found in Buildings as Basement walls and in bridges as Abutments. In both these cases, the stem is supported
by floor slab in Buildings and Deck in bridges. For the analysis part, the stem is considered
as a beam fixed at the base and simply supported or partially restrained at the top. Now lets understand failure modes of a cantilever
retaining wall. First is failure by Overturning. In this failure mode, the toe will act as
center of rotation and the wall would deform something like this. In absence of toe, the footing base below
the stem will act as center of rotation. All the lateral pressures will act as overturning
forces while the weight of the wall and soil on the heel will act as stabilizing forces. Second kind of failure is by sliding. All the lateral forces try to slide the wall. The resistance against sliding is mainly provided
by the friction between the base slab and the soil below it. The frictional force is given by 'mu' times
R where mu is the static friction coefficient between soil and concrete, and R is the resultant
soil pressure. When the lateral pressures are high and the
wall fails in sliding, a shear key can be introduced to increase the sliding resistance. The position of the shear key is decided in
such a way that the flexural reinforcement from the stem can be extended straight into
the shear key and it can create maximum counter pressure. The pressure generated on the shear key resists
the lateral forces. Third kind of failure occurs when the soil
below the wall fails in bearing pressure. A soil can bear a certain amount of allowable
pressure which is found by geotechnical study. Hence the width 'L' of the base slab must
be adequate to distribute the vertical reaction. Now let us see the forces which act on a simple
Cantilever Retaining wall. First let's assume a flat backfill with no
additional load. The soil will apply lateral pressure which
will vary with the depth of the soil, which means, top of the wall will bear no pressure
while bottom of wall will bear maximum pressure. This pressure is called active pressure, or
in short ‘Pa’ as it is actively trying to push the wall along it. The intensity of the pressure at bottom will
be Ka times 'gamma s' times 'H'. In this expression 'H' is the total height
of the backfill, gamma s is the unit weight of the soil. Here, Ka is the active pressure coefficient
based on Rankine's theory and is calculated via this expression. Here angle phi is the angle of shearing resistance
or angle of repose for the soil. When the backfill is sloped, the expression
changes to this, where additional angle theta is the angle of inclination of the backfill. For a typical granular soil such as sand,
phi is 30 degrees which makes Ka as 1 by 3. This Pressure Pa will act at the centroid
of the pressure intensity triangle which is at a distance of H/3 from the base. The value of this force will be the area of
triangle which is half times base into height. This was about the active pressure, there
is one more pressure which is called as Passive pressure which acts from the toe side and
helps the retaining wall. The coefficient of passive pressure 'Kp' is
calculated by this expression which comes out to be 3 for granular soil with phi of
30 degrees. This pressure is generally not included in
design calculations hence making design conservative. Next force which will act on the wall is the
surcharge if considered. This is also an active pressure. The surcharge load can come from various means
such as vehicular traffic or any other live loads. Let's consider this surcharge having intensity
of 'w' Newtons per meter square and uniformly distributed. This pressure will act laterally on the wall
with an intensity of Ka times 'w'. The force due to this surcharge is the area
of the rectangle which is Ka times w times height. Next Force which acts on the wall is from
the water present behind it. This case is a bit complex, so let's simplify
it slowly. When water does not have an escape route,
it will get accumulated and apply pressure on the wall. Let's consider the height till which the soil
is submerged in water as 'Hw'. The remaining dry soil will have a height
of 'H minus Hw' where 'H' is the total height. The dry soil will have a unit weight of 'gamma
s' and the submerged soil will have a unit weight of 'gamma submerged'. Let's make the pressure diagrams one by one. First will be the pressure from dry soil,
let's call it 'Pa1'. This dry soil will act as a surcharge for
the remaining portion of the wall. Let's call this pressure 'Pa2'. The maximum intensity of the pressure will
be 'Ka times unit weight of dry soil times the dry soil height'. Now let's talk about submerged portion. Here the first pressure, let's call it 'Pa3'
will come from submerged soil and will have an intensity of 'Ka’ times the unit weight
of submerged soil, times the height of submerged portion. The water pressure in this area, let's call
it 'Pa4' , will have an intensity of unit weight of water times the height of the submerged
portion. All the forces can be added by finding the
area under them to calculate total force acting on the wall. The water pressure can be avoided by providing
proper drainage facilities like holes and drains. Failure to do this can result in building
up of large amounts of water pressure. Now let's look at the typical reinforcement
in a cantilever retaining wall. The wall is provided with reinforcement to
resist various kinds of stresses produced in it. The main reinforcement in the stem is as shown
and is distributed with equal spacing along the length. The bars are lapped if required and have a
90 degree hook at the end. If the wall has a key, the main reinforcement
is extended into the key. On the compression face we also have reinforcement
which is extended into the base slab with 90 degree hook for sufficient anchorage. The base slab has reinforcement at both, top
and bottom faces to resist the tension and compression forces. Distribution bars are spread throughout the
stem and base slab to resist stresses and hold the main reinforcement in place. That's it for this video, if you enjoyed this
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