As a part of our geotechnical engineering
series, in this video, we will look into retaining walls and why they fail. Watching our previous video first may be helpful
to better understand some of the concepts presented here. In short, the previous video showed that soils
always fail by shearing or in essence sliding. Therefore, in general we as engineers are
mostly interested in the shear capacity of soils. However, the shear capacity is influenced
by the confining stress so we needed a more complicated model rather than just looking
up a single value. We looked into the so called Mohr-Coulomb
model. This model is described by the friction angle
as the single most important parameter that determines the strength of granular soils. The physical meaning of the friction angle
is related to the incline at which the soil slides. Keep that thought in mind because it is absolutely
crucial for retaining walls. In this video we will focus strictly on gravity
retaining walls which are the most common but just for completeness be aware that there
are several types of walls including cantilever, crib, buttress, sheet pile, tied-back and
several others. Soil reinforcement is also a very popular
design solution. Soil reinforcement or mechanically stabilized
earth is achieved by extending polymer reinforcing strips or other kinds of meshes into the retained
soil. All these types of walls have one thing in
common which is to retain earth behind them. The retained earth behind them is therefore
understandably the largest load they have to resist. This load manifests itself into a sliding
action or overturning of the wall which are the main two failure modes [1]. Other modes of failure are also possible but
they fall under stability or foundation failures and will not be investigated here but in some
of the upcoming videos on foundations and slope stability. If you are enjoying these video consider writing
a comment or clicking the like button this helps us greatly. Now onto the design considerations. The design of retaining walls can be approached
from two directions, either increase the resistance of the wall, or decrease the earth pressure. Let us first look at the resistance side since
it is fairly simple. Gravity walls use their own weight to retain
the soil mass. The weight of the wall induces a frictional
force at the bottom of the wall that prevents sliding. Embedding the wall into the ground can add
an additional component of resistance which greatly increase the resistance. Extending the base of the wall into the backfill
helps activate the soil mass as part of the wall and increase the base friction and resistance
to overturning. But this usually requires more excavation
and could increase the cost significantly. Alternatively increasing the size and therefore
the mass of the wall could do the job but that is also a costly solution. In practice engineers tend to pay more attention
to the soil behind the wall and reduce its effects. Soils act similar to water in the sense that
they exert pressure on its surroundings. This pressure increases with the depth of
soil. Unlike water, the pressure from the earth
depends on whether the wall can move or not. In almost all practical cases, the wall does
move and this helps relieve some of the pressure applied against it. Terzaghi, the father of soil mechanics, performed
a bunch of full-scale tests of retaining walls and found that even the most insignificant
wall movements in the order of a fraction of a millimeter could reduce the lateral earth
pressure applied to the wall by 50%. [2]
Theoretically this reduction has to do with something called the active loading case and
is related to the coefficient of lateral earth pressure. But instead of going into technicalities we
will try to explain this intuitively. When the wall moves, the soil wants to follow
it and it starts to slide or separate from the main soil mass. We saw this in the previous video. This separation helps us because the detached
soil wedge has less mass and exerts a lower pressure than before the wall moved. Just to be clear, this movement is not even visible by the naked eye, If you can visually see that the wall
has moved then you are probably looking at a failing wall rather than an active loading
case. Here you can see how increasing the friction
angle of the soil decreases the pressure exerted on the wall. Now a good question is how does one increase
the friction angle? The friction angle is intrinsic to the type
of soil. For example clay and silts have low frictional
strength. They use their cohesion or stickiness to carry
the loads. The cohesive strength is unreliable because
it could sometime disappear if the soil gets saturated. Besides, clays and silts have low porosity
and tend to trap the water which is not something you want behind your wall. Granular soils like sands and gravels make
for the best backfill material [3]. They have decent friction angles and high
hydraulic conductivity which allows the water to easily drain. But to get the best out of your backfill,
it is absolutely crucial to have the backfill densely compacted. As we saw in the previous video, compacting
the soils packs the particles tightly together which significantly increases the friction
angle and therefore the strength of the soil. If the gravel is also well-graded, meaning
the size of the particles is fairly uniformly distributed, then the strength is even better. The last point to make before moving on to
the actual wall scenarios is about drainage. Providing drainage to the wall is probably
the most important consideration. The effects of poor or no drainage will become
apparent shortly. Now, let us look at a retaining wall design
scenario and see how the soil parameters affect the factor of safety. If you are not familiar with the concept of
safety factor it basically means how much the resistance of the wall is bigger than
the driving force of the soil. A SF of one means the two forces are equal
and only a minor increase in the weight of the soil will cause a collapse. The investigated wall is to resemble a typical
wall often seen in residential area. The modeled case does not consider any surcharge
loads on top but only the weight of the soil. Four different scenarios of the same geometry
were consider. First, the soil was assumed to be densely
compacted resulting in a high friction angle (40 deg). The second scenario used the same soil as
the first but with geo-grid reinforcement added to the soil. The third scenario considers the same soil
but no drainage to the wall so there is build-up of water behind it. And the last scenario assumes a loose backfill
material with no compaction and a lower friction angle (30 deg). After running the analysis in Optum the following
safety factors were obtained. Unsurprisingly, the reinforced soil achieved
a SF of 2.9 which means the system is nearly 3 times stronger than the equilibrium condition. Let me know in the comments why do you think
that is. The case with no drainage achieved the lowest
SF of only 23% higher than the state where collapse starts and is also 75% weaker compared
to case number 1. The loose backfill also performed poorly but
still outperformed the wall with poor drainage. The topic of proper drainage and construction
of retaining walls have so much in them that is impossible to cover here. Let me know in the comments if you would like
a part 2 to this video or whether to move on to other applications such as foundations
and slopes. Or if you are too lazy to write, just click
the like button as a sign of support and we will keep producing more videos. Thanks for watching, see you in the next video.
