Mechanically stabilised Earth

1. Unit 7: Mechanically
Stabilized Earth
By
DM Bester
Faculty of Engineering and the Built Environment

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What holds it together:
•abc

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What holds it together:
•Friction
•Molecular Bonds
•Cement
•Love

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•abc

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Soil has endless sliding
planes:

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Look at this sample and
apply a vertical force

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Analysis a
horizontal
failure plane

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Analysis at a angle the failure plane – the
greater the failure plane the more the
vertical force contributes to shear stress and
the less it adds to shear strength

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If shear stress exceeds the strength
sliding occurs – material has failed:

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Granular materials cannot stand vertically –
the weight is enough to cause shear failure:

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Pore sand out on a table – the
pile forms a slope:

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Angle of the slope is called angle of repose – this is
the steepest angle at which the soil can naturally
rest – in other words this is slope at which the shear
stresses with in the soil due to its own weight are
exactly equal to the shear strength caused by
internal friction – any steeper and the soil will slide.

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Lets look at a sample of soil
and put in back into ground:

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Now the soil around our sample
can apply horizontal pressure:

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This is called confining pressure, and it
helps to balance out vertical forces like the
weight of the soil itself. This confining
pressure is the reason that granular material
can be stable at a slope, but usually won’t
be stable vertically.

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This can be a problem if you are trying to
build an earth structure for two reasons.

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1) It takes about twice as much material
than if you are using something that can
stand vertically.

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2) Space – in crowded cities, space is at a
premium. If you are building an eart
structure every meter you go up in height,
you have to go out that far as well, or even
further:

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So what’s a Geotechnical
Engineer to do?
• What if there was a way
to add confining pressure
to the soil, without having
to build on a slope.

20. Faculty of Engineering and the Built Environment
Answer - Reinforced earth!
Just like rebar in concrete, you
can create an incredibly strong
composite material with soil just
by adding reinforcing elements. A
wall created in this way is called
mechanically stabilized earth or
MSE.

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If you look closely, MSE
walls are everywhere:

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Example:

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Just soil = failure:

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MSE:
•abc

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So what is
happening?
The tension in the
reinforcement is
generating
confining pressure
in the soil.

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This pressure
acts
perpendicularly to
the failure planes,
increasing the
shear strength:

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Building an MSE wall works exactly
the same way, and they are primarily
used in highway projects:

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Compacted soil is added in layers with
reinforcing elements in between each
layer:

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Most MSE walls ahs a facing of interlocked
concrete panels usually with some kind of
pattern, and these facing systems are what
make them so recognizable:

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