Understanding soil mechanics is crucial for any geotechnical application. Specifically,
understanding the capacity (or limits) of the soil is what enables successful design for foundations
or earth retaining structures. This video will be one of a series of geotechnical videos explaining
retaining walls, shallow foundations, and stability of slopes from
an engineering point of view. However, before we dive into these geotechnical applications,
first we need to understand some basic principles of soil mechanics. Soils,
unlike other construction materials such as steel, timber, or concrete, are almost always under
compression. But interestingly, soils never fail in compression. All practical soil failures are
due to excessive shear stresses. Shear stresses are less intuitive so let s have a closer look.
The shear capacity of common engineering materials (such as steel, timber, or concrete)
is governed by the strength of their molecular bonds and therefore usually fairly high.
Soil, on the other hand, is a discontinuous particulate material made up of various size
particles. The interlocking of these particles is what gives the soil its resistance to shearing.
When the shear stresses along a particular plane or slide-surface are large enough to
make the particles roll and slide past one another, failure is initiated. In the field,
shear failures of a soil mass are usually confined to a narrow zone where rearrangement of the
particles occurs [1]. Without these interlocking forces the material would behave more like a fluid
rather than a solid. This condition where the soil turns into a fluid-like material does happen
sometimes and is known as liquefaction, but it is a specialized case and we will not worry about for
now. Beside the interlocking or frictional shear strength, some soils also posses cohesive shear
strength. Cohesion in practical terms represents the stickiness of the soil such as in clay,
but is usually zero for coarse-grained soils like sands and gravels. What makes things complicated
is the fact that the frictional strength of the soil is dependent on the pressure the soil is
under. This makes intuitive sense since if the soil is compressed, the particles get further
interlocked and sliding becomes more difficult. On the other hand, submerging the soil underwater has
the opposite effect since the buoyant force from the water helps the particles become partially
afloat. In more technical terms, the pressure from the water acts on the particles tending to
separate them and therefore reducing the contact forces and friction resistance of the material.
In practice, the strength of soils is represented by the Mohr-Coulomb failure criterion. The German
engineer Otto Mohr graphically represented the strength of the soil as a line on a graph with
the material s normal stress plotted on the x-axis and shear stresses on the y-axis. When the soil
is loaded it forms a semi-circle on this plot that is known as the Mohr Circle. The two ends
of the circle represent the normal stresses that the soil is subjected to. An alternative way to
look at these is as a confining stress on the low side and applied stress on the high side.
These stresses are also known as the principal stresses of the material. Notice, that to keep the
circle small and away from touching the failure envelope, we need to increase the confining stress
together with the applied stress. In theory, if the soil is well confined, its carrying capacity
can be increased indefinitely. Note that every point in the soil mass draws a separate circle
on the graph. As the soil continues to get loaded, the circles grow until one or more
of them touch the failure line. The point in the soil mass whose Mohr circle touches the failure
line first is where the sliding in the soil will occur. This usually happens simultaneously for a
whole plane in the material which initiates a sliding failure. The slope of the failure line
is represented by a parameter called the friction angle which is the single most important strength
parameter for coarse-grained soils. If the friction angle is high, then the soil can
resist higher shear stresses. The physical meaning of the friction angle is related to the incline
angle at which the soil slips during failure. This is particularly important in the design of
retaining walls and shallow foundations as we will see in the upcoming videos. For someone unfamiliar
with yield lines this may seem overwhelming. The most important takeaway from here is that
the parameter called the friction angle is what determines the strength of a soil. The
friction angle of a soil is influenced by the soil type, shape of particles, groundwater condition,
and void ratio among other things. So angular, well-graded, compacted soils exhibit high friction
angles and make up for the best type of soil to be used as backfill, or to build a foundation on.
Stay tuned for the upcoming videos where we will discuss geotechnical applications such
as retaining walls and foundations. Click the subscribe button and the bell icon if you want
to be notified when the next video comes out. Thanks for watching and see you next time.