Prof. of the United States’ soil composition. There

 

Prof.
Kuszmaul

GEN2060

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Assignment
2

 

 

Geotechnical
Engineering, the science that explains soil and rock mechanics and its
applications to the development of human kind. One of the main purposes of
geotechnical engineering is to establish the safety of infrastructure under
extreme pressure. Some of these structures may include buildings, bridges, dams,
etc. The idea is to have a full understanding of how the soil and rock will
support the structure in order to keep it safe under certain loadings. This
paper will detail how an earthquake loading can affect a structure and how a
geotechnical engineer’s job is to design it to maintain its integrity and
safety.  

Geotechnical
engineers study the earths ability to support structures. This gives them the
ability to investigate the earth’s sustainability of its slopes and manmade
soil deposits. It also lets them evaluate cuts into the earth as well as fills,
shallow and deep foundations, and tunnels that are all directly related to the
earth’s rocks, soil, and water.

There are many
things to consider when designing a structure that will be supported on soil or
rock. The two main things to consider are the bearing capacity of the rock or
soil and analyzing the stability of sliding. The main goal when looking at
these two points is to ensure the stability and safety of the structure while
accounting for all possible loadings. Some of the loadings include the dead
load of the structure itself, the superimposed dead load, the live load, the
wind load, the snow or rain load, and finally an earthquake load. The structure
must withstand the loads without having any major deformations to the structure
itself or to the soil/rock supporting the structure underneath. Another factor
to consider is the ability the structure may have to slide on top of the
foundation. If a structure has the ability to move on top of the earth’s
surface it will not be very safe. (Hamilton)

The Natural
Resources Conservation Service (NRCS) which is a branch of the United States
department of agriculture has mapped out many parts of the United States’ soil
composition. There are four main ways the geotechnical engineers perform tests
on the soil. These tests include test pits, trenching, boring and In-Situ
testing. A test pit is simply drilling a hole to the desired depth and
evaluating the sub-surface conditions. Trenching is very similar to test pits. The
difference between test pits and trenching is the variety of samples. While trenching,
the pit is dug larger to give the observers the knowledge of how the soil
changes as you move in each direction. This sometimes includes more manual
labor such as shoveling or the use of hand-driven augers. Borings are holes
which people can manually extract soil and rock samples to be tested. Many times,
boring does not give completely accurate information of the way the soil as a
whole would react to a structures pressure because the act of boring can
actually disturb the soil conditions. In-Situ testing include penetration
tests. Theses test penetrate due to a direct push onto the soil such as the
Standard Penetration Test (SPT). In these forms of testing, the soil is not
removed which takes away the need to have samples sent to a laboratory. (Applied
Research Associates)

 

Most soils can be
categorized by being a gravel, sand, silt or a clay. The grains are classified
based on their particle size and texture. Sand grains relatively have a rotund
shape and are usually formed from pieces of rock that have been weathered. This
is either from physical weathering or from chemical weathering. When sand is
used as a foundation it is important to take the sand-fill ration into
consideration. It has been proven that as the sand-fill ration increased, the
hydraulic stability of the sand also increased. Clay particles are flaky and
very thin, which gives them a very high specific surface values. (Mishra)

Many different
forms of mapping must take place to analyze the location of a possible build
site. One form of mapping is known as aerial mapping. While aerial mapping a
location, an engineer would consider the entire area that will be needed to
build the structure, along with surrounding areas that may influence the
project or could be influenced by the project. While mapping the area some
things to take into consideration are, faults that may be in the rock, rock
slope instability, ground-water levels, or pipelines used for other utilities
of surrounding buildings. (Weathering) The next form of mapping is site
mapping. The site mapping process includes the complete understanding of the
rock formation around the possible location of building. Construction Mapping
is also a very important form of mapping that involves recording as the
structure is being built. Construction mapping is focused on record pictures, a
very important detail as the foundation is being placed. This type of mapping
can be very useful when a structure needs to be edited or added to later on. (US
Army Corps of Engineers)

While exploring the
possible location of a structure, the soils are carefully analyzed, usually in
the form of borings. There are many different tests that allow geotechnical
engineers to assess the soil/earth structure along with its ability to support
large structures. One is the degree of weathering. A rock can be categorized by
the amount of weathering it has undergone. For instance, a rock can be
considered extremely weathered to the point that it has become a soil, or it
can be unweathered, showing no signs of weathering due to mechanical or
chemical reasons. Hardness is also a factor when describing a rock. A rock is
considered very hard if it cannot be scratched with a knife and a rock is
considered very soft if it can be deformed by hand. These are just two of the
many tests done on rocks to ensure its stability when creating a structure. (Hamilton)

After carefully
considering all of the rock’s properties and completing several tests, the
rocks must be considered for their bearing capacity. The unit weight of the
rock is a major factor to consider. This may not be directly related to the
structure having any deformations but it can show how much weight can be added
to the rock while maintaining its stability. Another factor to consider is the
depth of the rock foundation under the foundation of the structure. (Hamilton) Unit
weight can be a bit tricky due to the addition of a water table. The rock found
above the water table, for the most part, can withstand more pressure and
loading than rock found beneath the water table. (US Army Corps of Engineers)  When considering foundation depth it is
important to understand the earth. Most rocks located at greater depths are
known to be denser and stronger than rocks located higher above sea level.
Although most of the time this is the case, there are times that at greater
depths, there are more types of rocks. When there are many different layers of
rocks it could mean that the rock foundation is not very strong and weaker than
a rock foundation at a higher altitude. (Rocks and Layers)

Sliding failure is
one form of failure a structure can undergo. In a sliding failure, the
foundation of a structure can slip and slide along the stratum. The stratum is
a series of layers of rock in the ground. (Rocks and Layers) A structure can
fail along the discontinuities when the joint sets are parallel to the
structure. This could cause the structure to dip making it eventually fail. A
structure can also fail at the interface. This type of failure involves a wide
spacing of discontinuities which would result in a terrible orientation of the
building and the strata. A rock mass failure occurs directly within the rock
itself. This type of failure can be prevented or is less likely if the rock
foundation is analyzed correctly. To prevent these failures, rock and property
testing as discussed earlier must be performed to give the design engineer all
the necessary information he or she needs. If a rock foundation is deemed
stable enough to support a structure than this type of failure is less likely
to occur, excluding failure due to natural disasters. Buckling failure has not
been seen as the cause of a structures demise yet but it has occurred without
the structure failing. In this form of a failure, the rock foundation has a
thin bedding on its upper layers. The rock appears strong but the upper layer
is brittle and can crack causing a failure. This also relies on the person
analyzing the rock foundation to do their job correctly. (Hamilton)

Foundation
engineering is a branch of geotechnical engineering that studies the forces a
foundation can withstand. The foundation must be able to support both man made
and natural forces. This also includes loads that are not just in the direction
of gravity. Earthquakes and wind forces must also be taken into account when
designing a foundation. The most important part of each project is the site
investigation. It is highly unlikely that two sites have exactly the same soil
composition which makes it necessary to take a close look at the soil. This
branch of geotechnical engineering can be one of the most important if not the
most important due to how early it is in the construction process. If a geotechnical
engineer makes a major error this early, the entire project is set up for
failure. A lot of the work done by these engineers cannot be seen because it is
done primarily underground. For example, when constructing a high-rise
building, there must be a foundation embedded deep into the earth’s surface in
order to support such loading. (PBS)

Seismic sliding is
a very important case to take into consideration when designing a structure. To
calculate the factor of an earthquake loading to a structure, it takes a bit of
chance. There is no way to tell how large of an earthquake a structure will
undergo but there are equations to help show the largest earthquake a structure
can withstand. To do this engineers must think of the worst possible direction
for an earthquake force to be applied to the structure and calculate. The
formulas are where is the calculated
horizontal earthquake force, M is the mass of the structure, m is the mass of
the adjacent rock or soil and x is the horizontal earthquake coefficient.  For the vertical forces the formula is  where  is the calculated vertical forces of the
earthquake, M is the mass of the structure, m is the mass of the adjacent rock
or soil, g is the acceleration of gravity and y is the vertical earthquake coefficient.
(US Army Corps of Engineers)

            Earthquakes
can greatly effect a structures stability in just seconds as they can deform
the rock foundation. Dams have collapsed due to earthquakes causing the ground
to shake/shift. An important case to touch upon is the San Fernando earthquake
in California. The earthquake happened in 1971 and caused a liquefaction
failure at the Lower San Fernando Dam and Reservoir. A Liquefaction failure is
when soils and/or rocks form a viscous fluid beginning to flow. The damn had not
been designed to withstand an earthquake of that magnitude and as a result it
failed. Prior to the catastrophe, scientists did not realize that the intensity
of ground shaking near the epicenter of an earthquake is much higher than
expected. An earth core rock fill dam is a good solution to a dam that can
withstand large earthquakes. In this type of dam there are three main parts.
The damn includes a main rockfill, the impervious zone, and auxiliary members.
The rockfill is where most of the support comes from. The weight of the
rockfill can provide the dam with internal stability. The impervious zone is
where all the water is held back from escaping. The auxiliary members are the
support in the dam. They are used to either support the membrane where the
water is held or to support the rockfill itself. 

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