Hello this is Professor Kitch.
Welcome to this webcast on section 10.1 which introduces the topics of compressibility and
settlement.
This webcast is mostly a vocabulary lesson When you're done, you should be able to describe
the sources of the three settlement components: distortion, consolidation and secondary compression
settlement And describe how both the magnitude and rate
of these three components differ between cohesive and cohesionless soils
Before we discuss the details of consolidation, let's look at some examples of civil engineering
projects where consolidation has played a key role.
Our first stop is in Mexico City.
Much of Mexico City is founded on land reclaimed from Lake Texcoco and underlain by a thick
and highly compressible clay with average moisture content of 280% and void ratio of
a whopping 6.9!
The entire region settled 6 to 7 meters during the first half of the 20th century due to
groundwater pumping.
One of the most famous buildings in Mexico City is the Palacio de Bellas Artes.
The steel structure with a masonry facade was constructed over a three decade period
at the beginning of the 20th century By 1950 the entire structure had settlement
more than 3 meters This photo was taken during construction.
Notice the steps leading from the street level up to the main entrance
Here's a photo from 2012 taken from the plaza in front of the palacio.
There are still a few steps leading up to the entrance
But they're well below the original ground surface.
It gives new meaning to the term downstage.
Fortunately the settlement was relatively uniform across the entire area of this building
and it hasn't suffered significant structural distress
Perhaps the most famous case of structural settlement is the Leaning Tower of Pisa
Unlike the uniform settlement seen at the Palacio de Bellas Artes, the bell tower at
Pisa has suffered from differential settlement with the South side of the tower settling
significantly more than the North side.
The differential settlement started during the construction of the tower during the 12th
through the 14th centuries.
By 1990 the tilt had reached 5.5 degrees or about 17 feet out of plumb
The rate of tilt at that time was about 6 arc-seconds per year
The tower was close to collapse and closed to visitors in 1990
Many methods had be suggested to stabilize the tower and several had been tried unsuccessfully
After careful studies during the 1990s the method chosen was to carefully remove soil
from under the foundation on the North side of the tower using directional drilling.
The photo on the cover of your textbook shows the drill rig used to remove the soil and
the angled tubes used to guide the auger The figure on the right illustrates where
the soil was removed.
The remediation took place from 1999 through 2001
Approximately 50 cubic meters of soil was removed from under the foundation on the north
side This generated about 180 millimeters of settlement
and reduced the tilt of the tower by half a degree.
The tower owners did not want to straighten the tower because its tilt is part of its
charm and attraction.
The half a degree reduction is not noticeable by the general public but significantly reduces
the stresses on the south side of the structure Most importantly, the remediation reversed
the rate of tilt to less than two tenths of an arc second per year to the north.
This northern tilting is expected to decrease to zero over the next one to two decades and
the tower is expected to remain stable for at least 200 years.
If you're interested in learning more about the remediation of the tower there's an excellent
paper by Burland, Jamiolkowski and Viggiani listed at the end of this presentation.
It's available open source through the International Journal of Geoengineering Case Histories.
For our final case history we'll look at the Kansai airport island in Osaka, Japan.
This is an amazing engineering project and was named by ASCE as the Civil Engineering
Monument of the Millennium in 2001.
In the 1980s, the City of Osaka had outgrown its existing airport, but there was not land
available in this densely populated urban area for a new modern airport.
The solution was to put the airport 5 kilometers out into Osaka Bay.
This not only solve the city's space issue but eliminated concerns about aircraft noise
allowing 24 hour international air traffic The airport is built on two man-made islands
which constitute the fourth largest artificial island ever created at 8.1 square kilometers
or 3.1 square miles.
The ocean at this location is approximately 17 to 20 meters deep.
Construction on the first island started in 1987 and placement of the fill was completed
in Dec 1991.
Terminal was completed in June 1994 and the flight operations started in Sept 1994
The total height of the fill material from the ocean floor was 36.7 m
The second island was constructed to add an additional runway to the airport to increase
capacity.
Construction for this island started July 1999.
On Oct 2005 fill complete and runway construction started.
On Aug 2007 the second runway opened.
The total fill height on the second island is 40-43 m.
Osaka Bay is underlain by very deep deposits of marine clays and sands.
The depth to bedrock at the location of the islands is over 1000 meters.
This figure shows the cumulative settlement of island 1 since construction started in
1987.
The data in marked with diamonds shows the total settlement since start of construction
and the lower line marked with squares shows the settlement since opening of the airport.
The island has experience over 3 meters of settlement since September 1994, when the
airport opened.
Significant consolidation of the clay layers beneath the islands was expected and the height
of the fill was designed to accommodate the expected settlement.
However the settlements were under predicted and, more disturbingly, the settlements continue
today at rates higher than expected.
As seen in this figure, the settlement rate has been steady at 7 centimeters per years
since 2007.
The design called for the airport to be 4 meters above sea level at the end of consolidation.
Island 1 is now below that level and continues to settle.
There is no consensus among engineers about why settlements were under-predicted and why
they continue past the time predicted.
Some evidence points to a failure to properly consider secondary compression settlement,
a phenomenon we'll discuss in just a few minutes.
Additionally, the deeper sediments are compressing more than was expected.
In addition to the overall magnitude of settlement, the island has experienced differential settlement
This figure shows the elevation of 12 monuments located on island 1 with elevations ranging
from 0.27 to 3.59 m.
Differential settlements were anticipated and the designers took several measures to
reduce their impact.
For example the runways were constructed with asphalt pavement rather than concrete pavement,
because the asphalt is much less susceptible to cracking under differential settlement.
One of the major concerns related to differential settlement was with the large terminal building.
Differential settlements could prove very damaging to this structures.
Two measures were taken in response to expected differential settlements.
First the structure was founded on a single large mat foundation which tends to reduce
differential settlement.
A second, and more creative solution, involves the ability to jack up individual columns
in the building to reduce differential settlement as it happens.
The columns for the terminal building were designed with extra-long anchor bolts attaching
them to the mat foundation and with jacking points that allow raising the columns after
construction.
The procedure is to monitor the differential settlements and, when needed, the anchor bolts
are loosened, the column jacked up.
Steel plates are then placed under the column and the anchor bolts refastened.
This picture shows a jacking operation in progress.
The anchor bolts have been loosened and column jacked up.
The workers are placing the steel plates under the column.
When they're done, the jacks will be released and the anchor bolts retightened.
Here's the rare case where getting jacked up is a good thing.
The Kansai airport has a very informative webpage with more information about this project.
It's listed in the reference section of this presentation.
I hope these projects have given you some insight into how settlement can affect civil
engineering projects.
While not typical these projects illustrate why we need to be able to calculate settlement.
So let's start by investigating the physical processes causing settlement
When we apply a load to a footing, there will be some settlement, delta.
And if we examine an element of soil under the footing.
We'll find that it deforms and bulges laterally, but without any volume change.
It simply deforms, as illustrated by this phase diagram.
Neither the volume of the solids or the voids changes.
The element just bulges.
We call the settlement associate with this behavior deformation settlement.
It occurs because the soil deforms under load, but there's no volume change in the soil.
I like to think of this as Jell-O type settlement.
It's like putting a can of soda on top of a bowl of Jell-O.
The Jell-O won't compress, but it will squeeze out to the sides and up, allowing the can
to settle a little below the original surface of the Jell-O.
If we zoom in on the soil element, we'll see that it's composed of soil particles.
And, as we discussed in an earlier module, the soil particles form the skeleton of the
soil, and this skeleton carries the effective stress.
Initially, before the footing is loaded this soil element is carrying and effective stress
of sigma-prime-z-naught, as illustrated here.
When the footing is loaded, the soil feels an additional effective stress, delta-sigma-prime-z,
due to the induced stress from the footing, as illustrated here.
This additional stress on the soil skeleton will cause the particles to roll around and
rearrange themselves in order to carry the increased stress.
This process of particle rearrangement will generate a change in the void volume of the
soil as illustrated by the phase diagram shown here.
Notice that all the volume change occurs in the voids.
The solid particles are essentially incompressible.
We call this process of particle rearrangement and decrease in void volume, consolidation.
And the settlement associated with it consolidation settlement.
Or sometimes primary consolidation settlement.
Primary consolidation settlement occurs due to a change in effective stress.
The soil skeleton reacts to this change in effective stress by rearranging itself and
the void volume decreases.
There is a second settlement associated with volume change.
But this one is not caused by a change in effective stress
Here we see the condition after the soil skeleton has rearranged itself to carry the addition
effective stress induced by the footing load.
Even after rearrangement of the soil particles, there will be some additional volume change
without any change in effective stress.
This is a creep strain, that is, a deformation without additional loading.
In soils, we call this deformation without addition loading, secondary compression.
It's called secondary, because it generally occurs after primary consolidation, and because
it's nearly always less than primary consolidation settlement.
Secondary compression does include and change in the void volume, as shown here.
But the volume change due to secondary compression, delta-V-sub-v-s, is less than that due to
primary consolidation, delta-V-sub-v-p.
So the total settlement, delta, is due to all three sources
Deformation settlement.
Primary consolidation settlement.
And secondary compression settlement.
And we write that the total settlement, delta, is equal to delta-d, plus delat-c, plus delta-s
The three forms of settlement do not contribute equally to the magnitude of settlement, nor
do they occur at the same time.
First let's consider the timing of each form.
Since distortion settlement occurs without volume change, it occurs very quickly
Consolidation settlement, in contrast, requires a reduction in void volume.
This will take time, particularly if a soil is saturated and the voids are filled with
water.
The time required will be controlled by the how saturated a soil is and what it's hydraulic
conductivity is.
Sands and gravels have very high hydraulic conductivity, and even when saturated, will
undergo consolidation quickly.
In contrast clay and silt soils can have hydraulic conductivities many orders of magnitude less
than sands.
In these soils, consolidation settlement can be very slow at sometimes takes years or decades
as we saw with the Kansai airport settlement.
Since secondary compression is a creep phenomenon, it is always slow regardless of the type of
soil The magnitude of settlement will differ by
soil type.
In general we will get more settlement in clays and silts (particularly soft clays)
than in sands or gravels, for the same loading level.
This table from your text characterizes the timeframe and magnitude of each type of settlement
by soil type.
For clays and silts distortion settlement occurs in the short-term and generally has
a negligible to small magnitude.
Consolidation settlement is a long-term phenomenon and can be moderate to quite large.
Secondary compression is also a long-term phenomenon and is generally small, but can
be large in some cases For sands and gravels, both distortion and
consolidation settlements are short-term phenomena.
The magnitude of distortion settlement is generally negligible, but can be small at
times.
The magnitude of consolidation settlement is generally small but can be moderately large.
Secondary compression is the only long-term settlement in sands and gravels, and is generally
negligible to small.
So let's summarize Settlement is important to us as civil engineers.
It can be large in some project and a major design consideration
There are three sources of settlement Distortion settlement which occurs without
volume change, is always, fast and generally small
Consolidation settlement which is due to changes in effective stress.
It causes a rearrangement of particles in the soil skeleton and a change in the volume
of the voids.
It's the largest contributor to the magnitude of settlement.
Can be fast or slow depending on the soil saturation and hydraulic conductivity.
It's always fast in sands and gravels due to their high hydraulic conductivity, and
nearly always slow in saturated clays.
Secondary compressions settlement is a creep phenomenon and occurs without a change in
effective stress.
It's always slow and generally small, but can be significant in some cases.
We compute the total settlement as the sum of distortion settlement, consolidation settlement,
and secondary settlement Here are references where you can find more
information about the Leaning Tower of Pisa and the Kansai airport settlement, for those
who are interested.