The
most common causes for building settlement are from underlying deposits of
compressible fill or native soils. Compressible soils which are under unchanged
building foundation loading cause settlement to start immediately and taper off
over time. Therefore, if the settlement is not noticed until much later in time,
the presence of compressible foundation soils is not likely the culprit. One
cause, which can result in building settlement at any time, would be the
shrinkage of plastic clay soils. These clay soils will shrink when they “dry
out” and are problematic where they are subjacent to the foundation and have
significant initial moisture. Shrinkage of foundation clay soils is typically
associated with added landscaping which causes water to be “sucked out” of the
soils.
Another
fairly common source of settlement are foundation soils that can collapse when
exposed to moisture. Therefore, settlement of the structure would be noticeable
after significant precipitation and is likely to occur early after and even
during construction. Soils which would exhibit this behavior are loose, drier
fine sands to silts. More common in colder climates, another typically early
post-construction source is thawing soil. More specifically, building
settlement results from thawing of frozen soils left below the foundation.
Two
other more typical causes are less time dependent but are location dependent.
These are building settlement from land subsidence in karst terrain and underground
mining. In other words, there are only certain regions where either karst
conditions and/or underground mines are present. These karst and mine
subsidence events may occur at any time. These land subsidence events are
discussed in blogs entitled “What is Karst Subsidence” and “What is
Mine Subsidence”.
There
are some causes of building settlement which are more directly identifiable.
These include from underground tunneling, structures next to temporary or
permanent yielding retaining walls, earthquake shaking of mainly loose fine
sands which can contain some silt, and high extraction underground mining which
causes immediate ground collapse.
Red
herrings of building settlement, even to the professionals, can be building
foundation heave, and from subtle landsliding. Landsliding is discussed in “Landsliding
What to Do” and building heave will be discussed in an upcoming blog. Where
the building damage is apparently from settlement but requires proper investigation
a qualified geotechnical engineer expert in forensic analysis is recommended.
If
MEA can assist you with your building settlement problems, please contact us at
314-833-3189.
Karst
subsidence is land subsidence that is caused by cavities or voids in the
underlying bedrock which collapse or from soil filling them in from above
resulting in surface subsidence. Under normal circumstances, the voids or
cavities were created by the flow of groundwater in fractures in soluble
bedrock over a great deal of time. The most significant land subsidence effects
occur over voids which have been solutioned in limestone bedrock but also
result in other soluble rocks such as dolomite, gypsum, and halite. The most
typical land subsidence results from groundwater draining downward into these
solution voids carrying soil particles with it. This results in the ground
settlement in the form of a sinkhole to a more gradual depression on the ground
surface. Therefore, when downward drainage of groundwater is caused into open
bedrock voids, the potential for subsidence results. Some more common triggers
are: unlined surfaced drainage trenches, pumping of water wells, quarry pit
dewatering and retention/detention ponds.
Figures 1 and 2 are examples of this.
FIGURE 1: SINKHOLE CAUSED BY DOWNWARD DRAINAGE FROM DEWATERING OF NEARBY QUARRY PIT
FIGURE 2: IRREGULAR DEPRESSION WHICH FORMED FROM DOWNWARD SEEPAGE OF WATER STORED IN A RETENTION POND
Mine
subsidence is the collapse or settlement of the ground surface from failure of
an underlying mine. The most common mine subsidence events are from the
extraction of coal. However, it also occurs from underground mining of other
ores or natural resources as well. This would include mines in gold, iron, zinc,
trona, salt, gypsum, limestone, etc. The nature of the mining and depth play a
significant role in how the subsidence expresses itself on the ground surface.
Based on essentially these two factors the mine subsidence can express itself on
the ground surface as pothole sized to large sinkholes and small to very large
trough to bowl-shaped depressions.
The
mine subsidence movements can be very gradual to rapid depending on the type of
mine failure. Example of larger and smaller sinkholes are shown in Figure 1 and
2. Examples of smaller to larger sag depressions of the ground surface are
depicted in Figures 3 and 4.
For
more information on mine subsidence see: Establishing Mine Subsidence Risk. In selecting a mine
subsidence expert see: What to look for in a
Geotechnical Engineering Expert.
FIGURE
1 SINKHOLE FROM MINE SUBSIDENCE
FIGURE 2 LARGE SINKHOLE
FIGURE
3 SMALLER SAG DEPRESSION FROM MINE SUBSIDENCE
FIGURE
4 LARGER SAG DEPRESSION FROM MINE SUBSIDENCE
If you have a project which requires geotechnical engineering how can you determine which company best suits your needs? The most common means used to find the most appropriate company for your work is to ask an associate, colleague, or even a friend who is some how connected to your area of inquiry. If a company is found by some other means, or even such a company(s) is suggested, that company should be further vetted. Some of the more relevant company information to obtain when determining the geotechnical engineering company to select are:
How long has the
geotechnical engineering company been in service?
Does the company
specifically do the geotechnical engineering work you need done?
Given their expertise,
are they reasonably local, especially if there is significant site work
involved?
Experience in serving
your industry
The
last point can be over looked. Companies that have experience servicing your
industry generally understand your needs and wants and therefore can more
intuitively anticipate critical site conditions which may otherwise not been
properly emphasis, or even been ignored.
Geotechnical
engineering companies service different entities. This includes contractors,
commercial property developers, government agencies, mining companies, homeowners,
petroleum related industries, power industry, and co-professional (architects,
civil engineers, structural engineers, and environmental engineers). These
different entities have different needs and wants.
In
addition to having an appreciation for the industry it is more important,
however, to have sufficient expertise in the specific areas of need and
circumvent industry service.
However, when problematic, unexpected geotechnical conditions has resulted at the site, one is looking to perform a Geoforensics investigation and remediate the unwanted conditions. Examples of this are unexpected earthwork difficulties, excessive structure settlement or heave, earth retaining structure excessive movement, dam failures, pavement damage, and land sliding. If there is potential, this situation may result in some form of dispute resolution. Then this work becomes more individual dependent having the unique abilities of an expert witness. See finding such an expert please refer to What to Look for in a Geotechnical Engineering Expert and Traits to Dig for In An Engineering Expert.
When
a site is experiencing landsliding, it is a good idea to have some basic understanding
of what might have caused this ground movement in the first place. Landsliding
in soil occurs when the slope is weakened or loaded. Weakening typically occurs
when the soils weaken over time (i.e. weathering), and the slope’s vegetative
root structure, which was anchoring the soil, is removed. Undercutting the
slope either naturally (e.g. stream erosion) or by man weakened or reduced the
slope’s resistance to sliding. Loading the slope can occur when temporary or
permanent loads are added to or placed on the slope (such as storage containers
or stock piles of materials at the top of the slope), or when the soil slope
gets soaked by excessive precipitation or when previously submerged slope is
now exposed. Based on the above, it stands to reason that stripping and
steepening the slope during land development causes the greatest damage to the
slope and should be carefully evaluated.
Given
the various phenomena which can exist as discussed above, the rate of sliding
can vary significantly from a slow creep to a rapid failure. When dealing with
an abrupt/quick sliding event some actions that can be taken are the following:
Block off area to
reduce hazard
Can easily progress
upward (rarely expands sideways without some causation component) – consider
this as part of the hazard area.
Cover/seal ground
cracks from precipitation runoff
With
slowly developing slope events, there will be signs of instability. These could
include:
Settlement at the top
of the slope resulting in downslope tilting and separations in adjacent
structures and flatwork.
Cracking in the slope
especially if roughly along the slope (e.g. not random network of cracking)
Fence posts, poles,
etc. titled downslope, trees leaning down slope, or if very slow trees curving
upwards to compensate for very slow slope movement.
The
above is illustrated in Figure 1.
It
is important to note that any point in time a slow-moving event can turn
abrupt.
Of course, there are other phenomena and technical issues involved when there is a sliding event than is given above. To properly assess and understand the sliding conditions and any hazards and to know how to properly remediate the event, a geotechnical investigation should be performed. For information on how to select the appropriate geotechnical engineering companies see: What to look for When Selecting a Geotechnical Engineering Company. It is important to note that is not advertised that contractor be hired to provide the fix without adequate engineering.
Cofferdams are temporary water barriers which are created to allow for dry construction to occur in a specified area. The water barrier most typically consists of sheet pile and internal bracing. Single wall cofferdams which have sheet pile sufficiently embedded into the soil to resist the outside water pressure are called a cantilever design.
FIGURE 1: Example of constructed cofferdam.
Where internal lateral support is provided to assist to resist the water load, the cofferdam is considered a braced cofferdam. Where the retained water gets quite high, cellular cofferdams are used. Cellular cofferdams are connected cells, or sheet pile bins, which are filled with soil to provide dead weight and resistance against the external water pressure.
Once the cofferdam enclosure is completed it is pumped out and the specified construction can begin. Figure 1 shows photographs of some examples of the constructed cofferdams. Figure 2 is a photograph of a cellular cofferdam.
FIGURE 2: Photo of Cellular Cofferdam. Photo from C.J. Mahan Construction Company.
The most appropriate mine subsidence expert for your case depends on the nature of the problem to be investigated. Mine subsidence investigations can require various expertise depending upon the focus of the problem. Some of the questions that may need answers are:
• Is the damage from mine subsidence?
• Will the underground mine result in subsidence in the future?
• If subsidence were to occur, what is the range of movement you would expect?
• If subsidence does occur, how severe could the resulting damage be?
• Was the mine designed properly so that mine subsidence would not result in the future?
• How can we mitigate the subsidence risk to a tolerable level?
• Can you stabilize the mine and how much would that cost?
As you can see from the above, the subject of mine subsidence can actually involve a number of expertise depending upon focus of the investigation. Also, there is the context of the expert mine subsidence investigation: Is it being done for existing or new construction, mine design, review of a mining permit application, or is it for tort litigation? Therefore, in addition to having the technical know-how, and expert with oratorical skills may also be necessary.
