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
Karst subsidence are typically in the form of sinkhole to bowl-shaped depressions. They can occur unexpectedly and fairly abruptly and can cause significant damage. Because of their erratic geologic nature, karst it is often difficult to quantify the subsidence risk and associated damage potential. Therefore, the expert which is hired should be well versed in all the subsidence engineering aspects of interest.
More common questions the karst subsidence expert will be asked to answer are:
• What is the chance that there will be subsidence in the future?
• If there is a subsidence(s), how severe will it be?
• If there is subsidence, how much damage can we expect?
• What are my options to reduce the risk of subsidence in the future?
• A sinkhole has appeared, what do I do?
• Is there any way to virtually eliminate the risk of subsidence in the future?
Subsidence investigations in karst terrain are most commonly related to new construction or encountering unanticipated subsidence or other karst features which disrupt construction progress, insurance claims, or subsidence damage. Given the context of the investigation and the amount of risk which may be involved should determine the level of expertise that the subsidence engineer should have. If tort litigation is involved the karst subsidence expert should also be competent in this area. Also, with greater knowledge and experience in karst subsidence problems, the more cost effective the solution.
