So, on
Thursday I’m off to Torino, Italy for the IAEG 2014 Congress (IAEG is the
International Association for Engineering Geology and the Environment). This
conference only happens every 4 years, and it’s a big one with lots of really
good applied geology presentations. The theme of the conference is ‘Engineering
Geology for Society and Territory’.
I have a
poster presentation on Thursday (September 18) in the morning poster session.
My presentation is titled “Investigative Procedures for Assessing Subsidence
and Earth Fissure Risk for Dams and Levees”. The full abstract is posted below
for those that are interested. There is a paper included in the conference proceedings,
but it’s really just an extended abstract. My colleagues and I have a series of
papers that we authored for the USSD (United States Society on Dams) Conference
that was in Phoenix in 2013 that cover the topic in much greater detail.
One exciting
part of this meeting (other than being in Italy!) is that AEG is proposing to
host the next IAEG Congress in 2018 in San Francisco. I really hope that it’s
selected as it will be a great meeting to bring to the US for the first time
and a spectacular setting. I’ll post the results of the selection here and on
Twitter when it comes through.
Speaking of
Twitter, I plan to ‘tweet’ regularly while in Italy (probably at odd hours for
US folks), so tune in at @AEGFergason for updates.
Abstract for
the meeting below:
investigative
procedures for assessing Subsidence and earth fissure risk for dams and levees
Kenneth
C. Fergason, PG
AMEC Environment
and Infrastructure, Inc.
Michael L.
Rucker, PE
AMEC Environment
and Infrastructure, Inc.
Bibhuti
B. Panda
AMEC Environment
and Infrastructure, Inc.
Michael
D. Greenslade, PE
Flood Control
District of Maricopa
County
The
depletion of groundwater resources in many deep alluvial basin aquifers in the Western
U.S.A. is causing ground subsidence, as it does in many regions worldwide.
Ground subsidence can severely and adversely impact infrastructure by changing
the ground elevation, ground slope (grade) and through the development of
ground cracks known as earth fissures that can erode into large gullies. Earth
fissures have the potential to undermine the foundations of dams, levees, and
other pertinent structures and cause system failure.
Earth
fissures that have been exposed to flowing water will most likely have
observable surficial expressions such as ground cracking, piping holes,
vegetative and tonal lineaments, and similar features, however uneroded earth
fissures often do not have surficial expression.
Subsequent
to the performance of an evaluation of the overall subsidence experienced in
the vicinity of a subsidence-impacted structure, a detailed investigation to
search for earth fissures must be performed. Such an investigation must include
investigative techniques capable of detecting earth fissures that do not have
significant surficial expression. Utilizing the findings of subsidence
investigation, additional investigative methods for earth fissure search
include photogeologic (lineament) analysis, assessment of the capability of
near-surface soils to develop an earth fissure, assessment of the degree of
ground disturbance, detailed site inspection, seismic refraction profiling for
concealed earth fissures, and excavation of trenches.
Satellite-based
interferometry by repeat pass synthetic aperture radar (InSAR) provides unique information
about active land subsidence over large areas based on multiple radar images (commonly
about 100x100 km scenes) obtained from different time periods. The subsidence
or deformation image known as an interferogram can also reveal with proper
interpretation some preliminary subsurface information about alluvial basin
geometry, lithology and hydrology where active land subsidence is interpreted. However,
utilizing interferometry is very a complex task that requires synthesis of
available information to properly or best constrain an interpretation.
Effective
subsidence risk assessment and mitigation requires understanding and
quantification of historic subsidence, and estimation of potential future
subsidence that could impact the dams and levee infrastructure. A primary subsidence mechanism is increasing
effective stress due to groundwater level decline within saturated compressible
basin alluvium. Ultimate subsidence
magnitude at a given location is a function of change in effective stress,
compressible alluvium thickness and material modulus. Modulus is typically a function of depth and effective
stress. Subsidence rates are assumed to
largely be a function of rate of groundwater level decline, alluvium
permeability or hydraulic conductivity and distance from groundwater level
stress points (such as pumping wells).
Basin alluvium and bedrock interface geometry, and changes and
interfaces in basin alluvium lithology, profoundly influence patterns and the
degree of subsidence. Characterization
includes collection and synthesis of historic survey and well data, surface
geophysical methods for basin and bedrock characterization, and when available,
InSAR to document recent or current subsidence patterns. Utilizing a synthesis of this information,
subsidence modeling matching documented historic subsidence and estimating
potential future subsidence can be developed to assess potential impacts on dam
and levee infrastructure.
Utilizing
the results of the site characterization and subsidence modeling, a
finite-element stress-strain model can be developed to estimate past and future
ground strain. Estimated tensional strain values can be utilized to predict
where earth fissures are likely to initiate with future subsidence and reduce
the risk of failure.
Multiple
case studies from Central Arizona will be utilized as examples, including
McMicken Dam and Powerline Flood Retarding Structure which are operated and
maintained by the Flood Control District of Maricopa County, Arizona.
