Tuesday, September 9, 2014

IAEG in Torino, Italy

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.

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