seismic activity and undersea slope stability 1

[rennkafer]

I'm conducting research for a project which requires me to make some predictions on undersea slope stability in the event of an occurence of seismic activity. Apparently this will be an exercise in comparing and combining historical data with quantitative models, and an inexact science at best. As a mechanical engineer by training, and not a geologist, I feel as though I'm just scratching the surface of this subject, but I have discovered some useful information, mainly in the area of seismic charts and advanced seafloor mapping technology. I'd appreciate any suggestions (bibliographical references, contacts, etc) on how to quantify the information I collect in order to produce a numerical model of high risk areas on the seafloor. Thanks! Steven K. Madoski
Mechanical Engineer
Ocean Systems Division
NFESC, Code ESC 52
Port Hueneme, CA

 

[Luca]

Hi rennkafer,

I remember I read some paper about undersea slope stability
in the "FLAC and Numerical Modeling in Geomechanics (Proceedings of the International FLAC Symposium on Numerical Modeling in Geomechanics, Minneapolis, 1999),
C. Detournay and R. Hart, Eds. Rotterdam: A. A. Balkema, 1999".

If you have the opportunity to look this book, I belive you will find interessthing things.

See as well the following url:

http://www.itascacg.com

Hope this help,
regards,
Luca

 

[tomsklucki]

Look at British Geological Survey sites (BGS have a global seismology and geomagnetism group based in Edinburgh)
e.g. for a study of seismicity and earthquake hazard in the UK including North Sea

http://www.gsrg.nmh.ac.uk/hazard/hazuk.htm

also

http://www.gsrg.nmh.ac.uk/hazard/world_seismicity_database.htm

Try book by Harry Poulos (Univ. of Sydney) for references on the the geotechnical aspects

H.G. Poulos (1988)
Marine Geotechnics
ISBN 0-04-620024-X
published by
Unwin Hyman Ltd., London UK
Allen & Unwin Inc., Winchester, Mass., USA

 

[JacktheGeo]

I'm replying at this late stage in case someone else is browsing.
My MSc thesis was a quantitative analysis of submarine slope stability:

http://www.geocities.com/jafitzsimons/homepage.html

 

[toolpusher]

rennkafer,

The evaluation of slope stability and potential lateral displacements under seismic loading is a challenging task. In cases where there is little or no geotechnical information available for the seafloor sediments, then you are going to be incapable of arriving at any reliable conclusions regarding the potential for seismic liquefaction, or the magnitude and type of deformations that may develop.

Still, there are some things that can be done. Highly detailed bathymetric maps (multibeam sonar) are extremely helpful in identifying past mass movements and their likely points of origin. If you are near a major river mouth that is depositing large quantities of sediment, you may find the local area to be chroniclly unstable, even under aseismic loading conditions. Tidal drawdown is potentially capable of triggering liquefaction in some types of sediments, especially if they are gas-charged.

Mapping slope angles from whatever bathymetry data you have will help identify areas of greatest concern; you need to decide what the critical angle for static and dynamic stability is. One of the problems with marine sediments is that they tend to come to rest at a marginal level of static stability, if they are deposited on a slope, unless it is very flat (static shear stresses are balanced by the available shearing resistance of the sediment). For example, debris flowslides in sands can develop even under static conditions and travel considerable distances, since the remoulded shear strength is typically much less than the prefailure strength (unless they have already been remoulded!).

The addition of seismic accelerations can trigger liquefaction in some sediments, giving rise to similar types of mass movements. Clays tend to perform a bit better during seismic loading as they derive their shear strength from cohesive as well as frictional intergranular forces, although there are exceptions (you need to measure the peak and remoulded shear strengths to be sure using appropriate test methods).

At first you should try a simplistic approach, wherein you separately evaluate whether your sediments are capable of being triggered to liquefaction failure and under what loading conditions; then you should perform pseudostatic limit equilibrium slope stability analyses on the existing slope profiles and assign approprietly reduced shear strengths to the failure-susceptible zones you have identified. Liquefied sediments may

a) suffer unlimited deformation and develop flowslides
b) develop limited deformations that end once shaking stops
c) be resistant enough to remain in-place

This lengthy commentary is just the beginning of getting at a reliable answer - the best thing would be to involve an experienced marine geotechnical engineer in your project. A bit of study of the bathymetry and seabed features by an expert would likely be much more practical for you initially than embarking on a quest for some general (and unreliable) factors of safety against seismic liquefaction. A geotechnical engineer would be able to say with confidence which areas are of highest risk and how to address your needs accordingly.

Hope that helps!

 

[Focht3]

toolpusher has given excellent advice. I'm afraid your educational background (ME) ill prepares you for the daunting task that has been assigned to you. (My college roommate was a mechanical engineering student.) If engaging a consultant is out of the question, then you need to contact WES for assistance and guidance.

Let us know what happens -