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# Stability of Breakwater

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## Stability of Breakwater

(OP)
Hi all,

We are looking at a Breakwater, approximately 9m high with 4m above HAT plus climate change level. Hydraulic stability assessment requires rock sizes of up to 10-15 tonnes.

We have been asked to provide a preliminary assessment of seismic stability of the breakwater. To do this we are using limit equilibrium software SLIDE and the seismic force is inputted as a horizontal seismic acceleration.

Initially we were using the horizontal PGA of 0.23g. To achieve a factor of safety (FOS) of greater than 1 we need a breakwater side slope of 1 in 3. I have limited experience in breakwater assessments but I know they are typically set at 1 in 1.5 side slopes and there have been many breakwaters constructed in the region with this side slope.

As such this is leading me to think that our PGA is too high as I have found the following references:

1 - ASCE Standard 61-14: Seismic Design of Piers and Wharves
2 - PIANC Seismic Design Guidlines for Port Structures

which state that you cad adopted 50% of your PGA as your design seismic acceleration as these type of structures can absorb larger deformations.

This seems logical to me as a breakwater could probably deform up to 200mm and you would find it hard to notice! The question I am wondering is, is this approach commonly applied in the assessment of breakwaters?

### RE: Stability of Breakwater

So by doing a pseudo static analysis you have determined that the foundation materials will not be subject to liquefaction and reduce strength.

Pseudo static analysis is only a means to get a rough estimate of the horizontal seismic force required to get a fos of 1 which means 1 meter crest deformation.

You can consider using the methods by Dr. Bray for other estimates of deformations.

### RE: Stability of Breakwater

(OP)
Thanks GeoEnvGuy

Yes, the founding materials are very dense gravel / rock which will not liquefy.

How does a FoS of 1 mean 1 meter crest deformation?

I dont care about seismic deformations, not in the scope of our preliminary assessment and should be assessed by the final designer.

I need to get a reasonable slope that works under seismic conditions. A side slope of 1:1.5-2 is reasonable.

### RE: Stability of Breakwater

Last time I read into this was about 3 years ago. But how I understand it is that the seismic input values can range from less than 50 percent of the PGA up to 100 percent depending on the risk category of a structure, desired resiliency, and desired level of repairs after a seismic event. So conservatively you would use a factor of 1 if failure of the breakwater would cause some catastrophic results. If the breakwater can fail and be repaired without any issues, besides damaging the owner's wallet, then the PGA can be factored. I remember digging through USACE EM 1110-2-1902 and NCHRP Report 611 to find these recommendations. I also recall that it took me awhile to find any information regarding this subject. Not sure if anyone else has a different approach but this one seems to make sense to me.

### RE: Stability of Breakwater

To respond to how a pseudo static stability analysis is actually a deformation estimate.

Hynes Griffin 1984

Stability Analysis ...

... 5. For analysis of permanent displacements, the shearing resistance between the potential sliding mass and the underlying base is evaluated in terms of a critical acceleration N , defined as the acceleration (of the ground or embankment below the sliding surface) that will reduce the factor of safety against sliding to unity, i.e., that will make sliding imminent. The value of N , which is expressed as a fraction of gravity (g) , is obtained through a stability analysis similar to conventional pseudostatic stability analyses ...

... PART III: Conclusion
29. The results of analysis of earthquake strong-motion records using a sliding block model and a decoupled elastic response analysis show that permanent displacements for deep-seated sliding surfaces limited to less than 1 m can be assured if the ratio of critical acceleration to peak bedrock acceleration is at least 0.5. This value is considered to be very conservative and subject to downward revision as better understanding of elastic-plastic amplification response of embankments is developed.

### RE: Stability of Breakwater

(OP)
Thanks MTN.i found another reference to 50% of your PGA in the NCHRP report so I believe it’s a reasonable design assumption. I’ll know when I submit the report anyways!

Thanks GeoEnv, interesting

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