Seismic Surcharge Coordination

[Martino8]

In our region (Puget Sound WA), it seems like Geotechs keep increasing their seismic surcharge recommendations.
In the days prior to it being a code requirment, the few Geotechs ahead of the curve were commonly recommending about 8H (in psf)
Once it became codified, the design recommendations settled in around 8 to 12H for quite a while
In recent years, those values have crept up to as high as 15H, 18H, and even 25H!!

Whenever we try asking about it, we usually hear something like, ground motion maps have gone up. Yes but 2 to 3x???
Starting to wonder if what we're seeing is a wider and wider disconnect between Geotechnical and Structural considerations.
At those levels, the structural design of the retaining wall is severely affected so it's worth taking a closer look.

For example:
Building structures are not required to be designed for peak ground accelerations.
Instead we use a force reduction (or R value) for design and rely on good detailing to dissipate the excess energy.
Wouldn't it be consistent to apply that concept to reduce the seismic pressures resisted by the retaining structure?

 

[KootK]

Instead we use a force reduction (or R value) for design and rely on good detailing to dissipate the excess energy.
Wouldn't it be consistent to apply that concept to reduce the seismic pressures resisted by the retaining structure?

I would say no, it wouldn't be consistent for the most part. Consider:

1) In the case of a building basement retaining wall, the dissipation systems are above the basement and, as such, don't help for stuff below.

2) Soil isn't normally attached to the structures that retain it such that they are forced to move together under oscillation.

Consider the hypothetical shown below in which we would be able to use dissipation systems. And then imagine all of the ways in which an actual soil-structure system does not reflect such a model.

In my mind, seismic retaining loads kind of resemble throwing spaghetti at a wall. Just, you know, really heavy spaghetti.