March 16, 2012
by Paul Bodin
Seattle is now home of a new whiz-bang scientific monitoring gizmo! And PNSN operates it! And like all our monitoring gizmos--you can even watch it work.
This one is an array of accelerometers and piezometers (a fancy name for a pressure sensor) buried beneath a parking lot in SODO. We call it a "liquefaction array". It has been designed to study the process of soil liquefaction in the field.
Why should you care? Well, this liquefaction array is going to tell us what happens when strong shaking causes the soil at the site to lose its strength and capacity hold up structures--one of the most pernicious impacts of earthquakes in saturated soils. It will provide geotechnical engineers and scientists with the basic data needed to understand the physics of a difficult-to-observe process and to build safer structures. The particular issues that this array will address include two real puzzlers for the geotechnical community: 1) How deep does liquefaction occur? and 2) How is it that a site can liquefy repeatedly? I'm sure future blogs will discuss these and other import questions we can now begin to address.
The earthquake in 2011 in New Zealand provides vivid examples of liquefaction.
The array consists of accelerometers to measure the 3D shaking at the surface and at 5, 45, and 56 meters deep, and piezometers at 7, 23, 29, 42, 47 and 52 meters deep, as well as a surface barometer needed to "correct" the buried piezometer data. We can observe the shaking levels at these depths, and calculate the strains in the ground between them, as well as monitoring the ground-water pressure changes that cause the liquefaction.
The Seattle liquefaction array is in a site that has liquefied (wet sand squirting out over the surface!) during moderate earthquake shaking in 1965 and 2001, and probably in 1949. So it's a good bet that the site will liquefy again.
Also, thanks to a lot of geotechnical studies performed for the failed Monorail extension project, the site has been extensively characterized--we know a lot about what kind of material is where beneath the surface. This is extremely important to understand exactly what changes as the site liquefies.
Moreover, the sensors at the site are sensitive enough that any shaking--even from small earthquakes, and the trains that frequently run right next to the array, make useful signals that will be recorded. From now on, everything that happens at that site will contribute to a history of the changes that shaking causes in the site that lead up to a liquefaction episode.
This was a huge effort involving a lot of work from many individuals and organizations. PNSN's committee of expert advisors set us to the task of building a state-of-the-art geotechnical array several years ago. The US Geological Survey office in Seattle procured funding, bought the hardware, and coordinated the complex project. The Seattle school district provided access to the land and put up with a disruptive schedule of drilling and installation work. Colleagues at UC Santa Barbara and Fulcrum Consulting, who work with the Network for Earthquake Engineering Simulation (NEES) to operate several other geotechnical arrays (there are 3 in California and 1 in Anchorage) provided expertise in instrumentation and design. Geotechnical engineering firm Shannon and Wilson provided geotechnical data about the site and advice on sensor placement, and Gregory Drilling provided drilling services.
On a personal note. I've been interested in liquefaction since spending many years studying the New Madrid Seismic Zone in the central US, where large earthquakes 200 years ago covered huge parts (> 25% in many places) of the landscape with wet sand vented during strong shaking in the Mississippi River flood plain. In India after the 2001 Bhuj earthquake, I saw similar extensive liquefaction, including what started out as a sand geyser blowing sand 50 m away, and still flowed many liters per minute of silty water 2 weeks after the mainshock. To have a tool locally that will provide insight into the still mysterious processes that underly these phenomenon is a remarkable acheivement, and I can't express my gratitude sufficiently strongly enough to the folks that made it possible.
My e-mail trail about this specific project goes back to at least 2008, and includes a labyrinth of issues that all of the organizations noted above, with clear steadfast motivation and good will, worked together to overcome. We all owe the folks who persisted through the challenges an enormous debt of gratitude!
The two images below show data from ~55 m deep (~170 ft). Each is a "webicorder" style plot showing about 20 hours of data, 10 minutes per trace (line); traces chronological top to bottom. The acceleration record on the first figure is full of train signals (and a few trucks). The other figure shows piezometer data revealing how the trains cause the pore pressure to rise as the train passes over the array, and then fall back down. The "response time" already reveals the time constant to move water around at this depth.
Piezometer (water pressure)
An overenthusiastic spectator might interpret the lengthy signals to be long trains, the large bumps on the front (and sometimes the back) of the long vibrations to be the heavy engines, and the numerous short bumps to be single engines on the move.
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