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Lum Site to Benefit from Innovative ‘Geologic Raft’

Drive by the site of the former Lum Elementary School, and you’ll see a site buzzing with activity, as backhoes, bulldozers, and rollers prepare the ground for what will soon be a temporary campus for Wood Middle School students.

Alameda Post - A yellow vibra roller on the dirt at Lum School.
A vibra roller at the former Lum Elementary School. Photo courtesy AUSD

Construction vehicles are fascinating for small children, we know, but we think what’s happening beneath the ground will be of equal interest to adults, as it involves an innovative system to fortify the notoriously sandy soils in that area of the island.

As a reminder, Alameda Unified School District (AUSD) closed Lum Elementary School in 2017 due to the risk of a building collapse in an earthquake. That risk was a result of two factors: the highly liquefiable soil and the thin slab foundations on which the Lum buildings had been built. Soil and seismic engineers, as well as the Division of the State Architect (DSA), all deemed that combination of liquefiable soil and weak foundations to be unsafe. By contrast, the foundation for Wood Middle School, which is just across Rittler Field from Lum, has long piers that stretch down into more stable soil.

The site is now being prepared for a campus composed of temporary classrooms to first house Wood students and staff while the campus is rebuilt and then, two years later, Otis students and staff when their campus is rebuilt. Once the new Otis campus is built, AUSD plans to build a multi-sport athletic facility on the Lum site.

It’s important to note that the foundations and structures of modern temporary classrooms—also called “portables”—are completely different from those of the large classroom buildings constructed at Lum in 1959. The safety regulations that govern school buildings are also completely different now. In this case, the California Geologic Survey (CGS) has mandated that the sandy soils on the Lum site be remediated with a “geologic raft” in order to negate the risks attendant with earthquakes.

Alameda Post - A diagram of a geological raft.
Geological raft diagram. Image courtesy of AUSD

This raft consists of layers of “geo grid,” a very strong netting material, along with crushed rock and soil that are repeatedly compacted so as to create a stable foundation that resists liquefaction. (Readers familiar with sheet mulching may be familiar with this “lasagna” layering strategy.) The compaction is done with a “vibratory roller,” which looks much like a steam roller, but adds the element of vibration to the roller weight to push out the air between the rocks and make them fit closely together into very strong, stable layers.

The vibration is strong enough to be felt even on the sidewalks around the site, and several neighbors near Lum have reported feeling it at their homes. The district is aware that this can be unsettling—ironically, it feels somewhat like an earthquake. But we’d like to assure all community members that this vibration is normal, and this phase of the work will be done in several weeks. In the meantime, the crew will be using a lighter vibration setting on the rollers when they are working on the east side of the construction site, which is adjacent to houses.

The temporary campus will be completed by the start of the new school year. Over the next several months, we’ll provide periodic updates so the community can get a behind-the-scenes view of the facility being built from the ground up.

Susan Davis is the Senior Manager of Community Affairs for the Alameda Unified School District. Reach her at [email protected].

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