March/April 2013: Ask Dr. HOG

Ask Dr. HOG About Water and Earthquakes: Water Tanks in Seismically Active Zones

Dr. HOG recently visited a school in Northern California that wants to install a large, round 1,500 gallon tank on a hill above the playground for decent gravity feed. They felt that steel was a sturdy, durable choice. What’s wrong with this scenario?

The Problem
There’s a reason many steel tank manufacturers do not sell in California – and its all about rock ‘n roll. Well, actually its slip ‘n slide as the Pacific Plate and the North American Plate grind together. When the ground moves abruptly under a large or medium body of water contained in a tank the top section of water sloshes, and the bottom section moves with the tank body, effectively creating some very powerful shear.  A riveted tank might just pop its rivets, pouring around 6 tons of water down onto the playground.  A welded tank attached to a stiff foundation could shear in the middle.

The Solution
The volume the school wants to store is not a viable project for Rainwater HOGs, so I recommended daisy chaining two 660-gallon round Bushman tanks with a secondary fastening system to the ground, and allowing room for an additional tank when funding became available.  Daisy-chaining smaller tanks has a number of advantages to the monolith proposed:

1. It acts as a baffle to prevent the water mass gaining too much momentum in a seismic event, giving the system more chance of surviving an earthquake without spilling.

2. It facilitates a progressive installation as funds become available.

3. It prevents large scale failure – in the event that a tank fails (this is known to happen with some thinner walled tanks, although a Bushman quality tank means that it probably won’t), there is still functional storage in place.

4. It is less expensive and a more residential-scale installation of compacted gravel vs. the large scale concrete or other substrate that a big tank requires.

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