Creators of several nice modular block retaining wall systems. Their website has free design software for their products.
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Arizona Wells
A truly user-friendly website where volumes of groundwater and other well data are available for free. The interface makes use of the very popular Google Maps/Google Earth interface. Visit Arizona Wells
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Jacobs Associates SF
Jacobs Associates provides civil engineering consulting services with specialties in design and construction management for underground projects, as well as construction analysis and claims/disputes resolution. Visit Jacobs Associates SF
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Salt Cavern Information Center
Information about storage of non-hazardous oil field wastes in salt caverns. Visit Salt Cavern Information Center