Six Flags Marine World (Vallejo, CA) - Hultgren – Tillis Engineers has undertaken projects at Six Flags Discovery Kingdom for over twenty years, including the major renovation in 1998 that added roller coasters and other rides to the park. They have investigated site conditions and provided design criteria for recessed and above-ground aquatic tanks for dolphins and other marine mammals. Construction for the tanks often included temporary shoring and permanent retaining walls. Many of the roller coasters are supported on drilled piers that resist large lateral forces. Due the nature of this industry, new projects are generally not authorized until fall or early winter and must be completed by the following spring reopening. Hultgren – Tillis Engineers has helped plan and implement plans for wet weather construction conditions during the winter months.