Retaining structures hold back or “retain” soil and rock in grade separations. They also hold back water in some instances such as cofferdams and bulkheads. Retaining structures vary in type and material and HGA has experience in many types of design including cantilever and gravity walls, soldier pile walls, sheet pile walls, tie back walls, and soil nailed walls.

Detention structures temporarily hold back water to be released at a specified rate. These structures are typically utilized to manage both quantity and quality of storm water run-off from developed sites. Types of detention structures commonly used are detention ponds (typically constructed with earthen dams, cast in place concrete walls, gabion mesh walls, and segmental block walls), underground detention vaults, and cisterns