A building permit and engineered design is required for a retaining wall when the wall is over 4 feet in height measured from the top of the wall to the bottom of the footing or if supporting a surcharge. (See illustrations below to determine if a surcharge is present) Terraced or tiered retaining walls require a building permit and engineered design if the walls are over 4 feet in height or spaced apart at a distance less than two times the height of the lower wall. The following is required for submitting a retaining wall permit:
During construction of the wall(s), it is the responsibility of the permit holder to have the engineer of record perform an inspection(s) to ensure the construction is in compliance with their design and the approved plans. Upon completion of the retaining wall, copies of the Engineer’s inspection report or observation letter shall be submitted to the Douglas County Building Department to finalize and close out the building permit.
A surcharge is anything that could impose a lateral force on the wall. Any surcharge on the wall may require evaluation by an engineer.
As a general rule, any surcharge object must be at least two (2) times the total wall height away from the back of the retaining wall. This means if you have a total wall height of 4-ft. (buried height plus exposed height), the edge of any surcharge object should be at least 8-ft. away from the back of the wall
Sloping fill behind the wall is another type of surcharge that will impart a lateral load on the wall. Slopes are usually labeled by the horizontal (H) length for every 1-ft. change in vertical (V) elevation. So, a 10H:1V slope would increase 1-ft. in height for every 10-ft. in horizontal distance.
Terraced/tiered retaining walls:
A terraced or tiered wall is when two or more shorter walls are used in lieu of a single taller wall. The upper tiered wall exerts a surcharge on the lower wall and therefore they are treated as one wall unless the upper wall is offset (D) a minimum of two (2) times the height of the lower wall (H1). The height of the upper wall (H2) must be less than the height of the lower wall (H1).
Terraced wall section view
As an example, if there are two 3-ft. tall, tiered walls that are spaced 4-ft. apart, these are treated as one 6-ft. tall wall and will require a permit and an engineered design. If the two 3-ft. tall, tiered walls are spaced 6-ft. or more apart, they can be treated as independent walls. You cannot have more than one terrace on a non-engineered wall.