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24.70.085 Retaining Walls.

(Added by Ordinance No. 188884, effective April 4, 2018.)

A.  Retaining walls not regulated by the Oregon Residential Specialty Code or the Oregon Structural Specialty Code shall be designed in accordance with ASCE 7-16 and this section.

B.  Soil loads shall be determined in accordance with ASCE 7-16.  Retaining walls in which horizontal movement is restricted at the top shall be designed for at-rest pressure.  Retaining walls free to move and rotate at the top shall be permitted to be designed for active pressure.  Lateral pressure from surcharge loads shall be added to the lateral earth pressure load.  Lateral pressure shall be increased if soils at the site are expansive or the retaining wall will support an ascending slope.  Retaining walls shall be designed to support the weight of the full hydrostatic pressure of undrained backfill unless a drainage system is installed. 

C.  Retaining walls supporting more than 6 feet of backfill height, measured from the base of the footing to the top of the wall, shall incorporate an additional dynamic seismic lateral earth pressure.  When the Monobe-okabe method is used to calculate the active dynamic seismic lateral earth pressure, a horizontal acceleration coefficient equal to or greater than one-half (0.5) the design peak horizontal ground acceleration shall be used.

D.  Retaining walls shall be designed to ensure stability against overturning, sliding, excessive foundation pressure and water uplift.  Retaining walls shall be designed to resist the lateral action of soil to produce sliding and overturning with a minimum safety factor of 1.5 in each case.  The load combinations of ASCE 7-16 shall not apply to this requirement.  Instead, the design shall be based on 0.7 times nominal earth-quake loads, 1.0 times other nominal loads, and investigation with one or more of the variable loads set to zero.  The safety factor against lateral sliding shall be taken as the available soil resistance at the base of the retaining wall foundation divided by the net lateral force applied to the retaining wall.

Exception: Where earthquake loads are included, the minimum safety factor for retaining wall sliding and overturning shall be 1.1.