Side Hill Retaining Walls – Part 1


Side-Hill Retaining Walls: An Overview and Alternatives (Part 1 of 2)

By Randy Post, PE, GIT
Owner and Editor, GeoPrac.net
Project Engineer, NCS Consultants, LLC
Originally Published on March 4, 2009

Download the PDF version of Side-Hill Retaining Walls: An Overview and Alternatives by Randy Post (PDF document).

Part 2 of Side-Hill Retaining Walls: An Overview and Alternatives is now available.

Definition and Significance

Definition and Significance

Completed side-hill retaining wall for the SR 264 Second Mesa project in Arizona. Photo by Bharat Khandel, ADOT by way of NCS Consultants, LLCRetaining walls are by definition a grade separation structure. The ground surface behind the wall is higher than the ground surface in front of the wall. If the surrounding terrain is flat, you don’t need a retaining wall! (Figure 1:  Completed side-hill retaining wall for the SR 264 Second Mesa project in Arizona. Photo by Bharat Khandel, ADOT by way of NCS Consultants, LLC)

When a retaining wall is located on the edge of, or partway down a slope or embankment, these walls are frequently referred to as “side-hill” retaining walls. In most cases, we are referring to fill type retaining walls as opposed to walls in cut sections. In general, these particular walls are encountered in areas of steep topography, for projects involving site development or the widening of existing roadways or railways, and are often driven by constraints on property or right-of-way limits.

Soldier Pile Lagging Wall on US 70 near Ruidoso, NM. Photo courtesy of Mike Pegnam, Golder Associates, Inc.In the realm of roadway widening, you might have a project where the alignment of the roadway is already set and constraints of topography and right-of-way preclude you from considering an embankment slope, so you must use some kind of retaining wall to permit the widening. (Figure 2:  Soldier Pile Lagging Wall on US 70 near Ruidoso, NM. Photo courtesy of Mike Pegnam, Golder Associates, Inc.)

For a site development project for a subdivision, a developer with little knowledge of geology or engineering may have picked a parcel with steep topography that requires significant cut and fill (he probably got a great deal too!). But in the interest of maximizing the available land for house plots and not encroaching onto neighboring properties with the toe of a fill slope, retaining walls may be needed.

I worked on a design-build project that involved widening a US highway in a scenic river valley that was also part of an Indian Reservation. The toe of the existing roadway prism was right next to a network of historic drainage canals and the opposite side of the highway was in cuts at the base of the mountains. Those canals could not be touched and there was no way to construct the widening entirely with cutting into the other side of the highway, so some form of side-hill retaining wall system was needed.