Landscape Architect Articles

Fundamentals of Engineered Walls01-04-17 | Feature

Fundamentals of Engineered Walls
By Mike Dahl, LC/DBM

Depending on the locale in which they are built, retaining walls of a certain height require a building permit and structural wall design prepared by a qualified engineer according to Karen Nelson, engineering manager at VERSA-LOK???(R)???AE????, a manufacturer of segmental retaining wall solutions. Some site civil engineers choose to act as the engineer of record but other times the specifiers demand the plans be part of the construction package. In those cases, the contractor that is awarded the project hires a licensed engineer to generate the design. For large, complex or critical projects, the manufacturer suggests the engineer be retained directly by the owner or owner's representative to prepare the final wall design prior to construction bids. For this 25-30' wall at Weston Corners, a neighborhood mixed-use center in Cary, N.C., TerraTech Engineers prepared the plans for master wall builder Gib Lyttle, the owner of Delta Development.

Due to the rugged terrain, some of which was solid rock, at a new community development in Peoria, Ariz., called Northlands at Tierra Del Rio, the engineering team at Sargon Masonry Construction designed a segmental system that would not require cast in place footings. They chose to install the blocks with a near vertical stack, in a tiered system. In addition to traditional geogrid reinforcement, tiebacks attached to the granite with rock bolts further stabilize the walls.

In Merriam, Kan., BC Hardscapes and JE Dunn of Kansas City, Mo., installed 120,000 square feet of block to create this retaining wall with elements such as serpentine curves, terraced portions, pilaster bump-outs, columns spaced along the entire length of wall, and freestanding columns joined by aluminum fencing. Unfortunately for the developer, the mall that was supposed to be built on the site that the wall supported didn't develop as planned and the ghostly structure became known as the "Great Wall of Merriam" until a major retailer came in as an anchor store.

Engineered retaining walls are those that due to building code rules, usually based on the height of a wall, require a structural wall design prepared by a qualified engineer. These walls also typically necessitate a building permit.

Codes Are Key
The 2012 International Building Code mandates permits for retaining walls higher than four feet, measured from the bottom of the footing to the top of the wall – not measured by exposed wall. But more strict standards can be set out by individual states, counties and cities. The National Concrete Masonry Association reports that there are many locations without provisions for engineered design.

When different requirements apply, the more stringent of the two takes precedent. For example, in St. Louis County, Missouri, a permit is mandatory for any wall greater than three feet in height as measured from the top of the grade on the non-retained side of the wall to the top of the wall. However, in that county's town of Creve Coeur, permits are required for all retaining walls with a height of over two feet. So it is imperative to always verify building code standards with city and county officials.

When required, the final wall designs themselves must be prepared by independent civil engineers licensed in the state of the project according to Karen Nelson, engineering manager at VERSA-LOK???(R)???AE????, a manufacturer of segmental retaining wall (SRW) solutions.

The Man (or Woman) with the Plan
Nelson advises design professionals who aren't doing the structural engineering plan to retain an SRW engineer to provide it before bidding the project.

"Some site civil engineers choose to act as the engineer of record for retaining walls on their site and do the final wall plans in-house," she states. "Alternatively, specifiers often require the wall engineering plans to be provided as part of the retaining wall construction package. In such a case, the specifier provides the retaining wall product type, location, layout, grading and soils information. Bidding contractors (then) estimate the materials, labor and final engineering costs."

In this scenario, once a contractor is selected, they retain a qualified, licensed engineer to prepare the final wall design. For large, complex or critical projects, Nelson suggests an experienced SRW engineer be retained directly by the owner to prepare the final wall design prior to construction bids.

Building a Knowledge Base
The National Concrete Masonry Association provides numerous resources for SRW design and construction. One is the Design Manual for Segmental Retaining Walls, which is said to be the industry standard for residential, commercial and local agency projects (however a given project's requirements might call for different methodology specifications).

Sargon Masonry constructed the walls at Northlands at Tierra Del Rio over the course of two years with a crew that averaged 30-40 workers. About 100,000 square feet of blocks made up this three-tiered retaining wall system, which the developer wanted to allow for landscaped areas on the slope. On top of some of the wall is wrought iron fencing - nearly three miles of it.

Originally designed as a cast-in-place concrete wall with brick pilasters, the bids for construction of the Weston Corners wall came back drastically over budget. Delta owner Lyttle reached out to local producer Fay Block and GLA Architects to come up with the idea of a patterned retaining wall with a unique deeper faced unit to create the appearance of pilasters or columns that would provide the owner with the aesthetic they were looking for at a cost within budget. The block was installed in approximately six weeks after the blasting, water management solution, shotcrete and soil nail construction was finished.

Soil backfill was first used in the geogrid reinforced zones of the Great Wall, but as construction continued, that was switched to clean aggregate. More than 100,000 tons of gravel were used and some of the geogrids were as long as 40 feet. These tiered walls that support the roadway were built about 3' above grade to create a planter for landscaping. The structure also serves as an alternative to a traffic barrier or guardrails.

The Weston Corners segmental retaining wall presented the engineers and contracting company various challenges, including a gas easement near the back of the wall that prevented the typical use of geogrid soil reinforcement as that would cut into the easement. To avoid that and still stabilize the slope, a shotcrete wall was created and anchored with soil nails – steel rods driven into the slope's face at an angle. In addition, the site had to be excavated, through bedrock material, to an elevation approximately 30' below existing grade, which meant blasting had to take place. At the bottom, groundwater constantly filled the cut. The drainage plan included weep holes in the shotcrete, which tied into a PVC pipe that was encased with concrete, running behind the final wall and into the storm system.

Along with this manual, the NCMA also publishes an associated software application, Segmental Retaining Wall Software Version 4.0, which has a free 30-day trial mode for evaluation purposes. Nelson asserts that the program does not contain any product manufacturer's specific data but the design engineer can manually input that data or import it from a vendor data file.

Another resource from the NCMA is the Inspection Guide for Segmental Retaining Walls, which contains recommendations for designing, building and inspecting SRWs.

The guide defines these walls as, "gravity retaining walls, which can be classified as either: conventional (structures that resist external destabilizing forces"?(R)? solely through the self-weight and home decor ideas batter of the SRW units); or geosynthetic reinforced soil SRWs (systems consisting of SRW units in combination with a mass of reinforced soil stabilized by horizontal layers of geosynthetic reinforcement materials)."

Positive Reinforcement
The key to the stability of this classification of wall according to Nelson is that the design must allow for a reinforced mass large enough to resist external loads, and with enough geosynthetic (geogrids or geotextiles) reinforced aggregate layers to: keep this mass together, keep top units stable, and keep all the units coupled to the mass.

The NCMA recommends that reinforced fill should:
Be 1 inch minus, granular soil with less than 35% fines;
Be classified as SM or better;
Have a plasticity index of fines less than 20;
Be compacted to a minimum of 95% maximum dry density per standard Proctor moisture-density relationship (90 to 92 % Modified Proctor Density);
Be compacted in 6 to 8 inches maximum height lifts without exceeding the unit height;
Undergo one compaction test every 500 square feet of wall.

Other best practices from Nelson include:
An evaluation by the engineer of the subsurface conditions around the wall to identify stability issues;
Using a higher strength geosynthetic material or more layers to augment internal stability and facial stability;
Adding geosynthetic layers closer to the top of the wall to prevent crest toppling;
Lengthening the layers to reduce the risk of pullout or internal sliding, external stability and overturning;
Doing all three or improving the reinforced soil type to address what is known as internal compound stability.

The domain of engineered walls involves numerous details and comprises structures of many shapes and sizes - they can be as short as three feet high but with modern technology, can be over fifty feet in height. And the next time that a wall project calls for an engineer-generated structural wall design, you will now be a little better prepared to understand and manage that procedure.

As seen in LC/DBM magazine, January 2017.