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Retaining Wall Best Practices
Preparing and Building the Right Way

Building Big Block Retaining Walls

Landscape contractor, Big Bull Retaining Wall Systems, LLC, worked with Ghidorzi Construction Company to create a large engineered and tiered retaining wall needed to stabilize the slopes and grades at the newly built Ascension Medical Group Clinic in Wausau, Wisconsin. A big challenge was the site?? 1/2 s small size, which required careful coordination for block delivery on an as-needed basis to assure there was always ample room to work.


Building Big Block Retaining Walls

Materials supplier Oberfields LLC of Ohio helped transform this steep, unusable front yard with 800-square-feet of Kodah retaining blocks that imitate snapped limestone, to create terraces that turned previously abrupt slopes into an aesthetically pleasing functional space for the homeowner.


Taming aggressive slopes often takes intensive retaining walls. Installing these features successfully demands that appropriate building techniques be followed. First and foremost, design calculations should follow good engineering judgment and standard methodologies such as those from NCMA, AASHTO, or Allan Block, who recently put out an SRW best practices manual, which is the main source for this article and the principal tool in the manufacturer's Zero Wall Failure Program.

Following are some of the key takeaways from this manual.

Engineer Required?
Local codes and municipalities dictate, based on minimum height requirements, when engineering of a wall is a necessity. However, height is not the only factor. Others include, but are not limited to: soil conditions, number of terraces, steepness of slopes above or below, amounts of seismic loading and roadway surcharges.

Besides the wall layout and wall heights, a site plan should determine conditions above and below the wall, as well as locations and levels of live and dead loads. And it should point out locations of all existing and proposed utilities. A thorough geotechnical report on soil and groundwater conditions should also be obtained. Other issues to consider are future snow and/or storage loads and freeze/thaw concerns.

Digging Down
After a base trench is excavated, it should be compacted to the level specified in the geotechnical report and inspected by the onsite soils engineer prior to any base material being placed. The typical depth of the trench is calculated based on a minimum 6 inches deep of compacted wall rock base, and buried block depth equal to 1 inch per foot of wall height, with a standard minimum of 6 inches for commercial projects.



Building Big Block Retaining Walls

A total of 3,150 square feet of Outcropping large retaining blocks were supplied by R.I. Lampus Company of Pennsylvania on a complex project that required an 18-foot-high wall to support the earth around this tall RV garage. Small block Belvedere embellishments for a bridge leading into a governor?? 1/2 s driveway finished the project.


Building Big Block Retaining Walls

This diagram shows best practices for water management of retaining wall installations according to the Allan Block SRW best practices manual. Those practices include creating a swale on the backslope behind the wall so that water does not flow over the top of it, and making sure all drainpipes exit to daylight or to an underground drainage system.


The width of the trench is typically 24 inches wide but deeper facing units of course require more width. And, a larger trench, and therefore a larger base, may be needed if poor foundation soils exist.

Building Up
If deemed necessary, a layer of geogrid is installed 3 to 6 inches above the bottom of the base, with a minimum length of 60% of total wall height measured from the top of the base to the top of the wall. Additional layers are then placed after every other course of blocks.
Standard base material used is consistent with wall rock; well-graded compactible aggregate, 0.25 to 1.5 in in size.

It is recommended that facing units have an average (front to back) depth of at least 10 inches because units of this size or greater have a proven performance record. And it is recognized that as the unit depth decreases, so does the stability of the facing.

After placing the base course, a hand operated plate compactor should be used on the ground behind this course within the 3-foot-wide consolidation zone with a minimum of two passes. A proper testing protocol should be established to consistently achieve the minimum compaction requirements set by the design requirements.

Compaction of the other courses will begin by running the plate compactor directly on top of the block facing, and then compacting in parallel paths until the entire consolidation zone has been compacted.



Building Big Block Retaining Walls

The lower wall at the Wausau, Wisconsin, site creates an even grade halfway up the slope. The upper wall was built a full story high to match the highest point of the site, and it leveled out the property to accommodate a parking lot on the front side of the building.


The wall rock column depth behind the wall facing is typically 12 inches deep. But this depth will vary with wall height, if select/structural fill is not utilized in the reinforced mass. For walls from 10 to 20 feet tall, the bottom half is suggested to be increased to 24 inches deep and the top half to be the typical depth of 12 inches deep. For walls over 20 feet tall, the bottom half is suggested to be increased to 36 inches deep and the top half is divided in two; with a 24-inch-deep column and a 12-inch-deep column at the top.

It is common to place a horizontal layer of landscape fabric over the wall rock column at the top of the wall to protect the wall rock from being infiltrated by the topsoil placed to finish the wall.

Caps should be secured in place using a high quality flexible exterior grade masonry sealant.

This, of course, is not an exhaustive list of best practices. Many more are available at https://ncma.org, https://store.transportation.org and http://allanblock.com



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November 18, 2018, 5:03 am PST

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