On Solid Ground: Retaining Walls

There is 3/8-inch of mortar between each course of block in a basement wall, pointed out Jim Gruendemann, training coordinator, Versa-Lok Retaining Wall Systems, Oakdale, Minn.

“That’s why when it comes to preparing the base for a retaining wall, landscape contractors are better than masonry workers,” Gruendemann explained. “If the base of their basement wall isn’t level, masons can correct any errors made in leveling the base along the way as the wall increases in height because of this mortar joint. When landscape contractors build retaining walls, they don’t have the option of correcting these mistakes because retaining walls don’t have mortar joints. So, landscape contractors have to be as accurate as possible when preparing a retaining wall base.”

Ironically, however, when it comes to installing a retaining wall, landscape contractors generally rush through base preparation because it is so time consuming. Gruendemann noted that as much as 70 percent of the time it takes to install a retaining wall, depending on its height, should be spent on getting the base course down and level.

“If you don’t take the time to get the base course level, the retaining wall will look terrible,” Gruendemann advised. “Problems in the base course don’t go away as you build the wall. Instead, inconsistencies get magnified as the wall gets taller.”

By taking the extra time to make sure the base is at an adequate trench depth, compacted enough and level, landscape contractors can ensure their customers will get a strong, long-lasting, maintenance-free retaining wall.

TRENCH DEPTH. When excavating, contractors generally forget to dig an adequate trench depth for the particular height of the wall they are building, said Blaine French, regional sales manager, Keystone Retaining Wall Systems, Minneapolis, Minn.

“When there is inadequate trench depth, the wall doesn’t have a lot of protection from erosion and scouring and, eventually, the base will be exposed,” French warned.

The general rule, he explained, is to bury 1 inch of block for every 8 inches of wall height plus 4 to 6 inches of road-based material, such as crushed aggregate, and 1 inch or less of sand on the very top of the base to assist in the leveling process of the retaining wall blocks.

“So, for a 6-foot wall, you need 9 inches of block buried plus 6 inches of road-based material plus 1 more inch of sand,” French stated. “Some contractors forget about the 4 to 6 inches of road-based material needed.”

After figuring out the trench depth and beginning the excavation, French emphasized the importance of making sure the hole is free of loose organic material, such as soil, before and during the preparation of the base because organic material can settle over time, causing leveling problems after the wall has been installed.

COMPACTION. The 4 to 6 inches of road-based material in a retaining wall base can also settle if it is not compacted enough in the initial preparation.

“A wall could be dead level when it is installed and within one year the consolidation of the road-based material could result in an uneven wall,” French pointed out.

Although, according to French, 75 percent of landscape contractors use vibratory plate compactors to make sure their base is compact, Stephen Jones, president, Pave Tech, Bloomington, Minn., recommended vibratory trench rollers and reversible plate rammers because they are faster and apply a greater amount of force.

“For 3 to 4 inches of road-based material, a vibratory plate compactor will take four to six passes to get it all compacted,” Jones remarked. “For 12 inches of material, vibratory trench rollers or reversible plate rammers can do the job in just two passes.”

The ideal compaction for a retaining wall base is 95 percent standard proctor, Gruendemann said, meaning the material will be at 95 percent of its maximum density when compacted.

A trick to making sure the stone is consolidated into a dense mass is to cut the amount of the material being compacted at one time in half, said Dan Kaczmarek, sales representative, Paveloc, Marengo, Ill.