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Constructing Walls in Confined Areas

Explore alternatives for erecting a tall cut wall when excavation for geogrid is not an option.

By Tom Hatlen

Geogrid is considered to be an invaluable tool by engineer Bart Shippee. As he puts it, "Geogrid is fantastic. It's a highly efficient way to construct well-supported retaining walls. If there's space, make use of it."

However, what if you find yourself in a situation where space is limited and the requirement is to construct a taller wall? Bart has extensive experience designing walls that employ different techniques for stabilization.

"Generally, you're dealing with a cut wall and looking to optimize the usable space at the wall's base. Sometimes, you will encounter limitations on excavation for geogrid due to factors like property boundaries, roadways, parking lots, buildings, or established landscaping. Thus, we explore alternative solutions to minimize our physical footprint."

The three primary alternatives Bart employs for managing constricted spaces are:

• Implementing stabilized aggregate behind a small block SRW
• Utilizing large block retaining walls
• Employing earth anchors positioned behind a small block SRW

Enhanced Wall Mass for Increased Height

Stabilized aggregate consists of a permeable concrete mixture that is applied behind an SRW. Bart notes that this system is akin to forming a sizable block wall using standard-sized SRW blocks enriched by pouring the back section on-location. The porous concrete bonds with the block, resulting in a unified mass similar to a larger block. The heightened wall mass allows for constructing taller walls without relying on geogrid.

A basic guideline for the depth of either stabilized aggregate or large block walls ranges from 35% to 45% of their height, contingent on soil conditions, as well as the slope and load situated above the wall. Conversely, for an SRW that incorporates geogrid, the minimum requirement is that the geogrid extends at least 60% back from the wall's height.

For instance, an SRW measuring 10 feet with geogrid demands the geogrid to be located at least 6 feet from the wall face. In contrast, with the use of large blocks or stabilized aggregate, a common depth for a 10-foot wall would be around 4 feet.

Stabilized aggregate serves as a porous concrete mixture applied behind an SRW. Given that water permeates through the porous concrete, there's no need for additional drainage stone behind the wall. The stabilized aggregate adheres to the wall block, resulting in increased wall mass, thereby making geogrid unnecessary for the wall/stabilized aggregate assembly.

Excavation and Drainage Considerations

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The nature of excavation for large blocks and stabilized aggregate tends to differ for two main reasons:

• Large blocks are manufactured with fixed depths, while the depth for a stabilized aggregate system is tailored to the specified requirements.
• Stabilized aggregate inherently allows for drainage, eliminating the need for drainage stones behind it, unlike standard SRWs or large blocks. Bart explains, "The stabilized aggregate is akin to a concrete mix that incorporates angular drainage stone (e.g., #57 stone) within a cement paste, devoid of any fines. Its porosity ensures optimal drainage embedded within the mix, creating a superior draining wall system."

For an 8-foot wall that’s designed with a depth of 39 inches, if the nearest large block size available is 42 inches, excavation would be 54 inches (42-inch block plus an additional 12 inches for drainage). Conversely, excavation for a wall utilizing stabilized aggregate would be 39 inches (12-inch block plus 27 inches of porous concrete).

However, it's essential to note that the block depth diminishes as one ascends a big block wall, resembling a pyramid's form. If the first four rows of an 8-foot wall utilize a 42-inch deep block, the upper three rows may shift to more economical 30-inch blocks. In contrast, a stabilized aggregate wall would fully retain a consistent depth of 39 inches top to bottom.

Understanding Stabilized Aggregate

Bart rarely recommends stabilized aggregate walls that exceed 10 feet, although he asserts that constructs taller than that are possible. "As concrete costs escalate, alternative solutions become more appealing beyond a certain quantity per square foot. The economics are quite favorable up to 10 feet."

One significant advantage of stabilized aggregate lies in its selective usage. "In cases where there's just one location with spatial constraints, we can maintain geogrid usage for the remaining areas of the project, thereby containing costs. In approximately 75% of the projects where stabilized aggregate is implemented, the walls are a combination of both stabilized aggregate and geogrid."

Bart also considers hybrid wall designs when introducing utilities at a wall’s apex. "The lower section can be geogrid reinforced, while the top 4 to 6 feet can comprise stabilized aggregate."

For stabilized aggregate applications, Bart prefers utilizing SRW systems featuring open cores in the individual blocks and gaps in between to enhance adhesion. "During the pour, a piece of rebar is employed to insert concrete into the voids, strengthening the connection. However, with solid SRW units lacking cores, I might incorporate a small 24-inch strip of geogrid for structural integrity."

Typically, stabilized aggregate is poured in intervals of 2-foot lifts (three rows of 8-inch block), allowing the concrete to set for about 4 to 6 hours while the next set of rows is laid down. "Attempting to pour everything in one go could jeopardize the entire wall, resulting in a significant mess."

Of course, complications may arise when it comes to maneuvering the concrete truck near the wall. In such instances, contractors sometimes use plywood as a base for the truck, allowing skid steers to shuttle the concrete to the construction site.

Earth anchors can be strategically employed for wall stabilization. Bart mentions that a prevalent method for connecting earth anchors to the wall block utilizes geogrid and piping: "You attach a pipe to a row of anchors, wrapping geogrid around the pipe to reconnect to the wall's face." Image courtesy of Foresight Products.

Utilizing Large Blocks

In optimal scenarios, some large block walls can exceed 22 feet in height without requiring geogrid support, making use of 7,500+ pound blocks, each possessing a wall face area of 24 square feet. Block systems also range from weights of 300 to 1,200 pounds.

Yet, Bart predominantly works with large blocks weighing between 1,500 to 3,500 pounds, typically yielding about 5.5 square feet of wall face per block. He often specifies gravity walls within the 10 to 12-foot range, paralleling the specifications for stabilized aggregate walls.

While the installation of 1-ton blocks might seem daunting, Bart asserts that large blocks have surged in popularity due to their user-friendly nature. "You’ll want a reasonably robust excavator for placing 3,000-pound blocks, yet you don’t need to be a professional wall installer to manage a large block system. This user-friendly aspect has led many general site contractors to handle their installation. There’s no necessity for intensive compaction of backfill or extensive earthwork behind it. You simply excavate, position the blocks, and backfill with crushed stone. This presents a significant advantage."

John Franklin, a contractor, frequently collaborates with Bart on several wall projects (see page 16). John remarks, "Large blocks expedite the process, as they afford 6 square feet of coverage per block instead of a mere 1 square foot." The efficiencies in reduced excavation, minimized compaction, and gravity wall construction contribute to streamlined installation. Machinery can execute the bulk of the labor, preserving workers' physical well-being.

Large blocks find less application in residential contexts owing to their potentially disproportionate visual appeal in such settings. Bart estimates that costs associated with large block walls generally surpass those linked to stabilized aggregate walls.

Incorporating Earth Anchors

Bart has frequently utilized earth anchors to facilitate taller wall constructions without the necessity for geogrid. Helical earth anchors, which screw into the earth, and traditional earth anchors act similarly to oversized toggle bolts designed for hollow walls. "You drive a rod into the ground behind the wall to your desired depth, then leverage the rod until the head swings open like a toggle bolt, securing it within the soil."

Common earth anchors are usually driven in at intervals of 2 to 4 feet. Should boulders or bedrock be encountered, pre-drilling is carried out using a horizontal drill rig. Load testing tools are employed post-installation to validate that earth anchors will retain their grip.

Although earth anchors significantly lessen excavation need, securing subsurface easements is necessary if they extend into adjacent properties.

This large block wall, constructed by Franklin Construction adjacent to a rail line, was developed without interference with the tracks or associated utility poles, allowing uninterrupted rail traffic during construction.

Cast-in-Place & Soldier Pile Walls

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Bart has also made provisions for cast-in-place concrete and soldier pile walls, primarily for particularly challenging circumstances attributed to their high cost. He explains that cast-in-place concrete entails pouring concrete on location. "This could be your sole option when constrained by a property line. Rather than positioning the footing behind the wall as convention dictates, you might extend it out under the wall’s toe, keeping it within your property limits. Although it may be costly, it is feasible."

Soldier pile walls consist of drilling deep holes to install H-piles (wide I-beams) encased in concrete roughly every 10 feet. Pre-cast concrete panels are then fitted between the H-piles. While reportedly effective in tight spaces, soldier pile walls tend to incur steep costs. "Adopting soldier pile architecture can escalate expenses to over $100 per square foot relatively quickly."

Recently, a project originally designated for H-piles was evaluated, revealing a projected expense exceeding $350,000. Subsequently, shift to stabilized aggregate yielded a $160,000 savings for the property owner.

This wall, nearly 8 feet tall, necessitated an excavation depth of only 2.5 feet, which was filled with stabilized aggregate.

Various factors influence costs; however, in Bart's experience, a wall reinforced with geogrid generally costs approximately $20 per square foot, while stabilized aggregate, earth anchor, and large block walls typically range from $25 to $45 per square foot.

Bart Shippee, P.E., is a geotechnical engineer boasting over 24 years of expertise. Over the past two decades, he has designed in excess of 6 million square feet of retaining walls and regularly consults manufacturers on engineering product matters. He is acknowledged as one of the foremost specialists in SRW in the northeastern U.S.