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March 2019

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In this Issue:

Interchange 55 Logistics Park

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Interchange 55 Overview

When CT Realty decided to develop a roughly 120-acre site in Romeoville, Illinois for a new Industrial Park, they had many site considerations. With most of the flat development sites in the area used up, CT Realty worked with Jacob & Hefner Associates to fit two 650,000 ft2 (60,000 m2) buildings on the site with over 100 ft (30.5 m) of grade change across the property. As you can imagine, multiple retaining walls were required to provide the necessary parking and building area for the site. Clayco was contracted in order to streamline the process for the complex design-build site. Although there were 2 retaining walls on site that required careful considerations for adjacent properties, one particular retaining wall was especially unique as there was a roadway above that was required to be installed prior to the building of the retaining wall below it that would significantly reduce excavation capabilities and as a result would require a non-traditional design and build situation.

Design engineer, Ryan Freund of Virgilio & Associates began looking at various options for this retaining wall that would limit the excavation depth, as the road construction had just been completed. Throughout the process, Ryan looked at reinforcement and block options such as; geogrid reinforcement, no-fines concrete, big block, sheet piles, and finally settled on using a soil nail option with an Allan Block facing for this one challenging wall.

Earth Anchor Detail

Virgilio & Associates ran their design on the soil nail wall while using the common spreader bar application at the excavated cut to allow geogrid to be placed between the Allan Block units, wrap around the spreader bar, and return to the block facing 2 courses higher (Earth Anchor Detail above). Although soil nails were designed for the full height, Clayco’s design team looked at the site excavation and the location of the soil nails in 3D to find out where Classic Landscape could transition to geogrid reinforcement to keep the wall as economical as possible for CT Realty. Classic Landscape, an Allan Block Master Wall Builder, secured the project and successfully tackled the installation of all walls on site, including the soil nail wall.

For more detailed information on the Interchange 55 Logistics Park project, check out the case study here.

Various Wall Design Types

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How Do I Know What is Best?

A segmental retaining wall (SRW) often provides one of the most economical and aesthetic solutions for grade changes. The utilization of geogrid and concrete block is the most frequent solution due to this and the efficiency of the systems. However, because geogrid needs room behind the wall for installation, use may present a problem if space is limited (typical detail of a geogrid reinforced application seen below).

Typical Detail

In situations where geogrid may not be the right path (for instance a cut site with a neighboring property line directly behind the wall), there are other options. Other traditional solutions to grade differences include sheet pile, soil nails (as we saw at Interchange 55 – Logistics Park), earth anchors and rock anchors – these types are often referred to as tieback walls. The concern with many of these are cost, aesthetics and/or difficulties with design and installation.

Whether you have many walls onsite (like Interchange 55 Logistics Park) or a site with only a single wall, finding the BEST solution is not always easy. Reviewing your site, understanding the expectations of the owner and leaning on our Allan Block team may help you in your quest for the best solution. We offer free preliminary support and, such as the case with Interchange 55 Logistics Park, often see our block used with a variety of reinforcements to create the best solution. Here we saw Allan Block used with traditional geogrid AND soil nails to provide the best solution for the owner. Whether you are designing a traditional SRW, incorporating a tieback design such as a soil nail or earth anchor, we can assist you with finding the best solution.

Tieback Wall Designs

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How Are These Designed?

Retaining walls have been constructed with many different reinforcement options throughout the years from geogrid, no-fines concrete, soil nails, earth anchors, sheet piles, rock anchors, etc. Every site may require a different type of reinforcement option behind a retaining wall, but one of the main questions engineers ask themselves is how to design them?

The answer to this question is the design software called AB Walls 3D. If you have designed gravity walls, geogrid walls, or No-Fines Concrete in the past, you would know that AB Walls 3D design software is the industry leading software for all these options. Did you know that you can use the same AB Walls program when looking at an earth anchor, soil nails, or other tie back structure?

When looking at a tie back type structure, you have these main components:

  • Earth Anchor – Anchoring device that is typically made up of steel
  • Anchor Rod – Tensioned rod, typically steel that attaches to the earth anchor
  • Temporary Facing – Typically a shotcrete facing to stabilize the soil vertically during construction
  • Permanent Facing – This can be concrete, steel, or timber to provide the aesthetic for the wall structure

So how does AB Walls 3D help design these components?

Starting with a geogrid reinforced wall in AB Walls 3D, the program provides the designer the tensile force on each layer of geogrid within the internal stability.

With these loads from AB Walls 3D, you can now add a factor of safety to the loads at the discretion of the designer. These factors can be 1.5 – 3 and really depends on what the designer feels is necessary. Spacing is a requirement at this point as the designer will need to account for the tributary area applied to each anchoring rod.

AB Walls Internal Stability Results

First, we will start with the horizontal spacing. A common recommendation from the anchoring company can range from 4 – 8 ft (1.2 – 1.4 m) center-on-center. The spacing of the anchors may be dependent on the type of facing and how the facing will be attached to the temporary facing. Common applications use a spreader bar along the temporary facing that is secured to the anchoring rod. That spreader bar is typically the limiting factor when determining the horizontal spacing. The designer will want to check how much deflection takes place in the spreader bar to determine if the spacing needs to be reduced or if the deflection is acceptable.

Next is vertical spacing. Many anchoring companies will have recommendations on the maximum spacing, but you will want to think about the constructability of the facing. Even if the maximum vertical spacing is 3 – 5 ft (0.9 – 1.5 m), you may want to think about how that facing is attached. One such option is to use Allan Block as the facing and use geogrid to wrap around a spreader bar and return to the facing 2 – 5 courses above. As Best Practices for Segmental Retaining Walls recommends 16 inches (40 cm) vertical spacing, this would amount to 2 courses between each layer of geogrid which would place the spreader bar every 4 courses, see Facing to Earth Anchor Tieback Detail.

The question needs to be asked, “How long does the anchor need to be?” This length will be determined by the force that will be required for the rod to hold, but also needs to take into account the anchored depth beyond the active zone. The active zone is the fill material that will be placed behind the temporary facing and compacted during the installation process. Typically, this material is structural fill and rarely used as in-situ (site) soils unless approved by the geotechnical engineer. The passive zone is the soil that is undisturbed during construction and provides the majority of the resistance.

soil nail wall cross section

Since the height of the retaining wall and site soils will have the most influence on the length of the anchoring rod, it is difficult to make recommendations, but those lengths will commonly be 30-50% the height of the wall for typical applications.

Keep in mind, the anchoring depth is the depth beyond the active zone. This means that if a 6 ft (1.8 m) anchoring rod is required, but the active zone is 2 ft (0.6 m) at the vertical location of the rod, an 8 ft (2.4 m) earth anchor will be required to exceed the requirements. Like geogrid retaining walls, there is typically a minimum anchoring depth for most tie back systems. A 5 ft (1.5 m) minimum anchoring depth is common for traditional earth anchored retaining walls.

Once you have determined the spacing of the anchoring rods, you can establish a tributary area in which the load will be applied to the anchoring rod. There are many companies that provide different diameter anchoring rods that can be selected directly from the manufacturer or distribution companies. Those rods have recommended angles at which they will be installed which can range from 0 – 15 degrees from the horizontal. Since the loads are determined, the appropriate anchoring rod can be selected based on that factored load.

Is Geogrid a Tieback?

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To set the story right, geogrid is hardly ever used as a tieback like what we have discussed in the previous article.

When wall heights exceed the limits of a gravity wall (wall that relies solely on the weight of the block), geogrid can be added to provide a stable wall condition by creating a Coherent Gravity Wall.

Coherent gravity walls use a flexible synthetic mesh (geogrid) to stabilize the soil. Layers of geogrid inserted between the blocks and extending behind the wall interlock with the surrounding soil to create a cohesive soil mass. This mass uses its own weight and internal shear strength to resist both the sliding and the overturning pressures from the soil being retained.

Geogrid Roll

The wall rock in the Allan Block cores provide a positive connection between the layers of geogrid and the Allan Block wall, locking the two systems together. The reinforced soil mass becomes the structure and the Allan Block wall becomes the facing. The specific location and embedment length of the grid layers depends upon the site conditions, wall heights, and Long-Term Allowable Design Strength (LTADS) of the grid being used.

The Great Wall of China, dating back some 2,200 years, was built as a double-sided retaining wall. The soil between the two walls was a mixture of clay and gravel reinforced with tamarisk branches. Allan Block retaining walls employ “old technology with new materials.” Since the late 80s, modular block retaining walls have used geogrid reinforcement to stabilize the soil and resist the lateral pressure behind the retaining wall.

AB Walls 3D

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Having the capability to model an SRW in 3D may provide economic advantages such as the case with the Interchange 55 Logistics Park project. Understanding where grid may be interrupted by a road, building, property line or any other number of obstructions, could help provide the best solution and prevent a serious issue later in the project. That is why we have taken the time to incorporate AB Walls with 3D capability.

That’s correct, the software you have been using for years now has the ability to export your design into a 3D model. The best part of this is that it can do so with the click of a button. By using a 3D design platform, SketchUp, with the Allan Block 3D Modeling Tool extention, our AB Walls 3D software has never provided you more power.

AB Walls 3D cover

Whether you are just starting with AB Walls or an industry expert, 3D modeling is sure to provide value as it did for Clayco at the Interchange 55 project. In this case 3D modeling helped identify where they could transition away from the more expensive soil nail solution to the most economical solution with geogrid within the same wall. What was created was a Complex Composite Structure that resulted in a structural and most economical solution for the owner. Call us today to find out how this tool can help you on your next project!

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