On Retainer

In addition to providing beautiful landscaped spaces and creating opportunities for dramatic plantscapes, retaining walls can be lifesavers on tough sites. Whatever the job, retaining walls can be a great solution, but it's important to estimate the work profitably and, of course, know how to build the wall efficiently and properly.

The decision to incorporate a retaining wall into a landscaping project often comes from necessity: a difficult site with steep grades or the need to maximize the available land and keep as much of it as functional as possible. Another purpose might be an aesthetic focal point for the project that includes eye-catching, dramatic greenscapes.

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Retaining walls are the “go-to” solution when erosion control is an issue. A good commercial example is Wesley/Valley Square Commons, a housing project in Golden Valley, Minn. The project had a site that dropped at a steep angle to an adjacent creek. The developers wanted to protect the creek and enhance it as an amenity, while maximizing the number of housing units on the site. The retaining wall accomplished both objectives.

Retaining walls maximize the functional and aesthetic potential of a site and make a project possible when it otherwise wouldn't be. Word of Faith Church in Atlanta had grading so steep it would have been impossible to build a road to provide access to the building and its parking lot. It took a retaining wall 60 feet high to do it, but it made the church accessible and the project possible.

Of course, retaining walls have an aesthetic function that doesn't require steep grades and challenging sites. For example, a Jordan, Minn., project by Complete Landscapes used a two-tier retaining wall design to create planting areas that added a beautiful complement to the home's front façade.

ESTIMATING A RETAINING WALL PROJECT

Incorporating retaining walls into a project is limited only by your imagination, but it's also easy to imagine losing money if you don't estimate the costs properly.

Consider these factors when estimating a retaining wall project: materials quantities and costs; labor hours and costs; equipment costs; subcontractor costs; overhead costs; and profit.

Consider also the particulars of the wall, such as its height, slope, whether there will be a load (a car, for example) near or at the wall and whether there's any water nearby.

  • Materials quantities and costs

    Beyond the block and cap units, you'll need geosynthetic reinforcement if the wall will be higher than four feet, filter fabric, aggregate for the base and backfill, drain pipe, backfill material and adhesive for cap units.

    Once you've calculated the square footage of the wall and know the quantities you'll need of the other materials, straightforward math will get you the materials costs.

  • Labor hours and costs

    Estimate labor for a retaining wall job as you would any other component of your project. However, be sure to avoid unpleasant surprises by considering items such as time of year, difficulty of base prep, soil compaction, material staging (distance from the work site), access to wall, size of wall units and backfill volume.

  • Equipment costs

    A retaining wall project will require some or all of these items: forklift, skid-steer loader, compactor, assorted small tools (you likely have already) and transit costs.

  • Subcontractor costs

    Keep in mind that you may need the help of outsiders to get the project completed. For example, will you need to hire an engineer to draw a plan of the project to submit to the municipality or simply to show the client?

  • Overhead costs

    While your crew is on the job site, it's still costing you money to light the office, maintain the copy machine, pay the receptionist and print brochures, so include money to cover those costs in the equation.

  • Profit

    Assuming you're in business to make money, include your profit margin in the estimate.

BUILDING A RETAINING WALL

Once you've submitted the estimate and obtained the commission, it's time to start work. Following is a general overview of the steps involved in the installation process. If you're new to the installation process, work with someone who's experienced or get help via the block manufacturer's Web site. Established manufacturers often have a toll-free customer service help line.

Building a retaining wall is a nine-step process:

  1. Stake out the wall

    You'll need to mark where the wall will go. Do so with stakes, and then excavate for the leveling pad and grades shown on your plan. Take out enough soil behind the wall to accommodate the reinforcement material. The trench for the leveling pad should be a minimum of 24 inches wide and 12 inches deep.

  2. Prepare the leveling pad

    Make the leveling pad with a good aggregate of compactable base material of ¾-inch minus with fines. The pad should extend 6 inches in front of and behind the first course of block and be at least 6 inches deep. It's important that the leveling pad stays level over time, so be sure to compact the aggregate.

  3. Install the base course

    This step is critical, as you'll need this to be dead-on. Otherwise, you'll be creating problems that will haunt you as you add subsequent courses to the wall. Start by running a string line to align the wall units. Begin laying block at the lowest elevation of the wall, and, assuming you're using a retaining wall product with a rear lip to guide installation, remove it from the base course so the block can lie flat on the pad. Place the blocks flush against one another and be sure they're in full contact with the leveling pad. Make sure the blocks are level: back-to-back and side-to-side.

  4. Building the wall

    Clean any debris from the top of the blocks. Keep the wall bond by placing the units in a staggered relationship to the course below. For best results, use filter fabric directly behind the wall extending from the bottom of the base course to the middle of the top course. This will keep material from seeping between the blocks and creating opportunities for weed growth and other aesthetically undesirable consequences.

  5. Design the drainage

    Place drain tile as low as possible behind the wall so water drains down and away from the wall into a storm drain or an area lower than the wall. Ideally, you should cover the tile with a geotextile sock that will serve as a filter and reduce the chances of a clog in the tile. Cover the tile and fill the area behind the block with a drainage aggregate at least 12 inches from the wall. If drain tile outlets are required, space them no more than 75 feet apart and at low points of the wall.

  6. Compact the soil

    Shovel the in-fill soil behind the drainage aggregate and compact it with a hand-operated compactor. Make sure the aggregate is level with or slightly below the top of the base course. Bury the front of the base course with soil.

  7. Reinforce the courses

    First and foremost, remember that you'll need an engineer to design any walls higher than 4 feet. Your construction plan shows which courses will need reinforcement. Measure and cut the reinforcement based on the plan design lengths and lay it perpendicular to the wall, pulling it taut and pinning the back edge in place to keep it from wrinkling. Place the front edge of the reinforcement 2 inches from the face of the block and lay the next course to secure the reinforcement. Add drainage aggregate behind the blocks, then add a minimum of 6 inches of in-fill soil and compact it. This is the minimum depth of backfill required if you intend to operate vehicles on the reinforcement. If you do drive on it, avoid sudden turning or braking.

  8. Finish grade and surface drainage

    Protect your wall with a finished grade at the top and bottom of the wall. Use 6 inches of soil with low permeability. This will supplement your drain tile work and ensure proper drainage away from the wall.

  9. Clean and restore the site

    Remember that the job's not done until the site's cleaned up, so brush off the wall and pick up any debris.

LEARNING CURVE

A successful retaining wall project takes planning and the patience to stop and think things through at critical times in the construction process. And, as with most things, you'll get better at it — estimating and building — once you've done it a few times. You can shorten that learning curve by working with those who've done similar projects, by going to manufacturers' Web sites, and by accessing other knowledgeable resources along the way.

Based in Minneapolis, Minn., Anchor Wall Systems, Inc., provides its licensed manufacturers and end-use customers with concrete retaining wall solutions for commercial, municipal and residential landscaping projects worldwide. For more information, visit www.anchorwall.com or call 1-800-473-4452.

THE BUILD

Building a retaining wall is a nine-step process:

  • Stake out the wall
  • Prepare the leveling pad
  • Install the base course
  • Building the wall
  • Design the drainage
  • Compact the soil
  • Reinforce the courses
  • Finish grade and surface drainage
  • Clean and restore the site

THE BID

Consider these factors when estimating a retaining wall project:

  • Materials quantities and costs
  • Labor hours and costs
  • Equipment costs
  • Subcontractor costs
  • Overhead costs
  • Profit

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© 2014 Penton Media Inc.

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