Segmental Block Retaining Walls, continued

When we build a wall on the site of an existing building, our first question is whether the spot we're building on was cut or filled when the structure was originally built. We must be absolutely sure to place the wall either on virgin ground or on properly compacted fill.

Building a wall on uncompacted fill is an invitation to disaster. I remember one job site where a deck contractor built his deck on posts bearing on pad footings set below our area's 42-inch-deep frost line, just as code requires. Imagine his surprise when the deck settled 10 inches! No one ever told him that there was a 15-foot deposit of fill under his footings because the house was built on a steep slope. The soil under and around a retaining wall might experience forces greater than the weight of a deck footing. If that wall is placed on uncompacted fill, it's going to move.

Once we're sure the wall will rest on native soil, we still need to know the soil type, because that determines what kind of base preparation is required. For simplicity's sake, I'll talk here about the broadest soil categories: sand, gravel, and clay.

A well-drained gravel soil is ideal: It locks into place to provide good, stable bearing, and water percolates through it quickly. Clay, on the other hand, can seem firm and solid when you're digging in it, but if it absorbs water and expands, it can become a nightmare, making the wall shift up and down. Sand usually drains well, but it can be an unstable base.

Retained soil characteristics. The soil that the wall will be holding back also must be assessed to determine what engineers call its "angle of repose" — the angle at which materials are self-supporting.

Imagine three trucks dumping three different soils and making three cone-shaped piles of soil. The surface of each pile would naturally rest at a different angle with the horizontal — that particular soil's angle of repose. The steeper the soil's natural angle of repose, the less need there is to hold the soil back or to reinforce it behind the wall. And we always place the geogrid so that it extends past the line of that angle and embeds itself in soil that is self-supporting.

For strong soils like gravel, the angle of repose is about 45 degrees. But you can't assume that it's the same in every instance. In critical cases — walls that have to support loads imposed by decks, patios, driveways, or buildings — we remove all the natural material from behind the wall until we are sure that the only thing the wall has to support is our imported granular backfill.

Consulting an engineer. We can usually assess the site soil conditions ourselves, but not always. If we are uncertain about anything, we bring in a soils engineer. Good soils engineers are worth their weight in gold — they can help you prevent a problem today from becoming a nightmare later on.

I generally call an engineer if the wall needs to support any unusual load, such as a parking lot or a building. I also bring in an engineer if I feel the need for a little extra peace of mind. For example, I consulted one recently when a customer called me to repair a failing wall that another contractor had built incorrectly. The engineer confirmed my assessment of the situation, and together he and I designed a replacement wall. I probably could have figured it out, but since the job was already problematic, I appreciated the additional security. (If the original builder of the wall had followed an engineer's advice, of course, I would not have had to be there in the first place.)

Base Preparation

Once you know your soil conditions, you can move forward with preparing the subgrade, placing a compacted gravel base, and beginning to place block.

Subgrade. We always compact the existing soil in the bottom of our trench. You may need an engineer to specify the compaction method and to verify that you've compacted sufficiently — or at least to check the results you're getting with your compacting equipment at the beginning, until you're comfortable that your methods are working.

How we treat native soil depends on the soil type. Firm gravel is fine; we put our base stone right on top of it. If the soil is clay, we dig down at least double the recommended depth before placing our base material (if the manufacturer's literature calls for a 6-inch compacted stone base for a 4-foot wall, for instance, we'll dig a foot before placing the stone). Loose sand should be completely removed. But if it's too deep to practically remove, we dig extra deep and lay a soil-separating fabric, then place our stone on top of that.

Base material. Next we place the base material (Figure 3). In New York, we use "crusher run," a crushed stone mixed with stone dust to lock it solidly into place. We can tamp that nearly as solid as concrete with a walk-behind plate compactor.

Figure 3.A base of "crusher run," ground stone and stone dust, is placed in the footing trench and tamped hard to support the base course of block. The taller the wall or the less firm the subgrade soils, the deeper and wider the base of crusher run should be.

The base is always at least twice as wide as the wall blocks; the planned height of the wall determines the base material's depth. For low walls (less than 2 feet tall), we use about 3 to 6 inches of base thickness. Larger walls can have as much as 2 feet of base below them. We place the material in 3-inch lifts and tamp it down with walk-behind plate tampers to ensure proper compaction. Once the base is placed and compacted, you can begin to build.

We generally bury at least half of the base course of block below the original grade to restrain the toe of the wall from kicking forward under the pressure of retained earth. For taller walls subject to higher soil pressures, it may be necessary to bury one or two full courses of block or more. The engineering documents required for walls higher than 4 feet should specify the embedment depth of the wall's base. In the case where the downhill slope continues at the base of the wall, the calculations are more complicated and the required depth of the wall foot is typically greater (Figure 4).

Toe Embedment Anchors the Wall Base

Figure 4.The author likes to firmly bed his base course of block into the granular base by beating the block with the top of the pick handle (top). He prefers to use the heavy Versa-Lok units shown here because in his experience they stand up to this treatment better. The depth of the first course of block ("toe embedment") varies depending on site characteristics (bottom). If the ground slopes downhill from the wall base, manufacturer specs call for a deeper embedment to hold the base in place.

I like to pound my base course blocks into the layer of crusher run to make sure they're firmly seated. We smack them with the end of the wooden pick handle. That's one reason I personally prefer solid block units like Versa-Lok to hollow block systems — occasionally, I've broken hollow units by hammering on them.

Some companies make oversized base units for extra stability on very large walls. And if we're working with wall systems that use pins to hold segmental units together in the wall, we sometimes pin the base course blocks to the ground using 3/8-inch rebar, to help anchor the wall base to the earth.

Drainage. Without question, the most critical factor in retaining wall construction is drainage (Figure 5). Without it, walls will be exposed to hydrostatic pressure (which can double the load on the wall). In the North, poorly drained soils will expose the walls to frost action. If you see a wall that's bulging in the center, chances are that it's not properly drained; it's only a matter of time before it fails.

Figure 5.A drain tile in the granular backfill, sloped and run to daylight, is critical for retaining wall performance. It helps keep frost problems to a minimum and maintains soil behind the wall in a drained condition to reduce lateral pressure of soil against the wall.

We always bury at least a 4-inch perforated pipe behind the wall. The pipe should be wrapped in a landscaping filter fabric and run to daylight out of the wall and away from the base.

Backfill and geogrid. We backfill all walls with a #2 crushed stone, placed and compacted in lifts of 6 inches or less, and layered with reinforcing geogrid (Figure 6). It's important to lay landscape fabric on the slope behind the wall, so that dirt does not wash into the stone — silt material impedes drainage and can clog the drainpipe or its filter fabric wrap.

Figure 6.Space behind the wall is backfilled with 1-inch and 2-inch gravel, placed in 3- to 6-inch lifts and compacted in layers (above left). Geogrid at specified intervals (above right) locks the compacted fill into a single mass, which holds back the natural soil or fill behind it. The grid also holds the face block in place, either by interlocking with gravel placed into the block cores, or by engaging locking pins as in the Versa-Lok system shown at left.

The geogrid spacing and the distance it runs back from the wall into the slope are important factors in the wall's strength. The geogrid requirements are usually determined by the height of the wall, the soil type, and any additional load on the earth above the wall. Most walls get geogrid at least every 2 vertical feet in the wall and extending into the slope twice the height of the wall at the point where the geogrid lies. But the geogrid length varies in different soil conditions — the material needs to extend well past the line of the soil's angle of repose, so that it will stay firmly embedded.

Geogrid is locked into the wall face by pins, by friction between the block units, or by stone fill placed inside hollow block units, depending on the manufacturer. That's to hold the face block in place. But geogrid also separates the backfill into different layers, working to prevent the soil from slumping and increasing the stability of the wall. The grid effectively locks the backfill soil into a solid mass that acts as a heavy, bulky gravity wall.

Terraced walls. If you're building a terraced wall, where the upper wall will be built on the backfill of the lower wall, the geogrid becomes especially significant. You may need engineering in those cases. In general, terracing should be designed so that the lower wall does not have to support the upper wall. A rule of thumb is to set the upper wall back a horizontal distance at least one and a half times the height of the lower wall.

A Wall That Failed

Two of my favorite books are a matched pair by Mario Salvadori, the late Columbia University professor, titled Why Buildings Stand Up and Why Buildings Fall Down. Both are interesting, but I've always thought the second one teaches more. When it comes to retaining walls, I personally have learned more from structural failures than from structural successes. Here's one of our stories:

We were called in to propose a wall for a client. This wall was to be a terraced system of two walls, each to be about 4 feet of exposed wall. In addition to holding back soil, it would support the back of the home and a deck, which would be cantilevered over a 9-foot-tall stretch of the wall system.

After receiving three proposals from three different contractors, the client decided to go with the lowest price (which wasn't us). I cautioned the client to make sure he had structural diagrams and plenty of references from the chosen contractor. He said he did.

The construction started in November 1995 and was finished one month later. In January 1996, the client called me in to evaluate the wall, which had already started to fail (sections had started to fall over). I took a look at it and told him to call an engineer. After many long meetings with the client and the engineer, we decided to dismantle the wall and rebuild it. Here's what we found when we tore into it:

First, instead of digging down deep enough to install a 12-inch base of crusher run and two courses of 8-inch block (a total of 28 inches), the wall builders had set their base course of block on 4 inches of crushed stone and explained that they would fill in in front of the wall with 8 inches of topsoil. They did not understand that the base needs to be buried in the "inactive zone" of consolidated subsoil so as to create a "passive wedge" strong enough to keep the base course from sliding forward.

Second, along the 9-foot-tall section of wall supporting the house and deck, the builders had rolled up the geogrid behind the wall so they would not have to remove the existing railroad tie wall. They just built the new wall in front of the old wall, which had also been failing in the first place (that's why a new wall was needed). There was no locked-together soil mass to support the retained soil and the superimposed loads.

But here's what really made me scratch my head: Instead of running the downspouts from the back of the house into solid pipe directed away from the wall, the builders had tied them into the drainage pipe behind the wall! Five downspouts deposited most of the home's roof runoff into the wall. And they could hardly have made a worse choice for backfill: They used blowsand, a cheap, poorly draining material that holds about 19% moisture. In effect, they designed this wall to have continuously saturated fill behind it.

The three most important things that make a wall strong and durable are base preparation, drainage, and reinforcement. In this case, all three were accomplished either shoddily or not at all. With any of these items poorly done, the wall would fail eventually; doing all three of them wrong was a recipe for immediate failure.

We had quoted a price of $38,000 to build the wall originally. When all was said and done, it wound up costing the client $64,000. The moral of the story: Build it right the first time.