Anatomy of a Wall Failure
The near-catastrophic failure of a large retaining wall taught clients an expensive lesson-and provided one design/build company with an exceptional opportunity.
Nothing was staged here: This is how we found a Versa-Lok wall when the client called us in to inspect the work being done. It should be obvious that this is not the right way to build a wall.
Courtesy of Bruce Zaretsky
Allow me to begin with a disclaimer: While what you are about to read is true, the names have been changed to protect the innocent. And, frankly, to keep my company from getting sued. The facts are indisputable, and although the guilty have escaped without punishment, the clients have learned a valuable - and terribly expensive - lesson. Following my company's intervention, they also have a retaining wall that's as sturdy as the Great Wall of China.
Meet Bill and Kathy: Nice folks; easy-going and, until the winter of 1995, very trusting. They needed a retaining wall in their rear yard. Not just any retaining wall, mind you, but a big one. The project entailed removing an existing railroad tie retaining wall and replacing it with a precast segmental retaining wall system. The structure was to be a double-wall system, with a 4-foot (exposed) lower wall, a 4-foot (exposed) upper wall and two sets of steps linking the lower back yard with the upper. Keep that word "exposed" in mind. We'll need it later.
Like savvy, well-informed consumers, Bill and Kathy consulted three landscape companies to bid on the project. One of them was my company - Zaretsky & Associates Inc. - and one of them was a respected competitor. The last one was a company we'll call "We Don't Have a Clue Landscaping." The wall "designed" by WDHaCL Inc., which incorporated pre-designed plans provided by the manufacturer of the block system, was presented to Bill and Kathy with a quote that was significantly less than the other two. Guess who got the job.
The ABCs of wall construction
Before we go further, let's briefly review the basics. The construction of any retaining wall, no matter the size, is conceptually the same. There are three critical factors:
Please note that I didn't say "footer." The footing of the wall is the base material upon which the wall sits. The base of a retaining wall should be prepared by excavating to a predetermined depth (determined by the size of the wall) and placing crusher run to a predetermined thickness. Crusher run is the stone that forms the foundation of a driveway. Once the stone is applied, it is tamped to form a solid mass. (This is called compacting to 95 percent proctor density, a standard in the construction industry.) The base must be thick enough and stable enough to prevent settling of the wall.
Note: Bill and Kathy's wall called for 1 foot of crusher run below the base.
A long-range view of the wall shows the bowing; the pool the wall is supporting can be seen through the trees at left. Notice the storage shed is now leaning.
Deadmen or geogrid
When a timber wall is built, timbers are placed back into the slope, perpendicular to the wall. These deadmen use the weight of the soil to hold the wall up. (Picture your extended arm buried into a slope. Hard to pull out, right? Same concept with deadmen.) With a precast segmental wall, a product called geogrid is used for this purpose. The geogrid is a woven plastic product that extends from 4 feet up to 50 or 40 feet into the slope, depending upon the height and application of the wall.
Note: Bill and Kathy's project called for two levels of grid (6 feet and 8 feet long) for each wall.
In most climates, it is absolutely critical that water be kept from seeping behind the wall. We do this by backfilling the walls with crushed stone and perforated pipe; the pipes channel water away from the structure. Grading the area above the structure also serves to help keep the water away. In western New York, where we operate and where Bill and Kathy's project is located, the freeze-thaw cycle is a wall-buster. This wall, as any other, called for 4-inch perforated pipe, wrapped in fabric and covered with crushed stone to the top of the wall.
If a wall is built without any one of these factors, the wall will fail. Poor base preparation will cause the wall to settle. Insufficient geogrid will cause the wall to collapse from the weight of the soil behind it. Poor drainage will cause the wall to blow out from hydrostatic pressure and frost heave.
Sometimes failure will take time. It's sort of like an appliance warranty: After the warranty runs out, the appliance stops working.
Sometimes it happens pretty quickly.
Bill and Kathy's wall was built in November 1995. In January 1996, the wall started to fall. In just two short months, a $40,000 retaining wall began to crumble. What's wrong with this picture?
When Bill and Kathy asked me to take a look at the construction of the wall, I suggested they make two calls: one to an engineer and one to a lawyer.
Initial observation of the wall showed that there was no proper base preparation. Due to the size and configuration of the wall, it had been recommended that two courses of block be buried below grade, on top of 1 foot of crusher run. As installed, however, the base of the lower wall was placed on top of 4 inches of one-half-inch, crushed stone, which was set on top of the existing grade - not crusher run, not buried, not installed correctly.
The lower wall had not been backfilled with crushed stone, as specified; it was backfilled with blow sand, a cheap fill material, which has a moisture-retention rate of about 19 percent. The upper wall was placed on top of this sand, so it settled, and then fell down, because it had no geogrid. It was obvious that the original installers either didn't know or didn't care, but that's a moot point: The damage was already done.
Bill and Kathy had an engineer and me redesign the wall, according to the soil conditions and the manufacturer's recommendations. We quoted the project based upon this design. Our estimate to tear down and rebuild the wall with material already onsite was more than our original quote to build it with us supplying the material.
This wall is failing due to all of the reasons mentioned in the article. Water is being introduced behind the wall from over 1,500 square feet of impervious concrete paving behind the fence, in addition to running from the house's downspouts. Although the wall is over 8 feet tall, there is also no geogrid. The pea stone shown here used to be at the top of the wall-it has dropped because the wall is shifting out.
We expected teardown and excavation for the new wall to take one week. It took two. As we began demolition, we discovered that We Don't Have a Clue Landscaping Inc. tied four downspouts from the back of the house into their drainage. Remember the lesson on drainage? "Remove water from the back of the wall." Apparently WDHaCL Inc. was absent the day that lesson was taught. Their installation caused water from the whole rear roof of the house to drain into the retaining wall. (That one magnificent blunder amazes me more than anything else about this wall.)
And so you can imagine how wet it was behind the wall: There was the extra week.
As we continued the tear down, we found no geogrid on one whole side - this was the side that failed in two months. We did find grid on the other side, but because WDHaCL Inc. hadn't removed all of the railroad ties, the geogrid was folded up on itself 2 feet into the grade, not 8 feet as required. Wait ... railroad ties? Yes. The railroad ties were still there, because WDHaCL Inc. did not even excavate for drainage stone, which should extend an absolute minimum of 2 feet behind the wall - and preferably a lot more.
The wall and steps are now complete and ready for paving. Notice the #2 stone right up to the back of the wall.
Here's the scary part: This section of wall was 9 feet tall, and it was holding up a deck. After 18 years in this business, I thought I'd seen it all.
A happy ending
We finally removed all of the block and excavated for the lower wall to a depth of 28 inches below grade. This allowed for 12 inches of crusher run and two courses buried. Upon my recommendation, we had the engineer scheduled to inspect our excavation, our first layer of crusher, our second layer of crusher and the first base course. Bill and Kathy took no chances, and I don't blame them. Three weeks later, we laid the last block and cut in most of the caps.
After the first three courses, we backfilled with #3 crushed stone, tamping it in 6-inch lifts. It was critical to tamp the stone because the upper wall was going to sit on this stone. We laid in 6-foot lengths of geogrid and continued to build. After three more rows of block, we laid in 8-foot lengths of geogrid, on top of 1 foot of crusher run (base for the upper wall). We continued in this fashion until all 2,000 (give or take) blocks were installed.
Proper base preparation and drainage is shown here. Four-inch wrapped, perforated pipe is the standard drainage pipe used in these wall. Pipe and drainage stone are cheap: use lots of them!
The geogrid was then tied together at the back using #4 rebar, which was threaded through the grid and through helical piles, which were screwed 48 inches into the virgin grade well behind the wall and its backfill.
Each wall has two layers of drain pipe to drain away the water. We redirected the downspouts into their own pipes to carry the water far away from the base of the wall.
We took no chances with this wall, since one thing I didn't mention above was that not only was it holding up the deck, but it was also keeping a wing of the house from sliding into a ravine. I first quoted this wall at $38,000. When all was said and done, the homeowner paid $64,000 to rebuild it. That is a travesty.
A note about the photos
"Bill and Kathy," the clients whose retaining wall is featured in this article, weren't keen on having their epic property failure photographed. But we can still learn from the narrative, as well as a few photographic examples provided here. Good work is good work; the same principles apply.
It is now 16 years since we installed this wall. The last time I saw it was a year ago, and I am happy (and a bit proud) to say that it has not moved one millimeter, either vertically or horizontally. The lesson here is to know what you are doing, and act on that knowledge, not the desire to beat someone else out to get a job. Do it right the first time and clients and potential clients will notice.
Bruce Zaretsky is principal and owner of Zaretsky & Associates Inc., a design/build firm in Rochester, N.Y. He can be reached at firstname.lastname@example.org. And visit www.zaretskyassociates.com for samples of the company's work.