Stabilization doesn't have to be an uphill battle.
In
their natural habitat, hills and slopes can pretty much do as they please; they
can allow vegetation or not, they can erode, and they can remain
impassible.
However,
hills and slopes that coexist with civilization have to conform: No landslides!
No rockslides! Hills surrounded by prime real estate may also find themselves
molded to humans’ needs. Fortunately for humans, today’s erosion control
products allow us to make mountains into molehills fairly
easily.
Turning Peaks Into Parking
Lots
Most
of the nation isn’t flat. Making a site buildable often requires a lot of
earthmoving and securing. If the native site also includes wetlands that must be
preserved, the challenge increases. Soil Retention Systems of Carlsbad, CA,
recently faced such a task at the site of a Target store in California’s San
Diego County.
The
20-acre site was “a hilly area,” says Mike Nelson, Target property development
senior owners’ representative. “Our stores have a large footprint and require a
flat parking area. Soil Retention Systems created an engineered-wall design for
our store in Vista.”
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Photo: Soil Retention Systems |
| Tons of soil moving was required to make this a buildable site. |
Slopes
were just part of the challenge. “One of the property’s sections does get wet,
and it’s protected. The piece of property chosen is considered a wetland area, a
natural drainage area, and we had to build walls right up to that. The natural
lay of the land drops 60 feet to the bottom of the site, and we had to make it
flat. One side, 25 feet of cut, had to be made level. This rise was 15 feet from
the road—I’d say it was a relatively difficult site.”
Making
the site usable requires building walls with a stepped effect. “We have double
walls, 7 feet high, with a space between, and another 20-foot wall. We had to
erect nine different walls, ranging in height from 2 to 15 feet, by the
wetlands. The site also has two entrances, so walls had to be built to create
the roads coming up to the store—two walls all the way up the road. That’s $1.5
million worth of walls! The walls are designed to handle earthquakes, and
geogrid is installed behind each one. We actually needed two different types of
designs, to resist earthquakes and
erosion,” Nelson explains.
Soil
retention wasn’t the only thing on the menu—a great deal of soil moving was
required as well. “We had 250,000 cubic yards of cut and fill, and we had to get
rid of 80 cubic yards of fill. If not for the walls, that figure would have been
300,000 cubic yards to remove! In addition, when preparing the site, we had to
get easements from five different owners.”
Parts
of the project are still in the works, Nelson notes. “I’ve discovered we have a
couple areas with environment-sensitive plants; we may have to redesign the
walls [in those areas]. I’ll make that decision [based] on what type of walls
are used; maybe a zero-design, which goes straight up from the base, no recessed
slant, will work.”
A
good percentage of the walls used will be Soil Retention System’s plantable
Verdura walls. “I handle the site work; this is a self-performed project. I have
to decide what kind of wall I want, hire soil engineers, and so on,” says
Nelson. “I like Verdura. It goes up quicker than some brands, and we can do
heavy compaction with equipment right behind it—ease of installation. We like to
plant it; most communities like that. Not only does it look better, but it adds
some soil security with roots to hold the soil. If you have a wall that’s 20
feet high, planting hides some of that; plants make it look more
natural.
“This
is the second time I’ve used them. In the past, we’ve used a couple different
kinds of retaining walls, but for the ease of installation, I like Verdura. I
must have had 20 or 30 miles of them erected in Colorado. These go in much
faster than many other brands,” he adds.
The
work continues. “This is a 20-acre site, which will contain a
132,000-square-foot store. There are two more outspaces for other stores; we
will sell the parcels. The site has a stormwater pond and an underground system
that catches, holds back, and slows down ‘rushes’ of water, which is then
releasable to the storm system.”
He
adds that the only delays experienced at the site were from the required
archaeological surveys: “We had to dig 15 to 20 feet down into the sandy clay
soil, and some things were uncovered.”
When
all the digging and building is done, the area must also be planted. “The plants
to go in must be native to this area; most likely, we’ll use a native scrub
brush.”
He
says, “It’s taken $8 million for the whole contract for the site—widening
streets, putting in pavement, landscaping fees, utilities, etc. The store is
extra!”
Securing and Saving an
Attractive Slope
Retaining
walls hold back a slope, but in some instances, a hill can be aesthetically
pleasing, so another form of slope stabilization might be used. For a Centex
Homes subdivision, Pagnini’s Inc. of Antioch, CA, had the task of stabilizing a
2:1 slope that would end up as part of the “natural
landscape.”
“We
put approximately 20 rows of Earth Saver Rice Straw Wattles on that 400-foot
slope,” says project manager and estimator Dustin LaMantain. “The specs called
for no more than 18 feet of space between rows; this project took about 2,500
wattles. The slope not only worked to prevent slides into the homes below, but
it’s also going to be a permanent feature of the
neighborhood.”
The
slope required wattles on all sides: “Rainfall takes any natural direction in
which the slope is built,” he says. “Wattles slow down the water’s velocity and
filter any sediment that could get into the stormwater
systems.”
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Photo: Soil Retention
Systems |
| Walls of several different heights were constructed. |
LaMantain
is confident about his choice. “Earth Saver wattles are top of the line. The
company has a unique way of making them. They’re packed so tight [that] they
hold to the specification of 9 inches. I don’t think some other brands of fiber
rolls use enough straw. When you unload them, the bottom five wattles are
actually only 4 inches thick from being compressed. They’re not true to the
specifications that the state and water quality boards ask
for.”
Earth
Saver Rice Straw Wattles are made from recycled, naturally weed-free California
rice straw, and are available with three types of netting: biodegradable,
photodegradable, and burlap. They are available in three standard sizes: 9
inches by 25 feet, 12 inches by 10 feet, and 20 inches by 8
feet.
“Earth
Saver wattles stay true to the weight. Yes, they used to be costly, but the way
the market is out here, they’ve gotten competitive in their pricing, so there’s
no reason why builders shouldn’t be product-specific.”
The
wattles’ weight and density also help keep them in place. “Earth Saver wattles
will stay in place. We measure distances, make sure the wattle is parallel to
the slope, dig a trench or furrow, put the wattle in there, and stake it down.
They need to be butted, or overlap—one behind the other—a foot or two, to be the
most effective. Then we hydroseed the slopes.”
Any
special challenges while the work was being done? “So much emphasis was placed
on the safety of the crew,” LaMantain says. “Some times we did have to anchor
our crews, due to the slope’s steepness, because gravity doesn’t cooperate. In
addition, the sandy, rocky soil didn’t make it very easy to stand on the hill
areas.”
Were
any other options considered? “Silt fences were one option; the developer and I
decided wattles were a better solution. A silt fence on a 2:1 slope wasn’t
logical.”
 |
Photo: Soil Retention Systems |
| The terracing effect was needed, as the lay of the land drops 60 feet. |
Revegetation
played its part in the stabilization. “Plants help filter out sediments;
wildflower seeds were in the mix, as well as high- and low-growing grasses. The
wattles are biodegradable; in two or three years, they’ll become part of the
slope.”
The
Centex project itself is still not complete. “They’ve had to do $2 million worth
of grading alone. When the homes go in, of course, the cost of the development
will go way up. Pagnini’s part of the project cost approximately $75,000, which
includes the hydroseeding and fiber rolls. The project took us about one week,
with a crew of seven guys. It was mostly manual labor—we could not use machines
on that slope. You do what you can to get wattles up the hill, then do what you
can to get them settled and tied down.”
Avoiding “Rock” and
“Roll”
In
some parts of the nation, more than just soil has to be contained. Soil erosion
can cause the movement of large rocks, which not only can tumble down to the
road and block traffic, but also can injure motorists.
During
a six-month project, DBM Contractors Inc. of Federal Way, WA, repaired 350 feet
of the roadsides along Arizona’s State Route 89A—an area prone to dangerous
rockfalls. The route, which runs through the Oak Creek Canyon, connects Sedona
and Flagstaff.
 |
Photo: DBM, Boulderscape |
| Tieback anchors being installed |
 |
Photo: DBM, Boulderscape |
| After anchors and structural facing were completed, the wall was finished with architectural shotcrete facing. |
After
centuries of erosion, Oak Creek Canyon’s steep slopes had developed natural
draws; these channels allowed debris and rock an easy path toward the roadway.
The solution, known as the Banjo Bill Rockfall Containment Project, included the
installation of two 90-foot-long by 30-foot-high retaining walls, which were
placed at the bottom of these channels, creating a basin to trap falling
rocks.
In
addition to its $7.5 million cost, the project faced other challenges. Because
of the canyon’s very steep slopes, the Arizona Department of Transportation
(ADOT) consultants had to design the state’s first anchored micropile wall
system to facilitate construction on the steep slopes.
The
project used 136 micropiles, placed at a 48-foot depth, which were installed
every 16 inches, along with four rows of permanent tieback anchors. The
micropiles were also supported laterally by anchored shotcrete facing. All of
these supports were put in place to anchor the wall system. As heavy machinery
was needed to construct all these supports, DBM built reinforced earth access
benches at each wall location. As the anchors and shotcrete facing were
installed, the team removed the temporary construction benches.
After
the anchors and structural facing were completed, the wall was finished with
architectural shotcrete facing that had been stained and carved. The resultant
manmade “rockface” blends in with the local stone
formations.
 |
Photo: DBM, Boulderscape |
| The culvert near completion. |
ecause
of the area’s sensitive nature, all of the work was overseen by both the
Forestry Department and ADOT. Foliage trimming and removal was strictly limited
to the work areas. Oak Creek’s turbidity was regularly monitored, and the local
spring, which is a residential water source, was also tested for any
contamination, especially the cement grout used in the project. Even wildlife’s
needs were taken into consideration; by scheduling construction between
September 1, 2007, and March 30, 2008, crews avoided the endangered Mexican
spotted owl’s breeding season.
As
the project’s general contractor, DBM was responsible for the micropiles, the
permanent tieback anchors, the structural shotcrete, and all of the excavation
work. Boulderscape of Capistrano Beach, CA, completed the architectural
shotcrete fascia; NCS Consultants LLC of Tucson, AZ, and URS Corporation of
Phoenix, AZ, provided engineering and design for ADOT; and additional
subcontractors assisted with traffic control and
surveying.
In
the spring of 2008, DBM Contractors also completed a $6.2 million emergency road
repair on Arizona’s US Route 70. Hired by the site’s co-owners, ADOT and the San
Carlos Apache Indian Nation, DBM repaired a culvert and also built a retaining
wall to prevent future incidents.
As
the highway remained in use, DBM first had to develop a plan to divert traffic
to a single lane around the 1,200-linear-foot work zone. When construction
began, existing culverts had to be removed, and the area to be contained
required a soil nail retaining wall. Ingress and egress access roads also had to
be installed.
DBM’s
project team fabricated and slip-lined three new 13-foot-diameter by
90-foot-long culverts through existing and deteriorated 15-foot-diameter
culverts. In addition, 200 yards of annular grout was added to each culvert. DBM
crews anchored the area with 415 permanent soil nails ranging from 15 to 43 feet
long, installed 15,000 square feet of permanent shotcrete, and built wing wall
ends of 30 to 65 feet. The site was quite a challenge; it contained very steep
(1.5H:1V) slopes with loose rocks, boulders, and cavy soils. Any water needed
had to be transported from a well 7 miles away.
With
restricted access between both sides of the highway, most work was performed
from manlifts. The project was also complicated by the weather; crews toiled
during the potential flash flood season, enduring record-breaking, 100-plus
degree heat. Excavated earth was placed outside the Salt Creek Basin; DBM also
scarified and replanted the 3-acre work area and restriped US Highway 70.