Soil
Nailing at the Home Depot
Construction of a new Home Depot in Gladstone, MO, near
Kansas City, had an interesting foundation problem. The site was limited
and highly contoured. The building required a store entrance at grade
level and a rear shipping/receiving area about 30ft. below grade. A
1,000ft-truck ramp along the side of the building was needed that would
drop the required 30ft and provide rear area storage. However, because
the site was tightly limited, the ramp would come as close as 25ft from
the property line. Generally, the most cost efficient means of
excavating a foundation is cut and slope, but in this case, the surface
could not be cut back enough to avoid slope failures without disturbing
the neighboring property. Therefore the soil had to be reinforced to
modify and control its behavior so that a vertical retaining wall could
be built requiring far less land.
TRADITIONAL METHODS
Traditional thought in US foundation engineering has
been to retain a wall either by cantilever, anchoring a reinforced
concrete wall into the strata below and depending on its own mass to
retain earth, or by horizontally tying back a thinner wall with
pretensioned anchors. Tiebacks are steel bars equipped with an anchoring
device drilled through the soil past its failure plane into unaffected
soil that has the capacity, like rock, shale, etc, to withstand the
desired load. The tiebacks are prestressed to prevent movement and
designed to hold the load of the full height of the wall. Minimum load
is considered to be the dead load of the soil weight in the slip
plane of the full wall height and the bars are designed accordingly. The
failure ratio for this soil was determined to be approximately
1:1. For a 30ft wall, the anchors would have to be embedded over
30ft through the failure zone plus an anchor distance into more solid
soil. This meant anchoring in the neighbor’s site. Easements were
needed but could not be negotiated, so a tieback wall design was
abandoned in favor of a cantilevered wall. The cantilevered wall
was designed to be 5ft thick and keyed 25 feet deep into the underlying
competent limestone.
ALTERNATE PROPOSAL
The Judy Company, a Kansas City geotechnical specialty
contractor, bid on the foundation work in conjunction with Gernot
Ueblocker, a geotechnical engineer then with Golder Associates, and now
with Ground Enhancement, Inc. They proposed an alternate method of
securing the retaining wall by “soil nailing” , a procedure that
would allow a permanent, relatively thin wall built and secured without
crossing property lines and without the expense of the excavation and
construction of a massive cantilever wall. The proposal saved
$1,000,000 over bids using the cantilevered wall approach, and the
method was sound and proven, and the Judy Company was selected as the
foundation wall contractor.
CONCEPT
Soil nailing reinforces the soil within a failure zone
to make it behave as a monolith, much the same way as lumber acts after
having been nailed together. The process is done from the top down, that
is, soil is excavated and secured, or “nailed”, in small lifts,
starting at the surface. Residual stresses in the soil of the slip
plane act on the grouted mass of bar in grout and “nails” itself.
The great advantage of the technique is that it allows soil retention in
very tight areas at any configuration and at competitive costs to other
proven techniques such as tiebacks Once a lift is secured it becomes
self-supporting, able to permanently hold itself in position as the
excavation continues. The use of structural bars as "nails"
has grown extensively in use in Europe over the past twenty years and is
rapidly gaining acceptance in North America
To give the advantage of soil nailing perspective,
consider the more traditional approach of tying back a wall. Long steel
beams called “soldier piles” are drilled and grouted or driven
vertically into soil before excavation the length of the entire wall
height. As excavation proceeds, walers and wood lagging are placed
between the soldier piles. The load plane is taken from the entire
wall height, and the bar diameter, length, and anchor are designed
accordingly. The bars are prestressed to engage the anchor and prevent
lateral movement. As mentioned earlier, the anchors must reach
completely through the soil failure zone until competent rock is
located, because all loading is transferred through the anchor, ignoring
the soil residual stresses. Only the ability of the anchor is considered
for withstanding load.
CASE STUDY
On the Home Depot site, the initial wall excavation was
a 6ft cut into the slope for the 1000ft length which included an inside
and an outside corner. Although the wall lifts were to be 5ft, the
initial 6ft included 1ft of overcut to allow a tie-in to the next,
lower, lift. A mobile drill rig on tracks was brought in and drilled
8inch holes at an angle laterally into the failure zone and at the
spacing specified by the designer. Eight inch holes are considered
extra large but were chosen to be used with 1" diameter bars to
present a very large friction surface to the soil.
The holes were then filled with
sanded cement grout delivered from a nearby batch plant, and the bars
were inserted into the wet grout.
The soil nail bar type chosen
was “multiple-corrosion” protected, standard for permanent soil nail
applications. At the factory, bars are epoxy coated, and a corrugated,
plastic sheath is placed over it and grout is inserted between the bar
and sheath, constituting two measures of corrosion protection, with the
third being the grout in the drilled hole. The bars were fitted with
plastic centralizers to ensure even encapsulation by the grout. The bars
were placed in the holes with about 6 inches protrusion, and a steel
plate with headed studs and nut were installed to integrate the wall
with the bar.
Next, a vertical face drainage
system was installed, which prevents hydrostatic buildup Hydrostatic
pressure can quickly increase wall loading beyond design parameters. The
drainage system consists of manufactured panels cut into strips placed
in vertical and horizontal directions along the excavation cut face,
allowing water to enter from either the surface or subsurface and
providing drainage to grade. The drains are held in place by small soil
anchors hand pushed into the soil wall. (photo of completed drain system
with mesh)
Welded wire fabric mesh was
then applied along the vertical cut face. Recall that the initial cut
was made at 6ft to allow for a 12in tie-in to the next lift, so
the mesh extended down below this section of wall by 12 in. After the
mesh was in place, the lower 12-in. was backfilled, covering the mesh.
With the mesh and drainage
systems secured, “hard-facing” was applied. Facing is a surfacing
coating designed to keep the soil facing in place. On most retaining
walls, a hard facing is applied which is usually a sprayed concrete or
“shotcrete.” However, in the case of slope protection or earth
retention, a vegetative cover can serve as ”soft facing”. For
example, a geotextile fabric can be used that allows grass or vegetation
to grow through it, as in the case of roadway side slopes.
In this case, a structural wall
was required, so hard-facing was required. A 6 inch thickness of small
aggregate, 4000psi, shotcrete was sprayed on the 5ft depth, covering the
mesh and bar ends, and then troweled for a finished wall appearance. The
lift was finished and considered to be self-supporting.
The next lift was then
excavated 6ft, exposing the overlap mesh from the first lift and another
5ft for wall construction. New holes were drilled, bars installed, and
the process was repeated for 6 lifts.
Once a new lift is cut, the retained soil from the upper
lift is allowed to move, loading the nails in tension. Movement was
measured at a maximum of 0.1in. / lift, and the total for the entire
completed wall was 0.3 inches.
Total area for the wall was approximately 20,000 sq.ft.,
with a length of 1000 ft. Each lift averaged just under one week from
excavation to shotcrete, and the entire project was completed in 40 days
despite working during winter months.
SUMMARY
To summarize, soil nailing was chosen as the most
appropriate technique for several reasons, the most important being:
-
the technique can be performed on a very tight
building site
-
relatively inexpensive construction costs- soil nails are relatively
short because they act in friction, using the soil residual stresses
in the slip zone to reinforce itself.
-
soil nails use the entire length of the bar to act
upon the soil.
-
most cohesive soils or broken rock soils are
adequate
-
no prestresssing,
saving labor and time
-
no foundation base- retaining walls are secured
laterally into the side soil, so there is no need for piles or
foundation footers
-
excavation and wall construction proceed together,
saving schedule time
-
no independent shoring system is required.
-
no concrete foundation walls need to be formed and
placed- walls are hard-faced by shotcreting over bar ends and
reinforcing mats and trowel-finished.
-
Soils nails can be inserted through different strata
of soils, minimizing the impact of changing geologic strata
-
The drainage system employed prevents water buildup
(pressure), eliminating hydrostatic forces.
Soil nailing requires a great deal of craftsmanship and
geotechnical knowledge to design and construct a system, but with the
right soil and site conditions, it is a rapid and economical means of
constructing earth retention support systems and retaining walls.
