Practical Soil Nail Wall Design and Constructability Issues

Practical Soil Nail Wall Design
and Constructability Issues
by
Walter G. Kutschke, P.E.
URS Corporation, Pittsburgh, Pennsylvania
Fred S. Tarquinio, P.E.
Nicholson Construction Company, Cuddy, Pennsylvania
William K. Petersen, P.E.
URS Corporation, Fort Washington, Pennsylvania
Presented at:
The Broadmoor
DFI’s 32nd Annual Conference on Deep Foundations
Colorado Springs, Colorado
October 11-13, 2007
Nicholson Construction Company
12 McClane Street
Cuddy, PA 15031
Telephone: 412-221-4500
Facsimile: 412-221-3127
07-02-157

PRACTICAL SOIL NAIL WALL DESIGN AND CONSTRUCTABILITY ISSUES
Walter G. Kutschke, P.E., URS Corporation, Pittsburgh, Pennsylvania, USA
Fred S. Tarquinio, P.E., Nicholson Construction Company, Cuddy, Pennsylvania, USA
William K. Petersen, P.E., URS Corporation, Ft. Washington, Pennsylvania, USA
Introduction
Four significant soil nail wall projects in the eastern United States were recently
completed with a combined area of 175,000 square feet of finished shotcrete surface.
These projects required the use of innovative design and construction methods in order
to address various challenges, including slide-prone back slope materials, perched water,
highly erodible rock materials, curved wall alignments, very tight construction tolerances
and unexpected subsurface conditions. Special focus is given to issues such as bench
excavation, soil nail installation methods, shotcrete mix design, anticipated shotcrete
quantities, shotcrete nozzlemen qualifications and weather conditions, in order to provide
a lessons-learned database for future soil nail wall design and inspector considerations.
These projects also underscore the importance of design engineer and soil nail wall
contractor qualifications as well as effective communication with the owner.
The intent of this paper is not to reiterate the
design and inspector guidelines presented in
FHWA (2003) and FHWA (1994), but rather to
present issues that occurred during the
construction of four significant soil nail wall
projects in the eastern United States. Although
each project is different with regard to geologic
conditions and design, these projects each
experienced some similar situations. These
similarities indicate a trend in design,
specification and construction of soil nail wall
projects. It is in these situations that lessons are
learned and presented herein.
The first soil nail wall project referenced herein
was part of a larger project involving the
construction of a new railroad alignment in the
relatively mountainous terrain of western
Pennsylvania. This project involved the
construction of over 9,000 square feet of soil nail
retaining structure with a total of 13,800 lineal
feet of soil nails and 428 cubic yards of
shotcrete. In addition, the project also required
construction of a shotcrete slope protection
system which used very similar construction
techniques as those employed for soil nail walls.
This effort required the placement of
approximately 90,000 square feet of slope
protection with a total of 34,000 lineal feet of
rock anchors and nearly 2,800 cubic yards of
shotcrete. This system is believed to be the
largest known application of such a system to
date. Refer to Kutschke et al. (2007) for further
details regarding this work.
The second project involved the construction of
a soil nail retaining wall used for the support of
excavation for the new Chinese Embassy
building in Washington, D.C. This project
consisted of the installation of over 50,000
square feet of exposed shotcrete wall surface,
approximately 1,600 soil nails and over 1,600
cubic yards of shotcrete.
The third project involved the construction of a
soil nail retaining wall for the support of
excavation for a new retail development in
eastern Pennsylvania. This project involved
10,000 square feet of exposed shotcrete surface
requiring 400 soil nails and 400 cubic yards of
shotcrete.
The last project also involved the construction of
a shotcrete wall used for support of excavation
for a new retail development in southwestern
Pennsylvania. This project consisted of 16,000
square feet of soil nail wall with approximately
600 soil nails and 800 cubic yards of shotcrete.
These projects were all successfully completed
and are in service.
Subsurface Conditions
Soil nail wall construction is not appropriate for
all soil conditions (FHWA, 2003). In the authors’
opinion, well-drained cohesive soils, such as the

esidual clays and weathered bedrock
characteristic of the Piedmont and Appalachian
regions, are ideal for soil nail wall construction.
The importance of soil type cannot be
overemphasized, as illustrated by the following
examples.
The ground condition had been severely altered
at one of the referenced projects just prior to its
construction. Following completion of the field
survey obtained for design activities, the original
property owner had excavated a haul road into
the hillside for log truck operations, with the road
alignment corresponding roughly with the
planned alignment of the soil nail wall. In order
to re-establish the original ground surface to
preclude backslope failures, the general
contractor backfilled this haul road cut with
poorly-compacted material having significant
fines content. As excavation for the soil nail wall
proceeded through this loose fill, which reached
a maximum depth of about 6 feet at the wall
face, a large degree of sloughing occurred,
particularly in response to disturbance from
drilling the nail holes. This condition was further
exacerbated by a heavy rainfall event from the
remnants of Hurricane Katrina. The changed
ground elevations and slope failures created
numerous problems related to the as-designed
wall geometry, and additional survey and design
effort was required to adapt the wall to fit these
changed conditions. Furthermore, a very large
volume of additional shotcrete was required to
fill in voids where sloughing had taken place at
the wall face. Since the backfilling operation
occurred before the soil nail wall contractor had
mobilized to the site, there was nobody present
on-site that was able to foresee the dire
consequences of this decision. A detailed preconstruction
meeting, involving the general
contractor, subcontractor and the engineer may
have prevented this occurrence.
Groundwater, surface runoff or perched water in
the wall excavation are likely to cause stability
and drainage problems during construction. In
addition, the shotcrete may have a problem
adhering to the excavated face if surface water
is present. This was a major problem in a small
area of the wall in eastern Pennsylvania. As a
result, large quantities of shotcrete were
required in order to maintain the alignment of
this permanent wall.
Excessive seepage can be detrimental to newly
placed shotcrete because it acts to wash the
cement off of the aggregate and to create
additional load resulting in minor cracking and
sloughing to complete loss of shotcrete
adhesion to the wall face. The most effective
means to address seepage at the wall face is to
control and direct the groundwater flow. The
placement of additional drainage geocomposite
and / or the use of PVC drain pipe to capture
and direct the drainage away from the newly
placed shotcrete have both been effective, as
displayed by Figure 1, where drainage in excess
of 80 gallons per hour was occurring at select
Figure 1 – Additional Wall Drainage for Seepage
Control
locations along the shotcrete face. Although this
is an extreme event, it demonstrates the
effectiveness of this approach. The
geocomposite drains and PVC drain pipe
effectively collected and diverted the flow away
from the slope face and allowed the placement
of shotcrete in this instance. It is important to
note that when these drainage measures are
employed, they are self-supported by securing
them to the slope face or steel reinforcement
rather than relying on the shotcrete for support.
Bench Excavation
15± gals per hr
80± gals per hr
Bench excavation heights are not only limited to
the stand-up time of the ground, but
consideration must also be given to the
nozzleman’s abilities. In order for the proper
application of shotcrete, the nozzle must be
perpendicular to the slope face. As the angle
between the slope face and nozzle increases,
the degree of compaction decreases with a
corresponding increase in rebound. Bench
heights beyond about 5 to 6 feet place additional
burden on the nozzleman and can result in
quality problems as the upper reaches are

difficult to shoot and finish properly. As such, it is
the author’s experience that limiting the bench
height to 5 to 6 feet enables a nozzleman to
properly and safely shoot the wall face and
upper overlap area. Figure 2 displays a typical
Figure 2 – Challenging Application of Shotcrete
due to Excessive Bench Height Excavation
situation when the bench height approaches this
upper limit. The congested reinforcement zone
in the overlap area requires particular attention
that is difficult and burdensome for this
experienced nozzleman. Also the ability for him
to blend in the final shotcrete layer is further
exacerbated as the rebound will significantly
increase at the upper reaches (note the
significant rebound that has all ready
accumulated at the base). Design must utilize
appropriate vertical distances between lifts
considering not only soil conditions, but also
practical heights between lifts as well as address
maximum permissible bench lifts in the
specifications. For comparison purposes, Figure
3 indicates an appropriate bench excavation that
will be much easier for a nozzleman to work
with.
Figure 3 – Appropriate Bench Height Excavation
Nail Installation
Air-track drilling is an economical drill method
when drilling into materials that do not require
casing to support the hole. Figure 4 illustrates a
typical air-track drilling operation.
Figure 4 – Typical Air-Track Drilling
Soil nail drilling production rates are highly
dependent on equipment and driller, but rates of
1 to 2 feet per minute are typical values in hard
clay or weathered rock. Nail hole diameters are
generally limited to 4 to 5 inches with air-track
equipment. Although these hole diameters
theoretically provide sufficient grout coverage
between the nail and bonding strata, the ability
of the driller to consistently create a straight
shaft is debatable. This consideration,
combined with the natural sag of the bar as it
deflects under self-weight between the
centralizer support points, will significantly
reduce grout coverage locally along the bar.
Therefore, a centralizer spacing of less than the
10-foot industry standard is warranted in
environments that require long-term corrosion
protection. Furthermore, centralizers should be
secured to the soil nails by tie-wire; methods
such a taping do not properly secure the
centralizer and can result in bunching of the
centralizers as the nails are inserted into the
hole.
Although air-track drills are an economical
means of advancing a soil nail drill hole, the
drilling operation can create significant
disturbance at the slope face resulting in
sloughing and soil break-outs. If this condition
persists, a drill berm is highly effective, as
shown on Figure 5.

Figure 5 – Prudent Application of a Drill Berm
Figure 5 displays the slope face, in this case
consisting of cohesive residual soil, after drilling
and grouting. The dashed line noted in the figure
represents the back-of-wall face. The use of a
drill berm in this situation prevented the
sloughing and general drill disturbance from
impacting the back-of-wall face. There would
have been substantial detrimental impacts to
this structure had a drill berm not be used.
Telehandlers or similar machines are commonly
used to lift a bundle of soil nails for the labor
force to insert them into the grouted drill hole.
Although this practice is acceptable, care should
be taken as the nails are lifted from the forks. At
no time should the inspector allow the nails to
slide against the fork and into the hole. This
needlessly exposes the nail to abrasion that can
create holes in the epoxy coating. Nails should
be manually lifted and inserted into the hole.
Structural Materials
The most important aspects of material design
and quality control with respect to soil nail walls
are the nail grout and shotcrete. Typical nail
grout consists of a cement and water
combination with approximately 0.45
water:cement ratio, having 28-day compressive
strength of at least 4,000 psi. However, it is
crucial in the timing of most soil nail projects to
have 3-, 2- or possibly 1-day strength results.
Shotcrete is generally applied using the wet-mix
process (FHWA 2003). This process generally
results in a higher volume throughput with less
rebound. Wet-mix application rates for these
projects were typically about 6 to 7 minutes per
cubic yard of shotcrete. Similar to the nail grout,
it is critical to have shotcrete compressive
strength results at 3, 2 or 1 day(s) in order to
maximize production without comprising the
integrity of the wall.
Proper mix design and adequate drainage are
paramount to the longevity of the shotcrete face
due to freeze / thaw cycling. Shotcrete slump is
largely self-controlling; too wet and it will slough,
too dry and it will not pump. A combination of
proper air entrainment and a low water:cement
ratio help provide adequate freeze / thaw
durability. Published literature indicates that loss
of entrained air during the pumping and spray
application is typically 4-5% (FHWA 1998).
Typical wet mix shotcrete designs require a
water:cement ratio no greater than 0.45 with
minimum air entrainment of 7 - 10%, measured
at the truck. Pozzolans, such as fly ash,
improve pumpability and will produce a more
durable shotcrete by mitigating the alkali-silica
reactions, increasing resistance to sulfate attack,
and reducing ingress of potentially deleterious
materials such as chloride and water. However,
fly ash has the potential to impact air
entrainment. Hill (2006) indicates that as the
loss on ignition value of fly ash increases, the
dosage of air entrainment chemical will generally
increase. Suitable material selection is essential.
Proper aggregate distribution is very important
with regard to strength and durability of the
finished shotcrete face, but also is very critical
with regard to pumpability and the ability of the
shotcrete mix to adhere to the excavated face.
Figure 6 represents the recommended range of
aggregate size distribution for a good shotcrete
mix, which is in general conformance with
FHWA (2003).
Percent Passing by Weight
100
80
60
40
20
0
100
10
1
Sieve Size (mm)
Figure 6 – Recommended Shotcrete Aggregate
Proportions
0.1

Shotcrete reinforcement is based on the
structural requirements of the soil nail wall. In
addition to this reinforcement, an additional layer
of wire mesh-type reinforcement can be added.
The wire mesh opening should be no smaller
than 4 inches, since smaller openings will
generally act to interfere with shotcrete
placement. It is suggested that this mesh
provides additional confinement to minimize
shotcrete sag as a greater thickness of shotcrete
is placed; however, its benefit is questionable
and lift thickness should be limited to 6± inches
even where it is used.
Wall drainage is paramount to the longevity of a
soil nail wall system. Geocomposite drainage
panels or strips are often used to provide
drainage. These materials are generally tacked
to the slope face with a reinforcing pin and
installed in shingle fashion as the excavation is
lowered. Drains are daylighted by means of a
weephole, and care must be taken to avoid
creating a low spot for water to collect. Weep
holes must be covered during application of
shotcrete to avoid clogging the drain. Extreme
care must be taken by the nozzleman to avoid
placing shotcrete behind the drainage panel and
therefore render it useless. As such, it is
extremely important that the drains are securely
fastened against the slope face prior to
shotcrete placement.
Shotcrete Installation
On most projects, the general contractor
performs the bulk excavation, and therefore is
required to provide the finished cut soil/rock
faces onto which the specialty geotechnical
subcontractor will apply the shotcrete. In most
cases, the general contractors on these projects
found it challenging to excavate the weathered
rock to the planned angles without significant
overbreak, as exampled by Figure 7. As a result,
it was necessary to completely fill the overbreak
pockets, in some cases as much as 3 feet deep,
with shotcrete in order to leave a fairly uniform
finished surface. From this experience, it is
suggested that contracts include a pay item for
excess shotcrete. However, it is also important
to note that this item can be a source of
contention, and thus pay items should be
reviewed and accepted by the owner as readily
as possible. A separate pay item for plain
shotcrete is advantageous because it does not
include incidentals such as reinforcement,
Figure 7 – Overbreak
bearing plates, drainage strips, etc. It is
emphasized that the owner should periodically
review the work conditions in order to gain a
level of confidence that additional shotcrete is
necessary, and that quantities are not
unjustifiably increased. Bid quantities should
include a reasonable contingent value in order to
minimize financial impact to the project. It is
suggested that this value is approximately 30%
of the overall estimated neat shotcrete quantity.
Experienced Nozzlemen
For shotcrete installation, especially for
permanent shotcrete walls or temporary walls
with tight horizontal tolerances, it is extremely
important to have experienced shotcrete
nozzlemen. These individuals are ultimately
responsible for the final product, and this work
requires a high degree of craftsmanship. Preconstruction
test panels are necessary to
evaluate the nozzlemen qualifications, and the
preparation of shotcrete test panels (Figure 8) is
Figure 8 – Nozzlemen Test Panels

a standard Quality Assurance practice carried
out in order to evaluate the qualifications of each
nozzleman prior to the beginning of production.
It is important to note in Figure 8 that the panels
are at the same approximate angle as the slope
face. Both reinforced and unreinforced shotcrete
panels are prepared using the shotcrete mix
proposed for use on the project. The reinforced
panels are cored for visual observation to
assess whether the nozzleman’s technique
results in uniform shotcrete distribution around
the reinforcement. Figure 9 indicates shotcrete
Figure 9 – Shotcrete Test Panel Cores
cores obtained from a test panel. Note that the
left-most core exhibits a significant build-up of
aggregate (rock pocket) behind the
reinforcement. This test panel was created by
an inexperience laborer and he was not
permitted to serve as a nozzleman.
The cores taken from the unreinforced panels
are generally tested for unconfined compressive
strength and boiled absorption. It has been
observed that, even among personnel that have
been approved for a given project, different
nozzlemen can produce a wide range of
shotcrete quality depending on their individual
experience and technique. Therefore, it cannot
be assumed that just because a particular
nozzleman demonstrated adequate
qualifications per the project specifications that
he will consistently produce high quality
shotcrete in production. Inspection staff should
be aware of poor technique and the inferior
shotcrete qualities that develop as a result. It
should also be understood that even the best
shotcrete nozzlemen will not produce a
shotcrete face that looks like poured concrete.
Shotcrete faces in general will be rough and
non-uniformly colored unless followed by floating
and colored with pigmented sealers.
The nozzleman and inspector must also pay
close attention to the bearing plate area as this
will act as a barrier if the plates are mounted
prior to shooting. Figure 10 indicates an
experienced nozzleman. Note how the nozzle is
near perpendicular to the slope face and
relatively low rebound.
Figure 10 – Experienced Nozzlemen
Proper curing of the shotcrete during cold
weather is extremely important. Shotcrete not
cured properly according to project
specifications can result in low compressive
strength and surface deterioration. In addition to
curing, the receiving surface must be free of ice
or other deleterious elements. Figure 11
indicates one method to pre-heat a receiving
surface during inclement weather.
Figure 11 – Cold Weather Operations
Obstructed from view under the tarp are a series
of torpedo heaters. Also note the insulation
blankets, adjacent to the blue tarp, which was
placed on relatively fresh shotcrete. Test panels

shot under similar circumstances confirmed the
suitability of this approach.
Quality Control/Quality Assurance
There are two basic elements for quality control /
quality assurance for a soil nail wall project,
namely:
1. The soil nail elements, specifically
unconfined compressive strength testing of
soil nail grout cubes and proof/verification
testing of the soil nails.
2. The soil nail face, specifically the unconfined
compressive strength and boiled absorption
testing of the shotcrete.
Compressive strength testing of grout is
relatively straightforward. Figure 12 indicates a
typical scatter of nail grout data. It is important to
UCS (PSI)
14000
12000
10000
8000
6000
4000
2000
DATA CRITERIA
0
0 20 40
TIME (DAYS)
60 80
Figure 12 – Typical Grout Cube Test Data Plot
note that some data points fall below the criteria
line and could result in rejection of soil nails.
Although this was cause for concern during
construction, there is no trend to support low
grout breaks, and the probable explanation for
these outliers is a defective cube (i.e., improper
curing, cracked cube, et cetera). Significant
discussions could have been avoided had this
cube been identified as defective and not
suitable for testing. Handling, curing, storage
and transportation of grout cubes is very
important, and proper care must be adhered to
for accurate results of test samples. Specific
gravity testing of the mixed grout using a mud
balance is important to confirm the mix design of
the grout, especially when low compressive test
grout break results occur.
Soil nail testing generally involves verification
and proof testing as outlined in FHWA (2003).
Generally, soil nail tests should be performed to
assess the overall nail resistance. Separating
and testing various geologic strata within the
length of a single nail is not recommended
because it can create unnecessary
complications. The important parameter is the
overall resistance offered by the installed soil
nail as compared to the design resistance
required by the soil nail load diagram developed
for a particular design.
Figure 13 displays a typical soil nail test set-up.
Figure 13 – Typical Soil Nail Test Set-Up
It is important that the inspector and contractor
coordinate test activities. Typically, an observant
inspector will select test nail locations based on
drill rig response, review of cuttings, or some
other geotechnical concern. The design
engineer, inspector and contractor must
understand the type of test and test loads and
then use this information to select an
appropriate sized soil nail bar to avoid
overstressing the nail during a test situation, as
might happen if a production nail was tested
under a verification test load.
The review and interpretation of the nail test
data is done in accordance with the project
specifications. Typically, two plots are
generated, namely a movement vs. load plot, as
exampled by Figure 14, and a movement vs.
time plot, as exampled by Figure 15.

MOVEMENT (INCH)
0.00
0.10
0.20
0.30
0.40
TEST DATA CRITERIA
0.50
0 10 20 30 40 50
LOAD (KIPS)
Figure 14 – Soil Nail Test, Movement vs. Load
MOVEMENT (INCH)
0.316
0.312
0.308
0.304
0.300
0.296
TEST DATA
0.292
0 10 20 30
TIME (MIN)
40 50 60
Figure 15 – Soil Nail Test, Movement vs. Time
Summary and Conclusions
Four significant soil nail wall projects were
recently completed with a combined retained
area of 175,000 square feet. The lessons
learned from these projects were:
1. Bench Stability – Although soil nail wall
construction is extremely versatile, its use
should be limited to appropriate soil types.
Consideration must be given to soil stand-up
time and groundwater conditions.
Constructability reviews during design must
consider nail spacing and address bench
height limitations in the project
specifications. Innovation is the key to
success when encountering difficult
conditions and several possible solutions
were presented herein for difficult ground
conditions.
2. Shotcrete Over-Runs – All soil nail wall
projects will experience shotcrete overruns if
the neat area/volume is used in the bid
tabulations. Voids and slope sloughing are
inevitable. Paying for shotcrete overruns
can become a source of great contention
between the engineer, owner and
contractor. Project specifications should
consider the use of a bid item with
contingent quantities for extra shotcrete,
with extra quantities in the order of 30% of
the neat volume. The owner needs to
understand and accept these quantities as
they develop.
3. Nozzlemen Experience – Nozzlemen are
ultimately responsible for the overall quality
of the finished shotcrete product. Their
craftsmanship results in the final aesthetic
appearance of a wall (when specifications
require a gun finish) and their skill attributes
to the structural continuity of the wall. From
a contractor’s perspective, the nozzelmen
are given significant financial responsibility
and they rely on their skill to apply the
shotcrete in accordance with the tolerance
noted in the specifications. Establishing
their qualification prior to production is an
industry standard that should always be
adhered to.
4. Experience and Communication – The
experience that each team member brings
to the project is vital to the success of a
project. An experienced design engineer
and contractor understand issues that are
important. It is this experience and
communication that can maintain schedules
and limit financial risk.
The issues presented in this paper are those of
the authors based on the referenced project
experience. Other soil nail and shotcrete
projects may not have experienced similar
issues.
Acknowledgements
The authors would like to thank the people from
URS Corporation, Nicholson Construction
Company and Weidlinger Associates, Inc. who
were involved in the design and construction of
the referenced projects.
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