SOIL NAILING: WHERE, WHEN AND WHY
A PRACTICAL GUIDE
By
Thomas J. Tuozzolo, P.E.
Vice President, Moretrench
Presented at the 20th Central Pennsylvania Geotechnical Conference
Hershey, PA, 2003
SOIL NAILING: WHERE, WHEN AND WHY – A PRACTICAL GUIDE
Thomas J. Tuozzolo, P.E., M.ASCE1
ABSTRACT
Since its development in Europe in the early 1970s, soil nailing has become a widely accepted
method of providing temporary and permanent earth support, underpinning and slope
stabilization on many civil projects in the United States. In the early years, soil nailing was
typically performed only on projects where specialty geotechnical contractors offered it as an
alternate to other, conventional systems. More recently, soil nailing has been specified as the
system of choice due to its overall acceptance and effectiveness. However, although the
theoretical engineering aspects of soil nailing may be well understood, there is a far lesser degree
of understanding, even within the geotechnical community, as to the site conditions – where,
when and why – under which soil nailing should, and should not, be used. The purpose of this
paper, therefore, is to offer experienced-based guidelines to owners, engineers, designers and
general contractors trying to decide if soil nailing is the right system for their project. Typical
soil nail details, procedures, design, monitoring and testing considerations, and case studies are
presented as a tool to aid in making those decisions.
INTRODUCTION
Soil nailing is a method of providing temporary earth support and retention during excavation for
new construction. The technique is also used for construction of permanent retaining walls, slope
stabilization, underpinning, and protection of existing cuts (Tuozzolo, 1997).
The soil nailing concept was developed in Europe for the permanent and temporary stabilization
of natural slopes, for renovation of old retaining walls, and for repair of earth walls that had
prematurely deteriorated. The first recorded application was in France in 1972. Soil nailing was
also used as temporary shoring for basement excavations and as permanent and temporary earth
support for excavations associated with railroads and tunnels (Chassie, 1993).
1
Vice President, Moretrench American Corporation,
100 Stickle Avenue, Rockaway, NJ 07866. Tel: 973.627.2100. [email protected]
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The first recorded use of soil nailing in the United States was in the mid 1970s as a method of
providing temporary earth support during the construction of a new hospital (Chassie, 1993). The
major use of soil nailing in the United States to date has been for temporary excavation support
and earth retention during construction of buildings and other structures in urban areas.
FUNDAMANTAL CONCEPTS
Soil nailing is an economical, top-down construction technique that increases the overall shear
strength of unsupported soils in situ through the installation of closely spaced reinforcing bars
(nails) into the soil/rock. Typically, a structural concrete facing (shotcrete) is sprayed against the
excavated earth face to connect the nails and reduce deterioration and sloughing. A common
misconception is that this structural facing is the major element of the soil nail system. In fact,
the nails do the work.
Soil nails are passive elements and, unlike tieback anchors, they are typically not mechanically
pre-tensioned after installation. Rather, they become forced in tension when the soil they are
supporting deforms laterally as the depth of the excavation increases. Without soil movement,
the nails remain in a passive state. This is a fundamental difference between soil nails and
tieback anchors, and one that is commonly not examined closely enough or considered during
determination of proper use and monitoring.
The movements required to mobilize the nail forces are very small and generally correspond to
the movements that occurs in a braced system (Chassie, 1993). In some cases, specifications
require that the soil nails be tensioned and locked off, as a tieback would. This is not correct.
Some specifications even require production soil nails to have a free, or unbonded, length. This
is also not appropriate. In some cases, a small pre-tensioned load may be applied to limit the
amount of deflection of the soil that is required to mobilize the nail. However, a major posttensioning effort defeats the purpose and effectiveness of the nail.
CONSTRUCTION METHODS
Soil nailing is a specialty geotechnical technique. Owners, specifiers and general contractors
should ensure that the work is designed and constructed only by firms with the requisite
experience.
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Typical Construction Sequencing and Components
•
Excavate in Lifts
The execution of soil nailing consists of making a 1.2 to 1.8 m (4 to 6 ft) vertical cut
extending for a horizontal length to be stabilized and shotcreted the same day. The vertical
cut may need to be reduced or other measures may have to be considered if the face
stability of the soils is of concern. Typically, the cut is made to 0.3 to 0.8 m (1 to 2.5 ft)
below the elevation of the soil nails so that a suitable bench can be established for the
installation of the nails at the proper angle.
Usually, mass excavation is performed on the project with relatively large earth moving
equipment. Smaller equipment may have to be utilized to provide the necessary face
trimming prior to installation of the soil nail wall. It is important that excavation of lifts is
performed in a controlled manner so that over-excavation of the face, and sloughing of the
soils, does not occur. After the lift has been excavated to the proper elevation, a level
work area, or bench, is constructed in front of the wall to accommodate the drilling
equipment. Depending on the type of equipment used, this bench can be as small as 3 m
(10 ft) or as large as 12 m (40 ft), although 7.5 m (25 ft) is usual.
•
Install Nail
Typically, the next step is to install the soil nails. It is important to note that the shotcrete
can be applied prior to installing the nails if there is a concern with the standup time of
the soil and the possibility of sloughing of the soil. In the United States, common practice
is to use rotary and rotary/percussion drill rigs for the installation of the nail, although it is
quite common to utilize augers in cohesive soils. In Europe, some contractors utilize
machines to drive the soil nails into the ground (launched nails). Soil nails are typically
installed on a 1.2 to 1.8 m (4 to 6 ft) horizontal and vertical grid pattern, with the drill hole
inclination being 10 to 20 degrees from horizontal. The holes are either cased or uncased,
depending on the type of soil and are, on average, 100 to 200 mm (4 to 8 in) in diameter.
Once the hole is drilled, the nail is installed and the hole is tremie-grouted with either type
I, II, or III Portland cement and water. The water/cement ratio is normally on the order of
0.45 to 0.50. In the United States, it is common to use No. 25 to No. 36 (No. 8 to No 11),
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Grade 400 or 500 (Grade 60 or 75) threaded bar as the nail. The bar is centered in the drill
hole by means of plastic centralizers spaced every 3 m (10 ft) along the length of the bar,
as shown in Figure 1. The length of the bar is based on the design, but a common rule of
thumb is that the length of the first level of nails is typically 0.6 to 1 times the overall cut
height. Soil-to-grout bond values are used in the design to determine the final length of the
nail and the nail spacing.
1 ft = 0.3048 m
1 in = 25.4 mm
Figure 1: Typical Temporary Soil Nail Cross-section
•
Place Reinforcing & Drainage
Once the excavation is made and the soil nails are installed, reinforcing material, typically
a welded wire mesh, may need to be placed along the face of the excavation to reinforce
the concrete facing. It is not uncommon, however, to use reinforcing bars for the length of
the wall in lieu of the wire mesh. As an alternate to placing the reinforcing prior to
shotcreting, the reinforcing material can actually be added to the ready-mix concrete at the
plant. This is known in the industry as fiber-reinforced concrete. Some common types of
reinforcing material are steel or synthetic fibers.
When the bearing plate and nut are installed and the nail becomes loaded as the soil
deforms, this load is transmitted to the shotcrete facing. Because of this, it is important to
ensure that the facing can handle the punching shear induced from the nail load. It may be
necessary to strengthen this area directly behind the bearing plate with reinforcing. A
typical punching shear detail is shown in Figure 2.
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1 in = 25.4 mm
Figure 2: Typical Punching Shear Detail
If drainage behind the soil nail wall is a concern, a prefabricated geotextile drain mat, typically
0.6 m (24 in) wide, can be applied in strips to the excavated earth face of the wall between the
nails prior to applying the shotcrete facing (Figure 3). In most cases, drainage material is not
installed for temporary soil nail walls unless drainage is a major concern. For permanent walls,
the drainage material is extended from the top of the excavation to the bottom of the excavation
and tied into a drainage collection system at the bottom of the wall. If perched water is
encountered during construction, weep holes and small horizontal relief drainage pipes can be
installed.
1 ft = 0.3048 m
Figure 3: Soil Nail Drainage
•
Shotcreting & Installing Bearing Plates
Once the reinforcing and any required drainage medium are placed, the next step is to
apply the shotcrete facing. For most temporary shotcrete walls, this is accomplished by
applying a 75 to 100 mm (3 to 4 in) thick layer of 21 MPa (3000 psi) concrete. In a
temporary application, shotcreting is merely a method of tying the system together,
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providing a cover for the soil, and retaining the soil between the nails. The shotcrete can,
however, become more critical if the nail spacing increases. Immediately after the
shotcrete is applied, the soil nail bearing plates are installed on the fresh shotcrete facing
and their nuts are hand-tightened. It is very important to make sure that the bearing plates
and nuts are installed as soon as possible, but definitely before the next cut is made since
this is what provides the lock off of the nail as the soil deforms.
•
Repeat Steps to Final Subgrade
After the shotcrete has been installed and the soil nails have cured for three days, the
process is repeated in lifts until the predetermined subgrade elevation is reached. Typical
temporary and permanent soil nail details are presented in Figures 4 and 5.
1 in = 25.4 mm
Figure 4: Typical Temporary Soil Nail Detail
1 in = 25.4 mm
Gr. 60 = Gr. 400 SI
Gr. 75 = GR. 500 SI
Figure 5: Typical Permanent Soil Nail Detail
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Permanent Facing
Although soil nailing began as a method of providing temporary earth support, it has evolved as
an economical method of providing a permanent, finished wall. Since soil nailing is a very
flexible system in that there is no need to make 90° corners or straight walls, it is very easy to
provide a soil nail wall with curves and bends. If the final product calls for such geometry, more
than likely the permanent facing could be shotcrete (Figure 6). Once the temporary shotcrete
wall is constructed, a permanent facing typically involves adding another 100 to 150 mm (4 to 6
in) of shotcrete to the existing 75 to 100 mm (3 to 4 in) of shotcrete. The permanent facing will
be reinforced and a screed face finish can be provided. In lieu of using shotcrete as the
permanent facing, there are several other choices. A one-sided concrete wall can be poured
against, and attached to, the soil nail wall, or a variety of concrete block or precast panel systems
can be mechanically attached to provide a wall that will blend into any retaining wall system
(Figure 7).
Figure 6: Permanent Shotcrete or CIP Wall Facing Detail
1 in = 25.4 mm
Figure 7: Segmental Block Facing Detail
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Corrosion Protection
For temporary systems, single-corrosion protection for the soil nails is normally adequate. The
nail is simply a bare bar, with sufficient grout cover achieved by providing centralization of the
nail in the drill hole. For permanent soil nail systems, corrosion protection has to be examined
more closely. In non-aggressive soils, the nail should be provided with an epoxy or galvanized
coating as per standard manufacturer specifications. In more aggressive soils, fully encapsulated
nails are recommended. Typical tieback guidelines are normally followed to determine the
appropriate corrosion protection.
WHERE, WHEN AND WHY
Soil nailing is a very versatile geotechnical construction technique with many advantages over
other types of temporary and permanent retaining walls. Soil nailing can be used on numerous
sites to provide an economical and fast method of temporary earth support, underpinning and
stabilization. However, there are many factors to be addressed when determining if soil nailing
is the right system for a project, and it is very important to understand the dos and don’ts of soil
nailing before it is recommended. When evaluating where soil nailing is appropriate, the
following guidelines can be used:
The Right Underground Conditions
As with any geotechnical technique, the underground conditions are normally the paramount
consideration. A good geotechnical exploration and laboratory testing program is essential. As a
minimum, this information should include soil borings, groundwater data, sieve analysis, and
plasticity testing. Whenever possible, test pits excavated with a backhoe are recommended to
evaluate the stand up time of the soils prior to production work.
Underground Conditions that are Conducive
In general, soil nailing can be used in any soil where a vertical cut can remain stable for at least
24 hours. These cuts should be on the order of 1.2 to 1.8 m (4 to 6 ft), which is standard practice
for soil nail lifts. Cuts less than this reduce the economical advantage of soil nailing. Normally
soils that have some binder in them are considered the most favorable, such as:
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1. Natural cohesive materials, such as silts and low plasticity clays not prone to creep.
2. Glacial till.
3. Cemented sand with little gravel.
4. Find to medium sand with silt to act as a binder.
5. Weathered rock.
6. Residual soils.
Underground Conditions that are Not Conducive
The soil nailing system is not recommended if any of the following conditions are present:
1. Urban fills and loose, natural fill material.
2. Soft, cohesive soils that will not provide a high pullout resistance.
3. Highly plastic clays, since they are susceptible to excessive creep.
4. Loose, granular soils with N-values lower than 10 to 15.
5. Soils without any apparent cohesion and with high gravel content.
6. Expansive clays.
7. The presence of groundwater. Groundwater presents a problem with maintaining face
stability during the cut between lifts, and must be properly controlled in the
construction of a soil nail wall.
Most Effective Applications
Although soil nailing is a very fast and economical earth retention system, the most cost
advantages are realized in situations where other, more conventional, techniques have
limitations. Soil nailing offers the most advantages during the following applications:
Difficult Soil Conditions: In some situations, the underground conditions may not be conducive
to driving soldier piles for a conventional, tied-back, soldier pile and lagging wall. The presence
of cobbles and boulders or the presence of rock above subgrade may require the soldier piles to
be installed with large, costly drilling equipment. In general, this will greatly increase the overall
unit price, thus reducing the cost effectiveness of the system. However, since soil nails are
typically short, small-diameter, drilled-in elements, the cost of installation generally does not
significantly increase under these conditions.
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On-line Systems: On some projects, due to space limitations, the foundation wall for the
proposed structure needs to be within millimeters of an adjacent structure or property line. Since
most conventional earth retention systems have a general width of 0.3 to 0.6 m (12 to 24 in),
installation would be impossible under these circumstances. Other options, such as concrete
underpinning pits can be very costly, particularly in difficult, dense soils.
In the above
situations, soil nailing can provide large savings. Since temporary soil nail systems are normally
75 to 100 mm (3 to 4 in) thick, they will not encroach into the new structure as other systems
would, and since soil nail walls can be designed to withstand surcharge loads, they can be
installed adjacent to existing structures.
Low Headroom Conditions/Tight Access: On some projects, earth retention may be required in
low headroom conditions or within tight access. One such situation would be the widening of an
existing highway, where the new portion of the highway must be constructed while an existing,
overhead bridge remains active and the overhead bridge girders remain in place. With the
development of compact, low-headroom drilling equipment the soil nailing system can be more
easily installed under these conditions whereas previously a much more expensive system, such
as driving and splicing soldier piles or sheet piling might have been the only option.
Permanent Walls: While soil nailing has become a highly effective system for retaining cuts
during construction of new structures, its use for the economic installation of permanent
retaining walls is still evolving. Since soil nail systems are already designed to withstand
temporary lateral loading, designing the wall to withstand permanent lateral and vertical loading
is a rather simple process and although unit costs will increase, the overall wall cost will still be
less than the cost of installing a new foundation wall.
Soil nailed walls are much more “flexible” than conventional, rigid walls. Thus, they perform
better during a seismic event. These structures can conform to deformation of surrounding
ground and can withstand larger total and differential settlements in all directions.
Areas of Concern
In the following situations, soil nailing should be ruled out or additional measures should be
taken:
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Loose Fill & Sandy Soil Conditions: Since soil nailing is a top-down construction technique, it is
essential that the cut face can stand up long enough to install the nails and the shotcrete facing.
“Pushing the envelope” and installing soil nailing where this is not achievable is not
recommended and can result in face stability failure.
Below the Water Table:. The presence of flowing groundwater will limit the stand up time of the
cut face and can also cause face stability problems. In addition, if the installation is successful
there is the added consideration of hydrostatic loading on the soil nail wall and additional
drainage requirements.
Outside Temperature: As for any concrete application, and as per the ACI code, the shotcrete
should be applied when the ambient temperature is 4.5° C (40° F) and rising. The construction
of the soil nail wall below such temperatures is only achievable with an imported heating system
and should be a major consideration during evaluation of use of the soil nail system. Temporary
heated enclosures and tents can be constructed to keep the temperature above 4.5° C (40° F), but
this can dramatically increase the overall construction cost of the soil nail wall.
Utility Interferences and Easements: As for tieback construction, a major consideration when
specifying or choosing a system should be the location and proximity of adjacent underground
utilities and adjacent property limits. Existing utilities may cause interference during installation
of the nails, and easements may be required if the nails will extend onto adjacent property.
Expansive Clays: As stated previously, clays that have the potential for expansion or are
susceptible to creep should be approached cautiously. An additional monitoring and testing
system should be implemented prior to production work to verify the movement of the nails and
the wall during construction.
Large Surcharge Loads: Soil nail walls can be designed to withstand significant exterior
surcharge loads (traffic, equipment and building surcharges, and so forth). Nevertheless, it is
important that the potential impact of these loads is understood and addressed prior to design
finalization, since they can dramatically impact the overall design and the future performance of
the soil nail system. Large wall deformations can result in significant settlement, and this should
be considered in the design of the soil nail wall.
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DESIGN, MONITORING AND TESTING
Design
The installation of a soil nail wall results in the construction of a composite, coherent mass
similar to that of a mechanically stabilized earth (MSE) wall. Although soil nail walls and MSE
walls have inherent differences in both construction and performance, they are similar in the
design approach. The reinforced soil mass is separated into two zones, an active zone and a
resistant zone. The active zone, or potential sliding wedge, is close to the face, where the lateral
shear stresses are mobilized, thus increasing the tension force in the nail and increasing the
pullout force. The resistant zone, or steady zone, is where the nail forces are transferred into the
ground. Ground movement during and at the completion of construction is restrained by the
frictional bond between the soil and the reinforcing element. Therefore, the most important
design consideration is to ensure that the soil nail interaction is effectively mobilized to restrain
ground movements and to ensure lateral stability with the required factor of safety for the
particular project (Sakr and Kimmerling, 1995).
In the early years, the most common methods used to design soil nail walls were derived from
classical slip surface limiting equilibrium design methods of slope stability analysis, modified to
incorporate the additional shearing, tensile and pullout resistance provided by the nail
reinforcement crossing the failure surface. These methods of analysis evaluate “internal” factors
of safety along potential failure surfaces throughout the soil mass.
The failure surface is
assumed to be either bi-linear, parabolic, circular, or a logarithmic spiral. The most common
methods used were the German Method (Stocker et al., 1979), the Davis Method (Shen et al.,
1981) and the French Method (Schlosser et al., 1983).
In the United States, two other methods have been developed and are commonly used for the
design of soil nail walls. These are the SNAIL method, developed by the California Department
of Transportation (CALTRANS, 1991) and the GoldNail design method developed by Golder
Associates of Redmond, Washington. The SNAIL method uses a bi-linear wedge analysis for
failure planes existing at the toe of the wall and tri-linear forces for failure planes developing
below and beyond the wall toe. The Golder method can analyze circular failure surfaces. Both
methods consider the tensile resistance of the nails crossing the failure surface. These two
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methods are considered improvements over the other earlier methods in that they consider the
limiting pull-out capacity of the nails and take into account the wall facing attached to the soil
nails as part of the system.
Monitoring
The FHWA’s Manual For Design and Construction Monitoring of Soil Nails (Byrne et al., 1996)
estimates that for a typical, vertical soil nail wall, designed with a reasonable factor of safety, the
peak wall displacements at the top of the wall tend to vary from 0.1%H (cut height) or less for
weathered rock and very competent and dense soils, to 0.2%H for granular soils, and up to
0.4%H for fine-grained, clay type soils. These displacements are comparable to those
experienced by other types of retaining systems such as tieback walls, which generally average
maximum lateral movements of 0.2%H (Byrne et al., 1996).
As with any retention system, some type of monitoring should be conducted to determine how
the system is performing or if additional measures need to be taken. The type of monitoring
program that should be implemented is directly related to the type of system being installed. For
temporary systems, monitoring can be performed by simply conducting daily or weekly
surveying with a typical transit and level to determine the horizontal and vertical movements that
have occurred, and which allows an approximation of the deflection of the system. Typically,
this type of device is accurate to 1.6 mm (0.0625 in).
When permanent soil nail walls or deeper temporary walls are constructed, it is prudent to install
inclinometers in addition to normal survey monitoring to measure lateral deformations. The
probe is inserted into near-vertical, grooved casing of known orientation. Typically, the grooves
are set perpendicular and parallel to the object in question. Readings are taken at set intervals
while the probe is being extracted. The resultant data is a set of tilt readings at known depths
that can be plotted to show the overall lateral deflection of the casing and subsequent
surroundings.
In addition to monitoring for deflection, there are some circumstances where determination of
the actual stress that the nails are experiencing may be useful and important. Such information
can be obtained by using vibrating wire strain gages, small instruments used to measure strains in
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steel structures such as bridges, piles and tunnel linings (Figure 8). Measurements are developed
by plucking the internal tensioned wire at its natural resonant frequency. Variations in the
frequency correspond to variations in the strain, which is converted and displayed on a readout
box. This type of monitoring is costly. Thus, it is typically used only on critical structures and
for research and development.
Figure 8: Strain Gage Installation Plan and Section
(After FHWA-SA-96-069 Manual for Design and Construction Monitoring of Soil Nails)
Field Testing
It is also important to implement a field-testing program to verify design assumptions. Field
testing also verifies that the equipment used on site is providing the assumed specified design
parameters of drill hole diameters, drill lengths, shotcrete thickness, and so forth.
Nail Testing
Testing performed on soil nails includes pre-production testing, verification testing and proof
testing. The type of testing can be directly related to the nature of the soil nail wall. For
instance, pre-production testing may be required only on a permanent wall to aid in the final
design.
Although the type of testing may differ somewhat, there is always some type of
verification required. Nail testing is performed with a center hole hydraulic jack and power pack
pump, similar to that used in tieback testing. As for tieback testing, the load is incrementally
applied as the movement of the nail is determined by using a dial gauge capable of measuring to
the nearest 0.0254 mm (0.001 in).
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•
Pre-production testing is performed on non-production or sacrificial nails.
The pre-
production test helps to determine the ultimate adhesion capacity as it compares to the values
used in the design. It is sometimes not possible to achieve ultimate pullout of the nail.
•
Verification Testing should be performed on at least one nail per row on all projects. The
verification test (pullout test) is performed on a shorter, non-production nail. The verification
nail should have a short unbonded (free) length so that the nail can elongate during testing
and ensure that the load is transferred to the bond zone. The bond zone is purposely made
shorter so that better bond values can be achieved and possibly bond failure will occur so that
the ultimate values are achieved (see Figure 13). The verification test is performed
incrementally, generally to a factor of at least two times the design value. The results
achieved are compared to the design, and changes in nail length or spacing can be made if
required.
•
Proof Testing should be performed on production nails. Typically, five percent of the total
number of production nails should be proof tested. These proof tests are not measuring
elongation as a performance test of a tieback would, but merely verifying that the bond is
achieved and that creep is not a concern.
•
Long Term Creep Testing can be performed in addition to proof testing when creep is a
concern. Fine-grained soils or plastic soils should be creep-tested. Deflection versus log time
results are plotted on a semi-log graph and are compared against the acceptable criteria.
Shotcrete Panel Testing
In order to verify the compressive strength of the in-place shotcrete for permanent wall facings, a
test panel is prepared from which samples can be extracted, rather than extracted from the wall
itself.
ACI 506.2-95 Specification for Shotcrete recommends ASTM C 1140-98 Standard
Practice for Preparing and Testing Specimens from Shotcrete Test Panels. Forms are
constructed from either steel or wood having a minimum width and length of 0.6 m (24 in) and a
minimum depth of 90 mm (3.5 in). The panels are prepared using the same reinforcement,
equipment, mix design and shooting orientation (slabs, slopes, vertical, overhead) as the actual
application. Panels are shotcreted either before or during the production phase of the shotcrete
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wall. The panel is then cured in the field or in a moist room until ready for testing. Cores are
drilled from the panel and tested in compression perpendicular to the surface of the panel.
Sawed cubes and beams may also be prepared from the panel and tested either perpendicular or
parallel to the surface. Testing is done at 7 and 28 days, unless project requirements specify
otherwise.
CASE STUDIES
Robert Wood Johnson Hospital - Cancer Center Addition
New Brunswick, New Jersey
Underpinning
Construction of a Cancer Center addition to the Robert Wood Johnson Hospital, in New
Brunswick, NJ, required excavation to hard rock at up to 7.6 m (25 ft) below existing grade. The
46 m by 61 m (150 ft by 200 ft) excavation site was bounded on two sides by hospital buildings
and on the other two sides by an emergency access road and a busy thoroughfare. Earth retention
and underpinning to protect the integrity of the structures and roadways was an integral part of
the project design.
Conventional Approach
A drilled-in soldier beam and lagging system, in conjunction with conventional concrete
underpinning, is a common and effective approach to this type of work in the northern New
Jersey area. However, evaluation of the subsurface conditions showed a soil profile of 3 m (10 ft)
of residual overburden soil with good standup time, below which weathered rock extended to
bedrock. These conditions are ideally suited to soil nailing, where underpinning and excavation
support can be achieved in a single operation.
Value Engineered Alternate
For this site, the geotechnical contractor calculated that soil nailing would be as effective as
conventional support systems, yet less expensive and faster to install. Since the new structure had
to be constructed within 100 mm (4 in) of the existing hospital, the soil nail system offered both
an economical method of providing the necessary underpinning and an on-line earth retention
system into which the new foundation wall could be tied. The geotechnical contractor proposed
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this value-engineered, alternative design to the excavation contractor and the construction
manager for the project. After evaluating the constructability of the system, it was decided that
soil nailing would be an innovative and economical solution for the site conditions.
Design and Installation
In unobstructed areas of the site, soil nails were installed at 15 degrees below horizontal on a 1.5
m (5 ft) grid pattern. However, a 0.6-m (24-in) diameter sanitary sewer that served the entire
hospital complex ran directly beneath existing foundations where the soil nail system had to be
constructed on-line and act as the form for the new foundation wall (Figure 9). Compromising
this essential utility would effectively shut down all hospital services and lead to very large
consequential damages. The geotechnical contractor investigated the precise location and
elevation of the sewer and adjusted the angle of the nail installation accordingly. In addition, a
very loose sand backfill, most likely used during the construction of the utility, was encountered
during the excavation of the upper soil nail lifts. Since this backfill did not exhibit good standup
time, the lifts were reduced to 0.6 m to 1m (2 to 3 ft).
Elevations in
feet.
1 ft = 0.3048 m
Figure 9: Cross-section Showing Utility and Soil Nail Installation
Three to four tiers of soil nails were installed, depending on the depth at which hard rock had
been encountered at the time of the geotechnical investigation. For each tier, a vertical lift of 1.2
to 1.5 m (4 to 5 ft) was excavated, extending for a horizontal length that could be stabilized
17
during the workday. Other than where backfill was encountered, the cut face exhibited good
stand up time, allowing the soil nails to be installed first across the entire face of the cut. A
geocomposite drainage strip and reinforcing wire mesh was then placed, and a 75-mm (3-in)
thick shotcrete facing was applied to bridge the soil nails and complete the structural
underpinning and retention system. The 1300 m2 (14,000 ft2) soil nail system was successfully
completed three weeks ahead of schedule, resulting in significant savings.
Route 87 & 287 at Meadow Street
Tarrytown, New York
Bridge Widening
Upgrading of Route 87/287 from two to four lanes to accommodate increased traffic flow
required the installation of a replacement bridge abutment approximately 1 m (3 ft) in front of
the toe of the embankment supporting the existing west abutment at the Meadow Street overpass.
Since the new abutment foundation would be up to 4.3 m (14 ft) deeper than the existing
abutment foundation, an excavation support system would be required to support the
embankment soils beneath the existing abutment during this work. The geotechnical contractor
was awarded a subcontract by the general contractor for New York State DOT to design and
build the earth retention system.
Soil Nail Design and Installation
The embankment soils consisted of silty glacial till with large boulders. Given the presence of
these boulders, coupled with the low headroom working conditions beneath the overpass, the
geotechnical contractor elected to use soil nailing to support the soils. With the active abutment
bearing directly on soil rather than deep foundations, the soil nail wall was designed to take into
account the large vertical surcharge of the existing abutment as well as lateral earth pressures
from the embankment behind the abutment.
Three levels of soil nails were installed on a 1.4 m H by 1.5 m V (4.5 ft H by 5.0 ft V) grid to
support the 4.3-m (14-ft) deep, 61-m (200-ft) long embankment (Figure 10). Given the nature of
the embankment fill, the geotechnical contractor chose to first reinforce the cut face with wire
mesh and a 75 to 100-mm (3 to 4 in) thick layer of 20 MPa (4000 psi) shotcrete. After the
shotcrete had cured, the soil nails were drilled and grouted in place. Specialty drilling techniques
18
were required to penetrate the large boulders during soil nail installation. Sacrificial test nails
were used to verify bond values. The soil nail wall was installed in one mobilization, with no
removal of overhead superstructure required.
Elevations in
feet.
1 ft = 0.3048 m
Figure 10: Cross-section Showing Installed Soil Nail Wall
University of Medicine & Dentistry
Dental School Expansion
Newark, New Jersey
Earth Retention
Expansion of the Dental School at the University of Medicine & Dentistry of New Jersey
(UMDNJ) in Newark, NJ, included construction of a five-story, steel structure with a one-story,
below-grade basement. The site was bounded on one side by the existing Dental School, and on
the opposite side by 12th Avenue, a major access road for emergency vehicles en route to the
adjacent University Hospital. Parallel, underground, electric and telephone duct banks, each 1.5
m by 0.6 m (5 ft by 2 ft), ran along 12th Avenue. The telephone bank lay nearest the site, 1.5 m
(5 ft) below existing grade and within millimeters of the proposed building.
The proposed structure, approximately 85.4 m by 36.6 m (280 ft by 120 ft) in plan, entailed
excavations to a depth of 8.2 m (27.0 ft) below grade. In order to facilitate these excavations, a
temporary earth retention system was required for approximately 80.8 lm (265 lf) along 12th
Avenue and for 38.0 lm (125 lf) along the west side of the site. Also, a permanent retention and
underpinning system was required for 18.3 lm (60 lf) on the south side of the project, along the
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existing, 3-story Dental School, where the new building would be constructed on-line and tied in.
Preliminary excavation support system concepts included driven steel sheeting and soldier beams
and lagging along 12th Avenue and the west side, and a permanent, on-line, soil nail wall along
the existing Dental School. However, the proximity of the duct banks along 12th Avenue
precluded the sheeting and soldier beam options since both systems would encroach into the new
building and would require the structure be moved and redesigned. In addition, the soil borings
encountered rock at or just above the proposed subgrade elevation, which could impede the
driving of soldier piles or steel sheets and likely necessitate a more expensive, drilled-in system.
Engineered Solution
The pre-construction geotechnical investigation indicated that the subsurface conditions
consisted of 1.2 m to 1.8 m (4 ft to 6 ft) of fill, underlain by natural silts and silty sands
containing varying amounts of gravel, cobbles and boulders. Weathered sandstone bedrock was
encountered at or above the subgrade elevation of the proposed structure. After evaluating the
soil conditions, space limitations and the schedule, the geotechnical contractor offered a system
that entailed a temporary soil nail wall along 12th Avenue and the west side of the site, and a
combination of drilled-in minipiles and soil nailing along the existing Dental School. The
minipiles would be drilled through the existing column footings to underpin the building and
transfer the loads below the proposed subgrade. Soil nailing would be used to support and retain
the soil between the existing columns.
12th Avenue and West Side
The soil nail wall along 12th Avenue and on the west side of the site entailed three to five levels
of soil nails installed at 15 degrees from the horizontal on a 1.5 m (5 ft) grid pattern. A 75-mm
(3-in) thick layer of shotcrete, reinforced with wire mesh, was used to tie the system together and
retain the soils between the nails. Construction began with the excavating contractor making an
initial, sloped precut to locate the top of the telephone duct bank and remove the existing fill
soils which did not exhibit good ‘standup’ time. The precut also allowed the first level of soil
nails to be installed at a 15 degree angle below the duct bank, and did not require any redesign or
relocation of the proposed structure. Once the first level was installed, and the soil nails and
shotcrete had cured, the process was repeated and the remaining lifts were installed until
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subgrade was reached (Figure 11). A view of the soil nail wall along 12th Avenue is presented in
Figure 12.
Elevations in
feet.
1 ft = 0.3048 m
Figure 11: Cross-section Showing Soil Nail Wall Installation
Figure 12: View of Soil Nail Wall along 12th Avenue
The wall was designed to withstand the temporary lateral and traffic surcharge loads. The system
was also designed to withstand the surcharge loads associated with a large crane scheduled to be
placed on the sidewalk directly behind the soil nail wall, to aid in construction of the new
addition. Since the soil nail wall for this part of the project was installed on-line, the new
foundation wall was constructed using a one-sided form and poured directly against the soil nail
wall. A drainage composite and waterproofing membrane were placed between the soil nail wall
and the new concrete wall. This 935 m2 system (10,000 ft2) allowed the proposed foundation to
be built ahead of schedule and without the need for any design modifications.
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Soil Nail Instrumentation and Monitoring
In order to evaluate design assumptions and monitor the construction, a comprehensive testing
program was implemented, consisting of field pullout tests, proof testing, surveying, and strain
monitoring.
Verification Testing: At least one pullout test was performed on each soil nail row (lift) to verify
that adhesion values used in the design program were obtained in the field. The pullout test was
performed incrementally, generally to a factor of at least two times the design value, on a shorter
sacrificial nail (Figure 13). In addition, a minimum of five percent of the production nails were
proof tested to 1.33 times the design load to measure the bond and creep values.
1 ft = 0.3048 m
1 kip = 4.45 kN
Figure 13: Pullout Test Detail
Survey and Monitoring: Survey monitoring was implemented to measure horizontal and vertical
displacements. The deflection of the wall was generally 0.2% of the cut height, which is within
the range of the anticipated deformation for wall construction in these soils (Byrne, et al., 1996).
Strain Monitoring: In addition to the normal monitoring, a strain-monitoring program was
developed and implemented for the temporary soil nail wall to compare the in situ parameters of
a multi-tier soil nail earth retention system with that of the original design parameters,
information that would be valuable in the future. Strain gage monitoring is discussed more fully
in the Design, Monitoring and Testing section of this paper.
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Two pairs of vibrating wire strain gages were installed on each of three, predetermined soil nails
in a vertical plane in order to determine the average strain (Figure 14). The nails selected for
instrumentation were located on tiers 2 through 4 of the 5-tier system.
Elevations in feet.
1 ft = 0.3048 m
Figure 14: Instrumentation Layout
Strain readings were taken on a weekly basis for a period of two months from initial installation.
More frequent readings were taken immediately following nail installation. Readings were also
taken prior to and after each new 1.5 m (5 ft) lift.
Long term monitoring was not feasible; however, data trends shown in Figure 15 and Table 1
indicate that in situ field stresses were similar to or less than the theoretical stresses calculated
using the CALTRANS Design Program (SNAIL). As shown, the nail stress increases as the next
cut is made to a new lift. Results in the upper nail are closest to the theoretical design stresses,
more than likely because it was monitored longer than the lower nails. Of interest also is how
the nail stress spiked in the strain gages installed 1.5 m (5 ft) behind the wall immediately after
the crane used to set the steel structure was brought on site and erected approximately 2.5 to 3.5
m (8 to 12 ft) behind the soil nail wall. As previously mentioned, the geotechnical contractor’s
design took this loading into account. A summary of the stresses is presented in Table 1.
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Stress Behavior
All Soil Nails
25.0
Crane set up at top
of wall above gages
Excavation
of "E" lift
Soil nail Bar Stress (ksi)
20.0
B47-15-U
B47-15-L
B47-5-U
B47-5-L
C47-15-U
C47-15-L
C47-5-U
C47-5-L
D24-15-U
D24-15-L
D24-5-U
D24-5-L
15.0
Excavation
of "D" lift
10.0
Excavation
of "C" lift
5.0
0.0
11/1/02
11/15/02
11/29/02
12/13/02
12/27/02
1/10/03
1/24/03
2/7/03
2/21/03
Date
Figure 15: Composite Stress Plot
Table 1: Summary of Stresses
Strain Gage
Number
B47-15-U
B47-15-L
B47-5-U
B47-5-L
Bar Stress
(ksi)
16.28
18.99
16.24
17.26
C47-15-U
C47-15-L
C47-5-U
C47-5-L
7.93
7.25
N/A
14.06
D24-15-U
D24-15-L
D24-5-U
D24-5-L
3.63
N/A
6.91
6.52
Average Stress Theoretical Stress
(ksi)
SNAILZwin output
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17.64
24.287
16.75
18.854
7.59
20.094
14.06
17.137
3.63
15.901
6.72
14.053
1 ksi = 6.89 MPa
Existing Dental School (Understanding the Conditions)
During construction of the permanent soil nail wall along the existing Dental School,
unanticipated conditions were encountered during excavation of the first 1.5-m (5-ft) lift. A
loose, cohesionless fill with poor standup time was encountered directly below the existing first
floor slab. To assess the actual conditions, the geotechnical contractor installed a trial test
section of shotcrete and test nails to determine if soil nailing would still be viable. The shotcrete
did not adhere to the loose fill, causing sloughing of the material and “cave-in” of the test
section. Additionally, the test nails did not achieve the bond values that the design assumed.
Considering all these conditions, the geotechnical contractor recommended the soil nailing
system not be installed and offered a redesign, which consisted of on-line concrete pits, tieback
anchors and treated timber lagging to retain the soil between the columns. As originally
proposed, minipiles were installed through the column footing to underpin the column footings
and transfer the column loads below the new subgrade elevation.
SUMMARY
Soil nailing is an accepted technology, the theoretical aspects of which are well understood and
well reported in technical literature. However, research indicates that there are few practical
guidelines available that offer a comprehensive, experience-based insight into the construction
considerations that should be addressed before a soil nail system design is finalized and
implemented. This paper is intended to bridge that void, and is offered as a tool to aid owners,
designers, specifiers and general contractors, as well as those within the engineering community,
in making the informed decisions that result in appropriate and successful applications of this
versatile and economic technology.
ACKNOWLEDGEMENTS
The author extends his thanks and appreciation to the following people whose cooperation and
input has been invaluable, both on the jobsite and in the preparation of this paper: Steven Lacz of
Moretrench Geotec, John J. Peirce, P.E., of Peirce Engineering; and Ken Quazza, P.E. of SESI
Consulting Engineers.
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REFERENCES
Byrne, R.J., Cotton, D., Porterfield, C., Wolschlag, G., and Ueblacker, G. (1996). “Manual for
Design and Construction Monitoring of Soil Nail Walls.” FHWA-SA-96-069, Federal Highway
Administration, Washington, DC.
Chassie, R.G. (1993). “Soil Nailing Overview” and “Soil Nail Wall Facing Design and Current
Developments,” presented at the Tenth Annual International Bridge Conference, Pittsburgh, PA.
Sakr, C. T. and Kimmerling, R. (1995). “Soil Nailing of a Bridge Embankment, Report 2: Design
and Field Performance.” Oregon Department of Transportation Experimental Features Project
OR 89-07
Schlosser, F. (1983). “Analogies et Differences dans le Compartment et le Calcul des Ouvranges
de Soutenement en Terre Armee et par Clouage du Sol.” Proceedings of Annales ITBTP No.
418, Sols et Foundations 184, pp 8-23.
Shen, C.K., Herrman, L.R., Ronstad, K.M., Bang, S., Kim, Y.S., and DeNatale, J.S. (1981). “An
In Situ Earth Reinforcement Lateral Support System.” National Science Foundation, Grant No.
APR 77-03944, CE Dept., University of California Davis, Report No. 81-03.
Stocker, M.F., Korber, G.W., Gassler, G., and Gudehus, G. (1979). “Soil Nailing.” Proceedings
of the International Conference on Soil Reinforcement 1, Paris, Vol.2, pp 469-474.
Tuozzolo, T.J. (1997). “Stabilizing the Stacks - A hybrid soil nailing system provides permanent
underpinning for a historic college library.” Civil Engineering Magazine, December 1997.
CALTRANS (1991). “A User’s Manual for the SNAIL Program, Version 2.02.” California
Department of Transportation, Division of Technology, Material & Research, Office of
Geotechnical Engineering.
BIBLIOGRAPHY
Elias, V. and Juran, I. (1991). FHWA RD-89-198, “Soil Nailing for Stabilization of Highway
Slopes and Excavations.” Federal Highway Administration, Washington, D. C.
Singla, S. (1999). FHWA-IF-99-026, “Demonstration Project 103: Design & Construction
Monitoring of Soil Nail Walls.” Project Summary Report. Federal Highway Administration,
Washington, D. C.
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