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1 INTRODUCTION

The use of soil nails in stabilizing slopes and

excavations has substantially increased during the last

decade because it is a cost-effective alternative to

conventional retaining structures. In the United States,

the widespread use of the soil nailing technology was

due to the fact that soil nail walls have been

demonstrated to be technically feasible and cost-

effective, especially in top-to-bottom excavations for

temporary and permanent applications.

Soil nailing was first used in early 1960s as a support

system for underground excavations in rocks known as

the New Austrian Tunneling Method (NATM). This

technique involves a passive steel reinforcement (e.g.,

rockbolts, soil nails) encased in grout and the

application of reinforced shotcrete. The strength of this

support system relies on the mobilization of the tensile

strength of the steel, as well as the bond strength

between the grout and the surrounding ground.

In the succeeding years, soil nails have been

subsequently used in highway projects with high-cut

slopes, bridge abutment cut walls and deep foundation

excavations in both Europe and United States.

Some of the advantages of soil nail walls are: 1) can be

applied on both soils and weathered rocks, 2) there is no

need to embed structural elements at the bottom of the

excavation, 3) installation is relatively quick and uses

less construction materials as well as smaller equipment,

and 4) cost is relatively low compared to conventional

retaining structures.

With the objective of developing the soil nail

technology as support system in various geotechnical

DESIGN CONSIDERATIONS FOR SOIL-NAILED WALLS AND SLOPES

Gian Paulo D. Reyes 1

Regine Chloe S. Albea 1

Jenna Carmela C. Pallarca 1

John Erickson S. Delos Santos 1

Michael Paolo V. Follosco, MSCE 1

Roy Anthony C. Luna, MSCE 1,2

Ramon D. Quebral, PhD

1

Benjamin R. Buensuceso, Jr., PhD 2

AMH Philippines, Inc., Diliman, Quezon City, Philippines

2 Institute of Civil Engineering, University of the Philippines, Diliman, Quezon City, Philippines

Abstract :

The utilization of soil nails can be a cost-effective solution to stabilize cut slopes or grade-separated roads and

highways. This involves a passive steel reinforcement encased in grout and a shotcrete or concrete cover applied

on the slope or excavation face to provide continuity. Soil nail derives its strength by mobilizing the bond

strength between the grout and the surrounding soil. This technique is applicable to both soil and soil-like

materials such as soft rocks or weathered rocks.

In the absence of local codes and specifications governing the design and construction of soil nails, the provisions

of US Federal Highways Administration (FHWA) can be adopted.

This paper presents the various considerations in the stability analysis and design of soil-nailed walls and slopes.

Modeling and analysis using limit-equilibrium approach are discussed. Case studies involving a grade-separated

road project, and a steep soil slope are presented.

Key words : Soil nails, Stability analysis, Limit equilibrium approach, Retaining wall, Slope stabilization

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projects, the US Federal Highway Administration

(FHWA) developed a design guideline for soil nail

walls. The manual incorporates the design methods,

construction procedures, and construction monitoring.

In the Philippines, however, there are no local codes

and specifications governing the design and

construction of soil nail walls. The provisions of FHWA

can be adopted in establishing the stability and design

of soil-nailed walls in the country.

2 COMPONENTS OF SOIL NAIL

The basic elements of a soil nail wall are presented in

FIG. 1.

Fig. 1 Basic components of soil nail wall (FHWA)

1. Steel reinforcing bars are the main component of soil nail wall. Placed in pre-drilled holes and

grouted in place.

2. Grout are placed in pre-drilled holes to hold the steel bars and at the same time transfer stresses

from the ground to the nail.

3. Nail head, washers, and bearing plate are structural components that attach the steel nail to

the facing.

4. Temporary and permanent facing provides continuity of the structure and supports the exposed

soil.

5. Geocomposite strip drain provides collection and drainage of seepage water flowing from the

surrounding ground.

3 LIMIT STATES

The analysis and design of soil nail walls considers two

(2) limit states or limiting conditions, the Strength Limit

States and Service Limit States.

3.1 Strength Limit States

These refer to failure modes where the applied

loads induces stresses that are greater than the

strength of the whole soil nail wall system or its

individual components, which causes the structure

to be unstable or collapse. The strength limit states

are classified as:

External failure mode Internal failure mode Facing connection failure mode

3.2 Service Limit States

These refer to conditions where the normal and safe

operation of soil nail wall structure is considered.

For soil nail walls, the main criterion for safety is

excessive wall deformation.

4 POTENTIAL FAILURE MODES

There are (3) potential failure modes needed to be

considered in designing soil nail walls: External failure

mode, internal failure mode, and facing failure mode.

The external failure mode refers to the development of

potential failure surfaces passing through or behind the

soil nails. The internal failure modes refer to the failure

in the load transfer mechanisms between the soil, nail,

and grout. Potential failures may occur when the

induced stresses in the nail/s are greater than the bond

strength developed between the soil and grout or greater

than the tensile capacity of the nail. The facing

connection failure modes refer to the potential failure

modes at the facing-nail head connection and is

designed similar to typical structural calculations

involving concrete and steel reinforcements.

The design considerations are based on these failure

modes and are presented in the following sections.

5 EXTERNAL STABILITY

As with conventional retaining structures, the stability

of a soil nail wall system is assessed by three (3) types

of analyses: Global Stability, Sliding Stability, and

Bearing Capacity analyses.

5.1 Global Stability

Similar to typical slope stability analyses, soil nail

wall systems will be analyzed by Limit-

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Equilibrium Methods. The soil nail wall mass is

generally treated as a block, global forces and

moment equilibrium are established, and a factor of

safety that relates resisting and driving forces is

calculated.

Fig. 2 Limit-equilibrium analysis of soil nail walls

The Factor of Safety (FS) is expressed as the ratio of

resisting forces to the driving or overturning forces.

= (1) 5.2 Sliding Stability

This stability analysis considers the ability of the

soil nail wall to resist sliding along the base of the

retained soil/rock mass against the lateral earth

pressures behind the soil nails. In sliding stability

analysis, the soil nail wall system is considered as a

rigid block in which horizontal lateral earth forces

is acting.

Fig. 3 Sliding stability analysis of soil nail walls

The factor of safety against sliding (FSSL) is

calculated as follows:

= =(2)

5.3 Bearing Capacity

In soil nail design, the bearing capacity of the soil

is not typically a concern unless the soil nail wall is

excavated in fine-grained, soft soils. Bearing

capacity failure occurs when the unbalanced load

induced by the excavation causes the bottom soil to

heave. The factor of safety against heave (FSH) is

given by Terzaghis equation:

= (3)

Where = Undrained shear strength of soil = Bearing capacity factor (based on Terzhagi) = Unit weight of soil behind wall = Height of wall

6 INTERNAL STABILITY

The two (2) most common internal failure modes in soil

nail walls are nail pullout failure and nail tensile failure.

6.1 Nail Pullout Failure

Nail pullout failure occurs when the pullout

capacity per unit length of soil nail is inadequate

and/or the nail length is insufficient. The pullout

capacity of soil nails depends on the bond strength

developed between the soil-grout interface, as well

as the size of the nail.

The mobilized pullout strength is generally given

by the following equation:

= = (4) = (5)

Where

= Pullout capacity

= Maximum design nail tensile force

= Pullout capacity per unit length

= Ultimate bond strength

4

= Length of nail

= Diameter of drill hole

= Factor of safety against pullout failure

6.2 Nail Tensile Failure

Nail tensile failure occurs when the longitudinal

force developed along the soil nail exceeds the nail

bar tensile capacity, given by the following

equation:

= (6) = (7)

Where

= Nail bar tensile capacity

= Maximum design nail tensile force

= Cross-sectional area of nail bar

= Yield strength of nail bar

= Factor of safety against nail tensile failure

7 FACING CONNECTION STABILITY

The three (3) most common facing failure modes

are flexure failure, punching shear failure, and

headed-stud failure.

7.1 Flexure Failure

Flexure failure occurs when there is excessive

bending beyond the flexural capacity of the facing

and is considered separately for both temporary and

permanent facings. The main driving forces that

cause flexural failure in the wall facings are the

lateral earth pressures.

Based on yield-line theory concepts, the facing

flexure capacity, RFF, can be estimated as the

minimum of the following:

= !265

"# + #$ %&&

& ' (& )

*+# (8)

= !265

"# + #$ %&&

& ' (& )

*+# (9)

= (10) Where

= Factor that considers non-uniform soil

pressure behind facing

= Reinforcement cross-sectional area per

unit width in the vertical direction at nail head

= Reinforcement cross-sectional area per

unit width in the vertical direction at midspan

= Reinforcement cross-sectional area per

unit width in the horizontal direction at nail

head

= Reinforcement cross-sectional area per

unit width in the horizontal direction at

midspan

= Nail horizontal spacing

= Nail vertical spacing

= Tensile yield strength of reinforcement

= Factor of safety against facing flexure

failure

= Design nail head tensile force

7.2 Punching Shear Failure

Punching shear failure is a failure that occurs

around the nail head and is evaluated separately for

both temporary and permanent facings. For

temporary facings, the bearing-plate connection is

considered. For permanent facings, the headed-stud

connection is evaluated.

The equation for facing punching shear capacity is

given by

= !, (11) , = 330-[*+#]& (12)

= (13) Where

= Correction factor for support capacity of

soil (usually 1)

= Punching shear force

= Effective diameter of conical failure

surface

= Effective depth of conical surface

= Factor of safety against facing punching

shear failure

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7.3 Headed-stud Failure

Headed-stud tensile failure refers to the potential

failures that may occur in the connectors (headed-

studs) providing anchorage to the nail in the

permanent facing. The nail head capacity against

tensile failure of headed-studs, RHT, is given by

= (14) = (15)

Where

= Number of headed-studs

= Cross-sectional area of headed-stud shaft

= Tensile yield strength of headed-stud

= Factor of safety against facing headed-

stud failure

8 CASE STUDIES

8.1 Soil Nail Walls as Slope Protection for a Grade-Separated Road Project

A road project intended to connect a minor arterial

road to a major road is currently being constructed.

The construction of the road involves a number of

high-cut slopes and tunneling through the soil/rock

formation. The project site is generally underlain

by tuff formation, which is composed of weathered

rocks also known as adobe. The site subsoil is

generally composed of medium dense to dense

sands and sedimentary rocks. To stabilize the

slopes and tunnels, soil nails with reinforced

shotcrete as facing were applied. This slope

stabilization and protection method provides a

more cost-effective solution compared to the initial

solution of reinforced concrete retaining walls.

A vertical cut slope, at a maximum of 7m in height,

was to be stabilized. To establish stability and

safety of the slope, the external, internal, and facing

connection stabilities of the soil nail wall design

were carried out. The slope protection scheme was

to have three (3) layers of soil nails, 6m long each,

with shotcrete facing of 75mm thickness. The nails

are spaced at 1.5m both horizontally and vertically.

A uniform load of 20 kN/m was applied on top of

the slope to account for the traffic loading. Both

static and seismic conditions were analyzed.

The external stability was assessed by Limit-

Equilibrium Methods through the aid of a computer

program, Rocscience Slide v6.0. On the other hand,

the design of the soil nail walls were based on the

FHWA design manual and a computer program

(Caltrans SNAIL), which follows the provisions of

the FHWA manual, was used to facilitate the

computations.

Fig. 4 Global stability analysis by Limit-

Equilibrum Method (Rocscience Slide v6.0)

Fig. 5 Internal and facing stability analysis using

CALTRANS SNAIL

Results

TABLE 1 shows the summary of results for the

external (global, sliding and bearing capacity),

internal and facing stability analyses. No headed-

studs were included in the design. The stability

analyses yielded adequate factors of safety.

Table 1. Summary of results

Type of

Analysis Results

Global Minimum FS 2.40

Sliding Minimum FS 9.35

Bearing

Capacity Minimum FS 17.87

Seismic Minimum FS 1.67

Internal

(Facing)

Service Load @ Nail 49.2 kN

Flexural 66.0 > 49.2 kN

Punching Shear 146.6 > 49.2 kN

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8.2 Soil Nail as Slope Protection Measure

A landslide mitigating measure is being

recommended for an existing power plant switch

yard to prevent further slope failure on its vicinity.

The existing slope failure is located at the drainage

outfall of the site approximately fifty (50) meters

from the switch yard, and mode of failure is

rotational slip - caused by the build-up of

porewater pressure induced by infiltration,

saturating the slope and causing slope movement

(width of about twenty meters). This apparently led

to the slope failure at the outfall of the drainage

canal and caused the washing out of the cascading

canal downstream of it. As a mitigating measure,

soil nails with shotcrete facing were utilized to

prevent further slope failure.

The site is composed of intercalating layers of

dense to very dense silty sand, and andesitic rocks

were observed at approximately 20m depth below

ground. The slope is about 25m high with an

inclination of about 45. The proposed soil nail

scheme was to provide four (4) layers of soil nails

with 12m length spaced at 1.5m out-of-plane and

2m vertical.

As in the previous site presented, the external

stability of the proposed soil nail scheme was

assessed by Limit-Equilibrium Methods using

Rocscience Slide v6.0. The modeling took into

consideration static (or normal condition) loading

and pseudo-static loads (earthquake).

Fig. 6 Global stability analysis by Limit-

Equilibrum Method (Rocscience Slide v6.0)

Results

TABLE 2 shows the summary of results for the

global stability analysis. The stability analysis

yielded adequate factors of safety.

Table 2. Summary of results

Case ru* kh** FS

1 0.1 0.0 1.317

2 0.2 0.0 1.289

3 0.3 0.0 1.199

4 0.4 0.0 1.097

5 0.1 0.1 1.183

6 0.2 0.1 1.096

*ru = coefficient that models the pore pressure as a

fraction of the vertical earth pressure for each slice

**kh = horizontal pseudo-static coefficient

9 CONCLUSION

As demonstrated, soil nails can provide an

alternative slope stabilization method for both

grade-separated road project and natural slopes. A

local design guideline is evidently warranted for

uniform analysis and design.

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ACKNOWLEDGMENT

The authors acknowledge the assistance provided by the

technical staff of AMH Philippines, Inc. in providing

project data and references.

REFERENCES

Abramson, L.W., Lee, T.S., Sharma, S. Boyce,. G.M.

(2002). Slope Stabi;lity and Stabilization Methods.

2nd Edition..

Das, Braja M. (2006). Principles of Geotechnical

Engineering. 5th Edition.ines Press.

Federal Highway Administration (1998). Manual for

Design and Construction Monitoring of Soil Nail

Walls.

Lazarte, C., Elias, V., Espinoza, D., Sabatini, P., (2003).

Federal Highway Administration - Geotechnical

Engineering Circular No. 7 Soil Nail Walls (FHWA0-

IF-03-017).