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26

GROUND ANCHORS AND SOIL NAILS

IN

RETAINING STRUCTURES

ILAN

JURAN,

D.Sc.

Professor and Head

VICTOR ELIAS, P.E.

Department

of

Civil and Environmental Engineering

Brooklyn Polytechnic University

V. Elias

&

Associates, P.A.

Consulting Engineers

26.1 INTRODUCTION

Ground anchor and soil nail retaining systems are designed to

stabilize and support natural

and

engineered structures

and

to

restrain their movement using tension-resisting elements. The

basic design concept consists of transferring the resisting tensile

forces generated in the inclusions into the ground through the

friction

(or

adhesion) mobilized

at

the interfaces. These systems

allow the engineer to efficiently use the in-situ ground in

providing vertical or lateral structural support. They present

significant technical advantages over conventional rigid gravity

retaining walls or external bracing systems

that

result in

substantial cost savings and reduced construction period.

Therefore, during the past few decades, ground anchors, and

more recently soil nails, have been increasingly used in civil

engineering projects.

The use

of

these systems in permanent structures requires

careful evaluation of the durability of the structural elements

and

assessment

of

the long-term system performance. A variety

of inclusions, corrosion-protection systems, and installation

techniques have been progressively developed by specialty

contractors. This chapter briefly describes the construction

process and the main structural elements. I t presents the main

aspects

of ground-inclusion interaction, illustrates the observed

behavior of instrumented str\lctures, and outlines durability

considerations, performance criteria, and design approaches

that have been developed to ensure the internal and external

stability of these composite retaining systems.

26.2 PRINCIPLES, HISTORICAL DEVELOPMENT,

AND

FIELDS OF

APPLICATION

26.2.1 Permanent

Ground Anchors

Permanent ground anchors are prestressed cement-grouted

tendons used in soils or rock to restrain

and

control the

displacements

of

structural elements such as walls or slabs.

They have been developed mainly by specialty contractors

involved in temporary excavation

support

systems and in some

cases are p roprieta ry. The anchors a re installed in drilled holes

and prestressed

to

the design load in order to mobilize and

transfer the required resisting force from the ground to the

868

structural element. Temporary ground anchors are used for a

specified construction period

and

their service life is generally

less than 2 years. Permanent ground anchors are corrosion

protected to insure their long-term performance throughout the

design service life of the structure.

Figure 26 .1 shows a schematic diagram of a permanent

ground a nchor. The basic components of the ground

anchor

are:

• The tendon is made of prestressing steel wires, strands, or

bars and includes:

a. The anchor bond length-where the tendon

is

fixed in the

primary grout bulb

and

transfers the tension force

to

the

surrounding ground. The

anchor

bond length is designed

to

provide the required load pull-out capacity of the

anchor.

b.

The unbonded length-where the tendon

is

free to elongate

elastically

tran

sferring the resisting force from the anchor

bond length to the structural element (i.e., wall face, slab,

etc.). I t s designed

to

reach the underlying substratum or, in

Fig. 26.1 Permanent ground anchors.

H.-Y. Fang (ed.), Foundation Engineering Handbook

© Springer Science+Business Media New York 1991

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homogeneous soils, to locate the anchor bond length

beyond the potentially unstable soil mass adjacent to the

structural element.

• The anchor grout, also called primary grout, is generally a

portland cement-based mixture or a polymer resin and is

used to transfer the anchor force to the ground. Secondary

grout can be injected into the drilled hole after stressing to

provide corrosion protection for unsheeted tendons.

• The anchorage is

a device attached to the tendon that consists

of a plate and an anchor head (or threaded nut) and permits

stressing and lock -off of the prestressing steel.

During the past 50 years, permanent ground anchors have

been extensively used by contractors to provide vertical and

lateral support for natural and engineered structures. Typical

applications of ground anchors are illustrated in Figure 26.2.

They have found widespread acceptance in a variety of civil

engineering projects including cut slope retaining systems,

tied-back diaphragm or soldier pile walls, bridge abutments,

stabilization of natural slopes and

cliffs,

tunnel portals, under

pinning, repair or reconstruction of quay walls, dam spillways,

loading ramps, hangars, etc. They have also been frequently

used as tied own supports for dams, transmission towers, and

waterfront structures, primarily to resist uplift water pressures

and rotational loadings.

Tiebacks were first used to anchor structures in rock. The

earliest permanent rock tied owns were installed by the French

engineer Coyne for anchoring the Jument lighthouse (1930)

and raising the Cheurfas Dam, Algeria (1934).

By

the late

1950s,

use of permanent rock tiedowns had become common practice

in

renovation and construction of dams (Evans, 1955; Morris,

1956; Middleton, 1961) and towers (Weatherby, 1982). In the

1950s contractors began to use tiebacks for temporary supports

Fractured

sandstone

...........

/

Permanent

tiebacks

(a)

.D

Wall

(c)

Ground Anchors and Soil Nails in Retaining Structures 869

of deep excavations. The first permanent soil tiebacks in the

United States were installed in

1961

in a very stiff silty clay for

the construction of retaining walls for the Michigan expressway

(Jones and Kerkhoff,

1961).

However, in spite of long-term

European experience, permanent ground anchors had not been

in common use in the United States until the late

1970s,

mainly

because of engineering concerns with regard to long-term

performance, potential time-dependent (creep) movement, cor

rosion protection of the tendon, and the need to establish

reliable quality control testing procedures to verify the short

and long-term holding capacity. Technological efforts have been

continuously invested by specialty contractors to overcome

these limitations, develop efficient corrosion-protection systems,

improve grouting methods and installation procedures, and

increase the tension capacity of the prestressed tendons.

The rapid acceptance and growing use of ground anchors

can be attributed mainly to significant technical advantages

resulting in

substantial cost savings and reduced construction

period. Specifically, in urban areas the use of ground anchors

often allows significant reduCtion in right-of-way acquisition

and permits the elimination of temporary support systems,

external bracings, or the need for underpinning existing structures

near to excavation sites. The increasing confidence in ground

anchor use for permanent structures is primarily due to reliable

quality control procedures that involve routine performance

and proof testing of all production anchors under loads

exceeding the design load. Performance specifications and codes

of practice, based on experience and long-term observations of

permanent anchor installations, have been developed in Euro

pean countries (French Recommendations, Bureau Securitas,

1977;

FIP

Rules, 1974; German Standards, DIN, 1972, 1976;

PTI Recommendations, 1980) and more recently in the United

States (FHWA; see Cheney, 1984) to specify design, construction,

and monitoring procedures

(b)

(d)

Permanent

tiebacks

Existing

dam

• ' , . "

'-

... . F. ..

Permanent

tiedown

Fig.26.2 Typical applications of permanent ground anchors. (a) Concrete wall.

(b)

Landslide and tunnel portal.

(c)

Permanent tower

tiedown.

(d)

Dams.

(After Weatherby,

1982.)

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870 Foundation Engineering Handbook

26.2.2 Soil Nailing

Soil nailing is an in-situ soil reinforcement technique that has

been used during the past two decades, mainly in France and

Germany, in cut slope retaining systems and slope stabilization.

The fundamental concept of soil nailing consists of reinforcing

the ground by passive inclusions, closely spaced, to create in

situ a coherent gravity structure and thereby to increase the

overall shear strength of the in-situ soil and restrain its

displacements. This technique has emerged essentially as an

extension ofthe "New Austrian Tunneling Method" (Rabcewicz,

1964-65), which combines reinforced shotcrete and rockbolting

to provide a flexible support system for the construction

of

underground excavations.

Although soil nailing technology is relatively new, it has

been used in a variety of civil engineering projects including

stabilization of railroad and highway cut slopes (Rabejac and

Toudic,

1974; Hovart and Rami, 1975; Stocker et

aI.,

1979;

Cartier and Gigan, 1983; Schlosser, 1983); construction of

excavation retaining structures in urban areas, for high-rise

buildings and underground facilities (Louis, 1981; GassIer and

Gudehus, 1981; Shen et aI., 1981); landslide stabilization

(Guilloux et aI., 1983; Blondeau et

aI.,

1984); tunnel portals in

steep and unstable stratified slopes (Louis, 1981); and other

civil and industrial projects. Typical applications of soil nailing

are illustrated in Figure 26.3. Several nailed soil-retaining

structures have been instrumented to establish a data base for

evaluation of structure performance and development of reliable

design methods (Stocker et

aI.,

1979; GassIer and Gudehus,

1981; Schlosser, 1983; Plumelle, 1986). In North America, the

system was initially used in Vancouver, B.C., in the late 1960s

in

temporary excavation supports for industrial and residential

buildings (Shen et

aI.,

1981). Presently, soil nailing systems can

(a)

(i) Conventional

method ---

I'"

///t t

/

I

Reinforced ,

concrete

(c)

(ii) Austrian tunneling

method

Anchor pin

be considered for any temporary

or

permanent application

where conventional cut retaining systems, such as cast-in-place

reinforced-concrete walls or tied-back walls, are applicable. As

demonstrated by GassIer and Gudehus (1981), soil-nailed

retaining structures can withstand both static and dynamic

vertical loads at their upper surface without undergoing excessive

displacements. Therefore, they can be effectively used in the

construction of bridge abutments. Soil nailing also appears

to provide an efficient and economical technique for repair and

reconstruction of existing structures, particularly tie-back walls

and reinforced soil retaining structures.

In soil-nailed retaining structures, the inclusions are generally

steel bars or other metallic elements that can resist tensile

stresses, shear stresses, and bending moments. They are either

placed in drilled boreholes and grouted along thei r total length

or driven into the ground. The nails are not prestressed but

are closely spaced (e.g., one driven nail per 2.5 ft2, one grouted

nail per 10-

50

ft

2

) to provide an anisotropic apparent cohesion

to the native ground. The facing of the soil-nailed structure is

not a major structural load-carrying element but rather ensures

local stability of the soil between reinforcement layers and

protects the ground from surface erosion and weathering effects.

It generally consists of a thin layer of reinforced shotcrete (4- to

6-in thick), constructed incrementally from the top down. The

facing and the nails are placed immediately after each excavation

stage to restrain ground decompression and therefore to prevent

deterioration of the original mechanical properties and shear

strength characteristics of the native ground. Prefabricated or

cast-in-place concrete panels have increasingly been used

in

the

construction of permanent structures to satisfy specific aesthetic

and durability design criteria.

As with ground anchors, soil nailing has been primarily used

for temporary retaining structures. This is mainly due to the

----

(b)

2f}[

TESTB

~ t

.. .. 1

20

ft

J

(d)

Fig. 26.3

Typical applications

of

soil nailing.

(a)

Landslide.

(b)

Retaining structures.

(c)

Tunnel portal.

(d)

Abutments.

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valves located along the bond zone. Typical examples of

regroutable anchors are the Soletanche

IRP

anchor (Jorge,

1969), the TMD-Bachy anchor (Clement and Navarro, 1972),

and the TPT anchor (Mastrantuono and Tomiolo,

1977).

The

Soletanche IRP anchor and the tube-a-manchette packer are

schematically illustrated

in

Figure

26.5.

Single and multi-underreamed anchors are used in stiff cohesive

soils and weak rock. Their installation process involves the use

of an underreaming device that is basically a cutting tool. First,

the cylindrical anchor shaft is drilled, usually using a continuous

flight auger. Then, the cutting tool is mechanically expanded

at a controlled rate to the design size of the underream. The

soil is removed by water flushing; neat cement grout is

tremie-grouted into the drill hole; and the tendon is inserted.

Depending on the underreaming device, several underreams

I Steel tendon

Sleeve-pipe

2 Plastic sheath

~ ~ ~ S I

IT - ' - - 3

Annular space (epoxy-

or

cement-filled)

Anchor

Strand

Rubber

manchette

Grouting

pressure distends rubber

manchelle and forces grout through

sealing grout

sleeve grout to secure tube

it manchette in hole

(b)

Fig.26.5 (a) Schematic section of an IRP anchor. (b) Detail of

tube-a-manchette for pressure grouting control. (After Pfister et al.,

1982.)

can be cut simultaneously. The spacing between the underreams

is

selected to induce a shear failure along a cylinder passing

through the tips of the underreams.

The main structural element of each ground anchor

is

the

steel tendon, which may consist of bars, wires, or strands.

Strands and wires have advantages with respect to tensile

strength (ultimate tensile strength: 270ksi for strands and

240 ksi for wires), and ease of transportation, storage, and

fabrication. However, bars (ultimate tensile strength of 150 to

160

ksi) are more readily protected against corrosion and, in

the case of shallow, low-capacity anchors, are usually easier

and cheaper to install. Often, availability and cost

will

be the

determining factors. In the United States, bars and seven-wire

strands are the most commonly used tendons. High-capacity

tendons made of 18 strands with a diameter of 0.50 or 0.60

inch are also available for high-capacity tied own applications.

B.

Soil Nailing

The steel reinforcing elements used for soil nailing can be

classified

as

(a) driven nails, (b) grouted nails, (c) jet-grouted

nails, and (d) corrosion-protected nails.

Driven nails,

commonly used in France and Germany, are

small-diameter (15 to 46 mm) rods or bars, or metallic sections,

made of mild steel with a yield strength of 350

MPa

(50 ksi).

They are closely spaced

(2

to 4 bars per square meter) and

create a rather homogeneous composite reinforced soil mass.

The nails are driven into the ground at the designed

inclination using a vibropercussion pneumatic or hydraulic

hammer with no preliminary drilling. Special nails with an axial

channel can be used to allow for grout sealing of the nail to

the surrounding soil after its complete penetration. This installa

tion technique

is

rapid and economical (4 to 6 per hour).

However, it is limited by the length of the bars (maximum

length about

20

m) and by the heterogeneity of the ground (e.g.,

presence of boulders).

Grouted nails are generally steel bars (15 to 46 mm in

diameter) with a yield strength of 60 ksi. They are placed in

boreholes (10 to 15 cm in diameter) with a vertical and

horizontal spacing varying typically from 1 to 3 m depending

on the type of the in-situ soil. The nails are usually cement

grouted by gravity or under low pressure. Ribbed bars can be

used to improve the nail-grout adherence, and special perforated

tubes have been developed to allow injection of the grout

through the inclusion.

let-grou ted nails are composite inclusions made of a grouted

soil with a central steel rod, which can

be

as thick as

30

to

40 cm.

A technique that combines the vibropercussion driving and

high-pressure (greater than

20

MPa) jet grouting has been

developed recently by Louis (1986). The nails are installed (Fig.

26.6) using a high-frequency (up to 70 Hz) vibropercussion

hammer, and cement grouting

is

performed during installation.

The grout is injected through a small-diameter (few millimeters)

longitudinal channel in the reinforcing rod under a pressure

that

is

sufficiently high to cause hydraulic fracturing of the

surrounding ground. However, nailing with a significantly lower

grouting pressure (about 4 MPa) has been used successfully,

particularly in granular soils. The jet-grouting installation

technique provides recompaction and improvement of the

surrounding ground and increases significantly the pull-out

resistance of the composite inclusion.

Corrosion-protected nails generally use double protection

schemes similar to those commonly used in ground anchor

practice. Proprietary nails have recently been developed by

specialty contractors (Intrafor-Cofor; Solrenfor) to be used in

permanent structures. These corrosion-protection schemes are

described in a later section of this chapter.

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Fig. 26.6a Construction process of a soil-nailed wall illustrating

excavation, shotcreting, and nailing.

Fig. 26.6b Jet bolting: installation of reinforcing elements. 1.

Vibropercussion hammer.

2.

Sliding support.

3.

Reinforcement to

be inserted. 4. Sliding guide. 5. Fixed guide. 6. Soil to be treated.

26.3.2

Facing and

Structural

Retaining Elements

In an anchored retaining system the wall has a major structural

role. It has to resist the tensile forces transferred by the anchors,

the lateral pressure applied by the retained soil, and bending

moments. The wall has to be stiff enough to restrain the ground

displacement induced by the excavation process. The facing in

a nailed soil-retaining system has only a minor mechanical role.

The maximum tensile forces generated in the nails are signifi

cantly greater than those transferred to the facing. The main

function of the facing is to ensure local stability of the ground

between the nails and to limit its decompression. Hence, the

facing has to

be

continuous, fit the irregularities of the cut slope

surface, and be flexible enough to withstand ground displacement

during excavation. The structural elements used to build the

anchored wall are therefore basically different from those used

to construct the facing of a nailed soil-retaining structure.

A. Structural Elements

of an

Anchored Wall

An anchored wall can be constructed with a wide variety

of structural elements, using different installation techniques.

Selection of the structural element for a specific application will

generally depend on the subsurface soil (or rock) type, ground

water conditions, local construction practice, availability of

material and equipment, and performance requirements. The

Ground Anchors and Soil Nails in Retaining Structures

873

structural elements can be evaluated in terms of their stiffness,

ease of handling and installation, durability, water-tightness or

continuity, and ease of removal. The elements commonly used

can be broadly classified into four major categories: driven

sheet piles, soldier piles and lagging walls, cylinder walls, and

concrete diaphragm or slurry walls. Typical properties of each

system are indicated in Table

26.1.

Sheet-pile walls usually consist of interlocking steel sheets

driven into the ground prior to excavation. They are fairly

impervious and easy to handle and install in soft clays,

cohesionless silts, or loose sands. However, they are difficult to

use in compact granular soils containing cobbles or boulders.

As compared with other elements, they are relatively flexible

and the wall displacement

will

in general be larger. They

are commonly used for marine bulkhead construction (see

Chapter 12).

Soldier piles

and lagging (Figure 26.7a) usually consist of

steel H-beams that are either driven into the ground or placed

in predrilled boreholes prior to excavation. Concrete bored piles

with reinforcement or permanent casing have also been used.

As

excavation proceeds, the ground between these piles is

retained by lagging of wood planks, cast-in-place, or precast

concrete elements. H-beam soldier piles and lagging walls are

probably the excavation support system most widely used in

the United States for temporary supports. They are easy to

install in most types of soils, and present a significant advantage

specifically in compact or irregular strata that would obstruct

sheet piling. They can be readily adapted to different site

conditions and irregular wall

alignments. The main disadvantage

of this retaining system is that the wall is rather pervious and

subsurface water

flow

may cause local instabilities. A properly

lagged wall should permit drainage, draw down, and fluctuation

of water level without flow of the retained soil.

Cylinder walls consist of an array of cylindrical caissons that

are usually constructed of reinforced concrete or mixed-in-place

soil-cement and are closely spaced to form a continuous wall.

They can

be

cast-in-place and installed using several techniques

such as hollow-stem augers, rotary drilling equipment, deep

mixing methods, or jet-grouting. Depending on the stiffness of

the individual cylinders, such a wall may be rigid enough to

support lateral loads with limited deflection. To achieve water

tightness and properly retain the soil, shotcrete or lagging in

the space between the cylinders may be required. Alternatively,

the cylinders can overlap to produce a continuous, impervious

wall. In addition to their rigidity, cylinder walls offer the

advantage of adaptability to irregular site alignments and can

be used in a variety of ground conditions.

Slurry walls

or concrete diaphragm walls are generally

formed in a trench supported by viscous mud slurry (see Chapter

20). Concrete is tremied into the trench, displacing the mud

slurry upward. Reinforcement of the wall is made by vertical

steel sections, precast reinforced-concrete members, or cages of

reinforcing steel. Recent developments include the use of precast

concrete panels. These walls can be designed to achieve a

specified degree of stiffness and water-tightness, and can be

integrated in the permanent structure. They are often used

where lowering of the water table would adversely affect

adjoining structures. Their main disadvantage is the relatively

high cost and the need for specialized construction equipment

and experienced contractors. They also may present environ

mental problems pertaining to slurry disposal.

Cast-in-place reinforced-concrete panels

have been used in

the construction of multi tied-back walls (Kerisel et aI., 1981).

Figure 26.7b shows a schematic cross section of a 30-m deep

open excavation retained by ten layers of prestressed anchors.

This anchored wall was constructed from the top down with

successive stages of (1) excavation, (2) in-place casting of the

reinforced-concrete panel (2-ft thick, 9-ft high), and (3) anchoring.

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TABLE 26.1 PROPERTIES OF

STRUCTURAL WALL

ELEMENTS.

EI-KSF/F

System

x

10

3

3 to 50

VERTICAL

STEEL SHEETING

SWF

to 14WF

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6ft to 8ft c-

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26.3.3

Drainage

Groundwater (see Chapters 1 and 7)

is

a major engineering

concern relative to construction of anchored and nailed soil

retaining structures.

An

appropriate drainage system must be

provided to (a) prevent generation of excessive hydrostatic

pressures on the facing (or the structural wall element), (b)

protect the facing element and particularly shotcrete facing from

deterioration induced by water contact, (c) prevent saturation

of the nailed ground, which can significantly affect the structure

displacement and may cause instability during and after excava

tion. In anchored walls, prefabricated vertical drains, porous

engineering fabrics, or subhorizontal drains can be used for

drainage ofthe subsurface

flow.

In soil nailing, shallow drainage

(plastic pipes, 10 cm in diameter,

30

to 40 cm long)

is

usually

used to protect the facing, while subhorizontal slotted plastic

tubes are used for deep drainage of the nailed ground. In the

case of permanent structures with prefabricated panels, a

continuous drain such

as

a geotextile can be placed behind the

facing.

26.4

SOIL-INCLUSION

INTERACTION:

PULL-OUT

CAPACITY ESTIMATES

The load-transfer mechanisms between a grouted anchor (or

nail) and the subsurface soil (or rock) as well as the ultimate

pull-out capacity depend upon several parameters, including

installation technique, drilling and grouting method, grouting

pressure,

size and shape of the grouted inclusion, engineering

properties of the in-situ soil and specifically its relative density

(or overconsolidation ratio), permeability, and shear strength

characteristics (see Chapter 3).

The grain size and porosity of the in-situ soil govern the

grout conductivity. In sands, gravels and weathered rock, with

hydraulic conductivities of 10-

1

to

1O-

2

cm/sec, grout will

permeate through the pores or natural fractures of the ground.

In fine-grained cohesionless soils (silts and fine sands), with

hydraulic conductivity smaller than 10-

3

cm/sec, the grout

cannot penetrate the small pores but rather compacts locally,

under pressure, the surrounding ground. Increasing the grout

pressure

will

induce a greater grout permeation into the ground

and/or a more effective ground densification. Consequently,

under high-pressure grouting, high radial stresses are locked

into the soil surrounding the anchor, increasing its pull-out

capacity.

26.4.1 Load Transfer in Ground Anchors

Figure 26.4 illustrates the basic soil-inclusion interaction

mechanisms for the main types of ground anchors.

Tremie-grouted straight-shafted anchors,

which are more

commonly used in rock and very stiff to hard cohesive soils,

generate their pull-out resistance through the lateral shear

mobilized at the grout-ground interface. The pull-out capacity

of these anchors is often estimated by

P

=

n;'D'L"ult (26.1)

where ul t

is

the ultimate lateral shear stress at the ground-grout

interface (also called shaft friction), D and L are, respectively,

the effective diameter and bond length of the grouted anchor.

It should be noted that the effective anchor diameter

D

is

difficult to estimate, since it is highly dependent upon ground

porosity and grout permeability.

I t is

commonly assumed that

in

competent rocks (Littlejohn and Bruce, 1975)

'ul t

=

lO%'Sa

for

Sa

<

600 psi (26.la)


where Sa

is

the uniaxial compressive strength. In cohesive soils,

'ul t =

IX'

S.

(26.1b)

where

IX is

an adhesion factor, and S. is the average undrained

shear strength of the soil.

The adhesion factor (IX) generally varies (Tomlinson, 1957;

Peck, 1958; Woodward et

aI.,

1961) within the range of

0.3

to

0.75, with the lower values obtained for stiffer and harder clays.

Low-pressure grouted anchors

are installed under an effective

grout pressure lower than 150 psi (or, in cohesive soils, under

pressure that would not fracture the ground) most commonly

using hollow-stem augers or tremie grouting, an open hole in

cohesive soils or cored rotary-drilled holes in cohesion1ess soils.

The grouting pressure

will

induce an increase of the effective

diameter of the grout bulb by permeation or local compaction

ofthe ground. Therefore, the pull-out resistance ofthese anchors

is highly dependent upon the grout pressure. It

is

primarily

derived from the ultimate interface shear stress but an end

bearing resistance can be mobilized owing to an effective

increase of the grout bulb diameter. The pull-out resistance

is

commonly estimated using Equation 26.1.

For cohesionless soils,

'ul t

=

p'

A .

tan

cfJ

(26.1c)

where

p

is

the effective grout pressure, cfJ

is

the internal friction

angle of the soil, and A

is

a dimensionless empirical coefficient

smaller than 1 (Hanna, 1982). For practical applications

p is

generally limited to less than

50

psi or 2 psi per foot of

overburden (Littlejohn, 1970).

High-pressure grouted anchors

are installed under effective

grout pressures exceeding 150 psi often using postgrouting

techniques or pressure injection (feasible only in cohesionless

soils). Figure

26.9,

by Jorge (1969), illustrates the significant

effect of grouting pressure on the ultimate load-transfer rate

(or ultimate lateral shear stress) of multiphase postgrouted

anchors in different types of soils. The high-pressure grouting

25

20

.::

"

8/19/2019 juran1991

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Z

.><

~

~

'u

'

.

'

.>

01)

c

'>,

....

...

'

-0

'

.Q

u

;;;

'E

5

2000

1800

1600

1400

1200

1000

800

600

400

200

o

Bond-to-ground length L , m

dense

Typeo/soil

Density

SPT

blows/30 em

•

Gravelly

Very dense

120

-

•

sand

Dense 60

-

Medium

dense

43

•

U

= dr:Jd

lO

I- -

...

= 1.6/0.16

Loose

I I

x

Sandy gravel

U=

15/0.3

Very den e

>

130

... 11.3 = Diameter

of

grouted body D = 11.3

em

Sandy

gravel

U=5to10

Gravelly and

= = = U=8to lO

~ ~ ~ ~ a n d

~ ~ ~

Medium to coarse

sand

=-=.,.,,;,.,

(with gravel)

U = 3.5to 4.5

Diameter of grouted bodies

D

= 10to IS em

Fig.26.10 Ultimate load holding capacity

of

anchors in sandy-gravel and gravelly sand.

(After Ostermayer and Scheele, 1977.)

results in a grout root (or fissure) system that mechanically

interlocks with the surrounding ground, increasing substantially

the pull-out capacity of the anchor. Particularly, in dense

granular soils this interlocking phenomena generates high

tendency for the soil to dilate, which in turn results in a normal

stress concentration at the

grout-ground

interface. The effect

of pressure injection on the soil-anchor interaction is difficult

to evaluate. Empirical relationships were provided by Oster

mayer (1974) for estimating the ultimate lateral shear stress for

high-pres sure-grouted anchors, with and without postgrouting,

in

fine-grained soils (sandy silts to highly plastic clays). Oster

mayer and Sheele (1977) developed empirical curves, reproduced

in Figure 26.10, to estimate the ultimate pull-out capacity of

pressure-injected anchors in granular soils as a function of

anchor length, soil type, density, and uniformity. These curves,

derived from 30 pull-out tests on anchors installed under grout

pressures of about 70 psi, illustrate that the ultimate capacity

of the anchor is not proportional to its length.

Underreamed anchors,

which are mainly used

in

stiff to hard

cohesive soils derive their pull-out capacity from adhesion along

their shaft above the underreams, end bearing of the first

underream, and lateral shear along a cylinder established by

the tips of the underreams. For the cylinder to

be

effectively

established, the spacing between the underreams should not

exceed

1.5

times their diameter (Bassett, 1977). Estimate of the

pull-out capacity of these anchors (Littlejohn, 1970) is based

on empirical formulas that are conventionally used for pile

design in

cohesive soils.

The load transfer along pressure-injected and high pressure

postgrouted anchors has been investigated by several authors

(Bustamente,

1975,

1976; Ostermayer and Sheele, 1977; Shields

et

aI.,

1978; Bustamente, 1980; Davis and Plumelle,

1982).

Figure

26.11 illustrates, for ultimate pull-out loads, the distributions

of the lateral interface shear stress along pressure-injected

anchors in gravelly sands of different densities (Ostermayer and

Sheele, 1977). Similar results were reported for postgrouted

anchors (Bustamente, 1972) in river sands (Fig. 26.12a) and for

straight-shafted anchors (Feddersen, 1974) in highly over

consolidated, stiff, plastic clays. Figure 26.12b shows the results

Soil density

Bond/engtll

L, .,m

0

Very dense

2.0m

•

4.5m

0

Dense

3.0m

0

Medium

2.0m

•

dense

4.5m

'

Loose

2.0m

...

4.5m

300

1200

\Loo

1000

ME

900

Z

800

><

3

700..

c

.2

600

u

:E

SOO

c

:.;;;

til

Very

dense

• . ~ a x l

•

,

• 0 0

-: \ Medium

" • •

den

e

__

_

O - - = =

-

,;;;.:;.;-

- , -

• max

l ,

=

mean

"

~ D , ~ . . . Loose

~ - - - - -- -- - - -- -- -_ ...,

- - - - - ' _ ~ max

t , "" mean "

...

100

Length. m

L,.

= 2.0 m

L,.

= 3 .0 m

L,

= 4.5 m

Fig. 26.11 Distribution

of

the lateral interface shear stress along

pressure-injected anchors at the ultimate load.

(After Ostermayer

and

Scheele, 1977.)

8/19/2019 juran1991

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o

2

3

E

E. 5

o

6

7

8

9

10

train

200

400

(a)


o

E

Pull

-o

ut load distribution

Displace

ment V, mm

Mobilization of lateral friction

(b)

Fig.26.12 (a)

Distribution

of

deformation along the length

of an

IRP anchor.

(After Bustamante,

1972). (b) Mobilization of the lateral

friction along an anchor in a plastic clay. (Winnezeele, Bustamente, 1980.)

of a pull-out test on an instrumented anchor

in

a plastic clay

(Bustamente, 1980). The slope of the tension force distribution

along the anchor corresponds to the lateral interface shear stress

mobilized at a specific depth under the applied pull-out force.

As shown in Figure 26.12, the shear stress-upward anchor

displacement curves obtained for different depths indicate

overconsolidation of the subsurface soil layer and illustrate that

the anchor displacement required to

fully

mobilize the ultimate

shear stress is about 5 to 10 mm. The results of these studies

demonstrate that in dense granular soils and highly overcon

solidated clays the load-transfer rate along the anchor is not

constant and the pull-out capacity is therefore not proportional

to the anchor length.

The variation of the load-transfer rate along the anchor is

mainly the result of its extensibility during pull-out testing.

It

is primarily dependent upon the relative rigidity (or elastic

modulus ratio) of the anchor and the grout-soil interface and

is

particularly pronounced in highly dilatant stiff soils. Wernick

(1977), Schlosser and Elias (1978), and Plumelle and Gasnier

(1984) have shown that the restrained tendency of the soil to

dilate during shearing results in a normal stress concentration

at soil-inclusion interfaces that affects significantly the load

transfer rate. As shown in Figure

26.13,

vertical stresses as high

as four times the overburden pressure were measured in a

medium dense river sand at the anchor interface. The higher

the (anchor-to-soil) elastic modulus ratio the more uniform is

the load-transfer rate. Figure 26.11 shows that in loose to

medium dense granular soils the interface lateral shear stress

is

approximately constant along the anchor.

It is

of interest to

indicate that these results are consistent with those obtained

for ribbed metallic strips in Reinforced Earth structures

(Schlosser and Elias,

1978).

Several authors have attempted to analyze the load transfer

along the anchor using the "t-z" method (Coyle and Reese,

1966; Davis and Plumelle, 1982), which is commonly applied

in design of friction piles,

or

more complex interface soil models

(Zaman et al., 1984; Frank et al., 1982). However, rational

anlysis of the

ground-anchor

interaction requires appropriate

interface properties that are difficult to estimate.

3

100

200

Distance, d, cm

Fig. 26.13

Restrained dilatancy effect around a ground anchor.

(After

Plumelle

and

Gasnier, 1984.)

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26.4.2 Soil Nail Interaction

In soil nailing, similarly to ground anchors, the load transfer

mechanism and the ultimate pull-out resistance of the nails

depend primarily upon soil type and installation technique.

The pull-out resistance of

driven nails

in a dense granular

soil was correlated by Cartier and Gigan (1983) with design

recommendations for Reinforced Earth walls (Schlosser and

Segrestin, 1979). These recommendations use the concept of an

"apparent friction coefficient" that

is

derived from Equation

26.1

assuming

(26.1d)

where

y

is

the unit weight of the soil

h

is

the overburden height above the nail

/1*

is

the apparent friction coefficient

As

shown in Figure 26.14, the apparent friction coefficient

(/1*)

obtained from pull-out tests in a nailed granular wall corresponds

to the design value generally used for the ribbed metallic strips

in Reinforced Earth walls. At relatively low depth, owing to

the restrained dilatancy

effect, the value of /1* is significantly

greater than 1 and it decreases with depth to tan

(4)

).

Laboratory-scale pull-out tests were conducted

by

Elias and

Juran (1988) in a medium dense sand to evaluate the effect of

the nail installation process on the apparent friction coefficient.

Figure

26.15

shows that the construction process for Reinforced

Earth (i.e., placing the nail during the construction and com

pacting the sand around the nail) produces a substantially

higher apparent friction coefficient than nailing by driving the

nail into the compacted sand embankment. In the latter case,

nail driving

will

significantly reduce the restrained dilatancy

effect on the pull-out resistance. Therefore, design guidelines

for Reinforced Earth walls cannot be extrapolated to soil-nailed

structures.

Grouted nails

are generally gravity-grouted. Their pull-out

resistance is therefore expected to be approximately the same

as tha t of an equivalent straight-shafted anchor installed under

low (or no) grout pressure. The drilling of the borehole for the

grouted nail produces an unloading of the disturbed surrounding

soil that can significantly affect its mechanical properties. The

soil-nail interaction is primarily dependent upon soil recom

paction due to grouting. In cohesionless soils, grouting pressures

of

50

to

100

psi are commonly used to prevent caving as the

casing

is

withdrawn. This grouting pressure

will

induce ground

5

10

o

Z,m

t/(kN/m)

10 20

- , -

t/AVERAGE

17

kN/ml

_ _ 5

10

SOIL: Sand = 33° c = 10 kPa

NAILS: Driven profile

o

2

F

Specifications for

_ reinforced

earth

_

tan

-

,m

Fig.26.14

Soil-reinforcement

friction between a driven nail and

a granular soil.

(After

Cartier

and

Gigan, 1983.)

5

;:

;;

c-

.2

U

4

:E

'0

3

C

Q)

'u

it:

2

.,

2

0

u

C

..,

:;;

Q.

Q.

<

o

Depth of nail, cm

Fig. 26.15

Laboratory pull-out test results.

(1)

Nails placed

during backfilling.

(2)

Nails inserted during excavation. (After Elias

and Juran, 1988.)

recompaction associated with grout penetration into permeable

gravelly seams, thereby increasing substantially the pull-out

resistance of the nail. Apparent friction coefficient values as

high as 3 to 6 have been reported (Elias and Juran, 1988).

Figure 26.16a shows a cross-sectional

view

of an excavated

(a)

(b)

Fig.26.16

(a)

Cross-sectional

view of

an excavated grouted nail

in a granular soil, illustrating effect of

grout

permeation.

(b)

Sectional

view of

an excavated nail in

silty

clay soil.

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200

150

z

-'"

~ 100

50

o

~ L - I

mum:

D = 2in

100

kN/rn Lille Ch ack

L , rn

Fig.26.17 Variation of pull-out resistance of grouted nails

with

embedment length.

(After Louis, 1986.)

nailed soil, illustrating grout permeation into an alluvial soil

grouted under pressure ofless than

70

psi. Figure 26.16b shows

that in a fine-grained cohesive soil the tremie grouting results

in a rather smooth soil-inclusion interface. The presence of

water at the interfaces, specifically in plastic soils,

will

generate

a lubrification effect, decreasing substantially the pull-out

resistance of the nail. Figure

26.17

(Louis, 1986) shows a


summary of pull-out test results obtained with low-pressure

grouted nails in different types of soils.

Jet-grouted nails are installed under a grout pressure that

can exceed

20 MPa

and

is

sufficiently high to cause hydraulic

fracturing of the surrounding ground (Louis, 1981). Similarly

to high-pressure grouting of anchors, the jet-grouting installa

tion technique produces a mechanical interlocking between the

penetrating grout and the surrounding ground that results in

a substantial increase of the effective nail diameter.

I t

also

provides recompaction of the surrounding ground that signifi

cantly improves the pull-out resistance of the composite nailed

soil inclusion. Field pull-out tests on jet-grouted nails (Louis,

1986) yielded ultimate lateral shear stress values as high as

400 kPa in sands and 1000 kPa in sandy gravels.

26.4.3 Estimates

of

Pull Out Cpacity from In-situ

Tests

To date, estimates of the pull-out resistance of anchors and

nails are mainly based upon empirical formulas (or ultimate

lateral interface shear stress values) derived from field experience.

These formulas are useful for feasibility evaluation and prelimi

nary design. Table

26.2

provides a summary of estimated

ultimate interface lateral shear stress (or ultimate load-transfer

rate) values for soil nails and ground anchors as a function of

soil (or rock) type and installation technique. Recently, increasing

attempts have been made to develop field correlations between

the ultimate lateral shear stress ('t

ul t

) and the engineering

properties of soils obtained from commonly used in-situ tests

such

as

the Standard Penetration Test (Fujita et

aI.,

1977) or

the self-boring pressuremeter test (Bustamente,

1975, 1976).

Recognizing apparent similitude between the soil response to

high-pressure anchor grouting and to the expansion of a

TABLE 26.2 ESTIMATED ULTIMATE INTERFACE LATERAL SHEAR

STRESS

VALUES

FOR

GROUND ANCHORS AND SOIL

NAILS.

Grouted Nails

Construction

Method

Rotary drilled

Driven casing

Jet

grouted

Augered

Soil Type

Silty sand

Silt

Piedmont residual

Sand

Dense sand/gravel

Dense moraine

Sandy colluvium

Clayey colluvium

Fine sand

(medium dense)

Sand

Sand / gravel

Soft clay

Stiff to hard clay

Clayey silt

Calcareous sandy clay

Silty sand fill

Ultimate Lateral Shear Stress,

kips/

ft

Soil

Nailing

(Elias

and

Juran, 1988)

2

to

4

1.2

to

1.6

1.5 to 2.5

6

8

8 to

12

2

to

4

1

to

2

8

20

0.4 to 0.6

0.8

to 1.2

1

to

2

4

to

6

0.4 to 0.6

Permanent Ground

Anchors

(Cheney, 1984

r

5 to 9

7

to 13

10 to

20

3.5

to

4.5

b

4.5 to

8.5

b

8.5

to

11 .5

b

2 to 4

1.5

c

a

Cheney recommends a safety factor of 2.5 With respect to the ultimate lateral shear stress values indicated

in

thiS

table.

b Values obtamed for pressure-injected anchors by Jorge

(1969).

C Design value proposed by Weatherby (1982) for hollow-stem augered anchor (assuming a diameter of 6

mches)

in

both sandy and clayey soils.

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882

Foundation Engineering Handbook

(a)

0.8

0.7

0.6

o

00

Q

Q

v

o

v

v

~

0.5

:2

o

•

. :

~ y :

o

Q

j 0.4

v Q

~ 8 , Q ~ Q

v

..

\-t9 .. Y, ' /o /

.... v

••

. . . . lOXar

o

o

0.3

o

• • 00 . . 0

0.2

0.1

o.

y>y...... ;;.-"000 0

od.,,< 0 Type IRS: • Bustamante

et

al.

S:>'6"/ , . Fujita et al.

o Ostermayer and Scheele

K Koreck

~ - . . .

Type

IGU:

... Bustamante

et

al. v Ostermayer

Loose

Dense

Very dense

20

40

60 80 100 120

SPT(N)

(b)

0.3

IRS

«l

c..

• 0 0 0

~ - . '

o .

•

.7.

~ ..

• •

• •

0 1 ; ~ ~ 0 ' . . ·v

2 0.2

IGU

'1 -

0

'

' , v v-.-

....... ' 't:>.-

-

~

_

~ v : r Type IRS: • Bustamante et al. oOstermayer

.1

v. v "''''

,. .... _ _ _ _ V ' Type IGU: ... Bustamante

et

al. v Ostermayer

?-:..

.. ... v V '

Jones, Turner, Spencer

o

0.5 1.5 2

2.5

5 10

15 20 25 30

SPT (N)

Fig.26.18

Empirical relationships

for

the determination

of

the lateral interface shear stress.

(a)

Lateral interface shear stress

for

sand and

gravel. (b) Lateral interface shear stress for silty

clay

soils. (After Bustamante

and

Doix, 1985.)

pressuremeter cell, the French Central Laboratory of Bridges

and Roads (L.c.P.c.) has conducted an extensive research

program including 94 pull-out tests in 34 sites to provide a

data base for field correlations.

Figure 26.18 shows the empirical relationships derived by

Bustamente and Doix (1985) to estimate the ultimate lateral

shear stress values

(Tul

l

) in different types of soils and rocks as

a function of the limit pressure

PI

obtained from the pressuremeter

test or the SPT N value. These guidelines take into account

the improvement of the soil surrounding the anchors by different

modes of injection, considering single-stage pressure-grouted

(lGU)

anchors (grout pressure of O.5PI < P < PI) and multi

stage postgrouted (IRS) anchors (grout pressure> PI). Also

shown in Figure

26.18

is the wide scatter of the

field

data

obtained by the L.c.P.c. and other investigators (Ostermayer,

1974; Fujita et

aI.,

1977; Ostermayer and Sheele, 1977; Koreck,

1978;

Jones and Turner, 1980; Jones and Spencer, 1984) that

have been compiled by Bustamente and Doix to establish these

empirical relationships. The pull-out capacity of the anchor

is

estimated using Equation

26.1.

The effective anchor diameter

is

estimated using a correction factor

(a)

to allow for diametral

expansion due to high·pressure grouting. The

a

values for

IGU

type anchors range from 1.1 in weathered rocks, silty clays, and

fine

sands, to

1.4 in

highly dilatant granular soils, while the

a

values for IRS type anchors range from 1.4 in granular soils

and weathered rock, to 1.8

in

stiff clays and marls.

The available field data pertaining to the pull-out capacity

of nails

is

presently still too limited to substantiate development

of reliable correlations.

An

attempt has been made by Guilloux

and Schlosser (1984) and Louis (1986) to correlate the measured

pull-out capacity of both driven and grouted nails with the

French recommendations (L.c.P.c. and S.E.T.R.A., 1985) for

the determination of lateral shaft friction on bored and driven

concrete piles from pressuremeter test results. Figure

26.19

shows that in fine-grained soils (i.e., fine sands, silts, non plastic

clays) predicted Tult values correlate reasonably well with

pull-out test results, while in dilatant gravelly soils, compacted

moraine, or fissured rocks they may significantly underestimate

the measured ultimate lateral shear stress.

It

appears that further research and

field

testing could

significantly improve the database for estimating the pull-out

capacity of ground anchors and soil nails. The pressuremeter

test appears also to provide valuable data for grouting proced

ure, such as the maximum injection pressure that can be used

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884


5.

Durability considerations may impose severe limitations on

the use of metallic inclusions in aggressive environments.

. In addition, it should be pointed out that in a n a i ~ e d - s ~ i l

retaining structure a certain soil-to-reinforcement relatIve dIs

placement is required to mobilize their interaction and generate

the required resisting nail forces. Therefore, in urban areas the

potential use ofthis technique can be limited by the requirement

to prevent movement of structures in the i m m ~ d i a t e vicinity of

excavation sites. Monitoring of the structure dIsplacements can

be implemented to verify that they are compatible with the

required performance.

26.6 FEASIBILITY EVALUATION

To evaluate the feasibility and engineering use of permanent

ground anchors or soil-nailing systems, soi.l conditions and

existing physical constraints have to be consIdered.

The presence of utilities such as subways or other under

ground facilities and the need to obtain

n d ~ r g r o u n d e a s e ~ e ~ t s

may preclude installation of anchors (or naIls) and can slgmfi

cantly affect the project cost.

Durability considerations require an evaluation of the aggres

siveness of the ground and the pore water, particularly when

field observations indicate corrosion of existing structures.

Ground anchors and nailing should not

be

used for permanent

structures in corrosive soils (e.g., soils with high contents of

cinder, ash or slag

fills,

rubble

fills,

industrial or acid mine

wastes, etc.). The soil tests most commonly used to evaluate

ground aggressiveness are electrical resistivity, pH, and .sulfate

concentration. The critical values for ground aggressIveness

commonly associated with ASTM standards are outlined in

FHW A DP-68-IR, Permanent Ground Anchors, and are sum

marized in Table

26.3.

Assessment of the suitability of the subsurface soil (or rock)

to provide short- and long-term pull-out capacity of the

a n c ~ o r

(or nail) requires a determination of its engineering propertIes,

specifically, shear strength and creep characteristics. ..

In rock, the overall strength

is

controlled by the eXlstmg

joints or discontinuity system. Highly fractured rocks with open

joints or cavernous limestone are difficult to grout and therefore

potential

use

of ground anchors or soil nails should be carefully

assessed.

Permanent ground anchors and soil nails generally cannot

be cost-effectively installed in loose granular soils with SPT

blow count number

(N)

lower than

10

or relative density lower

than

0.30.

Nailing becomes practically unfeasible in poorly

graded cohesionless soils with a uniformity coefficient of less

than 2. In such soils, nailing would require stabilization of the

cut face prior to excavation by grouting or slurry wall con

struction.

In fine-grained cohesive soils, long-term pull-out performance

of the anchors (or nails)

is

a critical design criterion. Permanent

ground anchors and soil nails are, generally, not feasible in soft

TABLE 26.3 FEASIBILITY CRITERIA WITH REGARD

TO GROUND AGGRESSIVENESS.

Test

Resistivity

pH

Sulfate

Chlorides

ASTM

Standard

G-57-78

(ASTM)

G-51-77 (ASTM)

California DOT test 407

California DOT test 422

Critical Values

Below 2000ohm/cm

Below 4.5

Above 500 ppm

Above 100 ppm

cohesive soils with undrained shear strength smaller than

0.5

tsf,

or soils susceptible to creep. A number of national codes

(German Standards and French Recommendations) index the

creep susceptibility to the Atterberg limits and natural moisture

content of the soil. They preclude the use of permanent ground

anchors in organic soils, and plastic clayey soils with liquid

limit

(LL)

greater than 50 and liquidity index (LJ) greater than

0.2

(or consistency index

( I )

less than

0.9).

Soils with a plasticity

index

(PI)

greater than 20 must also be carefully assessed for

creep. In light of the limited experience with soil nailing in

clayey soils, the applicability criteria developed for ground

anchors are recommended for feasibility evaluation of soil

nailed structures.

26.7 SHORT- AND LONG-TERM PERFORMANCE

OF ANCHORS AND NAILS

The effective load transfer from the anchor to the surrounding

ground requires a relative displacement between these tW?

components of the retaining system.

For

ground anchors, thIs

relative displacement

is

generated by prestressing the anchor

immediately after installation. In the passive soil nails, resisting

forces are generated owing to ground displacement during the

construction. Evaluation of the short- and long-term performance

of ground anchors and nails requires determination of their

load-displacement-time behavior for the specific application

and site conditions. Short-term performance

is

defined by a

time-independent load-displacement relationship, while an

assessment of the long-term performance should account for

the effect of time-dependent phenomena such as creep and

relaxation.

26.7.1 Short-term Performance

A static loading of anchors or nails can cause several "short

term" failure mechanisms:

a. Failure of the steel tendon or nails.

b. Shear failure of the soil mass owing to insufficient depth of

anchor embedment.

c. Failure of the grout-tendon or nail bond.

d.

Failure of the soil-grout bond.

The engineering design of the anchored (or nailed) retaining

system for specific application and site conditions should

provide a proper selection of the inclusion (i.e., mechanical

properties, length, inclination, spacings, and corrosion protec

tion) to prevent any of these failure modes.

(a) Selection

of

tendon or nail section should insure that the

working stress in the inclusion does not exceed its ultimate

tensile strength with an acceptable factor of safety. The Post

Tensioning Institute (PTI) recommends limiting the working

tensile stress in prestressed steel to 60 percent of the ultimate

tensile strength for permanent structures and 80 percent for

temporary applications.

(b) To prevent a shear failure of the shallow soil mass

overlying the upper anchors, the bond zone should be located

at a minimum depth of embedment that

is

generally of the

order of 15 ft (4.6 m). This embedment length should also permit

high-pressure grouting without damage to existing facilities.

(c) To insure that the strength of the ground is

fully

mobilized

the grout-tendon (or nail) bond should not be exceeded. The

mechanism of grout- tendon bond involves three components:

adhesion, friction, and mechanical interlock. The neat cement

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grout generally used provides steel-cement bonding values of

the order of 1 to 2 MPa, mainly owing to the mechanical grout

interlocking against irregularities in the tendon surface (i.e.,

ribbing of the bar, threading of rebars, or stranding of wires).

(d) Failure of the ground-grout bond will result in sliding

of the anchor or nail. The bonded anchor or nail length should

be designed to ensure that the force mobilized in the inclusion

does not exceed its pull-out capacity with an acceptable factor

of safety. The empirical relations currently employed to estimate

the pull-out capacity (or the ultimate lateral shear stress) of

anchors or nails can only be used for a preliminary design.

Pull-out tests on soil nails are required to provide reliable data

for final design, and for anchored walls each production anchor

should be tested to ensure its load-carrying capacity throughout

the anticipated service

life

of the structure. For practical reasons,

a minimum bond length of 15

ft is

generally required for ground

anchors in soils. Experience has shown that bond lengths

exceeding

40 ft

do not efficiently increase the anchor capacity.

Anchor inclination

should be as small as possible. However,

steep inclinations may be dictated by practical considerations,

such as right-of way constraints, buried utilities, and soil profile.

A minimum inclination of about 10° to

15° is

generally required

to facilitate and insure effective grouting, particularly under

low pressure. Higher inclination of tieback anchors

will

result

in a transfer of significant vertical forces to the structural element

(i.e., concrete wall or soldier pile) and is generally used only to

reach deep bearing strata or to avoid existing structures.

26.7.2 long Term Performance

Long-term performance of anchors or nails depends primarily

upon the potential of the ground-inclusion system to creep.

Theoretically, creep can develop in the three basic components

of the system: the ground surrounding the bond zone, the grout,

and the steel (i.e., tendon and/or connections). However, in

practice, creep deformations of the cement grout and the steel

are found to be insignificant, while fine-grained clayey soils may

undergo large creep deformation that

will

result in time

dependent anchor displacement. Large creep displacements

have been reported for multi-underreamed anchors (Ostermayer,

1974) and pressure-injected anchors (Bustamente et

aI.,

1978;

Bustamente, 1980) in plastic clayey soils. Relaxation of the steel

tendon (i.e., stress decrease under constant strain) can also affect

the long-term performance. However, for a stress level lower

than the elastic limit of the steel the stress loss

will

generally

not exceed 5 percent of the lock-off stress and its effect on the

displacement will be negligible.

Creep

is

a time-dependent deformation of the soil structure

under a sustained loading owing to a continuous fabric reorien

tation. The creep potential of a clayey soil

is highly dependent

upon the composition and structure of its minerals, its de

positional (preconsolidation) history, and its natural moisture

content (or consistency index). Several investigators (M urayama

and Shibata, 1958; Bishop, 1966; Singh and Mitchell, 1968;

Edgers et aI., 1973) have shown that, as illustrated in Figure

26.20a, for most soils, under a sustained deviatoric stress, the

log of strain rate

is

linearly decreasing with the log of time.

Singh and Mitchell (1968) reported that the slope m of this

linear relationship appears to be a soil property and is indepen

dent of the deviatoric stress

level.

The m parameter, which can

be obtained from laboratory creep tests, can be used to assess

the creep potential of the soil. Values ofm smaller than 1 indicate

a relatively high potential for accelerated creep associated with

a strength loss that will induce a creep rupture. Bustamente

(1980) showed that Singh and Mitchell's creep theory appears

Ground Anchors and Soil Nails in Retaining Structures

885

to consistently describe the observed time-dependent anchor

displacement under a constant load. He therefore suggested

that the creep displacement under a sustained load can be

esitmated using Singh and Mitchell's type equation

Ae«T

L ll

= 1l

o

+

- - (t

1

-

m

- 1)

(26.2)

1-

m

where

T

and L llo are respectively the applied pull-out force and

the initial displacement prior to creep; A,

IX,

and m are interface

creep parameters that are obtained from the experimental

log

l - l o g

t and log

f,.l

- T curves, and

f,./ is

the displacement

rate.

Figures 26.20b and c (Bustamente, 1980) illustrate the creep

behavior of an anchor in a plastic clay and the determination

of the relevant interface creep parameters. The test results

indicate a steady increase of the creep displacement almost up

to failure, which

is

consistent with the m

=

1 value derived from

the experimental log f,.l-t curves.

In spite of the apparent similarity between the laboratory

creep test results and the soil-anchor interface creep behavior

observed in situ, more fundamental studies are required in order

to develop a rational creep model for anchors in plastic

fine-grained soils.

In practice, the critical creep load of an anchor or nail

is

obtained from a load-controlled pull-out test following a

standard testing and interpretation procedure (DIN 4125,1972,

1974; Bureau Securitas, 1977; Cheney, 1984). The French

standard testing procedure

is

schematically illustrated in Figure

26.21a. Figure 26.21b shows actual results of a load-controlled

pull-out test on an anchor in a plastic clay (Bustamente,

1980).

It consists of I-hour sustained load increments of O.lF (where

Fg

is

the elastic limit strength of the steel tendon at which

permanent elongation is

0.1

percent). For each load increment

the anchor displacement (s) is plotted versus log time (T). An

upward concavity of the creep curve indicates an accelerated

creep inducing failure. The slope of the s vs.log T line

is

plotted

against the applied pull-out load to determine the critical creep

load

Fc.

The allowable anchor working load F

w

is the smaller

of either 0.9Fc or

0.6F

g

• The loading increment period can

significantly affect the test result. Therefore, a second test

is

conducted that includes a 72-hour sustained loading stage at

0.9F

w

to verify the long-term anchor performance.

26.7.3 Repetitive

loading

Anchored structures are often subjected to repetitive (or fluctua

ting) live loads such as tidal variations, wind or sea wave

loadings, etc. Permanent ground anchors must be designed to

withstand such repetitive loadings throughout the service period

of the structure, which may include millions of cycles. Documen

ted technical data on the long-term performance of anchors

under repetitive loadings are still very limited. Repetitive

loading tests on anchors for a seawall in France showed (Pfister

et a\., 1982) that for peak cyclic load levels smaller than

63

percent of Po (where

Po is

the ultimate static pull-out capacity)

anchor displacement became negligible after

five cycles.

However,

for larger cyclic loads anchor displacement continued to increase

at a constant or increasing rate. Begemann (1973) reported that

repetitive uplift loads on steel H-piles in sand under cyclic load

amplitude as low as

35 percent of Po generated progressive

pull-out of the piles. Laboratory model studies of repetitive

loading on plate anchors and friction piles have been conducted

by several investigators (Hanna et a\.,

1978;

Andreadis et a\.,

1978; Hanna, 1982) and suggest some trends in the anticipated

anchor response to cyclic loading. Specifically, Al-Mosawe

(1979) and Hanna (1982) showed that displacement rate (per

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/; f..

/;

f..

/; f.. log

T

I ~ .

10

6()

71] ~ i l } ~ i l ]

(i)

Creep

curves

0.1

(ii) Critical creep load

(a)

2

- - - -e- - - -

lOOkN 200kN

E

-_e-e-e-e

300

ekN""e_-e-e-e___

E

- - e - e_ - e - e_e - e - e - . _e_e_e

-e -e ~ k N

"

. e e · _ ____e _ e

E

-e-e_e. 500kN- e - e

__

u

-.-e-.-e_e 00 kN -e-e- ._e

6

....

e_ 7iXJ

e

kN'".-e-e-__

.

8/19/2019 juran1991

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888


1.0

O.OOOOOll:-__

+

_ _

__

~ - - - - , ; ; ; - l ; : = - - - : ; ~ .

1

Number of cycles, N

(a)

Number

of cycles, N

(b)

Fig. 26.22

(a)

Effect

of

number

of

cycles on the

rate

of anchor displacement.

(b)

Effect of number

of

load cycles on anchor displacement.

P

u

= ultimate pull-out load.

(After AI-Mosawe. 1979.)

load

is

attained. In order to determine long-term creep potential,

each load increment

is

maintained until measured deflection

is

negligible (Le., displacement rate

is

smaller than a specified

displacement increment per log cycle of time) and a I-hour

creep test is conducted under the reference test load. Cheney

(1984) recommends that the reference test load should be

1.5

times the design working load in cohesionless soils and 1.25

times the design working load in cohesive soils. The performance

tests are conducted on the first anchors (minimum of two

anchors) to verify the selected installation procedure and

provide reference data for the proof tests.

The proof est consists of a single cycle of ncremental loading

to the reference test loads specified above followed by unloading.

Each load increment

is

maintained until measured deflection

is negligible. The test results are compared with performance

test results on an adjacent anchor.

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Three acceptance criteria have been established (Cheney,

1984):

1. To ensure that the load transfer reaches the anchor bond

length, the deflection of the anchor head should exceed 80

percent of the calculated elastic elongation of the unbonded

tendon length.

2. The total anchor deflection measured at the maximum test

load should not exceed the calculated elastic elongation of

the tendon length measured from the anchor head to the

center of the bond length. This criterion (not valid for anchors

in layered soils or for underreamed anchors) ensures that

the center of gravity of the bond stress· distribution has not

been transferred beyond the midpoint of the bond length.

3. Creep displacement should not exceed 0.08 inches during

the final log cycle of time.

26.8 DURABILITY CONSIDERATIONS

The long-term resistance of permanent ground anchors (or

nails) depends primarily upon their resistivity to corrosion,

ground aggressiveness, and groundwater compositions. Under

ground steel corrosion is induced when a difference in potential

exists between two points that are electrically connected and

immersed in an electrolyte. Electrochemical cells may develop

between the steel tendon and an external metal element or in

local regions of inhomogeneities within the metal surface of the

same tendon. In either case the chemical reaction between the

groundwater and anodic regions in the exposed steel tendon

results in time-dependent metal loss.

For

the corrosion process

to occur, oxygen has to

be

supplied to the metal and therefore

air-moisture solutions, specifically in industrial areas, and soil

layers containing high oxygen content are highly corrosive. The

major variables that affect the corrosion rate are:

1.

Ground aggressiveness: organic soils, and acidic or highly

alkaline soils that contain large concentrations of soluble

salts such as sulfates, chlorides

or

bicarbonates, are highly

corrosive.

2.

Groundwater composition: acidic, alkaline, or salt solutions

have high electrical conductivity, inducing high corrosion

rate.

3. Differential aeration: high oxygen concentration (e.g., in fill

or near the ground surface) results in a cathodic environment,

its local variation in the ground generates electrochemical

cells and thereby accelerates the corrosion rate.

4. High stresses or cyclic stresses in the steel tendon accelerate

corrosion and may generate, in anodic environments, brittle

stress-corrosion cracking.

5. Environmental hazards including bimetallic action, large

temperature changes, anerobic bacteria, and stray currents

in the ground (i.e., currents caused by a mass transit facility,

electrical transmission, or transport systems) will generate

a highly corrosive environment.

The corrosion process can develop through different mecha

nisms, such as uniform surface corrosion,localized pit corrosion,

stress corrosion, corrosion fatigue, and hydrogen embrittlement.

The type of corrosion

will

significantly affect the degradation

rate of the steel tendon and the efficiency of potential protection

systems.

Documented technical data on the long-term corrosion

performance of ground anchors are very limited since only few

permanent installations have been in service more than 25 years

(Weatherby, 1982). However, performance trends can be antici

pated on the basis of extensive research that has been conducted

by the National Bureau of Standards (Romanoff, 1957) on

underground corrosion.

Ground Anchors

and Soil

Nails in Retaining Structures

889

A detailed review of corrosion-induced anchor failures by

Weatherby (1982) yielded pertinent conclusions, specifically in

demonstrating that:

1.

Quenched and tempered prestressing steels have been in

volved in a significant number of tieback failures.

2. The unprotected portion of the tendon just behind the anchor

head

is

highly susceptible to corrosion.

3.

All reported failures occurred in the unbonded length of the

tendon, mostly near the anchor head, where poor corrosion

protection (or none) was provided.

Based on these conclusions, FHWA recommendations for

permanent ground anchors (Cheney, 1984) require that all

anchors used for permanent applications be corrosion-protected

in the unbonded length and at the anchor head. For routine

applications, only a single degree of corrosion protection is

required, which may consist of a grease-filled sheath along the

free

stressing length and grout cover (minimum 0.5-inch thick)

in the bond zone.

A variety of corrosion-protection systems have been devel

oped. They mostly rely on the following basic principles

(Weatherby, 1982; Hanna, 1982).

Simple corrosion protection relies upon cement grout to

generate a noncorrosive high-pH environment and protect the

tendon in the bond zone. Plastic sheaths filled with anticorrosion

grease, special epoxy pitch, or cement mix, and heat-shrinkable

sleeves are commonly used for corrosion protection along the

free stressing length.

Coating

with electrostatically applied resin-bonded epoxy

can be applied to increase the corrosion protection in the bond

zone. Intact resin-bonded coatings, being dielectric,

will

preclude

the formation of galvanic cells in areas affected by microcracking.

Complete encapsulation of the steel tendon

is

accomplished

by grouting it into a uniformly corrugated plastic or steel tube

to provide double protection. The annular space between the

tube and the tendon

is

usually filled with neat cement grout

containing admixtures to control bleeding of water from the

grout.

Compression steel tubes are used by European contractors

to protect the tendon in the bond zone. The tube, which is

high-pressure grouted into the ground, maintains the pressure

injected grout under compression, preventing microcracking.

The unbonded length

is

generally protected using a grease-filled

PVC tube.

Secondary grouting

is

commonly used to protect the unbonded

length of the tendon. First, the anchor (primary) grout

is

tremied

into the bond zone and the tendon is tested and locked-off.

Then, the secondary (antibleed) grout is tremied around the

unbonded length of the anchor, bonding it to the surrounding

ground. Cheney (1984) recommends that secondary grouting

be used only for semipermanent or low-risk applications.

Postgrouting technique can be effectively used to provide

repeated high-pressure grouting in the bond zone with corrosion

protection of the tendon.

For permanent applications of soil nailing, based on current

experience, it

is

recommended (Elias and Juran, 1988) that a

minimum grout cover of 1.5 inches be achieved along the total

length of the nail. Secondary protection should be provided

by

electrostatically applied resin-bonded epoxy on the bars with

a minimum thickness of about

14

mils. In aggressive environ

ments,

full

encapsulation

is

recommended. It may be achieved,

as for anchors, by encapsulating the nail in corrugated plastic

or steel tube grouted into the ground. For driven nails, a

preassembled encapsulated nail, shown in Figure

26.23,

has

been developed by the French contractor Solrenfor (Louis,

1986).

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Protective

tube

with injection

holes

Welding

Fig. 26.23

TBHA nail patented and developed

by

Solrenfor for

permanent structu res.

Anti-corrosion

grease

Anchorage

Anchor

head

Bearing plate

juran1991

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