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In Situ Ground Reinforcement

In situ ground reinforcement is a technique to stabilize existing unstable ground due to the

change of geotechnical conditions by nature and/or human activities.

For example,

intensive precipitation may destabilize existing slopes and induce landslides due to increased

soil weight, reduced soil strength, and water seepage.

Scour of a slope toe in a river may destabilize the river bank.

Excavation in the ground for wall or foundation construction (i.e., a cut situation) induces

unbalanced forces and results in ground movement and even failure if not properly designed and

protected.

Underground tunneling may also induce ground movement and even collapse.

Ground anchors, soil nails, micropiles, and slope stabilizing piles have been used as in situ

ground reinforcement techniques to mitigate the preceding problems.

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Ground anchors are also called tiebacks.

They rely on long prestressed steel tendons bonded in a stable mass at a greater

depth and distance to provide tensile resistance to the unstable mass near the wall or

slope surface.

The tensile force induced by the prestressing of the steel tendons provides additional

normal stresses to a critical slip surface so that the shear strength along the critical slip

surface is increased thus resulting in a higher factor of safety against sliding.

Ground anchors are cement-grouted prestressed tendons installed in in situ soil or rock

by transmitting applied tensile loads into ground to stabilize earth retaining structures or

to provide uplift resistance to structures.

GROUND ANCHORS

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Figure 9.2 shows the basic components of a typical ground anchor, which includes three parts:

• Anchorage set, which consists of an anchor head, a bearing plate, and a trumpet

• Unbonded pre-stressing steel tendon

• Bonded steel tendon with grout

The anchorage component is to transmit the prestressing force from the prestressing

steel to the ground surface or the supported structure.

The unbonded steel is prestressed and can have elastic elongation and transfer the

resistance from the bond length to a structure.

A smooth plastic sleeve as a bond breaker is placed over the steel tendon to separate

the prestressing steel from the surrounding grout.

The bonded steel with grout can provide a tensile load into the ground;

therefore, the bond length should be behind a critical slip surface.

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There are four types of ground anchors commonly used in practice as shown in Figure 9.3:

• Straight shaft gravity-grouted ground anchors

• Straight shaft pressure-grouted ground anchors

• Postgrouted ground anchors

• Under-reamed anchors

The details of the installation procedure for each type of ground anchor may be different and are

discussed in Sabatini et al. (1999); however, the general procedure is the same, which includes the

following:

• Drill a hole.

• Insert a steel tendon.

• Grout the hole within the bond length.

• Install the anchorage assembly.

• Prestress the steel tendon.

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Suitability :

Ground anchors are suitable for a variety of geotechnical conditions.

They can be used in situ soils, rocks, or other geomaterials.

Different techniques may be used to install ground anchors in different geomaterials, mostly

related to drilling and stability of the hole.

Ground anchors may experience excessive creep deformations when they are installed in organic

soil or soils with high plasticity.

Caution should be exercised when ground anchors are used under such conditions.

ApplicationsGround anchors have been used permanently or temporarily in anchored systems.

Permanent ground anchors are typically designed for a service life of 75–100 years.

Temporary anchored systems are mostly used for earth support before permanent structures are

installed.

The service life of temporary anchored systems depends on project needs, but commonly ranges

from 18 to 36 months.

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Temporary Ground Anchors:

Hollow bar is often the product of choice for temporary works.

Its installation method of combining drilling & grouting enables anchors to be installed into a

range of ground conditions,

sometimes unforeseen at the time of initial site investigation, and self-drilling systems are able to

overcome problems with collapsing soils.

Hollow bars are also able to offer a limited prestress function, through the inclusion of bonded

sleeves between the couplers.

It is not possible to provide bonded sleeves over the couplers, as rotary percussive installation

precludes such sleeves.

Furthermore, rod handling and rod release of sleeved hollow bar sections using jaw clamps

requires specialist measures.

Advantages and Limitations

Ground anchored systems have the following advantages over conventional earth

retaining systems, such as gravity walls (Sabatini et al., 1999):

• No obstruction for workspace during excavations

• Tolerable to large lateral earth pressure without a significant increase of wall cross

sections

• Used as temporary excavation support as well as a permanent structure

• No need for select backfill

• No need for deep foundation support

• Fast construction

• Less surface right-of-way issue

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Possible Failure Modes

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Design Parameters

The design parameters for ground anchors may include the following parameters (mainly for

anchored walls):

• Type of application (temporary or permanent, critical or noncritical)

• Project requirements (tolerable settlement, factor of safety against slope failure)

• Construction constraints

• Geometry of project (such as depth of excavation)

• Type of wall facing

• Site subsurface conditions (type and properties of geomaterial, groundwater table, aggressive or

nonaggressive for corrosion)

• Loading conditions (surcharge, water pressure or seismic)

• Number of ground anchor levels

• Method of installation of anchors

• Length and inclination of anchors

• Depth of the upper level of ground anchor

• Bond length

Design Procedure

The detailed design procedure for an anchored wall is provided by Sabatini et al. (1999) as

follows:

1. Establish project requirements including type of project (temporary and/or permanent), project

geometry, external loading (water, surcharge or seismic), performance criteria, and construction

constraints (right-of-way limitations, nearby structures, and existing utility lines).

2. Evaluate site subsurface conditions and relevant properties of in situ geomaterial.

3. Establish design requirements, including factors of safety and level of corrosion protection.

4. Based on type of geomaterial, select lateral earth pressure distribution behind the wall. Add

water pressure and surcharge for total lateral pressure calculation if they exist.

5. Calculate horizontal ground anchor loads by adjusting vertical anchor locations to achieve the

optimum wall bending moment distribution.

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6. Determine required anchor inclination based on construction constraints and geotechnical

conditions.

7. Calculate a vertical force component and a force along the anchor from each horizontal anchor

load.

8. Evaluate horizontal spacing of anchors based on wall type and experience. Calculate

individual anchor load.

9. Select the type of ground anchors.

10. Evaluate the embedment depth and cross section of the wall by calculating vertical and

lateral capacities of the wall below excavation base

11. Calculate factors of safety for internal and external stability of the anchored system and

check them against design requirements.

12. Estimate maximum lateral wall movements and ground surface settlements. Revise design if

necessary.

If any of the calculated values in Steps 10, 11, and 12 does not meet the design requirement,

adjust design parameters of anchors and/or the wall and repeat the above design procedure.

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SOIL NAILINGIntroduction

Basic Concept Soil nailing is a technique to install closely spaced, passive

structural inclusions to stabilize existing unstable ground due to the

change of geotechnical conditions by nature and/or human activities.

Common natural causes are precipitation and/or erosion, while the

common human activity is partial removal of the ground for project needs.

The basic procedure for installing a soil nail consists of drilling a hole in

the ground, placing a steel bar in the hole, and grouting the hole.

Soil nails can be installed on existing or cut slopes and walls during excavation.

Figure 9.16 shows a typical cross section of soil nailing, which includes multiple soil nails,

temporary and/or permanent facing, and drainage system.

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There are different types of nails in practice, which include

• Grouted nails

• Self-drilling nails

• Jet grouted nails

• Helical nails

• Driven nails

• Shoot-in nails

Most of nails are installed in inclination angles of 10∘–20∘ below the

horizontal.

Different from ground anchors, soil nails are not prestressed when they are

installed.

Tension develops during the service due to ground movements.

Sometimes, nails are installed vertically or perpendicularly to critical slip

surfaces, especially for slope stabilization.

This application is similar to that by micropiles and will not be further

discussed in this book.

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Typical temporary facing is installed by steel mesh with shotcreting (i.e.,

spraying concrete).

Cast-in-place, prefabricated concrete facing, or other type of facing may be

installed as the permanent wall facing in front of the temporary wall facing.

A bearing plate and washers

are used to fix the soil nail

on the temporary facing.

Steel reinforcements are

connected with the bearing

plate by studded heads

before the permanent

concrete facing is cast in

place on the temporary

facing.

Figure 9.17 shows the details of facing connection between soil nails, temporary facing

(shotcrete), and permanent facing.

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Suitability Soil nailing is suitable for vertical or nearvertical excavations in both soils and

weathered rocks.

It is also suitable for stabilizing steep unstable terrain of soils or weathered rocks.

The favorable geomaterials for soil nailing installation include (Lazarte et al., 2003):

• Stiff to hard fine-grained soils

• Dense to very dense granular soils with some apparent cohesion

• Weathered rock with no weak plane

• Glacial soils

• Ground that can stand unsupported on a vertical or sloped cut of 1–2 m for 1–2 two days.

The unfavorable geomaterial conditions for soil nailing are (Lazarte et al., 2003):

• Dry, poorly graded cohesionless soils

• Soils with high groundwater

• Soils with large boulders or cobbles

• Soft to very soft fine-grained soils

• Organic soils

• Highly corrosive soil (e.g., cinder or slag) or groundwater

• Weathered rock with unfavorable weak planes and karst

• Loess.

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Applications

Soil nailing has been used for different applications:

• Vertical or near-vertical excavations

• End slope removal to widen existing bridge abutments

• Tunnel portals

• Repair or stabilization of existing earth retaining structures

• Repair or stabilization of existing natural slopes.

Examples of soil nailing: (a) temporary shoring (Byrne et al., 1998)

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Examples of soil nailing: (b) roadway widening (Byrne et al., 1998)

Examples of soil nailing: (c) end slope removal (Byrne et al., 1998)

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Advantages

Soil nailing has the following advantages as compared with alternate technologies (Lazarte et al,

2003; Elias et al., 2004):

• Less right-of-way requirements than ground anchors

• Less concrete usage than conventional reinforced concrete retaining walls

• No need for deep foundations or structural elements below the base of excavations

• Rapid installation with less material than ground anchors

• Less disruptive to traffic and less environmental impact

• More accessible to a job site and low overhead requirements due to the use of light

equipment

• Higher redundancy because of the larger number of nails than ground anchors

• More flexible to deformation than conventional rigid structures

• Easy incorporation of temporary wall facing into permanent wall facing

• More cost effective than conventional rigid retaining walls.

Limitations

The possible limitations of soil nailing are (Lazarte et al, 2003; Elias et al., 2004):

• Not appropriate for applications with strict deformation requirements

• Restriction by nearby existing utilities and structures

• Requirement for permanent underground easements

• Difficult to install below the groundwater table

• Low nail capacity and large creep deformation in high-plasticity cohesive soils

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Failure Modes Similar to anchored walls, soil-nailed walls may have internal, external, and facing

failures as shown in Figures

The internal failures as shown in Figure 9.19 include nail–soil pullout, bar–grout pullout, nail

tensile failure, and bending and/or shear failure through the reinforced mass.

The external failures as shown in Figure 9.20 include compound failure, sliding failure,

and global failure.

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The facing failures as shown in Figure 9.21 include facing flexure failure, facing punching shear

failure, and facing headed-stub connection failure.

Quality Control and Assurance

Quality control for soil nailing installation typically includes the following procedures:

• Verification of the quality of all the materials used

• Inspection of corrosion protection of nails

• Inspection of nail bars free of damage and required length

• Verification of the stability of excavated wall facing

• Verification of the size and length of drill holes

• Verification of nails installed at the desired inclination, spacing, and length

• Verification of sizes and locations of centralizers

• Measurement of amount of grout used in each hole

• Verification of shotcrete placed to the required thickness

• Verification of proper placement of welded wire mesh, bearing plates, and other

connection

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Quality assurance during construction should ensure the following items (Lazarte et al.

2003):

• Construction completed in accordance with plans and specifications

• No excavation height exceeding an allowable value

• Not caved nail drill holes during nail installation

• Nail bars of the right size and type (i.e., steel grade, length, diameter)

• Appropriate corrosion protection systems

• Properly grouting, installation of facing rebar and mesh, and shotcrete

• Sufficient grout strength from grout cubes

• Sufficient shotcrete strength from cores

• Nail pullout capacity from field testing meeting the requirements

• Drainage properly installed

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