Sistemareforzamiento

8/10/2019 Sistemareforzamiento

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2

PREVIOUS RESEARCH

2.1 Introduction to Rockbolting

Rockbolts are now the primary means of roof support in modern underground mining,

replacing timber prop and crib methods. This has been attributed to the increased safety

and productivity gained in their use (Peng and Tang, 1984).

Rockbolts provide strata control through the limitation of deformation, resistance to

free-body movement and crack confinement within the rockmass.

Brady and Brown (1985) identified four objectives for the application of rockbolting in

the mining industry, these being:

The ensuring of overall stability of the mine structure

Protection of major service openings throughout their designed life

The provision of safe and secure access to working areas

Preservation of unmined reserves in a mineable state

Rockbolting is not limited to soft rock roof support, but also rib support in coal mining,

and drive support in hard rock mining. Applications are also found in civil and

construction fields as slope and structural control.


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2.1.1 Types of Rockbolts

While there are many different rockbolts currently on the market, all can be classified

based on the length of anchorage used. These are:

Point anchorage

Full-length anchorage

Table 1 provides a summary of different rockbolt techniques; including anchorage

method and the strata type that is suitable for that rockbolt.

Table 1: Types of Rockbolts - simplified (Peng and Tang, 1984)

Anchor

TypeAnchorage Method Suitable Strata Comments

Slot and Wedge Hard Primitive method

Expansion Shell Medium Common in USA

Expansion Shell -

Bail AnchorSoft

Po

int

Grout (Resin or

cementitous)All, esp. soft

Can be used in combination with

expansion shell anchor

Resin Cartridge All, esp. soft Very common method

Cementitous Grout

(Pumped into hole)

Most Disadvantages include shrinkage

and long setting time

Split Set Weak Cheap, but require specialised

installationFull-length

Swellex Hard Rock HP water used to swell tube within

borehole


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2.1.1.1 Point-anchored Rockbolts

Point-anchored rockbolts are common in competent ground conditions, such as those

encountered in hard rock mining applications. With the development of resintechnology, the use of resin rather than cementitous or mechanical anchorage has been

favoured where appropriate.

The two families of point-anchored support systems are mechanical and grouted

anchorage.

2.1.1.1.1

Mechanical Anchorage

This anchorage system relies on the development of physical interlock between the

rockbolt and the surrounding rock. Rockbolts using this system are slot and wedge

rockbolts, or more commonly, the expansion shell rockbolt.

A Point-anchored expansion shell rockbolt is shown in Figure 1 with components

labelled. An expansion shell rockbolt is anchored through the application of torque to

the tendon, and this then expands the serrated leaves of the shell into the borehole.

Figure 1: Point-Anchored Rockbolt (Stillborg, 1994)


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2.1.1.3 Full-length Anchored Rockbolts

Full-length anchored rockbolts have continuous contact, either directly or via grout,

with the borehole along the full length of the rockbolt. The mechanism of anchorage isdistinct to that of point-anchored rockbolts, and developments in resin technology have

advanced their use throughout the mining industry. The two major full-length

anchorage methods are friction and grouting.

2.1.1.3.1 Friction Rockbolts

Split Sets

Consist of a hollow steel tube slotted along the entire length and tapered at one end.

The tube is forced into the borehole, this being of a slightly smaller diameter than that

of the expanded tube. The tube then acts to generate a radial force onto the borehole

wall. Frictional resistance acts along the length of the tube, as shown in Figure 2.

Figure 2: Friction Rockbolt (Stillborg, 1994)

Swellex

These operate on the same frictional model as the split set, but the tube is placed into a

slightly larger borehole and water is pumped in to pressurise and expand the tube,

forcing the tube against the borehole. This provides immediate support, and unlike

split-set rockbolts, relies on hydraulic rather than physical energy for installation.


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However, susceptibility to corrosion and a small capacity for deformation limit Swellex

rockbolts to hard rock conditions.

2.1.1.3.2 Grouted Rockbolts

For this method, the rockbolt is grouted into the borehole with either a cementitous or

two-part resin grout, providing continuous contact between the encapsulated rockbolt

and the borehole surface.

Technological developments in grout chemistry, and the widespread integration of

grouted rockbolts into mining systems, have resulted in safety improvements, increased

production, improved ventilation and reductions in costs (Peng and Tang, 1984).

Grouted rockbolts have been developed to cope with severe roof conditions, and today

are the primary means of support of mine roadways (Fabjanczyk and Tarrant, 1992;

Wagner, 1997)

Grouted rockbolts may be characterised into one of three groups (Peng and Tang,

1984):

Untensioned

Pretensioned

Post-tensioned

Untensioned grouted rockbolts are most commonly used, as the time saved over

installation of tensioned rockbolts outweighs the minimal support benefits gained

through the tensioning process (Haas, 1975; Nitzche, 1976).

Cementitous grout has high strength and elastic modulus, but requires time to reach full

strength. Thus, application is limited to hard rock, and other competent strata

conditions (Wagner, 1997). An installed rockbolt is shown in Figure 3.


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Figure 3: Fully Encapsulated Rockbolt (Stillborg, 1994)

Polyester resin grout is suited to soft and weak rocks, where high strength and rapid

curing times are required. The resin is usually supplied in cartridge form, with the

action of the spinning rockbolt rupturing the plastic wrapping and mixing the resin

mastic and catalyst.


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2.2 Rockbolt Mechanics

This section details the mechanisms of interaction between rockbolt, resin and rockmass

that govern performance. The principle objective of support systems is to provide

support to the rockmass itself, and this depends on (Bieniawski, 1987):

Dimensions of the opening

Geotechnical properties of surrounding rock

Levels of acceptable deformation in the opening

2.2.1

Support Mechanisms

The objectives of strata control are (Wagner, 1997):

To prevent strata separation and uncontrolled roof failure

To maintain and enhance the strength properties of rockmass through

mobilisation of frictional forces

The support mechanisms through which rockbolting systems achieve strata control can

be summarised as follows (Whitaker, 1998):

Suspension of thin stratum from massive upper strata

Beam building (friction effect)

Formation of a rock arch

Pinning


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2.2.1.1 Suspension of thin stratum

Thin strata layers in the immediate roof can be supported through suspension by

rockbolts anchored in a stable strata horizon, such as an overlying massive strata, orstiffer stratum, as shown in Figure 4.

Figure 4: Cross-Section of Drive, suspension of thin roof slab shown (Wagner, 1997)

Design of a support system through the suspension mechanism must consider the

following factors (Wagner, 1997):

Rockbolt anchorage load capacity must be greater than the weight of the roof

layer to be supported

Support factor of safety must be appropriate

Rockbolt spacing must consider thin strata sagging between rockbolts

Critical length of anchorage must be recognised

Anchorage stratum must be competent, with consideration given to high contact

stresses around mechanically anchored rockbolts

Stable Horizon

Immediate Roof Slab

Supporting Tendons


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2.2.1.2 Beam Building

Beam building theory is applied where the strata are thinly laminated and a competent

layer is out of practical rockbolting range. By clamping together through rockboltingthese layers, multiple beams then become a single beam. This thick beam provides

increased effective stiffness and strength, as shown in Figure 5:

Figure 5: Cross-Section of Drive, Beam Formation (Wagner, 1997)

When the beam deflects, compression occurs in the upper layers, and tension in the

immediate roof layers. This results in differential movement between layers, thus

generating frictional shearing. Resistance to this mechanism is through the following:

Cohesion between layers

Frictional resistance between layers

Clamping force acting normal to the layers

As untensioned rockbolts rely on deformation to generate normal forces, it is

advantageous for the rockbolts to be pre- or post-tensioned, as this ensures that a normal

force will be acting to clamp layers together before strata deformation occurs. Thus,

resistance to horizontal shearing is increased (Snyder, 1983)

Laminated Stratu


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Panek (1956) applied beam mechanics to this mechanism, determining stresses in terms

of bending stress, x, and shearing stress, xy, at any point of the beam. The normal and

shear stress components are shown in Equation 1and Equation 2.

( )t

wLx

2max

2

=

Equation 1: Bending Stress

( )4

3max

wLxy =

Equation 2: Shear Stress

where:

w = unit weight of immediate roof rock

L = roof span (m)

t = thickness of formed rock beam (m)

As the roof span increases, tension cracks appear mid-span on the base of the beam and

near the ends of the beam of the top. Propagation of these cracks leads to failure. The

maximum displacement of the beam, , is determined with Equation 3.

2

4

32Et

wL=

Equation 3: Maximum Displacement of Beam

where:

E = Youngs modulus

Equation 3 demonstrates that displacement is highly susceptible to changes in the beams

dimensions, with displacement increasing as span increases and thickness decreases.


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2.2.1.3 Voussoir Arch

Compressive rock arches are formed in a jointed rockmass to provide a competent

rockmass surrounding the opening, shown in Figure 6. This mechanism relies on the

identification of critical blocks to be supported, and the systematic placement of

supporting rockbolts to establish a compressive rock arch.

In addition to rockbolts, cable bolts are also used to maximise support effectiveness, and

to increase the scope of the compressive arch. In blocky ground, shotcrete and mesh

can also be used to bind the surface together. It should be noted that this mechanism is

rarely encountered in soft rock mining, though jointing in stratum is possible.

Figure 6: Formation of Rock Arch (Wagner, 1997)

Wright (1973) discussed the Voussoir arch, explaining that the blocks in the immediate

roof become self-supporting, with load transferred to the walls of the opening. Thus,

load is distributed around the opening as an arch. Wright also explained how a cracked

mine roof will unload, and within the rock a distributed load arch will form. Hence, the

Voussoir arch method aims to enhance the natural load distribution arching effect.

Compression Zone


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2.2.1.4 Keying of Blocks

Keying of blocks is a secondary support measure, where spot bolting is carried out to

prevent perceived possible failure of unstable rock wedges and blocks. Examples ofconditions where keying is appropriate include zones of localised roof failure, in a

jointed rockmass and in locations where rib spall is likely (Whitaker, 1998). W-straps

and mesh can also be used to prevent roof surface movement.


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2.2.2 Anchorage Mechanisms

Rockbolts provide support and reinforcement to strata through transferral of their

strength and stiffness characteristics to the surrounding rockmass. The mechanism by

which rockbolts transfer support capacity to the rock is termed load transfer. The

driving factor in the growth of Fully Encapsulated Resin Bolt (FERB) use in mines in

the past three decades is a more effective load transfer capacity over point-anchored or

friction rockbolts in most geomechanical conditions.

2.2.2.1 Point-anchored Rockbolts

While a point-anchored rockbolt may be capable of sustaining tensile loading equal to

that of a resin-encapsulated rockbolt, it must be noted that this loading is transferred to

the rockmass only at the anchor point and borehole collar (Gray et al, 1998).

Also, often the rockbolt will be acting through different strata layers, each with

individual mechanical properties and stress conditions. Unlike resin-encapsulated

rockbolts, individual stratum stresses will load cumulatively along the entire length of

the rockbolt. The non-encapsulated portion of the rockbolt carries the load generated

between the anchor and the collar, and the total tensile capacity of the rockbolt may

never be achieved.

Thus, the performance of the system relies of the performance of the anchor and collar,

which must transfer loading onto the tendon, while localised contact stresses acting at

these points can lessen this performance. This is why FERB will perform better than

point-anchored rockbolts. Also, in situations where shearing between strata occurs,considerable displacement will occur before the rockbolt can provide restraint, due to

the absence of a grout between the tendon and the rock (Eaton, 1993).


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2.2.2.1.1 Mechanically Point-anchored Rockbolts

The anchorage mechanism of mechanical anchors is shown in Figure 7. An anchorage

force FH is created by the action of the expansion shell, as well as the axial force F Bacting on the tendon (Kovac, 1999). Thus, the anchorage force is proportional to the

axial force. The load capacity of the mechanical anchor rockbolt is governed by anchor

contact stresses and shear strength of the strata in which the anchor is placed. Load

capacity is independent of tendon length.

Figure 7: Mechanically Point-anchored Rockbolt Anchorage Mechanism (Wagner, 1997)

FB


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The relationship between anchorage and axial forces can be represented thus:

BH FF =

Equation 4: Relationship between anchorage and axial forces

where:

FH = anchorage force

FB = axial force

= Constant, dependent on expansion shell and rock strength

In effect, the mechanically point-anchored rockbolt functions as a clamp, acting at the

anchor point and the borehole collar. Thus, load capacity can only be transferred to the

strata through these points, and acquired load will be distributed evenly along the

rockbolt length. Hence, cumulative axial load can be measured at the collar, as shown

in Figure 8.

Figure 8: Mechanically Point-anchored Rockbolt Load Distribution (Briggs, 1996)


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2.2.2.1.2 Resin and grout Point-anchored Rockbolts

In the case of resin or cementitous grout anchorage, the anchorage force, F H, is

proportional to the encapsulation length (Wagner, 1996). This relationship can beexpressed as:

EH LF =

Equation 5: Relationship between anchorage force and encapsulation length

where:

FH = anchorage force

LE = effective encapsulation length

= Constant, dependant on load transfer characteristics of system

This relationship can also be interpreted as shown in Figure 9.

Figure 9: Relationship between Anchorage Force, FH, and Effective Encapsulation Length, LE

(Kovac, 1999)


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Thus, as strata separation occurs, a cumulative axial load will be placed on the un-

encapsulated tendon. This axial load will then diminish to zero at the top of the tendon,

through load transfer mechanisms. Cumulative load can be measured at the borehole

collar. This is illustrated in Figure 10.

Figure 10: Grout Point-anchored Rockbolt Load Distribution (Briggs, 1996)


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2.2.2.2 Fully Encapsulated Rockbolts

The main failing of point-anchored rockbolts is that system capacity is often dependent

on the performance of that anchorage point. Thus, while full load capacity for thetendon may not be reached, the system may fail through anchor failure, or the rock

surrounding the anchor may fail through excessive contact stresses.

A fully encapsulated rockbolt is considered more effective, as the mechanism of load

capacity may allow maximum support capacity to be achieved at multiple locations

along the tendon (Gray et al, 1998). Thus, the performance of FERB is dictated by load

transfer characteristics of the support system. The effective length of encapsulation, LE,

is based on the length of encapsulation along which load transfer occurs, rather than the

full length of the encapsulated rockbolt (Wagner, 1996). This will be discussed in

following chapters.

A comparison between forces generated on FERB and mechanically point-anchored

systems is shown in Figure 11.

Figure 11: Comparison between Support Capacity of Fully Encapsulated and Point-anchored

Rockbolts


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2.2.3 Loading Conditions

2.2.3.1 Weight-controlled Loading

The weight of the rock to be supported drives this condition. The rockbolts support the

rock by countering this weight, using the suspension support mechanism. Both point-

anchored mechanical and resin-encapsulated rockbolts can be used in this condition.

2.2.3.2

Displacement-controlled Loading

The driving component of this condition is deformation and buckling of strata layers

along bedding planes. In this condition, the application of rockbolts is to bind

individual stratum together, minimising separation and differential shearing. Fully

encapsulated rockbolts are more effective than point-anchored rockbolts, as they act to

clamp layers together, as well as providing shear resistance.


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2.3 Fully Encapsulated Resin Bolts (FERB)

2.3.1 Components of FERB

The widespread use of FERB has given the mining industry improved control over

strata conditions, increased safety and production, improved ventilation and reduced

costs (Peng and Tang, 1984).

The three general components of FERB are:

Tendon

Bearing plate (including nut)

Resin

2.3.1.1 Tendon

From an operations perspective, the tendon should be of low cost, as well as easy to

transport, store and install. The tendon itself dominates the performance of the support

system. Mechanical properties, relative dimensions and geometry all contribute greatly,

and variations within these factors will alter performance considerably.

2.3.1.1.1 Mechanical Properties

The mechanical properties of the tendon must be adequate for the loading conditions

expected. That is, the tendon must be able to withstand the stresses placed upon it by

the surrounding strata (Eaton, 1993). While the majority of tendons are steel,

specialised rockbolts of fibreglass and plastic composites are also available.


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Typical mechanical characteristics of rockbolt steel are shown in Figure 12, with points

of interest labelled.

Figure 12: Typical Stress/Strain Relationship for Rockbolt Steels (SCT, 1996)

where:

R = Elastic Strain

P = Yield Point

M = Ultimate Strength

B = Breaking Strength

C = Breaking Strain

Yield Strength

Yield strength is the stress at which the tendon no longer will behave elastically, and

after which plastic behaviour occurs. That is, beyond this strength, deformation will

proceed with little additional loading.

Elastic Modulus

The elastic modulus of steel used for rockbolt tendons is commonly 200-220 GPa.


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Ultimate Strength

This is the peak stress, after which failure of the bar begins through necking, which

sheds load and lowers stress.

Breaking Strength

This follows tendon failure through necking, and precedes the rapid, physical failure.

Thus, it is the stress after which the tendon physically fails, through applied load and

deformation.

Elongation

Measured at peak load, this allows determination of available elongation of the tendon,

after which the failure process begins.

Rockbolt manufacturers commonly offer a variety of steel strengths for each tendon

design. Table 2 details those offered in Australia, by Celtite and DSI Arnall, formerly

ANI Arnall (Celtite, 1999; ANI Arnall, 1995).

Table 2: Yield and Ultimate Strengths for a range of Rockbolt steel grades (ANI Arnall, 1995;

Celtite, 1999)

Strength (kN)Company Load Point

Standard Grade High Grade Extra High Grade

Yield 125 182 220Celtite

Ultimate 174 302 344

Yield 110 145 220

ANI Arnall Ultimate 165 230 310


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2.3.1.1.5 Tendon Thread

The design of the threaded end of the tendon is vital, as it governs the installation

process. Current standard is a torque nut, with either plastic resin insert or pin, fitted tothe threaded lower end of the tendon. This allows rotation of the tendon into the hole

for resin mixing. After sufficient time for the resin to set, this nut then has additional

torque applied, which allows the insert or pin to break out, and the nut is tightened up

the thread to the collar.

A summary of break out systems is shown in Table 3.

Table 3: Conventional Nut Break Out Mechanisms (Gray et al, 1998)

Type Advantages Disadvantages

Forged Head Simple No tensioning possible

Drive SquareSimple, standard nut is free

running

Requires separate forged drive head, requires

change of dollies

Crimped NutCheap, standard nut. Simply

screwed on end of rockbolt

High residual torque, unreliable break out,

damage to threads

Resin/PlasticPlug

Standard nut. Simply screwedon end of rockbolt

High residual torque, unreliable break out,debris in dolly

Double Lock Nut No debris in drive dolly Non standard nut, unreliable break out

Bulbed Bolt Standard nut. Simply screwed

on end of rockbolt

Unreliable break out, requires special thread

rolling of bolt

Crimped WasherSimply screwed on end of

rockbolt

Non standard nut, unreliable break out, debris

in dolly

Shear Pin Standard nut Unreliable break out, cost

2.3.1.2 Bearing Plate

The bearing plate acts to contain the rock surface surrounding the borehole collar,

providing a large, stable surface for the rockbolt to act against. Most bearing plate

designs include alignment correction mechanisms to compensate for non-perpendicular

installation.


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2.3.1.3 Resin

The primary grouting system for FERB (Eaton, 1993) is resin. This is considered

superior in performance to cementitous grouts as it gives a better anchorage for many

rock types with a shorter setting time. Developed in the early 1970s, fast-setting

polyester resin cartridges were readily adopted by mines, as fast curing times shortened

the installation cycle time.

The design of these cartridges has remained basically the same. However, advances in

resin technology have resulted in increased strength and elasticity, while curing times

have reduced.

A resin cartridge is shaped like a sausage and contains two compartments. The outer

compartment contains the resin mastic, and the inner compartment contains the catalyst.

During installation the cartridge is pushed into the hole, the plastic wrapping is pierced

and shredded by the spinning rockbolt, thus allowing mixing between the mastic and

catalyst. These react to form a hardened resin that bonds the rock to the rockbolt.

The mastic contains polyester polymer and styrene monomer, which react with the

introduction of the benzoyl peroxide catalyst. It also contains up to 75% by volume of

inert powdered limestone filler, which prevents shrinkage during setting, as well as

small amounts or accelerator and inhibitor compounds, which regulate the gel time

and maintain homogeny of the filler.


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2.3.2 FERB Strata Control

Until the early 1990s, support systems were designed to form reinforced rock beams in

the roof strata, using pre-tensioned rockbolts to hold the strata layers together, thus

stiffening the rock and minimising sag. However, advances in rockbolt monitoring

have confirmed that differing rock strata often behave as discrete units (Eaton, 1993;

Gale, 1998). Currently accepted theory is that failure will most likely occur in

individual stratum, determined by the mechanical properties of that stratum. Failure of

individual stratum will then lead to increased stresses on other stratum and so on. Thus,

the failure mechanism is progressive and cumulative.

While rock strength has been shown to increase with the application of active confining

forces, through the use of pre- or post-tensioning of rockbolts, the confinement offered

by any rockbolting system offers negligible measurable strength increase. Rockbolts

are thus a passive system, activated in response to strata dilation of any orientation

(Serbousek and Signer, 1987).

Post failure strength of rock can be increased with the application of relatively small

confinement forces. Rockbolts can generate this confinement after rock movement,

activating against small deformations and preventing significant further deformation.

Thus, FERB improves the load capacity of failing stratum by acting to clamp together

shear planes and cracks within the rock (Gale, 1998). In addition, as a FERB can

transfer its full load capacity at multiple locations along the rockbolt, a system can

actively contain failure at multiple strata horizons (Gray et al, 1998).


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The clamping mechanism is illustrated in Figure 13.

Figure 13: Clamping Mechanism of FERB (Gale, 1998)


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2.3.3 FERB Reinforcement Modes

FERB is a passive support system activated through rockmass deformation. This

deformation is aligned axially, through strata separation, or normally, through slippage

between stratums. The tendon provides axial restraint to deformation, while the grout

and tendon act to resist shear deformation (Peng and Tang, 1984).

2.3.3.1 Axial restraint

The mechanism by which a rockbolt resists axial loading is shown in Figure 14. As

strata layers separate along a parting, shear forces are generated within the resin, acting

in turn on the tendon (Gale, 1998). This mechanism is driven by adhesion and/or

mechanical interlock within the resin (Signer, 1990).

Figure 14: Mechanics of FERB (Gale, (1998)


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2.3.3.2 Shear Resistance

A FERB provides excellent resistance to sliding between roof strata layers. For a

parting to shear, the tendon and grout have to be deformed through bending, so that theresistance of the tendon to bending governs resistance of the system to strata slippage

(Gale, 1998). This is shown in Figure 15.

Figure 15: Mechanism of Shear Resistance (Gale, 1998)

Factors that influence rockbolt shear resistance effectiveness include (Wagner, 1997):

Tendon diameter

Rockmass characteristics

Joint plane friction properties

Grout annulus thickness

Resin characteristics


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2.3.3.3 FERB Failure Modes

Axial failure of a FERB can occur though failure of the tendon, grout, or rock, or by

failure of the interfaces between these components.

2.3.3.3.1

Tendon Failure

Provided anchorage length is sufficient, the full support capacity of the steel tendon can

be reached, after which the required support load exceeds the ultimate strength of the

steel, which then fails. This failure can be corrected through either design, by altering

support density or pattern, or by the use of a higher capacity tendon, either

metallurgically or physically.

2.3.3.3.2 Resin Failure

The resin may fail due to the proximity of the ductile steel tendon, whose deformation

places the resin in tension. As the resin is weak in tension, plastic deformation and

failure will occur. Alternatively, the length of resin encapsulation may be insufficient to

support the required load.

This failure can be minimised through optimising support efficiency, to reduce large

deformations and thus minimise tension loads on the resin.

2.3.3.3.3 Rock Failure

The loading of the tendon also places the rock in tension, which may cause plastic

deformation and eventually exceed the tensile strength of the rock. Again, this failure

may be prevented through support optimisation, to reduce deformation and minimise

tensional loading.


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2.3.3.3.4 Interface Failure

The shear stresses present at the tendon/resin interface are often of a greater magnitude

than those at the resin/rock interface, as the radial forces act on a smaller surface area.

Failure on the tendon/resin interface is possible if the resin and rock are of similar

physical properties and the anchorage length is insufficient, as the shear stresses acting

on this interface are greater. However, in soft rocks, the strength of the rock is often

less than that of the resin, and failure will then occur at the resin/rock interface.

2.3.3.4

In situ loading

While these failure modes are valid for ideal installation conditions, which are possible

in the laboratory, they are seldom found in the field. In practice, the following factors

may influence the failure mode (Fabjanczyk et al, 1998):

Incorrect resin installation, through under- or over-spinning

Variations in borehole size or length

Presence of particulate matter in borehole

Loss of resin into partings and voids in strata, thus lessening volume available

for bonding

Glove Fingering, where portions of the resin cartridge plastic wrapping are not

shredded during installation, and instead act to prevent bonding between tendon

and rock


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2.3.4.2.2 Tendon Diameter to Borehole Ratio

The diameter of the tendon in relation to the borehole sizing governs the size of the

grout annulus. Commercial tendons are supplied with core diameters ranging 5% of

the specified value (Fabjanczyk et al, 1998). This variation is due to production

techniques and profile design (Hocking, 2000).

Peel (2001) found that small variations in annulus thickness could slightly alter support

capacity, while large annuli (+5mm) exhibit substantially reduced capacity. Thus, if a

support system is designed for a certain annulus thickness, installation of tendons with

smaller than specified cores will result in reduced capacity support. If the designed

annulus thickness is 5mm or greater, this can result in dramatic reductions in support

capacity.

2.3.4.2.3 Tendon diameter reduction

As axial load is applied to a tendon, the diameter of that tendon reduces. Thus, during

loading the deformations, which act to provide confining force against the grout, are

actually moving away from that grout. Hence, confinement is reduced, while the

effective resin annulus thickness is increased (Fabjanczyk et al, 1998). Once the tendon

yields, this reduction is intensified, as the tendon proceeds to failure.

The magnitude of this reduction can be minimised metallurgically, through the selection

of steel tendons with higher yield strength.

2.3.4.2.4

Elongation CharacteristicsDiameter reduction is governed by the elongation characteristics of the tendon. Figure

16 details the stress-strain behaviour of a steel tendon under axial load. In the elastic

zone, the reduction is not considered sufficient to diminish the confinement generated

by the deformation profile.


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However, after yielding has occurred, the full length of the tendon experiences

diametric reduction at an increased rate. This acts to reduce the confinement offered by

the deformation profile. Once ultimate strength has been reached, localised necking of

the tendon occurs, which releases all confinement around that point.

Figure 16: Elongation and profile characteristics during loading (Fabjanczyk et al, 1998)


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2.3.4.2.5 Surface Finish

The surface finish of the tendon can affect the strength of bonding between grout and

tendon, and therefore the load transfer characteristics of the support system. Rustingand pitting alter this finish, as does the presence of contaminants such as grease, oil and

dust.

Figure 17 compares load-displacement characteristics of smooth and rusted tendons

(Fabjanczyk et al, 1998).

Figure 17: Effect of surface finish on load transfer performance (Fabjanczyk et al, 1998)


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Figure 18 compares initial load transfer performance between a rusted rockbolt, a clean

rockbolt and a clean rockbolt previously rusted. The increase in loading for the rusted

rockbolts was attributed to an increase in adhesion between resin and surface pitting.

Thus, a rusted surface performs better than a clean surface during initial loading.

However, once sufficient load is applied, this advantage is lost.

Figure 18: Effect of Rusted Tendon Surface on Initial Load Transfer Performance (SCT, 1996)

2.3.4.2.6 Quality Control

Commercial tendons are not always produced precisely to specifications, resulting in

variation from the designed support system. It is also possible to alter the performance

of the tendon through incorrect mine site handling and storage. Installing warped

rockbolts will result in poor resin mixing and varying annulus thickness, giving reduced

anchorage strength.

Poor storage may allow the tendon to come into contact with lubricants, chemicals and

water, and result in altered surface characteristics, corrosion and rusting. Hence,

strength characteristics are reduced.


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2.3.4.3 Resin Properties

The performance of the resin with the borehole surface or tendon is critical to the

system performance. Properties that may influence this performance include physical

properties, confinement properties and the annulus thickness.

2.3.4.3.1 Resin Performance

Maximising load transfer of a support system requires the following resin properties

(Eaton, 1993):

Resin strength greater than strata, allowing transfer of stresses to tendon duringstrata dilation, rather than failure

High compressive modulus, allowing stress transfer from resin to tendon before

significant strata movement occurs

Minimal creep properties over time

Low viscosity during installation, to maximise contact with irregular surfaces

Eaton (1993) compared the load-transfer characteristics of low and high performance

resin, keeping other elements of the support system identical. It was found, as shown in

Figure 19, that the high performance Celtite AT resin was effective in resisting strata

deformation, by keeping total deformation to 10mm, while the low performance resin

experienced 50mm total deformation.


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2.3.4.3.3 Resin Annulus

The size of the resin annulus is driven by economic and operational factors, such as drill

cycle times and volumes of resin used, as well as geotechnical factors. Eaton (1993)

advised that resin annulus should be minimised as the closer the tendon to the borehole

the more immediate the stress transfer within the system.

Gale (1990) found that for optimum performance, the annulus thickness should be

minimised to aid mixing during installation, and to improve load transfer between

tendon and rock through proximity. However, smaller annuli often contain air pockets,

formed on the tendon during the installation process.

Increased annuli resulting from larger boreholes display reduced shear stress capacity

(Fabjanczyk and Tarrant, 1992). Laboratory push tests found a 30% drop in the load

transfer capacity of a 22mm diameter tendon when the annulus was increased from

2.5mm to 3.5mm thickness. This downward trend is shown in Figure 20.

Figure 20: Effect of Hole Diameter on Load Transfer (Fabjanczyk and Tarrant, 1992)


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Gerdeen et al (1977) suggested that larger boreholes would provide improved anchorage

due to a greater surface area to distribute shear forces. However, use of a small

diameter rockbolt in this borehole would result in a large annulus thickness, thus

lowering anchorage capacity. Therefore, in large boreholes, larger diameter rockbolts

must be used to ensure performance is optimal.

Optimum resin annulus can be defined as the minimum thickness that can be applied,

given operational constraints, viscosity requirements and the need for adequate mixing

of mastic and catalyst. Thus, optimum annulus can only be identified through

consideration of the FERB support system.

Peng and Tang (1983) found that the optimum annulus was 3.2mm, or 6.4mm

difference between tendon and borehole diameter. This optimum was true for that

experimental series, and may not be universal.


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Peel (2001) conducted pullout tests of 21.7mm core diameter tendons anchored in a

variety of borehole sizes. Mix-and-pour resin was used in these experiments to avoid

anchorage inconsistencies due to the presence of cartridge wrapping. He found that

capacity was similar for annulus thicknesses of 2mm to 4mm, with a 25% capacity

reduction once annulus thickness reached 5mm. This reduction was attributed to an

alteration in the failure mechanism, with the resin playing a greater role.

These results are shown in Figure 21. While these results differed from those of

Fabjanczyk and Tarrant, it is likely this is due to different testing methods.

Figure 21: Effect of Hole Diameter on Load Transfer (Peel, 2001)


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2.3.4.4 Rock Properties

The third component of a support system is the rock into which the tendon has been

grouted. The behaviour of the rock under load contributes greatly to the performance ofthe support system as a whole.

Peng (1988) identifies rock types commonly associated with coal mining, in order of

decreasing strength, as:

Sandstone

Limestone

Sandy Shale

Shale

Clayey Shale

Clay

2.3.4.4.1

Physical Properties

Given that soft rock is largely heterogeneous, points of weakness often occur at manypoints along the length of tendon. These variables include:

Lithology

Bedding planes

Physical Properties, including

o Compressive Strength

o Tensile Strength

o

Elastic Moduli

Presence of Water

Bedding planes in particular will affect support systems, as these are frequently of much

different strength to the surrounding stratum. Weaker stratum will shed load and

deform easily, leaving stronger stratum to carry this load. Thus, partings will cause load

redistribution, resulting in increased loading of stiff stratum and deformation of weak

stratum.


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While a typical uniaxial strength of weak rock lies in the range of 20.8MPa to 48.2MPa,

the presence of water will greatly reduce this, into a range of 0.0MPa to 13.8MPa (Peng,

1994).

2.3.4.4.2 Weathering

Weak roof is especially susceptible to air weathering. Weathered roof surface rock will

fail between rockbolts, causing the roof beam to lose integrity. In addition, weathering

of the newly exposed rock will occur, furthering the failure process. This surface failure

can be controlled using straps or mesh, or avoided through the application of shotcrete.

2.3.4.4.3 Length of Encapsulation and Rock Strength

The demonstrated strength of a resin anchorage is related to the strength of the rock and

volume of included resin, which governs the length of encapsulation formed along the

tendon. Franklin and Woodfield (1971) found that weaker rocks required more resin, or

greater encapsulation length, to achieve the same strength found in stronger rocks. A

longer length of encapsulation allows greater length of load distribution, reducing peak

load and thus not exceeding rock strength.

2.3.4.4.4 Loss of resin into strata

Bedding planes and other discontinuities inherent in sedimentary rocks represent an

avenue of escape for resin during installation. While fully mixed resin may provide

some degree of restraint to a parting, unmixed resin is simply lost. This reduction in

volume results in reduced anchorage length, and consequently a reduction in support

performance.


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2.3.4.5 Installation Processes

In order for a support system to achieve full potential, the installation of the tendon

must follow the designed process. Resin and tendon manufacturers have developedprocesses to ensure optimum installation. However, in an operational environment

these cannot always be replicated, resulting in incorrect installation and diminished

performance.

2.3.4.5.1 Borehole Quality

Roughness

Karabin and Bebevec (1978) investigated FERB performance against borehole quality.

Their study indicated that the bond between resin and borehole surface was purely

mechanical, with the deformation profile of the tendon and the irregular surface of the

borehole providing physical resistance to displacement. Hence, shearing across these

surfaces would precede shear failure of the system.

Field observations of boreholes concluded that the condition of the borehole surface

significantly affected the load transfer characteristics of a FERB support system.

Gerdeen et al (1977) conducted a series of laboratory tests comparing anchorage

capacity between boreholes of varying roughness. From Table 4 it is clear that the

boreholes with random grooving out performed smooth holes by a factor of 3, and as-

drilled holes by a factor of 2. Thus, it is clear that roughness contributes greatly to

support system performance.


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Table 4: Borehole condition and anchorage capacity (Gerdeen et al, 1977)

Hole Diameter (mm)

19mm Tendon 25mm Tendon

Borehole

RoughnessHole Condition

25 28 32 38 32 38

Clean 56.0 36.6 32.3 75.2 59.7 -

Clean Wet 74.0 - - - - -As-drilled

Dirty 69.1 56.4 - - - 66.3

Clean 38.5 18.1 15.9 27.5 37.8 10.4

Clean Wet 31.4 27.3 - - - -

Dirty - - - - - -

Worked

Smooth

Cast Smooth - - - - - 24.3

Clean 116.2 178.8 - - - -Random

Grooving Dirty 89.9 90.9 86.4 109.6 189.8 87.7

Therefore, comparisons between FERB systems must include consideration of the

inherent variation due to borehole quality.

Presence of Water

Dunham (1973) compared load transfer performance of rockbolts installed in dry and

water filled holes. He found similar average anchorage capacities for dry and wet

boreholes, and concluded that the effect of water is negligible.

Gerdeen et al (1977) conducted laboratory testing after field observations indicated

water, dust and roughness were all influential factors. The results of these tests arereplicated in Table 4. No differentiation could be made between wet and dry holes,

indicating water does not adversely affect anchorage capacity.

Gray and Fabjanczyk (1992) suggested that the presence of water might reduce

anchorage capacity in clay rich rocks. The use of water flushing during drilling may

liberate clay particles from the rock, resulting in a thin film of clay depositing on the

borehole surface, which then would inhibit resin adhesion to that surface.


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Presence of Drilling Fines

Clay fines and other dust present on the borehole surface, released during dry drilling,

would inhibit resin adhesion and thus reduce anchorage capacity (Gray and Fabjanczyk,

1992).

2.3.4.5.2 Resin Mixing

During installation, it is vital that manufacturer specified spinning and setting times are

adhered to. Over- or under-spinning will alter final characteristics, diminishing load-

bearing capacity.

2.3.4.5.3 Glove Fingering

During installation, the rockbolt is spun to shred the plastic resin capsule between the

deformation profile and the borehole surface, and to mix the mastic with the catalyst.

Ideally, the plastic cartridge will be pushed to the top of the borehole.

If the installation is incorrect, the cartridge may not be shredded adequately. This then

may glove the tendon, preventing adhesion between grout, tendon or boreholesurface. This reduces the effective length of encapsulation, which in turn diminishes

load-bearing capacity.


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2.4 Load Transfer

2.4.1 Introduction

FERB is a passive reinforcement system, reliant on strata deformation to generate

support. As deformation occurs and is restrained, the tendon load is transferred to the

surrounding rock. Performance of the FERB support system is governed by the

efficiency of this load transfer mechanism. Thus, load transfer capacity is a measure of

the effectiveness of the system (Gray et al, 1998). The load transfer mechanism

generates and sustains reinforcing force in the tendon as a result of strata deformation

(Fabjanczyk and Tarrant, 1992).

The FERB support system is comprised of three elements: the grouting material, the

surrounding rockmass and the tendon, including the faceplate and nut. When installed

this system initially provides passive support. That is, the support system is activated

once bedding planes within the rock separate, which places load onto the tendon, which

is then transferred along the length of the tendon and separation is resisted.

The FERB may provide support at a single point, such as a parting, or multiple points

along the grouted length. The manner of this load transfer distribution has led to

conflicting linear and exponential transfer theories.

Load transfer is defined as the change in load with respect to the distance along the

tendon, giving a load transfer rate (Serbousek and Signor, 1987). It has also been

stated as a measurement of peak shear stress capacity and system stiffness (Fabjanczyk

and Tarrant, 1992), where peak shear stress is the average shear stress over anencapsulated length at the maximum applied load.

In these definitions, the support system fails when any of the components in the system

fail through shear or yield failure, due to the stresses generated by stratum separation.

The system will also fail if the rock/grout or grout/tendon interface shear strength is

exceeded. Shear stress capacity can be calculated using Equation 6.


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LF=

Equation 6: Shear stress capacity

where:

F = change in force (N)

= Diameter of rockbolt or borehole (mm)

L = length of distributed force (mm)

= Shear stress


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The objectives of optimising load transfer are (Gale, 1990):

To increase system stiffness by maximising load transfer, thus lessening tendon

length under load

To utilise the complete tendon capacity, by preventing resin or rock failure

To optimise support system through tendon length, support density and

placement

To manage collar loads


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2.4.3 Mechanisms

FERB provides strata reinforcement through axial restraint and shear resistance. As the

focus of this research project is axial restraint, and as the facility would require

significant modification to allow study of shear resistance, axial restraint will be the

mechanism assumed to be acting during load transfer. This assumption is accurate as

the applied load is purely axial, with any shear force due to weight of the jack negated

once the jack bears against the face of the core.

2.4.3.1 FERB Load Transfer Mechanism

The mechanism of load transfer in a FERB is driven by the behaviour of the support

system displacing under load relative to the resin and rock. Load transfer includes the

mechanisms of adhesion and mechanical interlock, which is the transferral of load

between tendon, resin and rock through contact surfaces (Signer, 1990).

FERB tendons are manufactured with a helical deformation pattern, providing a

significant contact surface. The surface of the borehole is generally irregular, a result of

the drilling process and geolithic variables. The resin acts to fill the space between rock

and tendon, forming an annulus connecting these irregular contact surfaces.

It is accepted that load is transferred from rock to tendon by shear resistance of the

grout, but the nature of this resistance has not been proven. It is believed that shear

resistance is created through adhesion or mechanical interlock, or a combination of the

two (Signer, 1990). Serbousek and Signer (1987) have demonstrated that in elastic

loading, mechanical interlock is the primary means of transferral of shear force betweenthe support components.

2.4.3.2 Factors Influencing FERB performance

The key components of the FERB are the tendon, grouting substance and the

surrounding rock. While the tendon is comprised of ductile, high strength and high

elasticity steel, the grout and resin are weaker, brittle materials of lower strength

(Serbousek and Signer, 1987).


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These components acting together govern the performance of the FERB, with the

tendon absorbing load and deformation, thus preventing the tensional failure of the

grout and rock. As long as there is a sufficient length of encapsulation, the ultimate

capacity of the tendon can be reached (Serbousek and Signer, 1987). This

encapsulation length is influenced by the following factors (Signer, 1990):

Material properties of tendon, grout and rock

Borehole smoothness

Grout annulus thickness

Installation quality, including grout mixing, tendon centrality and collar

condition

In order for optimal performance of the FERB to be achieved, the following conditions

are necessary (Gale, 1990):

Resin installed according to manufacturers instructions

Borehole surface free of particulate matter that may interfere with resin/rock

interface

Small resin annulus allowing efficient mixing and maximising loading

mechanism between rock and tendon


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Figure 23: Force distribution in FERB around strata dilation (SCT, 1996)

2.4.4.2 System Stiffness

Stiffness of the tendon/resin or resin/rock interface is the rate of shear stress generated

for a given strata displacement (Wagner, 1997), and is calculated using Equation 8.

l

AEk=

Equation 8: System Stiffness

where:

k = system stiffness

A = cross sectional area of tendon

E = Youngs modulus of tendon (GPa)

l = length of tendon over which the strata deformation is dissipated (m)


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For a FERB, l is related to the strata displacement, and the shear strength of the

tendon/resin and resin/rock interfaces. Figure 24 illustrates the benefit of using FERB,

as a longer tendon length allows greater distribution of load transfer, resulting in a

reduction of generated forces (Wagner, 1997).

Figure 24: Effect of active rockbolt length on support resistance (Wagner, 1997)


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2.4.4.3 Loading due to Strata Separation

Calculation of loading due to separation of strata layers as a function of system stiffness

and tendon length is shown in Equation 9 (Wagner, 1997).

l

ksFB

2=

Equation 9: Loading due to strata separation

where:

FB

= transferred load (kN)

k = system stiffness

l = tendon length experiencing dissipating load (m)

s = length of strata displacement (m)

Loading of a FERB experiencing a small amount of strata separation is shown in Figure

25. Load is applied to the tendon at the separation point, dissipating though the load

transfer mechanism. It should be noted from Equation 9 that system stiffness would

significantly influence the maximum sustainable force in the tendon.

Figure 25: Forces generated around strata separation in FERB (Wagner, 19970


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With the application of strain-gauged rockbolts, it is now possible to calculate load

distribution along the length of an installed FERB through the measurement of resultant

strains. This load distribution allows identification of individual stratum failing within

the roof, as these experience greater displacement, and thus greater strains are recorded.

Figure 26 illustrates this, with a series of diagrams of load profiles along tendon lengths,

with the position of weaker stratum bands noted. A comparison of the support

performance for good and poor load transfer systems is also made.


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Figure 26: Comparison between good and poor load transfer in FERB (Fabjanczyk et al, 1998)


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2.4.5 Stress Distribution Profile

2.4.5.1 Exponential Distribution

Studies using finite element mathematical modelling techniques of the stress

distribution around a tendon grouted into a cylindrical hole in rock (Coates and Yu,

1970) have shown that the majority of load is transferred through shear stresses rather

than through the base plate. Analysis revealed that up to 80% of load is transferred

through shear mechanisms, and that the load on the tendon decays exponentially away

from the loading point.

As the use of FERB became widespread, rockbolt knowledge remained confined to pull

out tests, which provided details on load bearing capabilities of the support system but

very little on the mechanisms within that system. Pells (1974) observed that some

rockbolts failed at a very low overall rockbolt strain. He deduced that the rockbolt loads

were confined to a small distance on either side of the loading point. Thus, the rockbolt

was failing due to this concentration of strain. Subsequent testing indicated that the full

length of the rockbolt might not be experiencing full loading.

Farmer (1975) further investigated FERB stress distribution, deriving a theoretical stress

distribution model showing exponential decay along the rockbolt. This theory was built

around a finite slice through tendon, grout and rock, based on a boundary element

processes.

= ax

X

2.0

0

1.0

Equation 10: Theoretical FERB stress distribution

where:

x = shear stress at distance x along the rockbolt (MPa)

0 = applied load (N)

a = annulus thickness (mm)

x = distance along rockbolt (mm)


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Figure 27: Theoretical Stress Distribution of FERB in rigid socket with thin resin annulus

(Farmer, 1975)

Farmer then sought to validate this theory through laboratory testing to compare

experimental with theoretical stress distributions. The testing consisted of pullout tests

of strain-gauged tendons from cores of cement, limestone and chalk. Farmer found that

the experimental results were consistent with theoretical distributions up to a certain

high load. Above this load, it was found that debonding was occurring at the rock/grout

and resin/tendon interfaces. It was concluded that the theory was valid for elastic

loading of the system.

Figure 28 and Figure 29 show the theoretical behaviour (broken lines) and experimental

results (solid lines) for encapsulated lengths of 350mm and 500mm respectively. At

low loads, there is reasonable correlation between both sets of results, but as load

increases, this correlation is reduced. In addition, at high loads a greater portion of the

rockbolt experiences loading, rather than the small length predicted.


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Figure 28: Theoretical and Experimental Stress Distribution - 350mm rockbolts (Farmer, 1975)

Figure 29: Theoretical and Experimental Stress Distribution - 500mm Rockbolts (Farmer, 1975)


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Haas and Nitzsche (1976) investigated the performance of a pre-tensioned FERB,

observing the effect of tensional confinement on bed separation, as it was recognised

that greater bedding displacements were occurring in the upper portion of the rockbolt.

A model was created using finite element methods assuming symmetry of stress

distribution into the immediate rock. This model showed that loading of the rockbolt

was not uniform and that the stress variation along the rockbolt was non-linear. This

behaviour was similar to that found in Farmers research, and is shown in Figure 30.

Figure 30: Variation of Rockbolt Load into Grout (Haas and Nitzsche, 1976)

It was concluded that almost 80% of the pre-tension was lost within 125mm (5 inches)

of the nut, and the residual load was only 5% 210mm (10 inches) from the nut.


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Therefore, pre-confinement offered to the upper portions of the tendon is insignificant,

and strata deformation in this portion will be unaffected by any pre-tensioning.

Hyett, Moosavi and Bawden (1996) used numerical and analytical methods as well as

Farmers equation to create a model describing a passive FERB with free ends, with

loading being applied at the centre of the rockbolt due to bed displacement. The

conclusions drawn using this approach were:

There is an exponential stress distribution, with peak load located at the point of

displacement (in this case bed separation)

Increasing displacement resulted in an increased load transfer rate

At low displacements the load is transferred along the whole tendon length

At high displacements load transfer is confined to the section of tendon closest

to displacement point, generating high stress concentrations

Whitaker (1998) derived two models of stress distribution along a FERB, these being

field and laboratory models. The laboratory model was based on the FERB Pull-test

facility at UNSW, and simulated a strain-gauged rockbolt, grouted in a concrete coreand confined within a biaxial cell. The tendon was loaded axially by use of a jack

acting against a nut on the end of the tendon, as well as against a plate positioned on the

face of the core.


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Figure 31: Exponential Load Distribution - Laboratory Model (Whitaker, 1999)

This model displayed an exponential stress distribution, and found that increases in

confinement resulted in increased load transfer rates. However, the field model

conflicted with this, because it described linear transfer rates, as discussed in the next

section.


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2.4.5.2 Linear Distribution

Gerdeen (1977) investigated the influence of tendon length on the load transfer process

through laboratory experimentation. Using 500mm (~20 inches) strain-gaugedrockbolts encased in a plaster material he found that the entire length of the tendon was

utilised to transfer loads, with stress distribution along the tendon decaying linearly.

This transfer rate increased with increased loading, as shown in Figure 32.

Figure 32: Linear Stress Distribution (Gerdeen, 1977)

Extensive field studies using strain-gauged rockbolts have sought to determine the stress

distribution in a mining environment, as opposed to a laboratory environment.

Radcliffe and Stateham (1980) observed 50 strain-gauged rockbolts in three different

mines with bedded strata. They found a linear stress distribution along the length of

encapsulation, and that load transfer rates were symmetrical around a parting located at


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the centre of a rockbolt. However, load transfer rates were not symmetrical about a bed

separation that did not occur in the centre of the rockbolt, as shown in Figure 33.

Figure 33: Linear Stress Distribution due to Bed Separation (Radcliffe and Stateham, 1980)

Patrick and Haas (1980) conducted similar tests at different mines and reached similar

conclusions. They also found that a rockbolt might experience both tensional and

compressive loading due to bed separation. Deflection of the lower layers placed the

region about the lower parting in tension, while deflection of the upper layers, if the

middle stratum remained stable, would put the region about the middle and upper

parting in compression.

Serbousek and Signor (1987) sought to prove that loading was confined to short

distances around partings or from the rockbolt head, as found by Pells (1974), rather

than the full length of encapsulation. Laboratory and field pull-out tests of strain-

gauged FERB found that anchorage length required to dissipate load remained constant

as applied load was increased. This is shown in Figure 34. They concluded that

increased loading generated a stiffer system allowing increased load transfer rates.


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Figure 34: Increased Load Transfer rate as applied load increased on constant anchorage length

(Serbousek and Signor, 1987)

The second finding of this research was that the length of tendon was insignificant to

the systems load transfer characteristics. It was found that a short tendon transferred

loads over a similar distance as a longer tendon. Thus, the shorter tendon generated astiffer system allowing increased load transfer rates. This is shown in Figure 35.

Figure 35: Similar load transfer rates for varied anchorage length (Serbousek and Signor, 1987)


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Serbousek and Signor concluded that 90% of applied load in a standard pull-out test

dissipates into the rock within 24 inches of the rockbolt head.

Strata Control Technology (SCT) carries out numerous field strain-gauged pull-out

tests. Results consistently demonstrate linear decaying stress distribution along the full

length of encapsulation. Results also show that transfer rates vary as the tendon passes

through strata layers of differing strengths, suggesting that peak transfer rate is related

to strata strength. This mechanism is shown in Figure 36, where stress is distributed

along the full length of the tendon as it passes through several different strata layers.

Figure 36: Field distribution of stress, highlighting loading around strata parting (SCT, 1996)


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2.4.5.3 Summary of Load Distribution

It is apparent from studying the literature that the mechanism of load distribution has

not been adequately proven. An exponential decay of load has been found in thelaboratory, in computer modelling and in the field, while linear decay has been

consistently found in field measurements. In addition, while some studies have

concluded a discrete distance is required to dissipate load, others have found that the

entire tendon length is required.

While these discrepancies may be attributed to differences in testing methods, clearly a

scientific study is required to isolate each conclusion, and to identify the root causes, so

that the load distribution mechanism can be fully defined.


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2.5 Deformation Profile

2.5.1 Introduction

While FERB load transfer mechanisms have been extensively researched and the result

of these investigations published, little public knowledge exists regarding the impact of

the design of tendon deformation profile. The studies available frequently neglect to

specify rib height or spacing, although these factors are credited as the driving

mechanism of load transfer. Thus, it is difficult to compare studies due to uncertainties

regarding the actual profiles.

Research by the manufacturers of strata control products has remained confidential.

This means that engineers designing support systems rely on empirical methods, with

expert consultation being the domain of the manufacturers.


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2.5.2 Tendon Profile Significance

Manufacturers commonly release the following basic information regarding their tendon

products:

Yield Strength

Ultimate Strength

Uniform Elongation

Density

Core Diameter

Bar Diameter

The deformation profile itself appears as a simple schematic, with no information

regarding geometric design. While it is possible to calculate the deformation height

from the core and bar diameter, other dimensions such as spacing, width and pitch angle

are determined through physical measurement only.

In order for engineers to be well-informed in designing support systems, the effect ofprofile design on support characteristics must be comprehensively understood. As the

profile is a support system component that can be extensively altered through design, a

sound knowledge of this variable would allow further optimisation of support system.


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Figure 38: Sample Deformation Profile Designs (Gale et al, 1995)

Figure 38 shows a range of deformation profiles, as compared by Gale et al (1995).

Threads A and D are combination designs (similar to the HPC rockbolt), threads B, C

and E are basic designs for comparative purposes, and thread F is similar to the designof many rockbolts in industry use.


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Figure 39: Load/Displacement behaviour variation between different deformation profiles (Gale etal, 1995)

The load transfer performance of these profiles is shown in Figure 39. It is clear from

these curves that different profiles behave in quite distinct manners. Thus, a clear

knowledge of profile performance mechanisms is vital in designing optimal support

systems.


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2.5.3 FERB design considerations

FERB deformation profiles are designed to assist in the installation process through

shredding of the resin cartridge and by mixing the mastic and catalyst. The profile is

also designed as an irregular surface, promoting adhesion with the resin. The geometry

of the profile is designed to assist in the generation of confinement, through the

mechanical interlock mechanism.

When comparing different tendon designs it is important that consideration is made of

the intended purpose. Different manufacturing processes allow various designs to be

categorised as follows (Gray et al, 1998):

Hot rolled, ribbed tendons (Y bars, T bars, J bars, HPC bars)

Plain tendons, bent into worm profile, providing economical rib with good

mixing ability (Wriggle rockbolts)

Plain tendons, cold worked, forming roughened profile (Videx bars)

Continuously threaded tendons


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2.5.4 FERB Manufacturing considerations

As tendons are manufactured with a deformed surface, the process of adding thread to

attach a nut is made difficult. A smooth, round bar would allow simple cold rolling of

the thread (Gray et al, 1998); this then provides the problem of adding a deformed

profile.

Thus, the following alternate techniques are used to add the thread to a deformed

tendon:

Skimming the ribs off, then cold rolling the thread on

Swaging the end of the bar, then cold rolling the thread on

Cold rolling the deformation and thread simultaneously onto a plain, round bar

Each of these methods is costly to the manufacturer. While large profiles have greater

load transfer performance, they are more expensive to manufacture due to the threading

process. Thus, cost as well as performance need to be considered in the design process.


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2.5.5 Profile Design Considerations

Given that deformation profile is a key component in the load transfer mechanism, the

relationship between design and effectiveness must be fully understood for engineering

use. That is, by understanding the role of deformation profile in load transfer, the

system may be optimised through engineering an optimal design.

Fabjanczyk and Tarrant (1992) carried out laboratory analysis of 50mm push tests,

finding that the deformation profile significantly affected confinement generation within

the resin annulus, with the height of the deformations being critical.

Further research (Fabjanczyk et al, 1998) looked at tendons of progressively reducing

deformation height, with an AX bar reduced from 1mm deformation height to zero

deformation in 3 steps. The resulting graphs, Figure 40 and Figure 41, demonstrate

significant loss of stiffness and load capacity, as well as loss of shear capacity due to

reduction in confinement.

Figure 40: Load/displacement Performance of AX Bar with reduced deform thickness

(Fabjanczyk et al, 1998)


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Figure 41: Load/Confinement of AX Bar with reduced deform thickness (Fabjanczyk et al, 1998)

Research (Gray et al, 1998) has also found that large deformations act to concentrate

stresses. Stress Raisers are formed at the base of the profile, at the joining of

deformation and tendon core. Rounding this join into a smooth curve can reduce this

stress concentration. This then reduces the chance of crack propagation in the tendon.


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2.5.6 Deformation Profile and Frictional Control

After failure has occurred, the rate of load transfer is governed by friction. Figure 42

demonstrates the process whereby the loaded tendon displaces relative to the resin,

causing the deformation profile to apply radial force to the resin, and thus to the rock.

Figure 42: Mechanism of Frictional Control in Load Transfer (SCT, 1996)

Assuming the resin and rock are of similar strength, failure will occur on the

tendon/resin interface. As the tendon displaces under load, the radial forces applied by

the deformation profile will fracture resin. As it is in a post-failure state, the strength of

the crushed resin is sensitive to the confinement generated by the tendon. Thus, if

generated confinement is reduced, support capacity of the resin declines and the load

transfer characteristics of the system are reduced (SCT, 1996).


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2.6.2 Standardised Methods of Testing

2.6.2.1 Field Pull-Out Testing

2.6.2.1.1

Apparatus and Procedure

The International Society of Rock Mechanics (ISRM) has guidelines for evaluating rock

anchor testing. Figure 43 details the apparatus recommended for this testing.

Figure 43: ISRM Rock Anchor Evaluation Apparatus (ISRM, 1985)

In this test, care must be taken to ensure the load is applied axially to the tendon by the

jack. Applied load is measured using gauges on the hydraulic jack, while displacement

is recorded using dial gauges to an accuracy of less than 0.1mm. A reliable datum is

required to measure displacement, and all surfaces must be clean of loose material to

ensure application of load is axial.

This test is not destructive, and the anchor is not pulled out of the hole. It is an

acceptance test, to determine if the anchor can sustain a specified load.


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2.6.2.1.2 Mining Environment Testing Considerations

The ISRM method is more suited to Civil Engineering applications than Mining

conditions, as it neglects to account for adverse conditions frequently encounteredunderground. Specifically the requirement for surfaces to be clean can be difficult to

maintain, and the use of specified equipment may not be feasible in the confines of a

mining environment.

Field tests in mining environments tend to concentrate on load transfer characteristics,

testing to failure of the system to include evaluation of post-peak load behaviour.

Apparatus for field-testing is shown in Figure 44.

Figure 44: Mining environment pull-out testing apparatus (SCT, 1996)

This test supplies the following data about the support system:

Peak load maximum load achievable

System stiffness in elastic, yield and post failure zones

Peak shear stress sustainable on resin-rock or resin-tendon interface


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2.6.2.2.2 Gun-Barrel Testing

The apparatus for this test is shown in Figure 46. This method replicates field

conditions of loading across a discontinuity, and utilises two thick-walled, internally

threaded steel cylinders, into which a tendon is grouted. The cylinders are then placed

into a tensile testing machine, and load applied. Displacement is measured across the

join of the cylinders.

Figure 46: Gun Barrel Pull Test (SCT, 1996)

This method removes many of the uncertainties involved with pull-testing, as free-end

elongation corrections and compressive face loads are avoided.


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2.6.2.2.3 Confinement Controlled Testing

Fabjanczyk et al (1998) examined a facility designed to replicate field conditions

without the associated inherent variables. This facility is the model from which theUNSW Pull-Test facility was constructed in Stage 1, and is shown in Figure 47.

Figure 47: UNSW Pullout testing facility

Field conditions are replicated through the incorporation of a biaxial cell, which

provides confinement to the test core during loading. This confinement simulates the

horizontal stress field acting on the rock in a mine roof.

The testing method employed by this facility is termed the Short Encapsulation Pull-Out

Test.


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2.6.3 The Short Encapsulation Pull-Out Test

2.6.3.1 Selection of Testing Method

Two laboratory-testing methods are available for determination of rockbolt

performance, these being the Push Test and the Short Encapsulation Pull-Out Test. The

Push test cannot be considered representative of field conditions, as the applied force

places the tendon under compressive load. This test is more suited to theoretical studies

of annulus thickness, deformation profile and resin characteristics.

The Short Encapsulation Pull-Out Test provides tensile loading, replicating the

diametric reduction of the tendon under load. The UNSW Pull-Test Facility allows the

following variables to be controlled:

Drill-bit rotation and advance speeds during drilling

Tendon rotation and advance speeds during installation

Confinement Pressure of core within biaxial cell

Applied Load during pull-out testing

Data Acquisition rates


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2.7 Conclusions of Previous Research

2.7.1 Load Distribution

Review of rockbolt literature has shown conflict between several core theories, these

being concerned with the form, and length, of load distribution along a tendon.

Exponential and linear load distributions have been shown both in laboratory and field

testing, and while some tests have shown that load is concentrated at specific points of

the tendon, others have shown the full length is loaded to some degree.

It is clear that these discrepancies are a result of two different loading mechanisms:

Jack bearing against rock face

Bed separation

Bed separation is the mechanism present in the field, whereas jacking is used in

laboratory and field pullout testing. Exponential load distribution is found using the

jacking method, and linear distributions found through the use of in situ strain-gauged

rockbolts undergoing bed separation. Thus, the additional confineme nt offered by thebearing surface to the rock may have some effect on load distribution.


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2.7.4 Research Objectives

Verification of load distribution mechanism for both field and laboratory loading

methods

Evaluation of the effect of inherent variables of support systems in influencing

system performance

Contribution to public domain knowledge of FERB support systems

Development of a standardised, systematic means of testing and analysing

pullout tests

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