Rock Bolting Final

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TERM PAPER

ROCK BOLTING


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Submitted to: SIR DR.S.M.JAMIL

Date: 22nd MAY 2012

Group:

Ameer Hamza BE-CE-13

Hassan Rauf BE-CE-150

Waseem Akram Khan BE-CE-144


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Rock Bolting

Introduction:

Rock bolting is a primary means of rock reinforcement used to stabilize excavated rock

in underground mines and tunneling.

Rock bolts are always installed in a pattern, the design of which depends on the type of

excavation. They are arranged in such a way as to transfer the load from the exterior of the rock,

to the stronger interior part of the rock. Rock bolting reinforcement and support is maximized

with the addition of a wire meshing system and shotcrete or revegetation.

Rock bolting is utilized to secure potentially unstable sections of the rock mass, improving long

term rock stability. Drilling is carried out using specialized drills and Spider Cages. Rock bolts

are installed using epoxy or high strength cement grouts. Rock bolts can be installed as tensioned

rock bolts or as passive dowels. All rock bolt and anchor test results are recorded in logs and the

records are filed.

Rock bolts have been used for many years for the support of underground excavations and a

wide variety of bolt types have been developed to meet different needs which arise in mining and

civil engineering.Rock bolts generally consist of plain steel rods with a mechanical or chemical

anchor at one end and a face plate and nut at the other. They are always tensioned after

installation. For short term applications the bolts are generally left un-grouted. For more

permanent applications or in rock in which corrosive groundwater is present, the space between

the bolt and the rock can be filled with cement or resin grout.

Rock bolts have been widely used for rock reinforcement in civil and mining engineering for a

long time. Rock Bolts reinforce rock masses through restraining the deformation within the rock

masses. In order to improve bolting design, it is necessary to have a good understanding of the

behavior of rock bolts in deformed rock masses. This can be acquired through field monitoring,

laboratory tests, numerical modeling and analytical studies.
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Since the 1970s, numerous researchers have carried out field monitoring work on rock

bolts installed in various rock formations [13]. Freeman performed pioneering work in studying

the performance of fully grouted rock bolts in the Kielder experimental tunnel. He monitored

both the loading process of the bolts and the distribution of stresses along the bolts. On the basis

of his monitoring data, he proposed the concepts of ``Neutral Point'', ``Pick-up length'' and

``Anchor length''.

At the neutral point, the shear stress at the interface between the bolt and the grout

medium is zero, while the tensile axial load of the bolt has a peak value.

The pick-up length refers to the section of the bolt from the near end of the bolt (on the

tunnel wall) to the neutral point.

The shear stresses on this section of the bolt pick up the load from the rock and drag the

bolt towards the tunnel.The anchor length refers to the section of the bolt from the neutral point to the far end

of the bolt (its seating deep in the rock). The shear stresses on this section of the bolt anchor the

bolt to the rock. These concepts clearly outline the behavior of fully grouted rock bolts in a

deformed rock formation. Bjornfot and Stephansson's work demonstrated that in joined rock

masses there may exist not only one but several neutral points along the bolt because of the

opening displacement of individual joints.

Bolting Theories:In general, rock bolting is very effective in a variety of geological and geotechnical conditions.

The main function of roof bolting is to bind together stratified or broken rocks such as

sedimentary rocks containing bedding planes, rocks consisting of natural joints and fractures, or

rocks with artificial fractures and cracks caused by the use of explosives (Peng 1984). The

theories used to explain the bolting mechanisms vary from place to place and sometimes are

elusive. However, it is broadly believed that bolt binding effects are accomplished by one or a

combination of the following three basic mechanisms,

Suspension Beam building Keying


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Suspension:

Whenever an underground opening is made, the strata directly overhead tend to sag.If not properly and adequately supported in time, the laminated immediate roof could separate

from the main roof and fall out. Roof bolts, in such situations, anchor the immediate roof to the

self-supporting main roof by the tension applied to the bolts. In some instances, it appears that

the immediate roof is suspended from the main roof by the bolts, as shown in Figure 2.2.1, or

weak strata are suspended from stable strata, as shown

In this case, the load carried by each bolt can be calculated as (Peng,

1984):

where

w = Unit weight of the immediate roof;


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t= Thickness of the immediate roof;

B = Roof span (i.e. entry width);

L = Length of immediate roof;

n1 = Number of rows of bolts in length L;

n2 = Number of bolts per row.

This equation holds only if the immediate roof would completely separate from the main roof

such that it is suspended entirely by the bolts, and the portion of weight of the immediate roof

supported by the abutments on both sides of the opening is ignored.

Therefore, this equation estimates the upper limit of load a bolt could bear while achieving the

suspension effect.

in Figure 2.2.2. The bolts have to carry the dead weight of the strata between bolt heads and

anchors.

In the second case mentioned above, the estimation of the load each bolt must carry is more

complex. It involves identifying a possible failure plane and then both the bolts tensile and shear

stresses. To ensure stability, bolts must provide sufficient axial force to increase shear force.

Suppose that a safety factor SFis required, then (Biron and Arioglu, 1983):


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Beam Building:

In order to create a suspension effect and stabilize the immediate roof or slope, the bolts should

anchor in a competent stratum at least 9 inches beyond the possible bedding interface.

In most cases, the strong and self-supporting main roof is beyond the reasonable distance that

ordinary roof bolts can reach to anchor for suspension. However, roof bolts can be applied in

such situation with great success. In fact, sagging and separation of roof laminate cause both

vertical movement and horizontal movement along the bedding interfaces. Bolts through these


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layers can prevent or greatly reduce horizontal movement, and the tension applied to the bolts

manually on installation or induced by rock vertical displacement clamps the layers together,

making all the layers have to move with the same magnitude of vertical displacement. On the

other hand, frictional forces, which are proportional to the bolt tension, are induced along the

bedding interface, also making horizontal movement difficult, as shown in Figure 2.2.4. This

bolting pattern is very similar to clamping a number of thin, weak layers into a thicker, strong

one, forming a fixed-end composite beam. Theoretically, assuming that all the thin layers are of

same material, the maximum bending strain at the clamped ends of the composite beam is

(Peng, 1984):

E= Youngs modulus;

L = Length of the immediate roof;

t= Thickness of the composite beam;

w = Unit weight of the immediate roof.

This equation shows that the thicker the beam, the smaller the maximum strain induced at the

clamped ends. In other words, the clamping action produces a beam building effect.


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Paneks research (1956) indicated that beam building effects increase with decreasing bolt

spacing, increasing bolt tension, increasing number of bolted laminate, and decreasing roof span.

In most situations, where the immediate roof consists of laminated strata, both suspension and

beam building effects coexist.

The bending strength of the bolted beam increases by n times compared to that of the unbolted

beam, while the bending stiffness increases by n2 times. The improvement of bending strength is

always good for roof stability. However, under certain conditions, increasing bending stiffness

may cause extra load from the overlying strata acting on the beam. The beam may not fail in

tension because of the increased bending strength, but may fail by shearing at the two ends once

the accumulated shear forces exceed the shear strength of the composite beam, as shown in

Figure 2.2.6. It is observed that this kind of failure has the following features:

The bolted composite beam falls out;

Failure planes at the two ends of the beam are nearly vertical;

The upper failure plane is exactly at the bolted horizon where pre-tension of the

bolts creates a tensile stress area around the anchor of each bolt; and

Sometimes using longer bolts just increases the height of roof fall.

Keying:

The keying effect mainly depends on active bolt tension or under favorable circumstances,

passive tension induced by rock mass movement. It has been shown (Gerrard, 1983) that bolt

tension produces stresses in the stratified roof, which are compressive both in the direction of the

bolt and orthogonal to the bolt. Superposition of the compressive areas around each bolt forms a

continuous compressive zone in which tensile stresses are offset and the shear strength areimproved, as shown in Figure 2.2.9.


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Types of Rock Bolts:

1.Mechanically anchored rock bolts2.Resin anchored rock bolts3.Swellex Friction Bolts4.Split-set Friction Rock Bolts


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1.Mechanically Anchored Rock bolts:Mechanically anchored rock bolts use expansion shells on the end of the shaft to connect the bolt

to the rock. The holes are drilled in advance, and the expansion shell is placed into the hole.Make the hole about 100 mm longer than the bolt. Once the bolt is inserted, pull on it sharply so

that it will expand and dig into the rock. Dig the bolt deeper into the rock by turning the nut on

the bolt. This kind of bolt is best for moderately hard to hard rocks. One of this system's pitfalls

is that it is prone to slipping in very hard rock.

One of the primary causes of rock bolt failure is rusting or corrosion and this can be counteracted

by filling the gap between the bolt and the drill hole wall with grout.

While this is not required in temporary support applications, grouting should be considered

where the ground-water is likely to induce corrosion or where the bolts are required to perform a

'permanent' support function.


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In general, threads on rock bolts should be as coarse as possible and should be rolled rather than

cut. A fine thread is easily damaged and will cause installation problems in a typical

underground environment. A cut thread weakens the bolt and it is not unusual to see bolts with

cut threads that have failed at the first thread at the back of the nut. Unfortunately, rolled thread

bolts are more expensive to manufacture and the added cost tends to limit their application to

situations where high strength bolts are required.

Tensioning of rock bolts is important to ensure that all of the components are in contact and that

a positive force is applied to the rock. In the case of light 'safety' bolts, the amount of tension

applied is not critical and tightening the nut with a conventional wrench or with a pneumatic

torque wrench is adequate. Where the bolts are required to carry a significant load, it is generally

recommended that a tension of approximately 70% of the capacity of the bolt be installedinitially. This provides a known load with a reserve in case of additional load being induced by

displacements in the rock mass.

The traditional method of grouting up hole rock bolts is to use a short grout tube to feed the grout

into the hole and a smaller diameter breather tube, extending to the end of the hole, to bleed the

air from the hole. The breather tube is generally taped to the bolt shank and this tends to cause

problems because this tube and its attachments can be damaged during transportation or insertion

into the hole. In addition, the faceplate has to be drilled to accommodate the two tubes, as

illustrated in Figure 2. Sealing the system for grout injection can be a problem.

Since the primary purpose of grouting mechanically anchored bolts is to prevent corrosion and to

lock the mechanical anchor in place. The grout should be readily pump able without being too

fluid and a typical water/cement ratio of 0.4 to 0.5 is a good starting point for a grout mix for this

application. It is most important to ensure that the annular space between the bolt and the drill

hole wall is completely filled with grout. Pumping should be continued until there is a clear

indication that the air has stopped bleeding through the breather tube or that grout is seen to

return through this tube.


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Resin-anchored Rock Bolts:

Mechanically anchored rock bolts have a tendency to work loose when subjected to

vibrations due to nearby blasting or when anchored in weak rock. Consequently, for applicationswhere it is essential that the support load be maintained, the use of resin anchors should be

considered

Resin anchored rock bolts, also called grouted rock bolts, are sealed using a resin and a catalyst.

A cartridge full of the resin is placed at the end of the hole, and the bolt is stuck in the hole after

it. The rebar is then "drilled" through the hole, puncturing the cartridge and causing the resin to

dry and seal the bolt in the hole. The resin is then released into the hole, and it slowly hardens

and keeps the bolt in place. This type of rock bolt is very common because it is very simple toinstall.

A typical resin product is made up of two component cartridges containing a resin and a catalyst

in separate compartments.

The cartridges are pushed to the end of the drill hole ahead of the bolt rod that is then spun into

the resin cartridges by the drill. The plastic sheath of the cartridges is broken and the resin and

catalyst mixed by this spinning action. Setting of the resin occurs within a few minutes

(depending upon the specifications of the resin mix) and a very strong anchor is created.

This type of anchor will work in most rocks, including the weak shales and mudstones in which

expansion shell anchors are not suitable. For 'permanent' applications, consideration should be

given to the use of fully resin-grouted rock bolts, illustrated in Figure 4. In these applications, a

number of slow-setting resin cartridges are inserted into the drill hole behind the fast-setting

anchor cartridges.


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Typical set-up for creating a resin anchored and grouted rockbolt. Resin grouting

involves placing slow-setting resin cartridges behind the fast-setting anchor cartridges and

spinning the bolt rod through them all to mix the resin and catalyst. The bolt is tensioned after

the fast-setting anchor resin has set and the slow-setting resin sets later to grout the rod in

place.

The high unit cost of resin cartridges is offset by the speed of installation. The process described

above results in a completely tensioned and grouted rock bolt installation in one operation,

something that cannot be matched by any other system currently on the market. However, there

are potential problems with resins.

There is some uncertainty about the long-term corrosion protection offered by resin grouts andalso about the reaction of some of these resins with aggressive groundwater. For temporary

applications, these concerns are probably not an issue because of the limited design life for most

rock bolt installations. However, where very long service life is required, current wisdom

suggests that cement grouted bolts may provide better long term protection.


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Swellex Friction Bolts

Swellex friction rock bolts are similar to split-set friction bolts. They are also made of collapsed

tubes except they expand through the use of water pressure. They are extremely simple to install.The main problem with them is their lack of durability.

Specifications:

Length up to 12 m Hole diameter = 32-52 mm Tensile load= 100-240 kN Inflation Pressure = 30 MPa Instant Full load bearing Capacity Fast Application Not sensitive to blasting Elongation range: 20-30%


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Split-set Friction Rock Bolts

Split Sets rock bolt is a bolt for temporary stabilization and consist of a tube and plate. The high

strength steel tube is slotted along its length. One end is tapered for easy insertion into a drillhole and the other has a welded ring flange to hold the bearing plate. The tube is driven into a

slightly smaller hole, using the same standard percussion drill that made the hole. The slot

narrows, causing radial pressure to be exerted against the rock over its full contact length.

Split-set friction rock bolts are placed inside predrilled holes. They are made of collapsed steel

tubing, which is placed within the hole and twisted. This twisting causes the tubing to expand,

which secures the bolt to the hole's wall. These bolts are simple to install, but they lack tension

and the bolts can't be anymore than 3 meters long.

The bolts can be galvanised for corrosion protection.

Advantages:

Split sets are relative cheap rock bolts. Split sets are easy to install Split sets have immediate full-column, full loading capacity. Available in different seizes


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Coupling between the bolt and the rock:

Windsor proposed the concept that a reinforcement system comprises four principal components:

the rock, the reinforcing element, the internal fixture and the external fixture. For reinforcement

with a bolt, the reinforcing element refers to the bolt and the external fixture refers to the face

plate and nut. The internal fixture is either a medium, such as cement mortar or resin for grouted

bolts, or a mechanical action like ``friction'' at the bolt interface for frictionally coupled bolts.

The internal fixture provides a coupling condition at the interface. With reference to the

component of internal fixture, Windsor classified the current reinforcement devices into three

groups: ``continuously mechanically coupled (CMC)'', ``continuously frictionally coupled

(CFC)'' and ``discretely mechanically or frictionally coupled (DMFC)'' systems. According to

this classification system, cement and resin-grouted bolts belong to the CMC system, while Split

set and Swellex bolts belong to the CFC System. When fully grouted bolts are subjected to a pull

load, failure may occur at the bolt grout interface, in the grout medium or at the grout rock

interface, depending on which one is the weakest. For fully frictionally coupled bolts, however,

there is only one possibility of failure decoupling at the bolt rock interface. In this study we

concentrate on the failure at the interface between the bolt and the coupling medium (either thegrout medium or the rock).

In general, the shear strength of an interface comprises three components: adhesion, mechanical

interlock and friction. They are lost in sequence as the compatibility of deformation is lost across

the interface. The result is a decoupling front that attenuates at an increasing distance from the

point of the applied load. The decoupling front first mobilizes the adhesive component of

strength, then the mechanical interlock component and finally the frictional component. The

shear strength of the interface decreases during this process. The shear strength after the loss of

some of the strength components is called the residual shear strength in this paper. For grouted

rock bolts like rebar, all the three components of strength exist at the bolt interface. However, for

the fully frictionally coupled bolt, the ``Split set'' bolt, only a friction component exists at the bolt

interface. For Swellex bolts, mechanical interlock and friction comprise the strength of the

interface.


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Conclusion

With the availability of technology and modern techniques, rock bolting is ever growing as an

effective method for retention and strengthening of rock masses. Due to its reliability, it isadopted in most cases involving rock. While some might consider this as a rather expensive

solution, the extent of application of rock bolting is superior to alternate methods.

Hence, we can safely conclude that rock bolting is not only a landmark in geotechnical

engineering techniques, but also a practical and reliable solution for excavations in rocky strata.

Rock Bolting Final

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