Rock Support and Reinforcement Practice in Mining, Villaescusa, Windsor & Thompson (eds) © 1999 Balkema, Rotterdam, ISBN 90 5809 045 0

Performance of the Swellex bolt in hard and soft rocks C. Li Luleå University of Technology, Sweden U. Håkansson Jacobson & Widmark AB, Sweden ABSTRACT: In general Swellex bolts strengthen the rock mass through a combination of friction and mechanical interlock at the rock-bolt interface. The anchoring mechanism of the Swellex bolt is actually different in hard and soft rocks. In hard rock, the secondary contact stress, induced by the mechanical interlock of the asperities at the borehole wall, plays a major role in the anchoring, whereas the primary contact stress, created by the bolt expansion does not contribute much. However, in soft rocks, it is the friction, and thus the primary contact stress, which determines the anchoring capacity of the bolt. 1 INTRODUCTION The Swellex is a type of rock bolt that strengthens the rock mass through a combination of friction and mechanical interlock at the rock-bolt interface. The use of the Swellex has grown rapidly in a world-wide scope during recent years, not only in hard rocks (high strength and high deformation modulus) but also in soft rocks (low strength and low defor-mation modulus). The Swellex is unique in provid-ing an instant reinforcement action in soft rocks where time is a crucial factor. Underground open-ings in soft rocks demand instant support following the excavation. The Swellex can satisfy this demand and practical applications have demonstrated its potential in dealing with stability problems in weak rock engineering. To achieve an optimum rein-forcement effect, the detailed knowledge of the performance of the bolt is needed.

The emphasis of this paper is put on the performance of the Swellex bolt in hard and soft rocks. The roles of the primary contact stress and the roughness of the borehole wall are examined in detail. The limits for the primary contact stress and the roughness angle of the wall are also discussed.

2 WORKING PRINCIPLES Swellex rock bolts were introduced by Atlas Copco in the 1980’s (Wijk and Skogberg, 1982). The Swellex bolt is made from a folded thin-wall steel

tube. Bushings are pressed onto both ends of the bolt, which are then sealed by welding. The lower bushing has a small hole through which water is injected into the bolt at high pressure to expand the bolt. During the expansion process, the Swellex bolt compresses the rock surrounding the hole and adapts its shape to fit the irregularities of the borehole, see Figure 1. After installation the bolt is held in the hole by the contact stress between the bolt and the borehole due to the elastic recovery of the rock material, as well as the mechanical interlock due to the roughness of the borehole (Stillborg, 1994).

High waterpressure

(a) (b) (c)

Water injectionhole

Foldedsteel tube

Bushing

Figure 1. Sketches illustrating the Swelllex bolt and the interaction between the rock and the Swellex bolt: (a) The Swellex bolt is placed in the hole; (b) The bolt is expanded under high water pressure; (c) The water pressure is released and the surrounding rock contracts, thereby providing the Swellex lock-ing effect. After Stillborg (1994).

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The reinforcement effect of the Swellex bolt can be represented by its pull-out resistance, Fpull, expressed as the pull-out load per metre. The failure of the bond between the rock and the Swellex bolt is either in the form of sliding along the asperities on the borehole wall, and/or in the form of breakage of the asperities. The pull-out resistance can be gener-ally expressed as

( )SRF fpull ,min= (1) where Rf refers to the frictional resistance at the rock-bolt interface, and S to the total strength of the sheared-off asperities. Expression (1) means that the pull-out resistance Fpull is the smaller one between Rf and S. The terms Rf and S can be expressed, respectively, as:

( ) ( iqqdR f ++= )φπ tan21 (2) and

AS τ= (3) where d = the diameter of the borehole, q1 = the primary contact stress at the rock-bolt interface, created by the installation, q2 = the secondary contact stress, induced by the mechanical interlock, φ = the friction angle between the rock and the bolt, i = the roughness (or dilation) angle of the borehole wall, τ = the shear strength of the rock, A = the total area of all sheared-off asperities. 3 EXPANSION OF THE SWELLEX IN THE

BOREHOLE In order to achieve an effective reinforcement the Swellex bolt must satisfy two requirements: firstly, a primary contact stress must be established between the borehole wall and the bolt; secondly, the bolt must fully fit the irregularities on the bore-hole wall after expansion. The first requirement is to enhance the frictional anchorage of the bolt, while the second is to achieve a mechanical interlock. The primary contact stress is built up due to the differ-ence between the stiffness of the bolt and that of the borehole. By considering the elastic recovery of the borehole as well as that of the bolt tube, the expres-

sion for the contact stress on the borehole wall is obtained as (Håkansson and Li, 1997):

( )⎥⎥⎦

⎤

⎢⎢⎣

⎡−−

+= ipm

f

ri

sr

s PPKKP

KKKq1 (4)

where

Ks = the radial stiffness of the bolt, fb

fbs KK

KKK

+= ,

Kf = the radial stiffness of the tube ring,

is

sf r

tEK 21 ν−= ,

Kb = the stiffness of the Swellex tongue (see Figure 2). It is a function of the expansion degree of the bolt.

Kr = the stiffness of the rock, r

rr

EKν+

=1

,

q1 = the primary contact stress at the rock-bolt interface, Ppm= the maximum pump pressure, Pi = the borehole pressure, i.e. the pressure on

the borehole wall during the bolt installation, t = the thickness of the bolt tube, ri = the radius of the borehole, Es = the Young’s modulus for steel, Er = the Young’s modulus for rock, νs = the Poisson’s ratio for steel, νr = the Poisson’s ratio for rock.

Expression (4) means that the primary contact stress on the borehole wall is a function of the stiff-ness of the rock and that of the Swellex bolt. The stiffness of the Swellex depends on the length of the bolt tongue, a short tongue resulting in a high stiff-ness. Therefore, the primary contact stress q1 is also a function of the degree of expansion. Figure 3

0 1 2 3 4Length of Swellex tongue, = a/b

0.1

1

10

100

1000

Stif

fnes

s co

effic

ient

ρ

Figure 2. The relationship between the stiffness coefficient Kb and the length of the Swellex tongue.

Kb E s

i tr ()

b

rit

a

3

104

Figure 3. Primary contact stress versus the expan-sion of the Super Swellex bolt. Parameter values used for the calculation: Es= 210 GPa, Er = 10 GPa, Ppm= 30 MPa, Pi= 15 MPa. shows an example illustrating the relationship between the primary contact stress and the tongue length. 4 PERFORMANCE IN HARD AND SOFT

ROCKS The primary contact stress is dependent on the Young’s modulus of the rock in hard rocks, but in soft rocks it is dependent on both the Young’s modulus and the strength of the rock, since yielding may occur in a limited area around the borehole during installation. As an example, Figure 4 illus-trates the theoretical solution of the primary contact stress when the Young’s modulus of the rock varies. In the calculations, it is assumed that the Young’s modulus is related to the uniaxial compressive strength (in soft rocks) by (Aydan et al., 1995). The primary contact stress increases with the Young’s modulus in soft rocks, while in hard rocks it decreases. It can be seen that it is in rela-tively soft rocks that high primary contact stresses are achieved.

4.180 crE σ=

As stated before, the anchorage of the Swellex rock bolts is achieved by a combination of friction and mechanical interlock between the borehole wall and the bolt. In soft rocks the asperities on the bore-hole wall are either crushed during the bolt installation or sheared off afterwards when relative movements occur between the borehole and the bolt. This means that the mechanical interlock makes only a small contribution to the anchoring of

0 5 10 15 20E modulus of rock (GPa)

0

4

8

12

Prim

ary

cont

act s

tress

, q

(MP

a)

50

Borehole diameterd= 52 mm

44

1

0 1 2 3 4Length of Swellex tongue, = a/b

0

2

4

6

8P

rimar

y co

ntac

t stre

ss,

q

(MP

a)

ρ

b

aq1

1

Figure 4. Primary contact stress versus the Young’s modulus of rock in hard rocks. Parameter values used in the calculations for the Super Swellex: D=54 mm, t=3 mm, b = 10 mm, Es = 210 GPa, νs = 0.3, νr = 0.2, Ppm = 30 MPa, Pi = 15 MPa, φ = 30o. the bolt, but it is friction that plays the main role in this case. The friction is directly proportional to the primary contact stress on the borehole wall. Thus, the primary contact stress is of vital importance to enhance the anchorage of the Swellex bolt in soft rocks.

In hard rocks, however, the asperities on the borehole wall are sheared off only with great difficulty, and therefore the mechanical interlock plays a significant role in the anchorage. It can be seen in Figure 4 that the primary contact stress is low in hard rocks. A secondary contact stress has to be provided in order to enhance the anchorage of the bolt. This secondary contact stress is achieved when the bolt tends to slide over the asperities on the borehole wall in hard rocks. Suppose the roughness (or dilation) angle of the borehole wall is i, see Figure 5. The radial contraction, u, of the bolt tube is related to the axial movement of the bolt, x, as follows:

ixu tan= (5)

ri

qi 2

Rock

Bolt tube

Figure 5. A schematic illustration showing the mechanical interlock between the borehole wall and the Swellex rock bolt.

105

On the other hand, it is known from elasticity theory that the radial contraction of the bolt will bring about a radial resistant stress, i.e. the secondary contact stress, q2. The relationship between the radial displacement and the secondary contact stress is given by:

s

i

Krqu 2= (6)

Combining the above two expressions, one obtains the expression for q2 as:

iKrxq si

tan2 = (7)

Let us look at the contribution of the roughness

of the borehole wall to the secondary contact stress through an example. Assume that a Super Swellex bolt is installed in a borehole with the radius ri = 24 mm. The roughness angle of the borehole wall is assumed to be i = 1o. The calculation using expres-sion (7) shows that the secondary contact stress q2 can be up to 0.9 MPa only with an axial relative movement of 1 mm between the bolt and the wall.

Figure 6 illustrates the relationship between the secondary contact stress, the roughness angle of the wall, and the borehole diameter. It can be concluded that in hard rocks it is not the primary contact stress but the secondary contact stress that dominates the anchorage capacity of the Swellex bolt. For this reason, the borehole wall must be rough enough to induce a secondary contact stress, as long as the bolt is subjected to an axial pull-out load. Figure 6. Secondary contact stress versus the diameter of the borehole at different roughness angles during a 1 mm axial movement of the Swellex bolt.

5 IN-SITU LOAD The load exerted on the in-situ rock bolts is caused by rock deformation. The distribution of the shear stress and also the axial tensile load along the entire length of the rock bolt can be calculated based on the rock deformation (Li and Stillborg, 1997). The loading process is the same for all types of rock bolts, but the characteristic of the Swellex bolt is that the shear stress on the bolt can be maintained at the level of the ultimate shear strength when slip is triggered at the rock-bolt interface. At this stage the maximum anchorage capacity of the Swellex bolt is reached. As illustrated in Figure 7, the shear stress on the bolt is directed toward the tunnel wall on the portion close to the wall, while it is directed toward the opposite direction on the portion located further inside the rock. The maximum axial tensile load in the bolt occurs at the neutral point where the shear stress on the bolt is zero. It is the anchor length of the bolt that determines the maximum axial load. When the rock deformation is large enough, sliding may occur on the anchoring section of the bolt, i.e. along the anchor length of the bolt. Figure 7. A sketch illustrating the shear stress and the axial tensile load along the Swellex bolt with a face plate, corresponding to the maximum anchorage capacity of the bolt. 6 CAPACITY

In contrast to traditional types of bolts, a limited amount of sliding movement on the borehole wall does not make the anchorage of the Swellex bolt fail, but on the contrary can activate the full load bearing capacity of the bolt. The evidence for this statement is displayed in the pull-out tests where the ultimate pull-out load of the Swellex bolt remains constant even after a long displacement. In this case the shear strength of the rock-bolt interface is mobilised along the entire length of the bolt. This

42 44 46 48 50 52Borehole diameter, d (mm)

0.1

1

10

Sec

onda

ry c

onta

ct s

tress

, q

(M

Pa)

* Super Swellex* 1 mm axial movement

i = 1o2345

2

Axial load

Neutral point

Pick-up length

Shear stress

Anchor length, l

106

feature of the Swellex bolt, which means that it can tolerate a long displacement without loss of its ulti-mate load bearing capacity, makes it unique in stabilising rock masses with large deformations. The ideal performance of the Swellex bolt should be that the bolt slides, instead of ruptures, in the case of large rock deformation. This performance demands that the anchor length of the bolt have a limit that depends on the bond strength, thus the total contact stress, at the rock-bolt interface. In the case where the asperities on the borehole wall are not sheared off, a general criterion exists for determining the anchor length of the bolt is: Max. pull-out load on bolt < Tensile strength of bolt

i.e.

( ) ( ) Tiqqdl tan21 <++ φπ (8) where the parameter l stands for the anchor length of the bolt, see Figure 7. In soft rocks, the roughness of the borehole wall as well as the secondary contact stress can be assumed to be zero, i.e. i = 0 and q2 = 0. Thus, the anchor length is obtained from expression (8) as:

φπ tan

1dqTl < (9)

In hard rocks it is obtained by assuming that q1 ≈ 0:

( )idqTl

+<

φπ tan

2

or

( ) tantan2

iixKTl

s +<

φπ (10)

Let us look at the influence of the primary contact stress and the roughness angle on the anchor length through an example involving the Super Swellex. The minimum breaking load of the Super Swellex is T = 200 kN. Assume that

Friction angle: φ = 35o

Borehole diameter: d = 48 mm Bolt tube stiffness: Ks = 340 MPa (calculated using the equation for Ks) In the soft rock, the anchor length of the bolt,

expressed by (9), is illustrated in Figure 8a versus the primary contact stress. For a primary contact stress of 1 MPa, an anchor length of about 2 m can

guarantee to make use of the maximum anchoring capacity of the bolt. In the hard rock, the anchor length of bolt, expressed by (10), is illustrated in Figure 8b versus the roughness angle of the bore-hole wall. For a roughness angle of 5.5o, an anchor length of about 1.2 m is long enough to make use of the maximum anchoring capacity of the Super Swellex. The pneumatic drilling can easily make a very rough borehole wall in hard rocks. This was proved by a field pull-out test in granite. In the test, the tensile strength of the Super Swellex was reached at an anchor length of 0.75 m. It is seen in Figure 8b that the roughness angle of the borehole wall is about 8o corresponding to an anchor length of 0.75 m if the primary contact stress is neglected.

0 2 4 6 8Primary contact stress, (MPa)

0

1

2

3

4

5

Anc

hor l

engt

h of

the

bolt,

(m)

q1

(a) In soft rocks (b) In hard rocks

Figure 8. Examples demonstrating the limit of the anchor length of the Super Swellex bolt with relation to the primary contact stress in soft rocks, and to the roughness angle i on the borehole wall in hard rocks. Parameter values used for the calculation: d = 48 mm, t = 3 mm, x = 1 mm, φ = 35o, T = 200 kN.

lLimit of the anchor length

0 2 4 6 8Roughness angle, (degree)

0

1

2

3

4

5

10

Anc

hor l

engt

h of

the

bolt,

(

m)

l

Limit of the anchor length

i

107

The two diagrams in Figure 8 are for the purpose of demonstration. The best way to determine the anchor length of the bolt, the primary contact stress as well as the roughness of the borehole is to perform field pull-out tests. 7 CONCLUSIONS (1) In soft rocks, it is the primary contact stress that

plays a significant role in the anchorage of the bolt.

(2) In hard rocks, the borehole wall must be rough enough and the bolt tube must be tightly matched with the irregular surface of the bore-hole wall. It is the secondary contact stress induced due to the roughness of the borehole wall that mainly determines the anchorage of the bolt in hard rocks.

(3) The Swellex bolt can provide a strong rein-forcement to the rock as long as a moderate primary contact stress, such as between 0.5 and 1 MPa, is achieved in soft rocks, or a relatively rough borehole wall, such as with a roughness angle of about 6o, is made in hard rocks.

Acknowledgements –The authors would like to tank Prof. Bengt Stillborg for his comments and review of the manuscript. Thanks are also extended to Mr. Pether Jonsson for providing the result of the pull-out tests. REFERENCES

Aydan, Ö., Akagi, T. and Ito, T. (1995). Prediction of deformation behaviour and design of support systems of tunnels in squeezing rocks. Report, Tokai University, Shimizu, Japan.

Håkansson, U. and Li, C. (1997). Swellex in weak and soft rock ⎯ design guidelines. Int. Symp. on Rock Support - Applied Solution for Under-ground Structures, June 22-25, 1997, Lillehammer, Norway, 555-568.

Li, C. and Stillborg, B. (1997). The mechanical behaviour of fully bonded rock bolts in tensile loading ⎯ A theoretical analysis. Int. Symp. on Rock Support - Applied Solution for Under-ground Structures, June 22-25, 1997, Lillehammer, Norway, 178-192.

Stillborg, B. (1994). Professional Users Handbook for Rock Bolting (2nd edition), Tans tech Publi-cations.

Wijk, G., and Skogberg, B. (1982). The SWELLEX® rock bolting system. 14th Canadian Rock Mechanics Symposium, Canadian Institute of Mining and Metallurgy, 106-115.

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