Rock Mech. Rock Engng. (2007) 40 (5), 519–524

DOI 10.1007/s00603-006-0094-7

Printed in The Netherlands

Technical Note

A Practical Problem with Threaded Rebar Boltsin Reinforcing Largely Deformed Rock Masses

By

C. C. Li

Norwegian University of Science and Technology, Trondheim, Norway

Received August 23, 2005; accepted April 24, 2006Published online June 16, 2006 # Springer-Verlag 2006

Keywords: Bolt failure, rebar bolt, rock reinforcement, rock support.

1. Introduction

Rebar bolt is probably the most commonly used rock reinforcement element in both

mining and civil engineering applications. The most commonly seen rebar bolt is an end-

threaded steel bar that is fully grouted in borehole with cement or resin, Fig. 1. A fully-

grouted rebar bolt is characterised by its high bond between the bolt and the grout because

of the ribs on the cylindrical surface of the bar. Rebar bolts are installed without preten-

sion. The load a rebar provides to the rock is developed afterwards when it is subjected to

rock deformation. After excavation, the country rock moves towards the opening with the

largest deformation at the free surface of the opening. Thus, it is at the surface of the

opening where the bolt is most loaded. On the other hand, the weakest part of a rebar bolt

is its thread which is located exactly at the surface of the opening. It is often seen in the

field that rebar bolts fail at the thread in case of occurrence of large rock deformation, for

instance in a weak rock mass subjected to high ground pressure. In this technical note,

failure of rebar bolts observed in the field is presented first. Then, the loading condition of

the bolt is examined for the purpose of showing how much the thread reduces the loading

capacity of the bolt. The aim is to point out the weakness of rebar bolts so that one is

aware of the direction to enhance the load-bearing capacity of rebar bolts when needed.

2. Observations of Bolt Failure

A metallic mine in Sweden is currently conducting its mining activities at a depth of

about 1000 m under the surface of the ground. The ground pressure at that depth is

quite high and the rock is chlorite-rich and weak. Therefore, rock deformation in mine

stopes is large. The wall-to-wall convergence in a 7 m wide stope often reaches more

than ten centimetres in a short period after excavation. The mine uses threaded rebar

bolts for rock reinforcement. The bolts are fully grouted in boreholes with cement

mortar. It was observed in the stopes that a number of bolts failed at the thread and lost

their support capacity. Figure 2 shows two of the rebar bolts installed in a mine stope.

These two photographs show how a bolt reacts to ground pressure. The rock moves

towards the opening under the ground pressure. The face plate of the bolt tends to

prevent the rock movement, resulting in a load on the face plate, as seen in Fig. 2a.

The load on the face plate is then transferred to the bolt through the nut and the thread.

If the bolt is properly grouted, the load in the bolt would become smaller towards the

far-end of the bolt because of the bond between the bolt and the grout (Li and

Fig. 1. A sketch illustrating a threaded rebar bolt cement-grouted in a borehole

Fig. 2. Rebar bolts in situ. a A heavily loaded rebar bolt, b a rebar bolt failed at the thread and sunk intothe rock

520 C. C. Li

Stillborg, 1999). It means that the bolt is subjected to the largest tensile load at the

thread. The result is, in an extreme case, that the bolt breaks at the thread. Figure 2b

shows such a case where the bolt broke at the thread and sank several centimetres into

the rock. The surface support of this bolt was completely lost after failure.

The nature of the bolt failure could be ductile or brittle, depending upon the

loading condition the bolt is subjected to. Figure 3a shows a typical ductile failure

of a rebar thread, in which the diameter of the thread became smaller than its original

dimension. This is the so-called necking phenomenon, which indicates a ductile fail-

ure. The bolt was located in a place where large rock deformation occurred. The

loading to the bolt was probably a progressive process, that is, the load on the face

plate of the bolt was increasing with rock deformation until the bolt failed.

Figure 3b shows a thread of bolt that was subjected to brittle failure. The brittle

failure is characterised by a clean failure plane without occurrence of the necking

phenomenon. With this type of failure, the bolt does not sink into the rock after failure,

as shown in Figure 3c. This implies that the bolt is subjected to a negligible rock

deformation when it undergoes failure. It was observed in the field that bolts that were

subjected to this type of failure were usually located in places close to work faces. It is

very possible that they failed due to dynamic loading induced by blasting at the work

faces.

3. Analysis

As mentioned previously, a rebar bolt is not loaded uniformly along its length owing

to its anchoring mechanism. The section close to the free surface of an opening is most

loaded and the bolt usually fails at the thread when the load is large enough. The

ultimate load of a rebar, given in the specification of the product, usually refers to the

Fig. 3. Failure modes of 20 mm rebar bolts in situ. a Ductile failure, b brittle failure, c a rebar bolt subjectedto brittle failure

A Practical Problem with Threaded Rebar Bolts 521

steel bar of the bolt, but not to the thread. The thread and the steel bar may be at

different stages of deformation when the thread undergoes failure. A closer look at this

problem would help one to have a better understanding of the behaviour of a rebar bolt

under loading.

Rebar is made of carbon steel. The mechanical behaviour of carbon steel is

characterised by its yielding load, ultimate tensile load and the ultimate elongation.

The typical values of the mechanical parameters for a few commercial rebar bolts are

listed in Table 1. The yielding load of a rebar bolt is in a range from 63 to 90% of its

ultimate load, and the ultimate elongation is from 6 to 20%.

The load-elongation behaviour of a steel bar is shown in Fig. 4. The elongation of

the bar linearly increases with the tensile load until the load reaches the yielding limit

of the steel. The load remains more or less at the yielding level for a certain elonga-

tion, and then increases again with increasing in elongation. This is the so-called

hardening of the material. The bar fails at its ultimate elongation. The stress-strain

Table 1. Mechanical properties of a few rebar bolts (Stjern, 1995)

Rebar bolt Diameter(mm)

Yielding loadPy (kN)

Ultimate loadPult (kN)

Py=Pult

(%)Ultimateelongation (%)

Østra round bar 20 65 100 65 8Østra rebar 19 120 150 80 6Østra rebar 25 220 250 88 –Østra CT-bolt 19 120 150 80 8Østra CT-bolt 20 140 170 82 –Østra CT-bolt 22 230 290 79 –Dywidag rebar 20 157 173 90 –Gasta rebar 20 140 173 81 6SCS threaded rebar 20 113 179 63 13SCS headed rebar 20 141 162 87 15Ares rebar 20 141 220 64 20DSI rebar 20 120 180 67 10

Fig. 4. Typical load-elongation curves of the solid bar and the thread of a rebar bolt. Py¼ yielding load,Pult¼ ultimate load

522 C. C. Li

curve of the material will be the same regardless of the diameter of the bar, but the

load-elongation curve will depend upon the diameter. Conventional 20 mm rebar bolts,

for instance, have a nominal diameter of 20 mm for the solid bar, while the inner

diameter of the thread (M20) at the end is only about 17.6 mm. The cross section area

of the thread is about 75% of the cross section area of the solid bar. The ultimate load

of the thread, therefore, is only 75% of the solid bar for the 20 mm rebar bolts.

A lower ultimate load in the thread section results in the thread undergoing

yielding and even failure earlier than the solid bar does. As shown in Fig. 4, the solid

bar of a bolt might be still under elastic deformation or just get into the stage of

hardening when the ultimate load of the thread is reached. Thus, the rock deformation

is mainly balanced by the elongation of the thread rather than the steel bar.

It is seen from the above discussion that the load-bearing capacity of a threaded

rebar is reduced about 25% compared to the solid bar of the bolt. In other words, a

threaded rebar with a nominal ultimate load of 200 kN would actually only be able to

carry a load of 150 kN because of the thread. The key to enhance the load-bearing

capacity of rebar bolts is to overcome the weakness of the thread. One measure to

achieve this is to enlarge the thread so that its inner diameter is at least equal to the

diameter of the solid bar. Another measure is to get rid of the thread and use headed

rebar bolts, as shown in Fig. 5. A headed rebar bolt is both stronger and stiffer than a

threaded rebar. Headed rebar bolts were used in many mines in Sweden until the

1970s, but today they have been completely substituted by threaded rebar bolts. This

may be due to the consideration of manufacture cost and other versatile uses of the

threaded rebar bolts in practice. In cases where the load-bearing capacity is of great

concern, headed rebars or rebars with enlarged thread may be considered.

The above analysis is limited to rebar bolts installed in weak rocks where rock

deformation is usually continuously distributed with its maximum at the free surface

of the opening.

4. Concluding Remarks

Rebar bolts usually fail at the thread in largely deformed rock masses. The thread

reduces the load-bearing capacity by about 25% for a conventional 20 mm rebar bolt.

Fig. 5. A sketch of headed rebar bolt

A Practical Problem with Threaded Rebar Bolts 523

In case that the load-bearing capacity is of concern, an enlarged thread or headed rebar

bolts can be considered as a substitute of the conventional rebar bolt.

References

Li, C., Stillborg, B. (1999): Analytical models for rock bolts. Int. J. Rock Mech. Min. Sci. 36(8),1013–1029.

Stjern, G. (1995): Practical performance of rock bolts. Doctoral thesis, University of Trondheim,Norway.

Author’s address: Prof. Dr. C. Chunlin Li, Department of Geology and Mineral ResourcesEngineering, University of Trondheim, NO-7491 Trondheim, Norway; e-mail: charlie.c.li@ntnu.no

524 C. C. Li: A Practical Problem with Threaded Rebar Bolts