New rockbolting methods for reinforcing tunnels against deformation

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Abstract

Some mountain tunnels suffer from perennial deformation induced by overburden and confining stresses. Conventionally rock bolts are being used as a countermeasure. Oftentimes the conventional rock bolts fail to deliver their optimum performance, owing to several potential causes. Such as: the ground around the tunnel is too plasticized while transmitting shear forces along with the solidified mortar; and the rock bolts are poorly anchored with grout at the rear end. Considering these facts, we develop a bonding of rock bolt, named CIB-bolt, a combined mechanism deals with ground Improvement, to improve the plasticized ground while preventing further deformation of tunnel and to achieve positive bonding of rock bolts. To confirm the effectiveness of the developed rock bolt which was utilized in the experiment model, a real field application and numerical analysis also were conducted. Observed results indicate that, the developed rock bolt exhibits excellent performance in transmitting axial stresses towards the hard layers of rock. This mechanism can prevent the yielding or plasticization of adjacent ground and control the deformations of tunnel lining.

Introduction

Tunnels in soft rock can be subjected to excessive ground pressure depending on the geology and geographic location in which they are constructed. Large overburden and high confining stresses can induce gradual deformation of the tunnel lining, attributed to the facts of squeezing and/or swelling of soft rocks.1,2

Squeezing is a phenomenon that accompanies the plasticization of the ground, and the ground strength ratio, which is the ratio of the compressive strength of the ground to the overburden pressure, is an effective index.3,4

The competency factor is often used in Japan as a stability indicator in major underground construction activities. It has extensively been used as a measure to indicate the level of plasticized ground and the requirement of additional supports to prevent the ground failure attributed to major deformations during excavation and maintenance.5

As for swelling, extreme expansion of calcium sulfate anhydrite has been observed in Switzerland, Germany and other countries.6 Meanwhile in Japan several issues have been reported related to mudstone, tuff and other rock masses which consists expansive clay minerals such as montmorillonite.7

In Japan, squeezing on the sidewalls of a tunnel and heaving are often seen to grow over the years at a constant rate of several millimeters per annum.8 This phenomena could be attributed to the influence of rock near the tunnel, which consists of expansive clay minerals also it can be slaked easily and gradually losing its strength and finally end up with plasticized ground.9 Rock bolting has proven to be effective for abovementioned deformations in many tunnels.

There are three major types of rock bolts: mechanically anchored, fully grouted and friction bolts.10 In Japan, fully grouted rock bolts are generally used with mortar as the grouting agent except the locations where spring water is abundant. When spring water is abundant, friction anchored method which expand the steel piping by the water pressure are applied. Although a special mechanism such as the D-bolt is required to prevent from major deformations, fully grouted rock bolts can protect the steel components against corrosion and enhance the durability compare to the other methods.11 Since the pioneering studies conducted by12 and13; the transmission of shear stress between fully grouted rock bolts and the ground has been extensively investigated by numerous researchers.

Rock bolts which are installed up on completion of a tunnel to control the deformation during its entire life, oftentimes fail to deliver their optimum performance owing to several possible reasons. Such as: the ground around the tunnel is too plasticized while transmitting the shear forces along with the help of solidified motar; and the rock bolts are poorly anchored with grout at the rear end, which eventually resulting poor anchorage and insufficient bonding between rock bolt and the nearby ground.14

Although the rock bolt anchors exhibit higher efficiency in a short term performance, weathering induced long term deformations of surrounding ground gradually deteriorate the strength of soft rocks and highly reduce the efficiency of rock bolts. This phenomenon increasing the all-around ground pressures on the tunnel lining and eventually lead towards large plastic deformations.

Considering the fact, in order to ensure positive bonding of rock bolts and improvement of plasticized ground while preventing further deformation of tunnel, a combined improvement and bonding of rock bolt (hereafter “CIB-bolt”) mechanism was developed.

This paper describes about the CIB-bolt, starting with an outline of the bolt. Subsequently it describes a model experiment which was conducted to verify the deterioration-retarding effect of “urethane” as a possible grouting agent for the CIB-bolt by simulating ground pressure associated with plasticization of model ground. Afterwards, Installation trials of CIB-bolt were carried out to confirm the workability, and the change in axial force after the introduction of prestress was evaluated by long-term monitoring. Finally, the influence of conventional and CIB-bolt on the performance of a tunnel under different scenarios also described using numerical analysis.

Section snippets

Outline and features

Tunnel deformation such as squeezing on sidewalls of the tunnel generally indicates that the ground around the tunnel is deteriorated or plasticized, meanwhile the ground away from the tunnel is in relatively good condition.9 Fig. 1 shows a schematic diagram comparing the conventional rock bolt and the CIB-bolt.

The conventional rock bolt, especially a longer one is likely to have a poorly anchored at the point. In addition, the ground near the tunnel may be deteriorated, and it is easy for the

Model experiment

A model experiment was conducted to verify the ground deterioration-and deformation-retarding effects of urethane as a possible grouting agent for the CIB-bolt by simulating ground pressure associated with ground plasticization.

Installation trial on an abandoned tunnel

Real field observations were made using the rock bolts which were installed on an abandoned, single-track, plain concrete lining tunnel with the cooperation of the Hokkaido Railway Company.

Numerical analysis

In this chapter, we try to evaluate the effect of CIB-bolt quantitatively by numerical analysis.

Conclusions

The main findings and the outcomes of this research are summarized as follows.

  • i)

    Development of the CIB-bolt

    • -

      The CIB-bolt was developed to offer positive anchoring of the bolt on the rear end also to improve the plasticized ground by grouting with the use of urethane as a possible grouting agent.

  • ii)

    Model experiment on a possible grouting agent for the CIB-bolt

    • -

      Plasticized ground pressure was simulated in a model experiment. It was found that, by using urethane as a grouting agent for the rock bolt,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The study covered in this paper was subsidized by the Railway Technology Development Program of the Japan's Ministry of Land, Infrastructure, Transport and Tourism.

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