Half tunnels along hill roads of Himalaya—an innovative approach

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Abstract

‘Half tunnels,’ which are excavated as overhangs within steep slopes of hard rocks, have an advantage over conventional full tunnels or open excavations in that they involve less cost and time. However, due to a lack of interest and their uncommon occurrence, the design and analysis of half tunnels have remained by and large unexplored. In the present paper, an effort has been made to study some of the half tunnels in the Middle and the Higher Himalaya in India. In addition to studying the geology of half tunnels, rock mass properties pertaining to Q and RMR for the rocks exposed around the half tunnel have been evaluated. The Markland test has been carried out to assess the probability of plane or wedge failures along the slopes in the half tunnel area. The results obtained have been again confirmed by analyzing the data with the help of a computer program uwedge. Both analyses suggest that the wedges are stable and that all the half tunnels analyzed are safe. Finite element analyses for different slopes and spans of half tunnels have been carried out to study the distribution of stresses around half tunnels. The range of maximum tensile stress concentration in the roof of half tunnels for different slopes is suggested.

Introduction

Road construction in the Himalaya, particularly in the Higher Himalaya, is a complex process due to steep and unfavorable slopes. Often huge excavations are involved with attendant slope stability problems in addition to the high cost of construction. Though tunnels are often used, the cost of construction is again comparatively high. It is here, that half tunnels can provide a viable solution to construction problems.

Half tunnels are excavated as overhangs of hard rock slopes along hill roads with one wall on the hill side and no wall on the valley side. These half tunnels in the Middle and Higher Himalaya have existed for more than 20 years—in spite of no artificial support for the tunnel roof. Half tunnels, which are excavated as overhangs of hill slopes, have the advantage over conventional full tunnels or open road excavations in that they involve much less cost and time. However, due to a lack of interest and their uncommon occurrence, the design and analysis of half tunnels have remained by and large unexplored. Half tunnels also offer the following advantages in addition to economy of construction—greater attraction for tourism and preservation of the local ecosystem—due to minimum disturbance of steep slopes.

Section snippets

Location of half tunnels

The Western Himalaya has a comparatively rugged topography with a complex geological setting. The National Highways, NH-21 and NH-22 traverse through the Western Himalaya along the Sutlej river valley and the Beas and Parbati river valleys. A survey of these National Highways indicated the existence of a number of half tunnels on their route. In all, 15 half tunnels were observed—seven half tunnels along NH-22 in the Sutlej valley and the remaining eight half tunnels along NH-21 in Beas and

Sutlej river valley

Seven half tunnels (T1–T7) are present along a 5-km-long stretch of road in NH-22. Massive granite gneiss rocks are exposed in this section of the road. The granites are hard, massive and traversed by only a few joints. The foliation is highly discontinuous in nature and is generally poorly developed in the entire area—though at places it is well developed locally. In addition to foliation, one set of near-vertical joints seems to be well developed. A few random joints are seen at places, which

Rock mass assessment

The rock mass quality of all the half tunnels have been evaluated using the Q-system of Barton et al. (1974) and the Rock Mass Rating (RMR) system of Bieniawski (1989).

The rocks exposed at the different sites have been studied and the individual rock parameters evaluated. The descriptions and corresponding ratings of the parameters and the final Q-value for all the half tunnels are given in Table 1, Table 2. The Q-values for the rocks of the half tunnel vary from 18.38 to 38.09 indicating that

Wedge analysis

The stability of the half tunnels has been checked through wedge analysis. The analyses have been carried out both with the help of the computer program uwedge (Hoek and Brown, 1980) and stereographically through the Markland Test. Half tunnels M1, M2 and M3, where only one joint set was observed, did not need to be analyzed by the program because cases with a number of joint sets less than two are automatically considered stable wedges. The factor of safety results of the wedge analyses for

Finite element analysis

Stress analysis using numerical modeling is increasingly being employed to study rock mechanics problems associated with excavations in rock. Excavation of a half tunnel in a geologically complex and tectonically active Himalayan terrain is a challenging task, which requires a proper consideration of in situ stresses. Since the stability and safety of the tunnel is of prime importance in the construction of half tunnels, it is essential to have an insight into the stress distribution of the

Stress distribution in half tunnel

Results of the finite element analysis indicate the presence of compressive horizontal (normal) stress in the floor of half tunnel. The magnitude of the horizontal (normal) stress decreases along the floor of the half tunnel face towards the side wall. Horizontal stresses near the slope–floor junction vary from 0.01 to 0.06 MPa (compressive) for various cases. It varies between 0.31 and 0.93 MPa (compressive) near the wall–floor junction. On the roof, the horizontal (normal) stress decreases

Conclusions

Half tunnels, which are excavated as overhangs within steep slopes of hard rocks, have an advantage over conventional full tunnels or open excavations in that they involve less cost and time. In the Higher and Upper Lesser Indian Himalaya, where road construction poses challenges due to steep slopes with overhangs, 15 half tunnels existing presently along the roads were chosen for detailed studies. Of these, seven are located in Sutlej valley and eight in Beas and Parvati valleys of western

Acknowledgements

The authors are extremely thankful to Prof. Raymond Sterling for his valuable suggestions and useful corrections.

References (5)

  • N. Barton et al.

    Engineering classification of rock masses for the design of tunnel support

  • Z.T. Bieniawski

    Engineering Rock Mass Classification

    (1989)
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