- 3401 -

Stability Analysis of A Railway Trench By Using Stereographical Projection

Seyed Vahid Alavi Nezhad Khaili Abad

Ph.D. Candidate, Faculty of Civil Engineering, Universiti Teknologi Malaysia,81310 UTM Skudai, Johor Bahru, Malaysia

Corresponding Author, e-mail: v_alavi_59@yahoo.com

Edy Tonnizam Mohamad

Associate Professor (Dr.), Faculty of Civil Engineering, Universiti Teknologi Malaysia,81310 UTM Skudai, Johor Bahru, Malaysia

Danial Jahed Armaghani

Ph.D Student, Faculty of Civil Engineering, Universiti Teknologi Malaysia,81310 UTM Skudai, Johor Bahru, Malaysia

Roohollah Kalatehjari

Post-Doctoral Fellow, Faculty of Civil Engineering, Universiti Teknologi Malaysia,81310 UTM Skudai, Johor Bahru, Malaysia

ABSTRACT An important step in designing engineering structures in rocky areas is evaluation the stability of rock slopes. Kinematic analysis as a purely geometric method is able to determine the possible modes of failure in jointed rock slopes. The required geological study is performed usually by utilizing walk over method as well as discontinuity surveys. This paper describes the stability assessment of a railway trench by analysis of the geological structure using stereographic projection technique. The studied trench is located at an entrance of a railway tunnel near Veresk in North of Iran. A scanline method was carried out to record the properties of discontinuities of the rock mass. Then after, daylight envelopes of stereographic projections were used to recognize the modes of failure for each major joint set in the rock exposure. A joint set was found in daylight envelope of toppling failure, while another joint set was defined to cause a planar failure. As a result, it was concluded that there are probabilities of both toppling, and planar failures along the trench face. Finally, some recommendations were advised in designing a supporting system with the help of observation and result of stereographic plot technique.

KEYWORDS: Slope stability analysis, Kinematic analysis, Railway trench, Rock slope, Stereographic projection.

INTRODUCTION Evaluation of the rock slope stability is a classic problem for geotechnical engineers, which

plays an important role in designing of trenches, dams, tunnels, and other engineering structures.

Vol. 18 [2013], Bund. P 3402 Moreover, identification of failure mode in rock slopes plays an important role in designing of an appropriate supporting system. In the past decades, some researchers have focused on stability assessment of rock slopes (Hoek and Bray, 1981; Goodman, 1989; Wyllie and Mah, 2004).

There are four primary modes of rock slope failure namely planar failure, wedge failure, rotational failure, and toppling failure. A detailed review of different types of rock slope failures has been presented by Goodman and Kieffer (2000). Influential factors in causing instability are structural geology factors such as joints, faults and folds (Alavi Nezhad Khalil Abad et al., 2011). Sliding of a rock mass on the joint/weak plane dipping away from the slope is termed as the planar failure. It generally occurs in hard or soft rock slopes with well-defined discontinuities and jointing, e.g., layered sedimentary rocks, volcanic flow rocks, block jointed granite, and foliated metamorphic rocks. When two distinct planes of weaknesses, joints or fault planes exist, the rock mass between these planes can slide down; this mode of failure is known as the wedge failure. According to Kliche (1999), rotational failures are little-deformed slumps, which are slides along a surface of rupture that is curved concavely upward. In slumps, the movement is more or less rotational about an axis that is parallel to the slope. Toppling failure takes place when a regularly spaced set of joints or bedding planes strike parallel, or nearly parallel, to the slope face and dip at a steep angle into the face.

Stability charts for soil slopes were first produced by Taylor (1937) and then draw the attention of many investigators to be used extensively as design tools (Hoek and Bray, 1981; Gens et al. 1988). Unfortunately, no stability chart is available in the literature to assess the stability of rock slopes based on rock mass strength criteria. Although the stability charts proposed by Hoek and Bray (1981) for Mohr-Coulomb material can be applied to rock slopes, this requires knowledge of the equivalent Mohr-Coulomb cohesion and friction for the rock mass. Besides, numerous studies have been performed to determine failure modes by stereographic projection technique (Goodman, 1976; Hocking, 1976; Hoek and Bray, 1981; Matherson, 1988; Markland, 1972; Cruden, 1978). In this study, stability assessment of a railway trench is performed by analysis of the geological structure using stereographic projection technique.

SITE LOCATION This study was carried out on a trench located at an entrance of a railway tunnel near Veresk

(Figure 1) in North of Iran (latitude: 35°54'26.73"N and longitude: 52°59'27.94"E).

This site was selected for the study due to high suspicious of failure in this area as it had specific geological structure. It is important to evaluate instability of the trench and find out the type of possible failure to design an appropriate supporting system for failure prevention. The mentioned area consists of multi layers of rock as marl, thin shale, and siltstone which based on geology map (Figure 2) belong to Barout zone. The geological age of the mentioned area is Precambrian. There is an anticline in the studied area. The geometry of the trench is as length of 130 m, height of 21 m, and slope angle of 78°. The orientation of trench is 6° from North. Figure 3 shows the overview of Veresk trench.

Vol. 18 [2013], Bund. P 3403

Figure 1: Site location of Veresk trench

Figure 2: Geological map of Barout zone with the scale of 1:250000

Vol. 18 [2013], Bund. P 3404

Figure 3: The overview of Veresk trench

METHODOLOGY OF THE STUDY The study consisted of surveying of weakness planes and joints in the site. The geological

study was performed utilizing walk over method as well as discontinuity surveys such as scan line surveys. Moreover, in this case study, the kinematic stability analysis of the rock slope was carried out. Kinematic analysis, which is purely geometric, determines which modes of slope failure are possible in a jointed rock slope (Wyllie et al., 2004; Mohamad et al., 2011). It refers to the geometrically-possible motion of a body without consideration of the forces involved. Also Angular relationships between discontinuities and slope surface are analyzed to determine the potential and modes of failures (Kliche, 1999).

Field study data such as discontinuities orientation, spacing were recorded by scanline method. Then dips software was used as a stereographic plot technique to identify the number of discontinuity sets and their orientation. Figure 4 shows the daylight envelope of toppling, planar, and wedge failure. These envelopes are used to recognize the modes of failure for each major joint set in rock exposure. Moreover, some of the geomechanical properties of the rock exposure were tested and the results are presented in Table 1.

Vol. 18 [2013], Bund. P 3405

Figure 4: Daylight envelopes on equal area stereonet

Table 1: Geomechanical parameters of Veresk trench. Density (gr/cm3) Cohesion (kPa) Friction angle (degree) σcm (MPa) Edm (GPa)

2.6 250 32 4.7 2.3

RESULTS AND DISCUSSION In order to evaluate the stability of Veresk trench, geological and structural geology data of

the site were recorded. To understand the mode of rock slope failure, analysis of the geological structure was performed using stereographic projection technique. Figure 5 shows three major joint sets as J1, J2, and J3 with dip/dip directions respectively as 80/195, 78/20, and 28/240. It should be noted that dip/dip direction of trench face is 78/6.

Vol. 18 [2013], Bund. P 3406

Figure 5: Plans of major joint sets and trench face

Figure 6 shows the major joint sets of the trench and daylight envelops for toppling failure. It can be seen that a joint set is located in daylight envelope of toppling failure. As a result, there is a probability of toppling failure along the trench face. This result was expected because the slope angle of trench face was less than the dip angle of J1.

Figure 6: Major joint sets and daylight envelops of toppling failure

Vol. 18 [2013], Bund. P 3407

Figure 7 shows the major joint sets of the trench and daylight envelops for Planar failure. It is seen that joint set number 3 is caused planar failure in Veresk trench. It is clear that this joint set is located near the black zone.

Figure 7: Major joint sets and daylight envelops of planar failure

Figure 8 shows the major joint sets of the trench and daylight envelops for wedge failure. As mentioned earlier, when two distinct planes of weaknesses, joints or fault planes exist, the rock mass between these planes can slide down as wedge failure. It is observed that there is no such condition to have wedge failure.

Figure 8: Major joint sets and daylight envelops of wedge failure

Vol. 18 [2013], Bund. P 3408

CONCLUSIONS AND RECOMMENDATIONS Slope stability evaluation of a railway trench near to Veresk in North of Iran was conducted

based on its geological data. For this purpose, discontinuities orientation condition was recorded by utilizing scanline method and the collected data were used to analyze the mode of rock slope failure. Dips software was used as a stereographic plot technique to identify the number of discontinuity sets and their orientation. Three major joint sets were identified as J1, J2, and J3 with dip/dip directions respectively as 80/195, 78/20, and 28/240. After evaluating all possible rock slope failures, it was concluded that toppling and planar failures can be occurred in the Veresk trench due to dip/dip direction of J2 and J3.

With the help of observation and result of stereographic plot technique, the following recommendations are advised in designing a supporting system:

In order to prevent water flow into the joints, a reinforced shotcrete layer with thickness of 5 to 10 cm should be applied.

In order to decrease the probability of both planar and toppling failures, a retaining wall with minimum height of 10 m (about half of trench height) should be constructed.

In order to collect the rain water, water channels should be installed in the upstream of the trench.

ACKNOWLEDGEMENTS The authors would like to thank Universiti Teknologi Malaysia for its financial support.

REFERENCES 1. Alavi Nezhad Khalil Abad, S. V, Mohamad, E. T., Jahanmirinezhad, H., Hajihassani,

M., and Namazi, E (2011). Zonation of landslide hazards based on weights of evidence modeling along Tehran-Chalos Road Path, Iran. Electronic Journal of Geotechnical Engineering, 16.

2. Cruden, D. M (1978) Discussion of G. Hocking’s paper ‘‘A Method for Distinguishing Between Single and Double Plane Sliding of Tetrahedral Wedges’’. Int. J. Rock Mech. Min. Sci. Geomech Antoine, M. M. and J. M. Bradford (1982) “Parameters for Describing Soil Detachment due to Single-Water Impact,” Sad Sam Soc. Am. J., 46, 36-840.

3. Gens A, Hutchinson J.N., and S. Cavounidis (1988) Three-dimensional analysis of slides in cohesive soils. Géotechnique 38:1, 1-23

4. Goodman, R. E (1976) Methods of Geological Engineering in Discontinuous Rocks. West Publishing, San Francisco.

5. Goodman, R.E (1989) Introduction to Rock Mechanics. 2nd edition. Wiley, New York. 562 pp.

6. Goodman, R. E. and D. S. Kieffer (2000) Behaviour of rocks in slopes. J. Geotech and Geoenvi. Eng., ASCE, 126:8, 675-684

Vol. 18 [2013], Bund. P 3409

7. Hocking, G (1976) A Method for Distinguishing Between Single and Double Plane Sliding of Tetrahedral Wedges, Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 13, pp. 225–226.

8. Hoek, E. and J.W. Bray (1981) Rock Slope Engineering Institution of Mining and Metallurgy, London.

9. Kliche, C. A (1999). Rock slope stability. SME. Littleton, CO.

10. Markland, J. T (1972) A Useful Technique for Estimating the Stability of Rock Slopes When the Rigid Wedge Sliding Type of Failure is Expected. Imp. Coll. Rock Mech. Res.

11. Matherson, G.D. (1988) The Collection and Use of Field Discontinuity Data in Rock Slope Design, Q. J. Eng. Geol.22, pp. 19–30.

12. Mohamad, E. T., and Alavi Nezhad Khalil Abad, S. V (2011). Assessment on Blasting–Induced Rock Slope Instability At Johor, Malaysia. Electronic Journal of Geotechnical Engineering, 16.

13. Taylor, D.W. (1937) Stability of earth slopes. J. Boston Soc. Civ. Eng., 24, 197–246.

14. Wyllie, Duncan, C. and C. W. Mah (2004) Rock Slope Engineering: Civil and Mining, 4th edition, Spon Press.

© 2013 ejge