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Proceedings of World Landslide Forum 3, 2-6 June 2014, Beijing

Giuseppe Delmonaco(1), Claudio Margottini(1), Daniele Spizzichino(1) and Bilal Khrisat(2)

Rock slope potential failures in the Siq of Petra (Jordan)

(1) ISPRA, Italian National Institute for Environmental Protection and Research, Via Vitaliano Brancati 48, Rome 00144, Italy.

(2) Department of Conservation Sciences, Queen Rania Institute of Tourism and Heritage, Hashemite University, Zarqa – Jordan.

Abstract The Siq is a 1.2 km naturally formed gorge that represents the main entrance to Petra (Jordan). Discontinuities of various type (bedding, joints, faults), mainly related to stratigraphic setting, tectonic activity and geomorphological evolution of the slope can be recognized. Structural condition determines a rock-fall potential activity that may involve unstable volumes, from 0.1 m3 up to over some hundreds m3. The latter can be catastrophic according to evolution of the movement (extremely rapid) and involved rock mass volumes. Slope instability, acceleration of crack deformation and consequent increasing of rock-fall hazard conditions could threaten the safety of people walking through the Siq. UNESCO, ISPRA and Jordan local authorities have implemented a project focused on landslide hazard assessment and risk mitigation strategies as a first step for the long-term conservation of the Siq. The paper reports preliminary data on landslide inventory, geomechanical properties of materials and assessment of landslide kinematics that affect the Siq of Petra.

Keywords Rock fall, Landslide kinematics, Cultural heritage Introduction

The Siq is a naturally formed deep gorge in the sandstone mountains that connects the urban area of Wadi Musa with the monumental area of Petra (SW Jordan) and represents, since Nabataean times, the main narrow entrance for some thousands tourists that access Petra every day.

Recent active and landslide processes involving the Siq and other parts of the archaeological area have arisen the attention on the geological conservation of the site as well as on the safety of visitors.

An international project, managed by UNESCO (Siq Stability – Sustainable Monitoring Techniques for

Assessing Instability of Slopes in the Siq of Petra, Jordan), has been funded by the Italian Ministry of Foreign Affairs, for the analysis of slope stability conditions of the Siq and implementation of an integrated remote and field monitoring systems aimed at the detection and control of deformation processes.

In the framework of the Siq Stability project activity, the paper reports a preliminary landslide inventory map of potentially unstable blocks and a geotechnical and geo-structural analysis with detection of the main potential slope failure types for a further landslide hazard assessment of the Siq area.

Geological and geomorphological setting

Petra is located on the eastern side of the Dead Sea Wadi Araba tectonic depression, a ca. 15 km-wide topographic low formed by shearing along the transform fault separating the Arabian and Sinai plates (Sneh, 1996; Ginat et al., 1998. The local stratigraphic succession (Quennel 1951; Bender, 1974) starts with ca. 50 m thick Salib Arkose arenitic formation, overlain by >500 m thick massive and poorly stratified Cambrian-Ordovician quartzarenites of the Umm Ishrin and Disi formations where the hand-carved rock monuments of Petra are entirely cut. The lithological and petrographic characteristics of the two formations are controlling weathering related conservation problems, affecting, as well, shear strength parameters of the rocks.

The Siq has a length of ca. 1.2 km with a general E-W orientation and a meandering course. It is the natural prolongation of Wadi Musa river before the Nabataeans diverted it through the Wadi Mudhlim tunnel. The difference in height between the beginning at the entrance dam and the current exit at the Treasury is 63 m, at least in the bottom part of the slopes. Actually, the difference in altitude between the path floor and the top of the slopes at the confluence with the Outer Siq is much higher (>200 m). This is due

G. Delmonaco, C. Margottini, D. Spizzichino, B. Khrisat – Rock slope potential failures in the Siq of Petra (Jordan)

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to the morphological setting of the area that presents a stepped-slope shape lower part, where visual survey is possible, and a middle and top sectors where slope faces are only partially observable from the Siq path.

Figure 1. Geological map of the Siq area

The Siq is surrounded by very steep slopes entirely formed by the Umm Ishrin Sandstone Formation that can be subdivided into three main units, according to texture, mineralogical composition and engineering classification (Fig. 1). The Upper Sandstone, called “honeycomb sand-stone”, is composed of white and mauve-red, coarse to medium grained, hard and massive sandstone, forming very steep slopes. It is characterized by typical cavernous weathering caused by solution of cement and consequent granular disintegration that form the typical honeycomb structures. The Middle Sandstone (“tear sandstone”) consists of multi-coloured, medium to fine-grained, well-bedded and friable sandstone.

Weathering is diffuse, especially by solution of the ferruginous and manganiferous layers and cements that cause change or the rock face from red-brown to yellow and grey. Cross-bedding structures and presence of interbedded silty and clayey sands are quite common. The Lower Sandstone (“smooth sandstone”) is made of white, medium to coarse-grained, hard massive sandstone.

The Siq geomorphology is the result of long and short-term factors such as tectonic up-lift, erosion due to runoff, differential erosion and weathering of sandstone materials. The slopes, as a general rule, presents a rupestral aspect, mainly massive. Nevertheless, discontinuities of various types, are present, mainly related to stratigraphical setting (bedding, generally horizontal), tectonic activity (faults, master joints, mainly sub-vertical), geomorphological activity (from vertical to medium-inclined joints). Sub-vertical and medium-angle dipping joints intersecting horizontal bedding are quite frequent in some parts of the Siq stratification and observed during field investigation. This situation may cause potential sliding of blocks, whose dimensions are depending on local, orientation, density and persistence of joints.

Rock slope failures and potential magnitude occurring in the Siq of Petra have been recognised (Delmonaco et al., 2012) considering the main failure type (fall, topple, slide). All the above geomorphological processes have been collected and elaborated through a geo-database and a preliminary landslide inventory map (Fig. 2).

Figure 2. Landslide inventory map of Petra area. 1) rockfall large; 2) rockfall medium; 3) rockfall small; 4) toppling large; 5) slide large; 6) slide medium; 7) slide small; 8) unstable debris; 9) scarp; 10) Siq Sector (numbered).

Proceedings of World Landslide Forum 3, 2-6 June 2014, Beijing

Geotechnical and structural analysis Field investigations have been conducted to

reconstruct the structural and geomechanical characteristics of the Siq slope-forming rocks with geotechnical field techniques and laboratory tests on representative core rock samples and blocks of the Umm Ishrin Sandstone Formation. The Siq has been divided into 15 different sectors, following morphological and structural criteria, in order to provide a zoning on potential landslide types and kinematical processes affecting each single sector.

The assessment of the strength parameters (friction angle/cohesion and UCS) and geomechanical indexes (i.e. RMR, GSI, Q system) as per ISRM suggested methods (ISRM, 1978; 1981; 1985) have been reconstructed through the execution of scanlines in each sector and: i) execution of Schmidt-hammer tests on discontinuities and intact rock and point load test with portable equipment for assessment of UCS; ii) tilt tests on core rock blocks for reconstruction of the base friction angle.

Figure 3. GSI distribution value for the whole Siq.

Laboratory tests on core and natural blocks have provided values of UCS, tensile strength and dynamic parameters.

In situ and laboratory analysis are reported in the following Table 1 and Table 2.

A rock mass classification (Hoek, 2007, Hoek and Brown, 1988 and Hoek et al.,1992) for the right and left flanks of each sector is given in Figure 3.

A geostructural characterization of discontinuities has been done in the Siq sectors in order to provide the reconstruction of the structural setting of the area.

Generally in the Petra area straight, individual joints, with mostly vertical planes, can be followed in the field for long distances (up to 500 m), mainly related to tectonics. The trends of the main high-angle joint systems in the area, mainly vertical are controlled by, firstly, the post-Ordovician pre-Lower Cretaceous stresses which affected the Palaeozoic rocks only, secondly, by the stresses which created the Dead Sea transform fault system (Miocene-recent) and, thirdly, by tectonics within the Ash Shawbak fold belt

(Miocene) (Barjous, 1987). Medium-angle joints are related to lateral unloading due to water erosion coupled with tectonic uplift as well as and to the typical petrographic and physical properties of the rocks (e.g. chemical and mineralogical composition, grain size, shape, thickness, homogeneity, porosity, permeability, type of cement).

Table 1 Geotechnical parameters estimated from in situ techniques

Natural unit

weight (KN/m3)

JRC UCS (MPA)

(Schmidt Hammer)

φ° (Tilt test)

UCS (MPA) (Point Load

test)

19.16-21.68

4-6 - 35-43 115 (hard

sandstone) - 4-6 25-35 35-43 30

(soft sandstone) GSI c (MPa) φ° Em (GPa) 72 0.4 43° 12

Table 2 Synthesis of geotechnical parameters from laboratory test

γdry

(KN/m3) Vp

(Km/s) Vs

(Km/s) σn

(MPa) Triaxial

tests

σt (MPa)

Brazilian tests

20.8 2.15 1.18 22.85 2.84 ρdry

(Mg/m3) Esec

(GPa) Etan

(GPa) Κ(σn/σt) νdyn

2.12 6.45 9.46 8.04 0.28 The intensity of jointing is variable depending on

distance from faults and strength of materials (i.e. higher within the siltstone sequences, lower within sandstone layers).

All data collected for blocks/slope areas in the Siq, have been analysed and represented with the geo-structural software DIPS® and reported in the Fig. 4.

Figure 4. Rose diagram (left) and plot (right) of joints surveyed in potential unstable sectors in the Siq.

The rose diagram shows that the major trend of joint systems ranges around a ENE-WSW directions

G. Delmonaco, C. Margottini, D. Spizzichino, B. Khrisat – Rock slope potential failures in the Siq of Petra (Jordan)

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(060–080° and 240–260°) with high slope angles (70–90°) exhibiting a trend parallel to local slope faces. These systems are in accordance with the dominant joint systems surveyed in the area that strike between 60–70° and 160–179° and are characteristic of the Umm Ishrin Sandstone and Disi Sandstone Formations (Jaser & Bargous, 1992). Also inclined joints, that represent the potential failure plane of sliding-mode blocks, are usually parallel to the main joint directions. The Figure 5 reports the main structural setting of the Siq area performed both through satellite image photo interpretation coupled with a field survey.

Figure 5. Sketch of main discontinuities of the Siq area performed by aerial photo interpretation and field survey. Legend: s (Pleistocene soil); Al (Alluvium and wadi sediments; Pl (Pleistocene gravel); DI (Disi Sandstone); uIN (upper Umm Ishrin Sandstone); mIN (middle Umm Ishrin Sandstone); f (fault); j (joint); # (sector number)

Kinematic analysis The presence of brittle sandstone in the Siq area

and in Petra, as a general rule, promotes a block composition of rocks due to high frequency of dis-continuities of various origin (i.e. faults, joints). All geomorphological processes and slope instability, acting along the Siq of Petra, are the results of different structural combination of the main joints families.

Rock slope potential landslides occurring in the Siq of Petra can be classified as toppling (base, direct and oblique), planar and wedge failure modes depending on the type and degree of structural control.

Slope processes differ along the Siq path both in terms of activity, namely their state (temporal evolution), distribution (spatial evolution), style (combination and repetition of different failure mechanism) and as magnitude (landslide intensity and potential volumes).

Toppling is widespread along the whole Siq area due to large presence of sub-vertical and opened discontinuities parallel to the slope faces, locally intersecting sub-horizontal bedding planes of weaker sandstone layers. Figure 6 is representative of a toppling failure mode involving a large volume block,

located in sector 5 of the Siq that is deemed as one of the most dangerous.

Planar failures in the Siq can occur along medium-angle dipping joints intersecting sub-vertical discontinuities (Fig. 7). For larger potentially unstable blocks in the Siq, planar sliding is generally coupled with toppling failure mode.

Wedge failures are not so frequent in the Siq slopes, although they can develop on slope faces where main joints, oriented generally E-W, encounter secondary joints with orientation NNW-SSE.

Potential unstable volumes of wedges are as a general rule <10 m3, though larger volumes can be involved (Fig. 7).

Figure 6. Potential toppling failure mode of a large block.

Figure 7. Planar sliding (left) and wedge sliding blocks (right)

Proceedings of World Landslide Forum 3, 2-6 June 2014, Beijing

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Figure 8. Example of kinematic analysis for sector 5, right slope. From upper left to lower right: direct toppling, flexural toppling, planar sliding, wedge sliding.

A kinematic analysis has been conducted for the

right and left slopes of sectors 1-14 (sector 15 is related to the Treasury area, out of the Siq) using a specific software (DIPS®). The main rock failure types analysed are: block and flexural toppling (Goodman and Bray, 1976; Matheson, 1983; Hudson and Harrison, 1997), plane and wedge failures (Hoek and Bray, 1981).

Slope face angles of right and left sectors have been calculated by analysing 10 m sections of the Siq, elaborated by Zamani Group - University of Cape Town, with a laser scanning technique. A general friction angle of 42°, derived by field analysis and laboratory test, has been considered as representative of sandstone materials in the Siq. In some sectors, characterised by different modes of slope angles, distinct elaborations have been produced. A total of 120 simulations have been calculated for the visible part of the Siq slopes where geostructural scanlines have been carried out (Fig. 8) and a synoptic table of potential failure types

elaborated for further landslide hazard assessment of the Siq sectors.

Conclusion Geomorphological, geo-structural and

geotechnical evidences of the Siq of Petra slope-forming rocks suggest that actual and/or potential instability processes in the Siq are the result of a combination of different predisposing factors in the area mainly depending on presence, frequency and orientation of discontinuities vs. slope orientation.

Potential instability processes and mechanisms observed in the Siq slopes can be referred to three main failure modes (or their combination): planar failure; wedge failure and toppling/flexural failures. Direct toppling and planar sliding are the most common and can develop large blocks.

The volume of blocks are generally > 15 m3 (medium to large blocks) with evidence of blocks exceeding 500 m3. Nevertheless, the presence of many

G. Delmonaco, C. Margottini, D. Spizzichino, B. Khrisat – Rock slope potential failures in the Siq of Petra (Jordan)

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isolated blocks, mostly with a volume < 1 m3, are widely diffused in the upper part of the Siq and not visible from the ground. Those blocks, along with loose debris outcropping in lateral channels, removable by flowing water produced by heavy rainfall, have to be taken into account for the assessment of hazard and induced risk. In addition, seismicity is another important landslide triggering factor that may cause the falling of rock blocks characterised by stability conditions near to failure.

The implementation of monitoring systems by satellite (SqueeSARTM, TeleRilevamento Europa) and site techniques (crack gauges) has provided preliminary results on slope deformation of major faults/joints systems, affected by negligible displacement in the last 10 years (< 1-2 mm). Site direct monitoring of cracks in the Siq set up since April 2011, displays very little displacement (< 0.2 mm) mostly connected with seasonal effects of temperature and rainfall on cracks movement.

In this regard, the recent installation of integrated direct monitoring systems (total station, terrestrial photogrammetry and wireless sensor network) on potential unstable blocks and slope sectors can provide an effective tool in order to control the evolution of slope processes and detect the most active parts of the Siq where to address further analysis and suggest landslide hazard mitigation measures.

References (in the alphabetical order)

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Bender, F. (1974) Geology of Jordan. Gebrueder Bornstraeger. Berlin. 196p.

Delmonaco, G., Margottini, C., and Spizzichino, D. (2012) Rock-fall hazard assessment in the Siq of Petra, Jordan. The Second World Landslide Forum, 3-9 October 2012, FAO, In: K. Sassa, P. Canuti, C. Margottini (eds.) Landslide science and practice Vol. 6, Inventory and hazard assessment. Springer, Heidelberg, pp. 441-449.

Ginat, H., Enzel, Y. and Avni Y. 1998. Translocated Plio-Pleistocene drainage systems along the Arava fault of the Dead Sea transform. Tectonophisics 284:151-160.

Goodman, R.E. and Bray, J.W. (1976) Toppling of rock slopes. Proc. Spec. Conf. on Rock Engineering for Foundations and Slopes, ASCE, Boulder (Colorado), 2, pp. 201-234.

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Hoek E. and Brown E.T. (1988) The Hoek-Brown failure criterion - a 1988 update. Proc. 15th Canadian Rock Mech. Symp. (ed. J.H. Curran), Toronto: Civil Engineering Dept., University of Toronto, pp. 31-38.

Hoek, E., Wood D. and Shah S. (1992) A modified Hoek-Brown criterion for jointed rock masses. Proc. Rock Characterization, Symp. Int. Soc. Rock Mech.: Eurock ‘92, (Edited by Hudson J.A.), London, Brit. Geotech. Soc., pp. 209-214.

Hoek, E. (2007) Practical Rock Engineering. E-book, Rocksciences, 247p.

Hudson, J.A. and Harrison, J.P. (1997) Engineering Rock Mechanics: an introduction to the Principles, Elsevier Sciences.

ISRM (1978) Suggested methods for the quantitative description of discontinuities in rock masses. Int. J. Rock Mech. Min. Sci. Geomech. Abs., 15:319-368.

ISRM (1981) Rock Characterisation Testing and Monitoring, ISRM Suggested Methods. Salzbourg, 1981. 211p.

ISRM (1985) Suggested methods for determining point load strength. Int. J. Rock Mech. Min. Sci. Geomech. Abs., 22:51-62.

Jaser, D. and Bargous, M.O. (1992) Geotechnical studies and geo-logical mapping of ancient Petra city. Town Mapping Project, Bulletin 1, The Hashemite Kingdom of Jordan, Ministry of Energy and Mineral Resources, Natural Resources Authority. 62p.

Mart, Y. 1991. The Dead Sea Rift: from continental rift to incipient ocean. Tectonophisics 197:155-179.

Matheson, G.D. (1983) Rock stability assessment in preliminary site investigations – Graphical Methods, Transport and Road Research Laboratory Report, LR 1039.

Quennel, A.M. (1951) The geology and mineral resources of (former) Trans-Jordan. Colon. Geol. Mineral Resour. Lond. 2:85-115.

Sneh, A. (1996) The Dead Sea Rift: lateral displacement and down-faulting phases. Tectonophisics 263:277-292.

Giuseppe Delmonaco ( ) ISPRA, Italian National Institute for Environmental

Protection and Research, Via V. Brancati 48, Rome 00144, Italy

e-mail: giuseppe.delmonaco@isprambiente.it Claudio Margottini ISPRA, Italian National Institute for Environmental

Protection and Research, Via V. Brancati 48, Rome 00144, Italy

e-mail: claudio.margottini@isprambiente.it Daniele Spizzichino ISPRA, Italian National Institute for Environmental

Protection and Research, Via V. Brancati 48, Rome 00144, Italy

e-mail: daniele.spizzichino@isprambiente.it Bilal Khrisat Department of Conservation Sciences, Queen Rania

Institute of Tourism and Heritage, Hashemite University, Zarqa – Jordan

e-mail: bilal@hu.edu.jo

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