Geological map of the Siq area 

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... 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. 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 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. 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). 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. (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 ...