Haibat Sultan Mountain is a long range with elevation of about 860 m (a.s.l.); the PilaSpi Formation forms its carapace in Koya vicinity, with relief difference of about 300 m from Koisanjaq plain. The PilaSpi Formation consists of well thickly to massively bedded dolostone and dolomitic limestone with thickness of about 120 m in Koya vicinity. The main trend is NW - SE being a limb of Bustana anticline representing part of the southwestern limb, with dip amount that ranges from (15 - 30). On 11th of November 2015 a landslide had occurred after a heavy rainfall along Koya - Dukan main road. The type of the slide was plane sliding due to daylight slope, which was formed after the road cut. The length of the slide area: along the road is 90 m with height of 40 m forming almost a parallelogram shape; the thickness of the slid beds is about 2.5 m. The estimated volume of the slid mass is 9000 m 3. The main cause of the landslide is the presence of daylight slope, high slope angle; more than the dip angle, presence of old crack surfaces which are filled by reddish brown clayey residual soil. After the he heavy rain fall, which lasted for 20 hours, the infiltrated rain water in the bedding planes in the well bedded, cracked, and jointed beds has increased the pore pressure and decreased the internal friction angle; therefore, the sliding has occurred. The root of the slid mass is below the base of the paved road; therefore, that part which is above the paved road has slid. The remaining part is highly cracked and partly accumulated in the base of the slid mass. Fortunately, the height of the slid mass is only 40 m and the relief difference between the crown area and the toe area is about 50 m; otherwise the slid mass would be larger than the present slid mass. From the field inspection, it is very clear that the involved area is very unstable and in critical equilibrium. The presence of daylight bedding above the crown area, clayey soil in the fractures and open joints and steep slope all are very favorable conditions for triggering the unstable slope, consequently developing of another landslide with larger mass.
On the 11th of November 2015, a landslide had occurred after a heavy rainfall along Koya - Dukan main road, north Iraq, Kurdistan Region, and it will be called Koya Landslide in the current study. The sliding had happened due to daylight slope, which was formed after the road cut. The main road was blocked by the slid mass for overnight.
The studied area is located northeast of Koisanjaq town, along the main road between Koisanjaq town and Sulaimaniyah city via Dukan town (
The main aim of this study is to record the landslide event and describe the details of the slid mass and other parts of the landslide with investigating and discussing the reasons of the sliding. Moreover, to recommend relevant preventative actions that should be considered in order not to trigger the involved area, this is in very critical equilibrium states.
Studies concerning the study area are very rare; however, as general mass movements studies are concerned;
few are present. Generally, regional studies are available; not for individual events; hereinafter are some of those studies.
Sissakian and Ibrahim [
Sissakian and Ibrahim [
Sissakian and Ibrahim [
In order to achieve the main aim of this study, which is to describe and discuss the reasons of Koya Landslide, the following materials were used:
・ Topographic and geological maps of different scales.
・ Google Earth and FLASH Earth images.
・ Different geological published articles and reports.
・ Historical books.
・ Field observation data.
Using the available topographical and geological maps of different scales with the help of FLASH Earth and Google Earth images, the location of Koya Landslide is determined and the stability of the area and near surroundings is evaluated. The parameters of the landslide were measured during a field work that was carried out on the second day of the event. The exact limits of the landslide were mapped, significant parameters of the landslide; such as crown area, shear plane, dimensions were measured and documented by tens of photos.
The geomorphology, structural geology and tectonics, and stratigraphy of the study area are given briefly; hereinafter, depending mainly on best available data [
The main geomorphological units in the studied area are:
・ Anticlinal ridges: The thickly well bedded massive carbonates of the PilaSpi Formation have formed obvious anticlinal ridges along the southwestern limb of Bustana, BanaBawi and Khalikan anticlines
・ Alluvial fans: Many small alluvial fans are developed in the study area and near surroundings, especially when the valleys cross the PilaSpi Formation towards the Gercus Formation.
・ Flat irons: These are well developed due to the thickly well bedded and massive carbonates of the PilaSpi Formation (
・ Mass movements: Many small phenomena occur in the study area, like toppling, mud flows, landslides and rock fall.
The study area is located on the southwestern limb of Bustana anticline, which is an NW - SE trending double plunging anticline. The length is about 11 Km, whereas the width is about 3.5 Km. The southern limb is steep; up to 55 Km, whereas the northeastern limb is very gentle; not more than 20 Km (
Tectonically, the studied area is located within the High Folded Zone of the Outer Platform (Unstable Shelf) of the Arabian Plate [
In the study area, only the PilaSpi Formation is exposed; however, the overlying and underlying formations are described hereinafter; briefly depending on best available data [
・ Gercus Formation: The formation consists of red clastics; mainly claystone and sandstone with some fine conglomerate. The presence of thick claystone beds may form slippery plain for sliding.
・ PilaSpi Formation: The formation consists of thickly well bedded carbonates; dolomite, dolomitic limestone with rare marl and marly limestone beds in the lowermost part of the formation.
・ Fatha Formation: The formation, in the study area consists of thickly bedded claystone, siltstone with very rare limestone and gypsum in cyclic nature.
At the night of 11th of November 2015 a landslide (
The length of Koya Landslide area; along the road is 90 m with height of 40 m forming almost a parallelogram shape (
Koya Landlside is a rock bedding slide that had happened along the bedding plane of the well bedded carbonates of the PilaSpi Formation (Figures 2-4). Two and locally three bedding planes are acted as sliding plain.
The crown area of Koya Landslide has not a continuous arch form (
The shear plane of Koya Landlside is very clear (Figures 3-5); since the landslide is of a plane slide type. The
shearing traces are very clear along the bedding planes of the slid mass as vertical grooves formed parallel to the dip direction, which is the same of sliding direction.
The toe area of Koya Landslide was removed in the second day of the sliding; since it was bocked the main road between Koisanjaq and Dukan towns. It was in form of accumulated blocks on the paved road (
In Koya Landslide, multiple types of causes had developed the landslides these are explained hereinafter.
Among these causes are the presences of well bedded carbonates encountered rarely with marl horizons (
ding planes.
Among morphological causes, the main one is the fluvial erosion by the rain water, besides the freeze-and-thaw weathering and shrink-and-swell weathering that have acted after continuous raining for 20 hours and drop in daily temperatures during November where the temperature may fall to zero and even below.
Moreover, the heavy rain shower and the presence of open cracks, fishers, joints; all filled with clay, may had caused formation of piping phenomenon by means of which the formed slurry in the opened voids has triggered and accelerated the sliding; acting as lubricant agent.
Among the human causes, the excavation and blasting of the rocks during widening of the road had formed fishers and cracks in the well bedded carbonates of the PilaSpi Formation (
The narrow one lain paved road with very steep gradient, also may acted as artificial vibration due to the passage of heavy trucks along the road. Such vibration also may trigger and accelerate the sliding [
As in each mass movement phenomenon, Koya Landslide area is a prone area for different types of mass movements, especially landslide, toppling and rock fall (
For each type of mass movement, there are certain types of precautions to control the movement [
1) Digging a ditch surrounding the upper part of the landslide and near surroundings (
2) Decreasing the slope of the daylight slopes initiated by haphazard cutting of the slope during widening of the road.
3) Bolting and anchoring the massive carbonate beds (Figures 2-5). After performing special study to choose the proper length of the anchors and/ or bolts, intensity of the anchoring; including the distance between them, angle of the anchoring and location of the anchors [
4) Increasing the vegetation on the upper limits of the landslide area and near surroundings.
5) Construction of relevant concrete protection wall [
Slope saturation by water is a primary cause of landslides. This effect had occurred in Koya Landslide form intense rainfall, where there is a distinct zone of weakness that separates the slide material from more stable underlying material (
The presence of clayey materials in between the open joint and bedding planes, and previously existing fractures (
brown clayey materials were formed as residual soil due to the weathering of the carbonates of the PilaSpi Formation.
The location of the Koya Landslide was suffering from slope stability problems. An old landslide and active rock fall (
The height difference between the crown area and the toe area is only 40 m; therefore, the landslide area is a small one. This is attributed to the fact that the bedding plane along which the slide has occurred; terminates just with the paved road (
The following can be concluded from the current study:
The Koya Landslide is a block-plane slide of translational slide type in which the moving mass consists of a single unit that gas moves downslope as a relatively coherent mass.
The main cause of the landslide is the rain water: after the heavy rain showers on the date of the sliding that has increased the pore water pressure and decreased the internal friction angle.
The previously existing unstable slope: due to the presence of old landslide and rock fall phenomena in the involved slide area.
The presence of daylight slopes due to haphazard cutting of the slope during the road cut and road widening has accelerated and triggered the sliding.
The absence of the vegetation also has played a significant role in the sliding.
The presence of clayey materials between the bedding planes, open joint planes and fractures has played as slippery plain after being oversaturated by the rain water.
Sakar A. Sadiq,Arezh S. Muhmed,Hozan Govar K. Haris,Dastan M. Hamma,Mohanad M. Abdwllah,Hozan H. Bibani,Hawkar A. Mustafa,Varoujan Sissakian,Nadhir Al-Ansari,Hamza K. Muhealddin, (2016) Mechanism of Haibat Sultan Mountain Land-slide in Koya, North Iraq. Engineering,08,535-544. doi: 10.4236/eng.2016.88050