S. Triantos

Geomorphological observations and

landslide hazard mapping in the west part

of Cyprus

S. Triantos

School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, United Kingdom

Corresponding author (e-mail: ee08st@leeds.ac.co.uk)

Abstract: Swelling clays occur in a large part of west Cyprus characterized by

poor geotechnical properties and intermediate to very high swelling potential,

causing significant damage to the present infrastructure, forcing the authorities to

re-site villages in the past and necessitating expensive remedial. Areas with

complex geology and topography, where the most adverse conditions occur,

were investigated in an attempt to assess the way of creation and the magnitude

of the failure mechanism. The field survey and the carried out slope stability

analysis suggest that instability problems occur in order of increased magnitude

in the following swelling clays formations; Kathikas Mélange, Superficial Mélange

and Kannaviou Formation. The most common failure mechanism is creeping

evolving into translational sides and/or rotational failures. The presence of an

overlying competent caprock (e.g. Lefkara Chalk, Kathikas Mélange), is

considered to protect from the erosion but promote instability. The produced

Factor of Safety map indicates landslide prone areas.

The highlands west of the Troodos range, an outcropping ophiolite sequence, characterize a

topographical zone with an elevation ranging from 400 to 800m asl. This zone has been

extensively examined in the past since the outcropping cohesive soils of high swelling potential,

(Cavit Atalar et al, 2006), combined with the complex geology-topography and Cyprus climate-

seismicity, cause instability issues and extensive damage to the island’s infrastructure.

These “problematic” soils, (i.e. in the study area; Kannaviou Formation, Kathikas

Melange, Superficial melange-weathered Mammonia), are exposed in an extensive area and are

frequently overlain by a competent caprock (Lefkara, Pakhna Formations), which generally

deteriorates the failure magnitude. The study area has been seriously affected by landslides in the

past, (1953 and 1969), due to a combination of catastrophic events, earthquake shocks of 1953

(M = 6.4), related with the rupture of the central part of the Paphos Transform Fault, (Papazachos

and Papaioannou, 1999) and the occurrence of swelling clays.

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Fig. 1 Sketch map with the occurrences of swelling clays in the study area, (Lapierre et al., 2007).

TOPOGRAPHY AND CLIMATE

The island of Cyprus is located in the south east part of the Mediterranean Sea centred at 35°E

and 33.5°N. The topography is characterized by great contrast, with plains lying between ridges.

The central part of Cyprus Island is dominated by an anticline structure formed by the Troodos

ophiolite, surrounded by lowland areas; Mesaoria plain to the east and the coastal plain of Paphos

region to the west. The north-east part of Cyprus Island is dominated by the Kyrenia Range. A

network of Winter Rivers rises in these ranges and flows out towards the coastal areas.

The climate of Cyprus, an intense Mediterranean type, is characterized by hot dry

summers and generally mild winters but local variations occur due to the topography and the

marine influences. During the summer the mean daily temperature ranges from 29 to 36 oC on the

central plains and from 22 to 27 oC on the Troodos Range, (G.S.D website, 2009). During the

winter the mean daily temperature ranges from 5 to 10 oC and 0 to 3 oC in the central plain and the

Troodos Range respectively, (G.S.D website, 2009).

The average annual total precipitation ranges from 450 to 1100 mm at the coastal plain and the

top of Troodos Range respectively.

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GEOLOGICAL BACKGROUND

The complex geology of the study area can be divided into three geological zones: (1) the

Troodos ophiolite, (2) the Circum-Troodos autochthonous sedimentary rocks and (3) the Mamonia

Complex and related sediments.

The Troodos ophiolite

In the central part of the Cyprus Island crops out an anticlinal structure which forms the Troodos

Range. This is a complete and underformed ophiolite sequence, with composition characteristic of

an arc or forearc environment. The Troodos Ophiolite consists of the following stratigraphic units,

in ascending order; Plutonics (mantle sequence and cumulates), Intrusives, Volcanics and

Chemical sediments, (Robinson and Malpas, 1990).

Circum-Troodos autochthonous sedimentary rocks

Around the Troodos range the Zone of the autochthonous sedimentary rocks emerges, ranging in

age from Upper Cretaceous, (Kannaviou Fm), to Pleistocene, (Fanglomerates-alluvium), (67 Ma

to recent). This sequence consists of bentonitic clays, volcaniclastics, mélange, marls, chalks,

cherts, limestones, calcarenites, evaporites and clastic sediments (G.S.D website, 2009).

The Allochthonous Mamonia Complex and related sediments

The Mamonia Zone or Complex occurs only on the south-western part of Cyprus and is consisted

of a diverse and structurally complex assemblage of volcanic, sedimentary and metamorphic

rocks, ranging in age from Upper Triassic to Upper Cretaceous (c. 235-100 Ma). These rocks,

were placed over and adjacent the Troodos ophiolite during the Maastrichtian, (after the

deposition of the Late Cretaceous Kannaviou Fm and before the deposition of the Late

Cretaceous-Early Tertiary Lefkara Fm), (Lapierre, 1975).

SWELLING CLAYS

Swelling clays occurred as a result of alteration of Troodos ophiolite and as pelagic sedimentary

deposits that took place post Cretaceous period, (Cavit Atalar et al, 2006). The lithologies that

crop out in the area are: (1) The grey coloured Kannaviou formation which is considered to have

been derived from a late Cretaceous volcanic pile, which was part of a volcanic arc generated by

subduction of oceanic crust in that period, (Robertson, 1977), comprising mostly of non

calcareous bentonitic clays and radiolarian mudstone. (2) Kathikas melange for the melange

which emerges in southwest Cyprus and is consisted of blocks of Mamonia Complex in a silty clay

matrix (Robertson, 1977). (3) Superficial melange consisting of deformed – weathered mamonia

complex rocks (Northmore et al., 1986).

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GEOTECHNICAL INVESTIGATIONS

A detailed geotechnical investigation was carried out in the south-western part of Cyprus by

Northmore et al. (1986). Undisturbed and disturbed samples were taken and tested by the

Geological Survey Department of Cyprus and British Geological Society laboratories.

Laboratory tests

Several tests were conducted in the obtained samples in order to determine and assess the

swelling clays geotechnical behaviour.

Particle size analysis: The tested Kannaviou samples showed a clay content ranging from 20-

60% depending on the lithology. The tested Kathikas and Superficial mélange samples showed

variable behaviour, depending on the depth of the tested samples ranging from 24 to 58% for the

deepest and the shallowest samples respectively.

Moisture content: The Kannaviou clays contain a moisture content ranging from 20 to 40%, at

16m depth and near the surface respectively. The Kathikas Mélange samples moisture content

ranges from 16 to 39%.

Specific gravity: The specific gravity of Kannaviou clays and Superficial Mélange was

determined to be 2.59 and 2.7 respectively.

Attemberg Limits: According to the derived liquid limit values, Kannaviou clays can be classified

as high to extremely high plasticity, Kathikas mélange samples can be classified as intermediate

to very high plasticity and Superficial mélange samples can be classified as low to intermediate

plasticity.

Mineralogy: A total of 149 samples from all the formations were analyzed by qualitative X-ray

diffraction. All the tested samples were found to contain calcium Montmorillonite (Smectite), with a

content ranging from 7.5 to 64.3%, for Kannaviou Formation, 6 to 38% for Kathikas mélange and

1 to 20% for Superficial Mélange.

Permeability: Values of permeability for Kannaviou and Kathikas Mélange samples were derived

from consolidation and triaxial tests, ranging from 6x10-14 to 6x10-11 m/sec and 1x10-10 to 4x10-9

m/sec, and 8x10-11 to 3x10-10 m/sec and 1x10-10 to 4x10-9 m/sec, respectively.

Consolidation: The results indicate an average value of overconsolidation ratio (OCR) of 15 for

the Kannaviou clays and a coefficient of volume Compressibility (Mv) ranging from 0.001 to 0.05

m2/MN, classified as very low. The coefficient of Consolidation (Cv) ranges from 0 to 4 m2/year.

Shear strength: The values obtained from the triaxial and ring shear tests are shown in tables 1, 2.

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Total - effective strength parameters

Formation cu (kPa) φ (degrees) c’ (kPa) φ’ (degrees)

Kannaviou clays, sandstones

10-40 13.5-25.6 13-50 18.3-29.7

Kathikas Melange 15-35 19.8-11.3 7.5-35 20.5-13

Table 1 Total - effective strength parameters of the swelling clays, effective residual strength parameters of the swelling clays, (values taken from Northmore et al. 1986).

Residual strength parameters

Formation cr’ (kPa) φr (degrees)

Kannaviou clays, sandstones

6-13.2 5.7-29

Kathikas Melange 3.3-19.3 10.8-22.3

Stratified Mamonia 6 15

Superficial Melange 15 8.2

Table 2 Effective residual strength parameters of the swelling clays, (values taken from Northmore et al. 1986).

Geomorphological and engineering geological description of selected

landslides

The study areas were centered near the villages; Mamoundali-Ano Panayia (Grid reference:

34o55’12 N, 32o37’05 E), Old Choletria-Nata (Grid Reference: 34o46’27 N, 32o35’02 E), Ayios

Dimitrianos-Phyti (Grid Reference: 34o54’56 N, 32o33’12 E), were previous workers have reported

landslide issues.

During the field investigation active and non active processes were spotted and the

following mechanisms are considered to pose a threat under a combination of catastrophic

events:

Deep/shallow seated rotational slide involving chalk/talus debris, Kathikas Mélange and Kannaviou formation.

Block sliding in chalk masses underlain by superficial mélange Toppling-rockfall-wedge failure due to the conjugated fractures in chalk cap Translational debris slide in Kathikas mélange generally of small magnitude. Translational debris flow and creeping phenomena within Kannaviou Formation.

Despite the evidence of movement (tilted pylons-abandoned dwellings), the field survey

observations don’t indicate that the damages were caused by earthquake induced landslides and

the scenario of topographic amplification and poorly constructed masonry is proposed.

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Failure mechanism – geotechnical behaviour

The most common failures within the mélange are shallow translational slides. The weakness

plane is the depth of the seasonal moisture change, (Northmore et al, 1986). Deep seated

rotational failures involving mélange and the chalk caprock and/or Kannaviou Formation are also

frequent and occur in the steepest slopes. In general terms Kathikas Mélange exhibits the best

behaviour among the swelling clays. The Superficial Mélange exhibits worse behaviour since it

consists of weathered and tectonic disturbed Mamonia rocks.

The most common failures within the Kannaviou formation are shallow translational slides

associated with creeping phenomena. Soil creeping is frequent in the study areas.

Morphologically it appears with the form of terracettes, of narrow width and short height

(approximately 1 m) and it can be evolved into rotational failures, or translational slides. The

affected slopes have an average slope angle greater than 20o.

Landslide Hazard Mapping

A Geographic Information System (GIS), analysis was carried out using the infinite slope model

based on algebraic manipulations of various raster data layers, in order to produce a Factor of

Safety map. The area used to verify the map’s credibility was Phyti-Statos, were engineering

maps prepared by Northmore et al, (1986), exist.

Method of Analysis: The stability of the slopes was assessed with the use of the infinite slope

model. This model is particularly appropriate for slope failures with planar slip surfaces, such as

translational slides, which are common in this area. Taking into consideration that in the study

area the present activity is due to reactivation of previous movements, (Northmore et al, 1986),

residual values are used in the infinite slope analysis.

The Factor of Safety is given by the equation (1):

Where FS is factor of safety, Cr’ is the residual effective cohesion, φr’ is the residual effective

friction angle, α is the slope angle, γ is the material unit weight, γw is the unit weight of water, t is

the slope-normal thickness of the slope element, and m is the proportion of the element thickness

that is saturated.

This report attempts to spot landslide failures of great magnitude with large

displacements, where the role of the residual effective cohesion is limited and the movement is

controlled by the residual friction angle. Therefore the effective residual cohesion cr’ is assumed

to be zero.

Then the Factor of Safety is given by the equation (2):

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For purposes of simplicity the mean unit weights of the Kathikas mélange and the

Kannaviou formation are used, the unit weight of the Superficial-Stratified Mélange is assumed to

be the same with Kathikas Mélange, due to the lack of data, which implies a minor error factor:

γmélange=20.4 kN/m3, γSuperficial mélange=20.4 kN/m3, γKannaviou=17.8 kN/m3.

Slope Map: A triangulated-irregular-network (TIN) model was constructed, using digitized

contours of 20 m spacing, provided by the Geological Survey Department of Cyprus. The origin

and the credibility of the provided data are unknown. This TIN model was used to produce a slope

map with 5 meters grid centers, given the scale of the used digital topographic map (1:25000).

Geologic Map: The used geologic map of the area was prepared by the Geological Survey

Department of Cyprus at a scale of 1:25.000. The origin and the accuracy of the provided data are

unknown.

Strength of Slope Materials: The estimated representative values of the swelling clays were

chosen after assessment of the test results given by Northmore et al, (1986) and in correlation to

the field observations. The chosen residual strength parameters were adapted based on the

following criteria; Borehole samples were preferred from the surface samples, the preferred tests

were from areas within the study area where the field survey spotted shallow translational failures,

the results had to be verified with the proposed values in the existing literature.

In the provided digitized geology there is no distinction between Kannaviou lithology,

therefore the natural mixture of Kannaviou siltstones and Mélange is chosen to represent

Kannaviou Formation considering the great extent of outcropping in the study area.

Ring Shear tests

Geologic unit

Φr’ (degrees)Best linear envelope

σn = 76 (kPa)

σn = 150 (kPa)

σn=248 (kPa)

σn=497 (kPa)

cr' (Kpa)

φr' (degrees)

Kannaviou siltstones-melange 21.4 18 16 13.6 15.9 12

Kathikas Melange 24.4 23.2 22.9 22.3 3.9 22.1

Stratified Mamonia/Superficial

Melange 21.7 19.2 16.3 15.3 12.4 14

Table 3 Proposed effective residual strength parameters for the outcropping swelling clays

For the Kathikas Mélange the field observation showed that its clay content is low and is

expected to behave frictionally. For the Superficial Mélange, the field investigation showed that

failures occur within highly weathered rocks with higher clay content. The chosen values are in

agreement with the proposed by Skempton (1984) relationship between residual friction angle and

clay content.

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As previously said effective cohesion cr’ is assumed to be zero in the slope stability

analysis, so the used φr’ values are those obtained from each stage of the ring shear test. Since

the field observation showed that the majority of failures occur in shallow depth the Factor of

safety map is calculated with a φr value at a level of 75 kPa (i.e. 3.5m).

Sensitivity Analysis: Prior to the creation of the Factor of Safety map, a slope stability analysis

was carried out, using the selected effective residual strength parameters. The analysis showed

that for a mid-level water table and for shallow slip-surfaces (~3.5m), the swelling clays will fail if

the slope angle exceeds: 15o for Kannaviou Fm, 17o for Superficial Mélange and 20o for Kathikas

Mélange. For greater depths (~12m), this limiting stability angle is even lower, (3o approximately).

Assuming the saturated condition, where m=1 and a shallow slip-surface, the swelling

clays will fail if the slope angle exceeds: 10o for Kannaviou Fm, 12o for Superficial Mélange, 14o for

Kathikas Mélange. For greater depths (~12m) this limiting stability angle is even lower, (2o

approximately).

Plotting the values of FoS against the depth of the shear surface and for a slope angle of

12o (typical slope angle for translational failures in the study area according to Northmore, 1986),

it can be seen that Kathikas mélange is not affected due to the fact that it more frictional than the

other formations.

FoS Sensitivity Analysis for m = 0.5

0.00

0.50

1.00

1.50

2.00

2.50

0 5 10 15 20 25 30

Depth (m)

FoS

val

ues Kathikas melange

Superficial melange

Kannaviou Fm

FoS Sensitivity Analysis for m = 1

0.00

0.50

1.00

1.50

2.00

2.50

0 5 10 15 20 25 30

Depth (m)

FoS

val

ues Kathikas melange

Superficial melange

Kannaviou Fm

Fig. 2, 3 Factor of Safety values against depth of slip-surface assuming m = 0.5 and m = 1.

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Factor-of-Safety Map: The Factor of Safety was calculated for every 5 meters grid cell (i.e.

25m2), by solving, with the use of ArcGIS software, the infinite slope analysis equation 2; the

effective residual friction angle (φr’), and the slope angle (α), were derived from the slope map

and the friction angle map respectively. The credibility of the produced Factor of Safety map was

verified with the existing engineering geology maps and it was found to be in good correlation.

Fig. 4 Factor of Safety maps when the water table is at the surface level. Northmore’s engineering geology maps (1986) are also plotted as a reference point.

Conclusions

The field survey showed that the western part of Cyprus is affected by the presence of the

swelling clays. Failures of great magnitude (i.e. rockfalls, rotational slides, deep translational

slides) occur and pose a threat to the human life and the present infrastructure but the

accumulating damages are caused by the swelling clays due to their swelling and shrinkage.

Creeping phenomena occur within the weakest lithology, (Kannaviou formation), which

quite often evolve into deep rotational slides. The occurrence of creeping is more frequent than

any other failure mechanism in the lowlands and creates accumulating damage to the present

infrastructure.

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The prepared Factor of Safety map is a basic approach to the stability issues of the area

since it takes into consideration the soil parameters, the slope angle and the depth of the water

table. A more detailed engineering mapping with the use of more than one slope stability models

can be more accurate.

References

Atalar, C., and Kilic, R., 2006. Geotechnical properties of Cyprus clays. IAEG2006 419, 1-7.

Geological Survey Department (GSD) 2009. Geological Survey Department. [online]. [Accessed 03/06/09]. Republic of Cyprus, Ministry of Agriculture and Natural Resources and Environment. Available from World Wide Web: http://www.moa.gov.cy/moa/gsd/

Lapierre, H., 1975. Les formations sedimentaires et eruptives des Nappes Mamonia et leurs relations avec le Massif du Troodos (Chypre Occidentale), France. Mem. geol. Soc, 123, 1-132.

Lapierre, H., Bosch, D., Narros, A., G. H. Mascle, G. H., Tardy, M., and Demant, A., 2007. The Mamonia Complex (SW Cyprus) revisited: remnant of Late Triassic intra-oceanic volcanism along the Tethyan southwestern passive margin, Geological Magazine, 144, 1–19.

Northmore, K. J., Charalampous M., Hobbs P. R. & Petrides G., 1986. Engineering geology of the Kannaviou, Melange and Mamonia Complex Formations. Phiti/Statos area, SW Cyprus. BGS Report No EGARP-KW/86/4, G.S.D Report No G/EG/15/86.

Papazachos, B. C. and Papaioannou, Ch. A., 1999. Lithospheric boundaries and plate motions in the Cyprus area. Tectonophysics, 308, 193-204.

Robertson, A. H. F. 1977. The Kannaviou Formation, Cyprus: volcaniclastic sedimentation of a probable Late Cretaceous volcanic arc, London. Journal of the Geological Society, 133, 447–466.

Robinson, Ρ.Τ and Malpas, J., 1990. The Troodos ophiolite of Cyprus: new perspectives on its origin and emplacement, in Ophiolites: Oceanic Crustal Analogues (eds J. Malpas, Ε.Μ. Moores, Α. Panayiotou and C. Xenophontos). Cyprus Geological Survey Department, 13-36.

Skempton, A. W., 1985. Residual strength of clays in landslides, folded strata and the laboratory, Geotechnique 35.

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