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Geotechnical hazards

 

The main geotechnical problems that occur in Cyprus are (a) slope failures and landslides (Fig. 4), (b) rock falls (Fig. 5) and (c) foundation settlements (Fig. 6).

 

Slope failures and landslides:  This term includes phenomena of partial or total instability or failure of natural or manmade slopes.  Landslides may occur due to (a) the mineralogical composition of the ground (presence of clay minerals), (b) the presence of faults, especially when their dip is parallel to that of the slope, (c) the dip of the sedimentary layers (parallel to the dip of the slope), (d) frequent and heavy rainfall resulting in erosion and mechanical weakness of the soil or abrupt increase of the water table, (e) intensive groundwater pumping resulting in lowering the water table, internal erosion and reduction of the soil cohesion, and (f) earthquakes and manmade intervention.

 

In Cyprus, relatively large and active slope failures and landslides occur in the western part of the island (Paphos District), where rocks of the Mamonia Complex and the Kannaviou Formation are exposed.  Characteristic examples of landslides are observed in the villages of Mamonia, Kannaviou, Statos, Pentalia, Kritou Marottou, and Anadiou.  In the southern part of the island (Limassol District), landslides relate chiefly to the Lower Marls of the Lefkara Formation as well as the Moni Formation (bentonite).  Examples can be seen in the areas of the villages of Kilani, Silikou, Doros, Korfi, Trimiklini as well as Moni and Pentakomo.  Also, in northwest Cyprus, landslides can be observed along the roads from Morphou to Skylloura and from Myrtou to Panagra.

 

The long mining history of Cyprus has resulted in extensive waste dumps in many areas such as Kalavasos in the south, Limni in the northwest, Mitsero and Mathiatis in central Cyprus and Troulli in the east.  In all these examples the waste dumps have been deposited without a proper design.  They are characterised by high porosity and high compressibility, low density and limited strength.  Thus, under certain conditions the slopes of these dumps may develop landslide phenomena.

 

A number of measures can be taken either separately or in conjunction with one another to face the problem of slope failures and landslides.  Such measures include the decrease of the dip of the slope, the unloading of the land-slit mass, the construction of berms or terraces, the controlled pumping of ground water to maintain a stable water table, the construction of drainage system and retaining walls etc.

 

Rock Falls:  Rock falls are observed mainly in the mountainous areas of Cyprus and in natural and manmade slopes.  They are mainly related to one or more causes according to the geological, topographical and rainfall conditions.  Faults and fractures play an important role, especially their density and network.  Earthquakes often contribute to the fracturing of the rock mass and eventually to rock falls.  Rock falls directly affect the safety of the people.  Examples of rock falls have been observed in Lemithou, Akrounta, Pelentri, Prodromos and along the road from Nicosia to Kyrenia.

 

Depending on the characteristics of each case of rock fall, different measures can be taken to resolve the problem.  The most frequently used method for facing the problem is the installation of anchors and rockbolts.

 

Foundation settlements:  Settlement is the subsidence of a civil structure that is caused by its weight and the relative compressibility and deformation of the underlying soil.  Thus, foundation settlements are highly related to the geological conditions and the mechanical properties of the ground.  Marls, clays and gypsum are the main soil types in Cyprus that are related to foundation settlements.

 

In Cyprus, marls are found in different types according to their geological age and lithological characteristics.  Marls are found either as thin interlayers in the Lefkara and Pakhna Formations or as thick layers in the Athalassa and Nicosia Formations.  The most extensive and significant type of marl, from the geotechnical point of view, is that of the Nicosia Formation.  The mechanical behaviour of marl depends significantly on its moisture and clay content.  Thus, the mechanical behaviour of marl is very sensitive to the effects of water, which gives rise to swelling and shrinkage phenomena leading to cracks in buildings.

 

Some clayey geological formations that contain the mineral montmorillonite, and thus belonging to the bentonitic soils, are characterised by high plasticity and swelling as well as shrinkage under certain water-content conditions.  In summer, for example, they shrink due to dryness and cause volume reduction leading again to the development of cracks in civil structures.  In winter, on the other hand, they expand (swelling effect) due to the absorption of water and become more plastic which reduces their strength and increases their compressibility.

 

The most characteristic swelling clays in Cyprus belong to the Kannaviou and Moni Formations (bentonitic clays), although the Lower Marls of the Lefkara Formation also exhibit significant swelling properties.  The problems that are related to swelling clays are mainly seasonal movements of buildings (subsidence and lifting) and other linear civil structures such as roads and drainage systems.  Swelling clay effects are observed in areas around the villages of Pentakomo, Moni, Kannaviou, Kritou Marottou, Pentalia, Marathounta, Nata, Kythrea, Myrtou, Gastria and Kornokipos.  Also, landslides are observed in areas that are composed of bentonitic clays.

 

The solution to the problems relating to foundation settlements due to the presence of marls or bentonitic clays is difficult, expensive and often of limited effectiveness.  The main remedy includes measures to ensure stable moisture content on the foundation level.  Raft foundation is also designed as well as pilling.

 

Foundation settlements or subsidence of civil structures can also be caused by the development of void spaces or caves in the ground called sinkholes.  In Cyprus, sinkholes are formed in lithological units that contain either gypsum or limestone and they are the products of the dissolution of these two lithologies by water.  Sinkholes in gypsum occur all over the island, in particular in the areas of Pissouri, Maroni, Aradippou, Kathikas, Kalavasos, Nisou, Pergamos and between Lefka and Galinoporni.  The sinkholes are rather small in size, may have an irregular shape and constitute foundation “traps”.  When their roof collapses due to the weight of the civil structure on top, sinkholes may cause subsidence problems.  Remedial measures are often very difficult and include filling up the sinkhole with grout.

 

Limestone is another rock type that sinkholes or caves can be developed in due to its dissolution by water.  In Cyprus, the dissolution of limestones appears to be rather limited, forming small, irregular surface caves or complex underground voids and drainage systems leading to the development of karstic springs (karst is a term derived from the Carso area in the Dalmatian Alps in Yugoslavia relating to rock dissolution).  Such karstic springs are found in the northern part of the island along the Pentadaktylos Range.  Underground dissolution voids are difficult to identify from the surface and constitute a serious potential problem in the stability of civil structures.

 

 

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