Features / Publishing

The destructive power of a slow-moving landslide

Recent research published in the Quarterly Journal of Engineering Geology and Hydrogeology provides insight into the destruction caused by ground movements at Pissouri Cyprus

The term ‘landslide’ conjures images of dramatic, fast moving events; earth and rock sliding rapidly or flowing down a steep slope, destroying everything in its path. But landslides can also move incredibly slowly – some only centimetres a year. Nevertheless, these ‘slow-moving landslides’ are capable of causing considerable damage – as the residents of the village of Pissouri in Cyprus have discovered first hand.

‘The term ‘landslide’ can have an emotive meaning to many people, given that press coverage of such events is usually associated with fast-moving and destructive displacements of the ground, usually accompanied by loss of life’ says Dr Gareth Hearn, an independent consultant who has been studying the area. ‘Nevertheless, the accepted geological definition of a landslide is ‘the movement of a mass of rock, earth or debris down a slope’, irrespective of its rate of movement. This is what is occurring at Limnes in Pissouri and the fact that the recorded rates of movement are ‘very slow’ is neither here nor there: they are landslide movements nonetheless.’

The old town of Pissouri occupying the higher ground in the distance with Limnes in the foreground and middle ground.

Pissouri is a village in the Limassol District of Cyprus. With a population of just over a thousand, about half of the residents are Cypriots, the rest from overseas – mainly the UK.

The recent ground movement at Limnes in Pissouri is not the first the area has experienced.

‘There is geomorphological evidence at Pissouri for a large, deep-seated landslide having occurred in the pre-historic past and it is conjectured that this might have happened during the late Pleistocene-early Holocene, i.e. following the last glacial period’ says Dr Hearn.

Significant ground behaviour problems began in the winter of 2011/12, which saw heavy rainfall and the start of noticeable damage to buildings and urban infrastructure.

‘Rainfall increases soil moisture and raises groundwater levels’ says Dr Hearn, ‘and this reduces effective stress and hence shear strength. But the drainage condition of the Limnes slope at Pissouri may not be solely due to rainfall. Contributions from domestic sources and other man-made water discharges, including possibly urban runoff from the old part of Pissouri located on the slopes above, might be exacerbating the situation.’

Villagers began noticing cracking and displacement to the ground, pavements, roads, walls and buildings. Eventually, houses became uninhabitable, some literally split apart by the ground movement.

Damage to housing at Pissouri

A recent report by the BBC’s Inside Out South documented the effect the destruction has had on British residents at Pissouri, many of whom have lost their homes. They have also been unable so far to claim compensation, with insurance not covering the damage, and the Cypriot Government yet to agree on its cause.

‘Proving a landslide: Ground behaviour problems at Pissouri, Cyprus’ was published earlier this year in the Geological Society’s Quarterly Journal of Engineering Geology and Hydrogeology, concluding that the ground movement at Pissouri cannot have been caused by anything other than a landslide.

‘Of course, the geotechnical properties of the soils exposed in Pissouri have caused foundation problems for structures elsewhere in the country, and these are unrelated to landslide movements’ says Dr Hearn, ‘but these problems cannot explain the fact that the entire Limnes area is moving downslope at an apparently accelerating rate. The angle of the slope at Limnes is very gentle and it is this fact that has probably led other observers to seek alternative explanations.’

A summary of recorded damage

It is perhaps because we associate landslides with dramatic, rapid ground movement that some have questioned the explanation. ‘First’ Dr Hearn explains, ‘a landslide does not have to be actively moving to qualify as a landslide. There are many landslides that were triggered during pre-historic times that remain dormant during the present day. The same situation applies to other geological phenomena, such as faults and volcanoes, for example.’

Dr Hearn was called in by the Pissouri Housing Initiative Group (PHIG) to examine the area in early 2017, having previously been involved in a landslide mapping project in neighbouring Paphos District, as well as environmental studies for the Paphos-Polis highway.

‘The assignment involved a review of available ground investigation information, an examination of available remote sensing data, including stereo aerial photography and drone imagery, a field reconnaissance and discussion with geological and geotechnical specialists familiar with the ground conditions in Cyprus. After an initial stage of investigation, we commissioned the analysis of InSAR satellite observational data and instigated a surface movement monitoring programme. A follow-up site visit was made in early 2018 to examine the progress of ground movement and structural damage.’

Cracking to buildings, roads, pavements and swimming pools

The research showed that there are active movements taking place at Pissouri at relatively shallow depths compared to the pre-historic landslide – up to approximately 15 metres below ground level. The team carried out surface monitoring in the winter of 2017-18 and found that there was an average downslope movement of approximate 60mm per month during that period. The InSAR satellite monitoring data collected between 2014 and 2017 demonstrated that much of the slope showed movements of 60 mm per year, with parts of the slope moving as much as 180 mm per year.

Pissouri is not the only place in Cyprus to be affected by ground movements – there have been several cases of major structural damage resulting from landslides, some resulting in entire villages being evacuated. Says Dr Hearn: ‘These case studies demonstrate how sensitive these low-strength soils are to fluctuations in groundwater and possible changes to drainage regimes brought about by changing land use. Although not the case at Pissouri, toe erosion by rivers and streams, and the effects of excavations along roads and highways, for example, can also be important factors in triggering or accelerating ground movements.’

The Cyprus government’s own investigations into what has happened at Pissouri have been ongoing for six years – if the conclusion is that a landslide is not responsible for the damage, there will be no compensation for those who have lost their homes.

Journal reference: ‘Proving a landslide: ground behaviour problems at Pissouri, Cyprus
Gareth James Hearn, Hayley Larkin, Kleopas Hadjicharalambous, Artemios Papageorgiou and Georgia Elina Zoi, Quarterly Journal of Engineering Geology and Hydrogeology, 22 May 2018, https://doi.org/10.1144/qjegh2017-134

Further reading: 

 

 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s