Earthen berms and energy-absorbing rock anchored-barriers have been recommended by a firm of consultants to prevent damage from further landslides on the slopes of Dooncarton and Barnachuille Mountain in Co Mayo.
According to a report which has been presented to the Government on the devastation caused by the September landslides, there has been continued movement on the mountainside over the past month.
P.J. Tobin Consultants, which compiled the report, warns the destabilisation of remaining deposits is such that even less extreme rainfall carries the risk of moving larger masses of material. Technical options which attempt to stabilise this material are not recommended because of the nature of the overburden and the ability of peat to creep around restraints.
The report identifies where berms (ledges at the bottom of a bank or cutting to catch earth that may roll down the slope or to strengthen banks and barriers) are required to protect infrastructure and adjacent properties at risk from further landslides. Three classes of necessary protection works have been identified in the report, which will be distributed to residents as a discussion document.
These are infrastructural works to graveyards and public roads, including replacement and upgrading of culverts, as well as wing walls, retaining walls and protective barriers at hazards. This cost is estimated at more than €2.8 million.
Earthworks and drainage are estimated at more than €685,000 and the construction of energy-absorbing barriers could cost more than €1.5 million. "These works are necessary," the consultants say, "in order to reduce risks to public infrastructure and property to levels which will permit normal life in the community to confidently resume."
They say the primary cause of more than 40 landslides in Pollathomas was rainfall of such intensity as to overwhelm natural drainage systems in the peat and weathered rock, thereby mobilising sections of overburden through buoyancy and gravitational forces.
Analysis suggests that at least 80 mm of rainfall fell on the mountain in less than two hours.