North Face inspection continues, but risk appears low
The Technical Services Department is in the process of carrying out a very detailed inspection of the North face of the Rock following the rock fall last March 2, but early indications are that the risk of a repeat incident on that scale is low.
Immediately after the rock fall the Technical Services Department carried out a visual inspection of the North Face together with a team of Specialist Geotechnical Engineers from the UK.
The inspection aimed to determine the possible causes of the rock fall and identify immediate and longer-term solutions to mitigate the risk of further rock falls in the area.
“The initial assessment has concluded that the rock fall has occurred from a similar area to the rock fall which occurred in 2014 and it appears that the residual risk of a further major rock fall from that location is small,” a Government spokesman told the Chronicle.
The “department is working closely with its geotechnical consultants and is in the process of carrying out a very detailed inspection of the North face in order to determine the likelihood and size of further major rock falls in the area.”
It is hoped that the detailed survey will provide information on the type and extent of mitigation measures that can be put in place to reduce the impact of further rock falls.
As the inspection is being carried out, timber hoarding will remain in place to protect to reduce any impact of fly rock should a further rock fall happen.
Looking towards long term solution, these potentially “could include the provision of catch ditches and fences but it is not possible to know the extent and cost of this until such time as the studies have been completed,” said the Government.
At present, only authorised persons can access the area and they are all aware that given the proximity of the cliff face there is always a risk of rock falls in the area.
Rock Falls in Gibraltar
The Technical Services Department is responsible for the monitoring and development of cliff stabilisation schemes in line with Government’s on-going cliff stabilisation and rock fall protection programme.
The technical services department explains “the geomorphology of the Rock is such that it is susceptible to rock fall hazards not just at the Eastside but over the whole of Gibraltar.”
It states that generally, rock falls in Gibraltar have been minor with localised damage.
However there have been a number of events, which caused significant impact.
“Other than the last major rock fall at the Aerial Farm there was a major landslide in Catalan Bay in 1942, a landslide in Camp bay in 1996, the rock fall which regrettably led to a fatality in Dudley Ward Tunnel in 2002 and a major failure of the scree cliffs by Catalan Bay Quarry in 2010,” the Government said.
A new high capacity rock catch fence was constructed just to the south of Both Wolds recently and was a continuation of a scheme which started last year to provide added protection in that area.
“Unfortunately, the surface area of the rock and the number of differing trigger mechanisms makes predicting when rock falls will occur an almost impossible task,” the Government said.
“Notwithstanding this, there is a considerable amount of knowledge of areas where rockfalls generally occur and the focus is on mitigating the risk of rock falls in areas where the impact of these can be greatest,” it added.
There are two approaches for dealing with the hazard namely Active and Passive.
“With the Active approach a potential rock fall is stopped from occurring in the first place.
This is normally achieved by either the installation of rock bolts or by the installation of mesh to hold back the rock face,” said the Government.
“Unfortunately these solutions are not always possible due to the height of the cliffs or environmental and technical considerations.”
“In these circumstances a Passive approach is employed whereby the rockfall is allowed to occur naturally but the effects of these are mitigated against to reduce the impact.”
“Examples of this approach include the construction of rock catch fences, catch ditches and bunds amongst others. There are numerous of examples of both these approaches being used around Gibraltar,” it added.
Why do rock falls occur?
“The mechanisms that trigger rock falls are often complex and include both natural and man-made origins. The most common causes include heavy rainfall and strong winds, differential erosion between differing rock strata, tree roots, burrowing animals and natural cavern development,” the Government said.
“Quarrying for rock for use in construction over the years has also led to problems in some areas in Gibraltar where the old quarry sites are now abandoned,” it added.