1. Introduction The cliffs in central Portugal’s coastal area are exposed to severe erosion processes and need particular attention regarding stability conditions, not only because of their geological and geotechnical characteristics but also concerning the potential constraints for social and economic activities in the case of a stability related. Preconditioning and triggering factors such as wave exposure, rainfall, water percolation, among others, can progressively lead to states of rupture or pre-rupture, namely, rockfalls, the decay of protection structures or others, and to hazardous situations. The particular case of the 15m high cliff in Portinho da Areia do Sul, Peniche (Portugal) falls within the above- mentioned environment. The occurrence of a minor landslide with rockfall, after a period of heavy rainfall, pointed out the need for an overall stability assessment since slope mass movements, even if small in proportions, can cause injury to people, threaten infrastructures, cause activity restraints and elevated costs in repair or rebuilding works. Although the referred event didn’t cause any damage or casualties, the existing risk could not be ignored. Therefore, mitigation measures should be undertaken in order to reduce and/or restrain hazards and, consequently, reduce or eliminate risks. Then, in order to support the selection of the mitigation measures to be implemented, a phased problem approach was considered which included a risk analysis performance. Cláudia Santos 1, Alexandre Santos-Ferreira 2, Elisabete Dias 3 1 post-graduate student at Faculdade de Ciências e Tecnologia, Univ. NOVA de Lisboa / Trainee at DGRM (former IPTM, IP.), csns.mail@gmail.com; 2 DGRM (former IPTM, IP.) / CICEGe - Faculdade de Ciências e Tecnologia, Univ. NOVA de Lisboa, asf1954@netcabo.pt; 3 DGRM (former IPTM, IP.), elisabete71@gmail.com. A coastal cliff stability study in Peniche (Portugal) XII International IAEG Congress Torino, September 15 th – 19 th, 2014 Study area The coastal cliff of Portinho da Areia do Sul is located in the south coast of Peniche’s peninsula (Portugal). The cliff studied corresponds to a near vertical slope that is 15m high at its highest section and extends for about 70m. The current cliff configuration result, and many of the observed fractures, result from rock blasting, due to the usage of this location as building material source, during the construction of Peniche’s fishing harbour. Geological and geotechnical characteristics: The intersection of variable attitude discontinuities (including the sub-horizontal bedding plans), along with erosion processes that gradually enlarge the discontinuities plans, cause block formation, some of these without support. Risk management and risk analysis After a small landslide, through visual surveys to the site location, the stability problems of the cliff were recognized. Although sea action (which can be an effective agent of slope mass movements) has little influence on the cliff, the rocky mass steepness and geotechnical quality along with several triggering factors increase the potential inherent risk at site. Also, the proximity of the building and the access road ascertained the need to implement a mitigation plan that met the cliff security requirements, given the area context. In order to support the referred plan development, a risk analysis was carried out within the risk management scope. Risk reduction measures and expected results To reduce the associated risk of any given hazard, the measures to be implemented can be aimed at the probability of the hazard to occur (p), the severity of the consequence (S) or both. Given the resulting evaluation of “Serious” (level IV), for an immediate risk reduction, was strongly advised: for the rockfall hazard, the interdiction to the public of the areas nearby to the foot of the cliff; for the people’s falling hazard, the placement of a guard fence in order to prevent the access to the edge of the cliff. In order to achieve a more permanent solution, more complex, time consuming and costly technical approaches were required. Also the considered options were very dependent upon the existing built structures at the base and top of the cliff (the restaurant and the access road), so the risk mitigation strategy should include a stability solution. Taking into account the rock mass stability analysis results, two rockfall mitigation options were proposed: the removal from the slope of detached and/or unstable blocks; and the use of rockbolts on blocks which could be subject to that intervention. Additionally, in order to stabilize very fractured or weathered areas and to prevent the erosion effects in the detrital layers, a drainage system installation and the application of fiber reinforced shotcrete facing, were recommended. Red clayish deposits (terra rossa soil) toping alternating weathered limestone and detrital layers (Lower Jurassic) PROBLEM APPROACH thorough visual surveys to identify potentially unstable areas to understand which factors led to the stated situation to monitor the stability conditions throughout time recognition of the general cliff conditions RISK ANALYSIS mitigation measures karst dissolution features multiple sub-vertical faults and discontinuities Overhanging sections differential erosion block formation, some of these without support landslide Along with the geological and geotechnical characteristics, the existing infra-structures are important to frame the local context. Located at the foot of the cliff, there is a building (about 2m from the slope) functioning as a restaurant and, at the top of the cliff, there is an access road to a residential area. The distance between the above- mentioned road and the cliff edge is variable and in some points it is reduced to about 1m. There is no sidewalk and there are no access barriers to the edge of the cliff, although there are vertical signs alerting to rockfall hazards. Those signs can also be seen at the foot of the cliff. Acknowledgments. The authors thank IPTM, IP. for the kind permission to use the data in this paper. PROBLEM APPROACH assess the impact of each identified risk RISK MANAGEMENT ADOPTED PROCEDURE determine the risk (R= p x S) identify potential hazards identify risk reduction measures Plan risk control To perform the risk analysis, five levels (levels 1 to 5) were established for the probability of the hazard to occur (p), the severity of the consequence (S) and the level of risk (R). The choice of five levels was suitable for this analysis since the classification wasn’t too narrow or too detailed, ensuring the reliability of the expected results. The cliff risk (R) assessment for identified hazards was obtained by the multiplication of the level of probability of its occurrence by the level of the potential consequence quantified in terms of severity, i.e. (R = p x S), considering that higher levels contribute to risk assessment in a higher grade than lower levels. So, the risk levels were set as presented. Final remarks The fairly easy visual access of the near vertical limestone coastal cliff in study enabled technical inspections and an extensive photographic survey to assess the rock mass stability conditions of the cliff. Based on the survey results, a risk analysis was performed and proved to be a practical and useful tool to support the proposed mitigation measures and to foresee their suitability in terms of risk reduction. The proposed mitigation measures included the restriction of public access to the hazardous area, the unstable blocks removal, the rockbolts installation and the reinforced shotcrete facing application. Due to the height of the cliff, the unstable blocks’ size and the constructions’ proximity to the foot of the cliff, the stabilization works were considered challenging and dangerous, requiring crane-mounted equipment and proficient teams. To keep the implementation works at low risk, a quality control program comprising a careful supervision by qualified technicians was advised. Previously implemented stabilization works in sites with similar conditions, comprising the described solutions, showed that this procedure would be able to achieve successful results, both technically and in the restrained visual impact. A second risk analysis results showed that by implementing the proposed measures, probability of the hazard to occur (p) would attain a “Unlikely” rate (level 2) and a “Highly unlikely” rate (level 1), for the rockfall and the people’s falling hazards, respectively, reducing favorably the risk rate as intended and as shown. Note that a total security status is very difficult to achieve due to technical and budget restraints; in fact, in every geotechnical intervention there is a certain residual risk.