GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 2.2 353 provide the general framework and guidelines for establishing a rapid assessment of the seismic risk for these type of dams. A preliminary criterion to identify the most critical structures that need a priority in planning further investigations and analyses has been defined. It is based on a simplified procedure developed to classify the large number of small earth dams. The proposed methodology is based on the compilation of data sheets that lead to a preliminary classification of these dams in term of their seismic vulnerability. More accurate and advanced approaches of seismic vulnerability assessment based on probabilistic method will be then developed. In this respect, a geophysical and geotechnical characterisation campaign has been performed on four of these dams to increase the level of knowledge. The simple approach of seismic risk assessment of small earth dam, and a description of the first steps toward the development of advanced tools are presented in the present note. Simplified approach of seismic vulnerability assessment. A detailed analysis of the seismic vulnerability can be performed only for a limited number of well-characterized dams. When a large number of structures are under consideration and the technical information are lacking, a simplified approach should be adopted to identify the critical structures. The proposed approach is based on basic information that can be obtained through a direct survey and a collection of technical data from available documentation. Following this approach, the vulnerability of small earth dams is estimated through an index V structure defined as the sum of four parameters for the limit state corresponding to a 10% exceedance probability in 50 years (SLV for the NTC 2018): V structure = V condition + V liquefaction + V settlements + V displacements (1) where: V condition is the vulnerability of the dam due to its general state. It is defined through a direct in situ inspection. This parameter varies between 0.2 and 1, and it increases as the deterioration phenomena affecting the body on the dam are more evident. V liquefaction is the vulnerability due to liquefaction phenomena. The value of this parameter is 0 if one of the exclusion criteria proposed by NTC (2018) is satisfied, and 1 in other case or in absence of direct observations. V settlements is the vulnerability due to the potential crest settlements. This parameter is defined as the ratio between the predicted settlement, evaluated through the relationship (Eq. 2) proposed by Swaisgood (2003), and a threshold value of 0.02%, corresponding to a moderate levels of damage following the same study of Swaisgood. (2) V displacements is the vulnerability due to the possible slope displacements. Also this parameter is defined as the ratio between the predicted slope displacements and an admissible value. The prediction values of slope displacement can be computed following an empirical formulation, like the one propose by Blake et al. (2002): (3) where D 5-95 is the significant duration of shaking in second (i.e. 9-95% normalized Arias intensity), k max = PGA/g (where g is gravity acceleration), and k y is the yield acceleration of slope (i.e. the horizontal seismic coefficient that reduces the factor of safety for the slope to unity). This last parameter can be preliminarily estimated on the basis of the static factor of safety (FS) as: k y = (FS-1)·sin a, where a is the average angle of the failure surface with the horizontal. In the proposed methodology, k y = 0 is assumed if no specific slope stability analyses are available. The value of D 5-95 is related to magnitude and epicentral distance (r in km) of earthquake through the relationship proposed by Abrahamson and Silva (1996):