Ernesto Motta

Seismic response of saturated cohesionless slopes

Abstract The serviceability of a slope after an earthquake is controlled by deformations, consequently a stability analysis that predicts slope displacements is desirable. In cohesionless satured slopes the seismic loading may produce an increase in pore pressures, which in turn, reduces significantly the effective state of stress. Depending on the seismic intensity and on the effective stress conditions before the seismic loading, the induced excess pore pressures may produce the slope failure. Slope displacements may occur even for seismic accelerations lower then the initial critical acceleration, because of pore pressure build-up. In this paper, a simplified procedure for the seismic response of infinite slopes is described, and the role-played by relevant parameters such as soil relative density and initial pore pressure ratio is pointed out.

The effect of travelling seismic waves on the dynamic response of earth dams

Abstract A theoretical investigation of the dynamic response of earth dams to the travelling seismic waves is presented. The earth dam is simplified as a truncated two-dimensional elastic wedge. The dam body consists of an isotropical linear viscoelastic material with homogeneous elastic modulus and density. The seismic waves travel along the longitudinal direction of the earth dam. The numerical calculations show the following. (i) For the longitudinal mode of vibration, the greater the ratio ( H L ) of the height to the lenght of the complete wedge, the more the natural transverse period of vibrational of the two-dimensional wedge is less than that of the one-dimensional wedge. Especially for the first two natural transverse period, this influence is large. The decrease of the ratio of the natural transverse period for a two-dimensional wedge to that for a one-dimensional wedge with the ratio H L is rapid for the higher than for the lower longitudinal modes. (ii) Comparing with the one-dimensional wedge, the natural transverse periods for the two-dimensional case in the complete wedge are lower, and they will increase as the coefficient of truncation, h H increases. (iii) When the frequency of forced vibration is less than the natural transverse frequency for one-dimensional wedge, the amplification is less for a two-dimensional wedge than for a one-dimensional wedge. (iv) When the phase difference of ground motion at both ends of the dam equals π, the amplification for a two-dimensional wedge is less than that for a one-dimensional wedge, but when the phase difference equals nπ, (n > 1), the situation is reversed. (v) As the coefficient of truncation, h/H, increases, the displacement model partecipations decrease monotically. (vi) In general, the displacement caused by an earthquake is greater for a one-dimensional wedge than for a two-dimensional wedge when considering the seismic waves travelling, but the acceleration response of a two-dimensional wedge with long length of dam to travelling seismic waves with long dominant period is greater than that of a one-dimensional wedge. When the length of the dam is short enough, the response of a two-dimensional wedge without considering the influence of travelling seismic waves always gives the greatest value.

Seismic Bearing Capacity of Shallow Foundations

The seismic risk mitigation is one of the greatest challenges of the Civil Engineering and an important contribution toward this challenge can be given by the Geotechnical Earthquake Engineering. Lesson learned by recent destructive earthquakes (January 2010 Port-au-Prince region of Haiti and March 2011 Tohoku Japan), confirms that local soil conditions can play a significant role on earthquake ground motions. Earthquake-induced damage in Port-au-Prince was devastating and widespread. Yet, there were clearly areas of the city where little to no damage occurred, and areas of the city where an overwhelming majority of the buildings were severely damaged or destroyed. These types of damage patterns are common in earthquakes, and a wide number of factors need to be considered in order to conclusively piece together the causes. For a given earthquake, these factors include, but are not limited to: (a) relative distance from the fault rupture plane, (b) construction type and quality, (c) local soil conditions (i.e. strength/stiffness of the soil foundation, depth to bedrock, impedance contrasts, geology), (d) topography (topographic and basin effects), and (e) near fault effects (rupture directivity, fling step, hanging wall effects, polarity effects, etc.). Often several of these factors work together and it can be difficult to identify the primary cause of damage. Design of foundations in seismic areas needs special considerations compared to the static case. The inadequate performance of structures during recent earthquakes has motivated researchers to revise existing methods and to develop new methods for seismic-resistant design. This includes new design concepts, such as, performance-based design (PBD) (Priestley et al., 2005) and new measures of the structure performance based on energy concepts and damage indexes (Park et al., 1987; Moustafa, 2011). Similarly, the widespread damage and inadequate performance of code-designed structures during the 1994 Northridge (California) and the 1995 Kobe (Japan) earthquakes have prompted seismologists and engineers of the essential importance of characterizing and modelling near-field ground motions with impulsive nature (Moustafa & Takewaki, 2010). For foundations of structures built in seismic areas, the demands to sustain load and deformation during an earthquake will probably be the most severe in their design life. As stressed by Hudson (1981) the soil-structure interaction is a crucial point for the evaluation of the seismic response of structures.

Attenuation laws of seismic intensity in the regions of Sicily and Calabria

Abstract The attenuation laws of seismic intensity in the regions of Sicily and Calabria (an area of Italy between latitudes 36° 30′ and 40° 0′ N, longitudes 12° 0′ and 17° 30′ E) have been studied. Three methods have been utilized: (1) under a certain magnitude M , to consider the relationship between the seismic intensity I and the epicentre distance δ (or the hypocentre distance R ); (2) under a certain imaginary intensity I 0 at the focus of the earthquake, to consider the relationship between the seismic intensity I and the epicentre distance δ (or R ); (3) to consider the relationship between the decay of seismic intensity, i = I 1 − I , and the epicentre distance δ (or the hypocentre distance R ). All the earthquake s in the area studied were divide into two groups, the first containing the earthquake with the depth of their source greater than or equal to 15 km, and the second containing the earthquakes with their depth of focus less than 15 km. The results show that: (a) in the case when the depth of focus of the earthquake is not considered, the rate of attenuation of the seismic intensity for the two groups of earthquakes with the epicentre distance δ is approximately the same; (b) the rates of attenuation of seismic intensity for the two groups of earthquakes with the hypocentre distance R are quite different — in general, the rate of intensity attenuation for the first group of earthquakes is greater than that for the second; (c) in comparison with earthquakes in the north and central areas, the earthquakes in the regions of Sicily and Calabria show a more rapid rate of attenuation of seismic intensity; this fact reflects one of the essential characteristics of the earthquakes in the area studied.

The local site response for upgrading the existing buildings against seismic hazard

The analysis of seismic ground response at the site was conducted using a numerical method, which is developed in three main phases: the definition of the geometric, geological and geotechnical model of the subsurface, the definition of the seismic input, the choice of one or more computer codes to use and the processing of the results. The reconstruction of the geological and geotechnical model of the subsurface has highlighted a morphology quite irregular especially with regard to the covers. In order to assess the effects due to the local stratigraphy, it has been decided to carry out seismic 1-D response analyses, using the computer codes EERA and STRATA. The 1-D analysis was also used for the calibration of computer codes by the modelling of simplified systems and comparing the results obtained by the different codes in order to verify its adequacy. The site response analyses have been developed for some sites in the city of Catania (Italy). On the sites some strategic buildings are localised to upgrade against seismic risk. In situ investigations were carried out in order to determine the soil profile and the geotechnical characteristics for the sites under consideration, with special attention given to the variation of shear modulus and damping with depth. Seismic Dilatometer Marchetti Tests (SDMT) have been carried out, with the aim of evaluating the soil profile of shear wave velocity (Vs). Moreover the following investigations in the laboratory were carried out on undisturbed samples: Resonant Column tests; Direct shear tests; Triaxial tests. The available data obtained from the Seismic Dilatometer Marchetti Tests results enabled us to evaluate the shear modulus profile.

GIS Techniques in the Evaluation of Pipeline Networks Seismic Hazard

To evaluate seismic risk, it must be taken into account that modern towns depend daily on lifelines and utility systems, that become essential after natural disasters, but are often without any earthquake threat. To evaluate lifeline networks seismic vulnerability, we usually refer to damage models, requiring parameters dealing with pipe features, soil behavior, and seismic hazard of the studied area (peak ground acceleration or velocity, PGA, PGV, or permanent ground displacement, PGD). In this work, models evaluating seismic hazard in a studied area and expected seismic damage for pipeline networks will be applied. A model is shown to assess earthquake induced slope displacements. Some attenuation laws will be selected to evaluate PGA, PGV and PGD. Finally, Repair Rate will be calculated for pipes of an important Italian water network feeding 20 towns of Etnean area, referring to three seismic scenario events. Applications will be developed in a GIS environment.

Kinematic interaction of a single pile in heterogeneous soil

The behaviour of deep foundation under static loads has been widely investigated and the available calculation procedures can be considered suitable for the current engineering applications. However, pile behaviour under seismic loading is more complex and less known, because one can find the contemporary action of inertial forces rising from the over-structure (inertial interaction) and of the soil deformations rising from the seismic waves (kinematic interaction). Italian code DM 14/01/2008 requires the dynamic soil-structure interaction in seismic foundation design to be taken into account, but it does not give any information about kinematic interaction strains evaluation criteria. Experimental evidences and theoretical considerations of many authors show that simply the kinematic interaction may induce high stresses on piles, especially near an alternation between a soft and a rigid soil layer interface. In this work pile behaviour due to kinematic interaction will be examined. An approach based on the differential equation proposed by Kavvadas and Gazetas (Kinematic seismic response and bending of free-head piles in layered soil. Geotechnique, 43, N.2, 207-222, 1993) will be used. The analysis is focused on the response of a single pile in a heterogeneous three-layer soil profile.

Lifeline Seismic Hazards: A GIS Application

In every urbanized area, lifelines and essential facilities play a very important role and they become essential after natural disasters, such as earthquakes, floods, landslides and so on. The purpose of this research is to develop a working tool to assess lifeline seismic risk, overlaying information about the studied area’s seismic hazard (referring to a seismic scenario) and lifelines that could expect damage. In damage models parameters are required, some representing pipes, others representing the soil behaviour and finally, at the very least a synthetic parameter representing the seismic hazard of the studied area (PGA, PGV or PGD). The evaluation of the network intrinsic vulnerability will be done in terms of a synthetic parameter called the Repair Rate. PGDs will be evaluated referring to attenuation laws and to earthquake induced slope displacements according to the Newmark approach. An application of the proposed model, developed by GIS techniques, will be applied to the case of a Sicily (Italy) important water network.

Site seismic response for a strategic building in the city of Messina by two-dimensional finite element analysis

It is well recognized that local seismic effects can exert a significant influence on the distribution of damages during earthquakes. Traditionally, these effects have been studied by means of simple one-dimensional (1-D) models of seismic wave propagation which take only the influence of the stratigraphic profile and soil/bedrock properties into account on the seismic response. Conversely, local effects derived from surface topography such as ridge, cliffs etc., which are typically two-dimensional (2-D) and three-dimensional (3-D) problems, have received less attention because of computational time, lack of experimental data and the need of more refined models. It is therefore of great interest to quantitatively evaluate the relative contribution on seismic response of stratigraphic as well as of topographic effects, which can be very different depending on the specific morphological conditions and geotechnical characteristics of the site.

An Analytical Expression for the Nonlinear Evaluation of the Soil-Pile Interaction

Pile foundations are commonly used for many types of structures and vertical and/or lateral loading design considerations are very important because the performance of the piles significantly influences the integrity of the structures supported by them. Current design methods of pile foundations often involve an estimation of a constant stiffness as an input, to model the soil-foundation interaction. Nevertheless, pile lateral response is nonlinear. It is therefore desirable to analyze the behavior of the foundation considering this nonlinearity. In the paper an analytical expression to predict the nonlinear behavior of a pile foundation is presented. To validate this closed form solution, computed results were compared with measures derived from loading tests, and it has been shown, that this solution can be used successfully for a provision of the nonlinear response of a soil-pile system.