Newmark Model Research Articles

Time probabilistic evaluation of seismically induced landslide hazard in Irpinia (Southern Italy)

A recently proposed method, which incorporates the Newmark model to evaluate the earthquake-induced landslide hazard at regional scale, was applied to Irpinia, one of the most seismically active regions of Italy. The method adopts a probabilistic approach to calculate values of critical acceleration a c representing the minimum strength required for a slope not to fail at a fixed probability level in a given time interval. Regional probabilistic hazard maps were generated for the two failure types most common in Irpinia (slump–earthflows and rock falls). The results suggest that quite moderate critical acceleration (0.05–0.08 g) could suffice to keep the slope failure probability low. However, the available data indicate that potential slide surfaces with a c below these values could be common in Irpinia, where, perhaps in relation to particular geo-environmental conditions, a relative large number of marginally stable slopes might survive other destabilising actions and fail even on occasion of not particularly strong earthquake shaking.

An Approach to Time-Probabilistic Evaluation of Seismically Induced Landslide Hazard

The need for effective strategies for preventing and mitigating damage caused by earthquake-induced landslides has stimulated recent developments of techniques for the assessment of seismic-landslide exposure at a regional scale. However, for a rational risk management, a crucial element is represented by the time horizon of the hazardous events. A new way to incorporate the time factor in seismic-landslide hazard assessment is proposed here. It consists of evaluating the temporal recurrence of seismically induced slope failure conditions inferred from the Newmark's model: First, by adopting Arias Intensity to quantify seismic shaking, well-established methods of seismic hazard assessment are employed to obtain the occurrence probabilities for different levels of seismic shaking in a time interval of interest. Then, some empirical relations, based on the Newmark's model, are employed to evaluate the slope critical acceleration a c for which a prefixed probability exists that seismic shaking would result in landslide triggering. The obtained a c values represent the minimum slope resistance required to keep the probability of seismic-landslide triggering within the prefixed value. Therefore, the space distribution of the calculated a c values can be promptly compared with the actual in situ a c values of specific slopes to estimate whether these slopes have a significant probability of failing under seismic action in the future. An example of this approach, applied to an area in southern Italy (Daunia region), shows that the introduction of the time factor significantly modifies the representation of the spatial hazard and allows evaluation of the relevance of seismicity as a landslide triggering agent.

Seismically induced landslide displacements: a predictive model

Newmark's model for predicting earthquake-induced landslide displacements provides a simple way to predict the coseismic displacements affecting a sliding mass subject to earthquake loading. In this model, seismic slope stability is measured in terms of critical acceleration, which depends on the mechanical soil properties, pore-pressure distribution, and slope geometry. The triggering seismic forces are investigated in terms of energy radiation from the source, propagation, and site effects, based on 190 accelerometric recordings from 17 Italian earthquakes with magnitudes between 4.5 and 6.8. The method is based on the calibration of relations having the general form of an attenuation law that relates the energy of the seismic forces to the dynamic shear resistances of the sliding mass to propagate the expected landslide displacements as an inverse function of the distance from the fault rupture; the amount of displacement computed through these relations provides a criterion to predict the occurrence of slope failures. Finally, maps showing, in a deterministic and a probabilistic way, the potential of seismically induced landslide displacements are displayed as a tool to provide seismic landslide scenarios and earthquake-induced landslide hazard maps, respectively.

An examination of simplified earthquake-induced displacement procedures for earth structures: Discussion

The Discusser has read this paper with considerable interest, since its subject matter pertains to our work in embankment dam engineering at the U.S. Bureau of Reclamation. We generally use Newmark's sliding block model to estimate dynamic deformations of embankment dams and feel reasonably comfortable with the results. However, the Authors suggest that the procedure can give significantly unconservative results and that it should not be used. The Discusser questions some of the reasons for the concerns and recommendations against the use of the Newmark procedure for estimating seismic deformations of embankment dams. As pointed out by the Authors, there are many viewpoints, opinions, and ideas for estimating seismic deformations for earth structures. Therefore, it is essential to consider details of the problem under study and those of possible solution procedures in selecting an appropriate solution strategy and procedure. As engineers, we all tend to tamper with the rigors of mathematical models to include some of the entities considered important for our problems or exclude some of the entities considered not so important, for simplification. However, considerations for consistency across various aspects of engineering work are essential, especially when the results of one analysis feed into another. (1) An embankment dam is a deformable earth structure, whereas the Newmark model is a rigid block on a rigid inclined plane. However, it is consistent with other aspects of slope stability models as explained in item 2 below. In addition, the stick-slip type of movements envisioned in the sliding block model have been monitored in rock movements in open pit mines during blasting (Oriard 1972). For its performance on embankment dams during earthquakes, the sliding block model was used for displacement calculations in studies performed to analyse the observed performance of La Villita Dam, a 197 ft (60 m) high earth and rockfill dam in Mexico, which had been subjected to six major earthquakes of magnitudes 4.9–8.1 located at epicentral distances of 6–75 mi (10–121 km) from the dam, and for which seismically induced permanent deformations were recorded and also detected in recorded acceleration responses. Deformations in the dam were significant and the crest acceleration records asymmetric. Details of the dam performance data and those of the experimental and numerical models used to study the observed responses are given in Elgamal (1992) and Elgamal et al. (1990). The asymmetric character of the acceleration responses at the dam crest were attributed to stick-slip type of deformations in the dam and it was learned that plastic yielding does not allow acceleration peaks to exceed yield acceleration, that is, a yield acceleration imposes a physical barrier on the magnitude of inertial forces that can develop in a sliding zone. Response of structures to vibration effects using steady state harmonic motion depends on how close the period of the structure is to the period of the harmonic motion. Thus, agreement of a numerical model results with shaking-table experiments using sinusoidal input motions is necessary but may not be sufficient for gaining confidence in its application to earthquake loading. It would be helpful to apply the Authors' procedure to La Villita Dam and compare the results. (2) Earthquake forces result from erratic vibratory motion of the ground on which the structure is supported. For a rigid structure, rigidly coupled to its foundation, force on the structure equals the mass of the structure times acceleration of the ground (F = M × A) at any instant; for a flexible structure which deforms slightly, F may be less than M × A for short periods of time; and for a very flexible structure subjected to a ground motion whose period is near that of the structure, F may be much greater than M × A. In a slope stability model by the limit equilibrium method, a rigid soil mass rests on a curved slip surface with a rigid plastic glue which conforms to a shear strength criterion properties of the slide mass above the slip surface are not related to those of the soil it represents and the materials below the slip surface provide a rigid, nonyielding support to the slide mass. In this model, no deflection occurs before failure and when failure takes place, the slide mass slides slowly downwards without accelerating. Yield acceleration for the slide mass is

Modified Newmark Model for Seismic Displacements of Compliant Slopes

Newmark sliding block analyses are widely used for estimation of permanent displacements of slopes in earthquakes. The conventional Newmark model, however, neglects the dynamic response of the material above a potential failure surface. Decoupled procedures have been developed to account for that response, but they neglect the effects of permanent displacements on the response. A modified Newmark analysis that considers the dynamic response, including the effects of permanent displacements, of the material above the failure surface is presented. The modified Newmark analysis shows that the decoupled approach produces somewhat conservative estimates of permanent displacements for stiff and/or shallow failure masses, but that it may produce unconservative estimates for failure masses that are soft and/or deep. Many slopes of large, lined landfills may fall into this latter category. The notion of a slope spectrum, which illustrates the effect of the natural period of a potential failure mass on permanent slope displacement, is also introduced.