Newmark block model of seismic displacement of a slope. A valid model for slopes restrained by structural elements?

The sliding block model proposed by Newmark (1965) is an accepted methodology for
assessing the accumulated displacement of a soil slope during an earthquake. For slopes restrained by elastic structural elements, such as geotextile reinforcement or ground anchors, the assumptions behind the Newmark block model may not be valid. The Newmark model assumes the resistance against acceleration remains constant with displacement, however, the restraint provided by an elastic structural element increases with displacement as it stretches. This paper considers the significance of this in design of restrained slopes and discusses possible modifications to analyses to allow for this elastic behaviour.