|Session 4.2 - Retaining Walls and Embankments|
This paper describes a new calculation method for the seismic displacement of retaining walls with embedment. A macroscopic failure surface and potential for plastic displacement in the general load space are considered in the method, to evaluate the subgrade reaction force from foundation ground. The method is capable of calculating not only horizontal, vertical or rotational displacement alone, but also their combined effect. The method is validated through comparison with centrifuge test results of a gravity retaining wall with dense backfill sand subjected to strong base shaking. The calculated displacement components (vertical, horizontal and rotational), agreed well with those measured.
Keywords: seismic, retaining wall, displacement, shallow foundation, bearing capacity
Seismic design of large, reliable, economic retaining structures can be achieved by allowing limited displacement, provided their likely displacement performance during earthquakes is known. Design is commonly based on pseudo-static methods using horizontal peak ground accelerations, with no consideration given to vertical ground motions.
The research examined the effect of significant vertical earthquake motions on the displacement of retaining structures. Numerical time history analyses using a finite-difference program FLAC were carried out for a typical reinforced soil wall. Four earthquake records with different characteristics were used, and the results are presented.
The research showed that peak ground accelerations are a poor parameter for the prediction of wall displacements. The sum of the power spectral density, representing energy content, correlated better with the displacements calculated. Vertical shaking and the frequency content of the earthquakes had a significant effect on wall displacements. The results led to a hypothesis that vertical shaking increases the flexibility of the retaining structure, modifying its natural period, and where this shifts the period to a frequency with significant earthquake energy, resonance and larger displacements result.
The research confirmed the importance of energy, frequency content and vertical shaking of earthquakes to the displacement performance of retaining structures, particularly in near-field areas with significant vertical shaking.
Keywords: earthquake, retaining walls, displacement, vertical motions, numerical analysis, energy
Correlating seismically induced permanent displacements to parameters characterizing the intensity of the earthquake's strong ground motions, allows the dynamic response computation to be decoupled from the seismic hazard evaluation in a probabilistic seismic displacement analysis. Using an earthquake database of over 1400 records, seismically induced permanent displacements were calculated using a linear and an equivalent-linear coupled stick-slip generalized single degree of freedom model. Linear and nonlinear regression analyses were performed on these results to identify Intensity Measures (IMs), which may be classified as being period-dependent or period-independent, that correlate best to the computed displacements. Optimal IMs were identified based on the efficiency and sufficiency criteria. The analyses demonstrated that the optimal IM depends on the dynamic response and strength characteristics of the earth slope. It is useful to categorize slopes as stiff or ductile and as weak or strong. The benefit resulting from the use of vectors of IMs as opposed to a scalar IM for displacement prediction depends greatly on the slope properties. An example is shown that demonstrates the benefit for using Arias Intensity as the optimal IM in the case of estimating seismic displacements of stiff slopes.
Keywords: earthquakes, ground motions, probability, seismic displacements
Southeast Missouri experienced the largest magnitude (estimated 8.0-8.3) earthquakes in recorded history (1811-1812). In a future major earthquake, the reopening of critical emergency vehicle access routes into St. Louis, Sikeston and Cape Girardeau would be a top priority. The extent of damage and survivability of these critical roadway features in the event of a major earthquake is not fully known. On basis of a study on detailed earthquake assessments at two bridge sites along designated vehicle access route, it was found that under an event with PE of 2% in 50 years, these routes will be rendered unserviceable. In this paper a detailed study on the displacements on top of the abutment due to sliding and rotation and considering non-linear soil properties has been estimated.
Keywords: abutments, seismic displacements, non-linear analysis, dynamic SSI, bridges