SITE RESPONSE ANALYSIS

The yield acceleration (Ky) obtained from the slope stability analysis represents the maximum horizontal acceleration which the design can accommodate before failure. Because the PGA was significantly greater than Ky, a site response analysis was performed to evaluate the amount of permanent seismic displacement.

A one-dimensional site response analysis is typically used for MSW landfills. A two-dimensional site response analysis may be performed for critical projects or when the 1D analysis indicates potential problems. A Finite Element (FE) analysis is used for complex geometries or when results of the 2D analysis are inconclusive. The degree of conservatism is reduced and more realistic results are obtained with each increasing level of sophistication (i.e. from 1D to 2D). 2D and FE analyses are rarely used in the analysis of MSW landfills.

The program most widely used for a 1D analyses is SHAKE91 (Idriss and Sun, 1992). However, this program may overestimates the MHEA when the input acceleration is greater than 0.40g (Kavazanjian and Matasovic, 1995). Because the PGA generated by the MCE for this project exceeded 0.8g, EBA performed a one-dimensional site response analysis using the non-linear program D-MOD_2 (Matasovic, 1995). The landfill was modeled by creating a representative 1D profile of waste dynamic material properties to the total depth of the landfill. Dynamic material properties required for the analysis included unit weight, shear wave velocity, and the relationship of shear modulus and damping with shear strain (modulus reduction and damping curves). Actual measurements of dynamic material properties for MSW are limited and have been estimated by others from empirical strong motion and field data collected from Southern California landfills (Augello et al., 1998; Matasovic and Kavazanjian, 1998).

Using the most recently published attenuation relationships which relate magnitude and distance to PGA, the target (design) spectrum was plotted for significant seismic sources (Figure 3). Strong motion records were then scaled up to the target spectrum with particular attention given to the period range bracketing the fundamental period of the site (Figure 4). Amplification of peak acceleration can occur when the fundamental period of the waste is close to the predominant period of the input or bedrock motion. Results of spectral amplification caused by the coincidence of resonant frequencies include building damage in Mexico City from the 1985 earthquake and collapse of the Cyprus structure in the 1989 Loma Prieta earthquake (Richardson & Kavazanjian, 1995).

Due to the close proximity (less than 1 mile) of the site to a major fault, near-field sources were determined to produce the most severe ground shaking. However, large magnitude earthquakes occurring on distant faults and producing long period ground motions which may correspond to the landfill's fundamental period were also considered.

Inherent in the site response analysis is the assumption that the waste mass above the failure surface (previously determined in the slope stability analysis) moves as a single mass and responds to the strong motion record with its own time history represented by the mean horizontal equivalent acceleration (MHEA) described by Bray and Repetto (1994). The time history of the MHEA was used to evaluate the permanent seismic displacement.

References

• Augello, A.J., Bray, J.D., Abrahamson, N.A. and Seed, R.B., 1998, Dynamic Properties of Solid Waste Based on Back-Analysis of OII Landfill, ASCE Journal of Geotechnical and Geoenvironmental Engineering, Vol. 124, issue 3, pg. 211-222.
• Bray, J.D. and Repetto, P.C.,1994, Seismic Design Considerations for Lined Solid Waste Landfill, Geotextiles and Geomembranes.
• Idriss, I.M. and Sun, J.I., 1992, SHAKE91: A Computer Program for Conducting Equivalent Linear Seismic Response Analyses of Horizontally Layered Soil Deposits, Center for Geotechnical Modeling, Department of Civil and Environmental Engineering, University of California, Davis, November.
• Kavazanjian, E., Jr. and Matasovic, N., 1995, Seismic Analysis of Solid Waste Landfills in Proceedings of the Geoenvironment 2000 Specialty Conference, ASCE, Vol. 2, pp. 1066-1080, New Orleans, Louisiana, 24-26 February, 1995.
• Matasovic, N., 1995, D-MOD_2, A computer program for Seismic Response Analyses of Horizontally Layered Soil Deposits, Earthfill Dams and Solid Waste Landfills, December 1995.
• Matasovic, N. and Kavazanjian, E., Jr.,1998, Cyclic Characterization of OII Landfill Solid Waste, ASCE Journal of Geotechnical and Geoenvironmental Engineering, Vol. 124, issue 3, pg. 197-210.
• Richardson, G.N., Kavazanjian, E., Jr. and Matasovic, N., 1995, RCRA Subtitle D (258) Seismic Design Guidance for Municipal Solid Waste Landfill Facilities, EPA/600/R-95/051, April 1995.
• Schnabel, P.B., Lysmer, J., Seed, H.B., 1972, SHAKE: A Computer Program for Earthquake Response Analysis of Horizontally Layered Sites., Report No. EERC 72-12, Earthquake Engineering Research Center, University of California at Berkeley, December.

Toland Road Seismic Analysis - Table of Contents