Keywords: earthquake, seismic behaviour, landfills, landfill liner systems, liquefaction, excess pore pressure, geotechnical.

Seismic behaviour of landfills is a major concern especially when the landfills are located in the vicinity of densely inhabited areas. Following the Northridge earthquake of 1994 in Los Angeles, Madabhushi (1994), one of the main worries was the dynamic behaviour of various Waste Containment Structures storing many kinds of toxic and non-toxic chemicals. The post earthquake behaviour of these structures need to be investigated to establish their effective functionality over their full intended design life which may extend to a period of 70 to 80 years. In other words, post earthquake integrity of the landfill liners must be ensured.

The leakage through landfill liners either simple clay liners or more modern multi-layer systems using geomembranes, geotextiles and GCL's has been well researched by Rowe (1998) and more recently by Foose et al (2001). However these studies concentrate on either the natural contaminant migration rates through intact barrier systems or accelerated rates due to defects in liner systems and imperfect contacts. However, during earthquake loading we are more concerned about the integrity of the landfill liner rather than the defects in fabrication of the liners.

Earthquake loading induces additional stresses in the liner systems. This may be due to the failure of the side slopes forming the landfills or due to the additional down drag stresses induced as the solid waste in the landfill settles down during the earthquake loading. Singh and Sun (1995) and Singh (1996) studied the behaviour of the municipal solid waste landfills when subjected to earthquake loading.

This paper aims at understanding the failure mechanisms of landfill liner systems following earthquake loading with emphasis on the liquefaction scenarios. In this paper we present the analysis of experimental data from centrifuge tests where the landfill liner consisted of either a single clay layer or a clay liner-geomembrane system, overlying a liquefiable soil deposit. The aim of these series of tests was to establish that dynamic centrifuge modelling can be used as an effective tool to study the complex problem of clay liner integrity following liquefaction of foundations soils, Madabhushi and Singh (1999,2000). The model clay liner was subjected to either earthquake induced liquefaction or generation of excess pore water pressures and partial liquefaction. In this paper we present the pore pressure contours that were plotted using the Matlab routines and the experimental test data.

The hydrostatic pore pressures in the foundation soil below the landfill are recorded well by the pore pressure transducers. The contours plotted from these recordings confirm this and match satisfactorily with those estimated using density of water, depth of the model and the g level. The same contour routines are used to plot the excess pore water pressures at various instants of the earthquake loading. These contours show the generation of excess pore pressures in different regions in the foundation soil below the landfill. Based on these plots we can see that the soil near the side slopes of the landfill becomes vulnerable first as this region has a lower total stress compared to the soil below the base liner. Subsequent redistribution of excess pore pressures as the earthquake loading progresses was also picked up very well with these excess pore pressure contours. The contour maps were also used to see the effects of breaching in the landfill liner on the redistribution of the excess pore pressures. The centrifuge test data shows that the breach has happened sometime after the end of the earthquake loading. The contour maps show very clearly that following the breach the excess pore pressures concentrate around the base of the side slope where the breach was observed (and confirmed with post test site investigation). Conclusions were drawn on integrity landfill liner and stability of the side slopes of the landfills under the action of the earthquake induced excess pore water pressures.

1

Foose, G.J., Benson, C.H. and Edil, T.B., (2001), "Predicting leakage through composite landfill liners", ASCE Jnl of Geotechnical and GeoEnvironmental Eng, Vol. 127, No.6. doi:10.1061/(ASCE)1090-0241(2001)127:6(510)

2

Madabhushi, S.P.G and Singh, S., (1999), "Seismic behaviour of waste containment structures, New Frontiers and Challenges", Proc. Civil and Environmental Engineering Conference, Asian Institute of Technology, Bangkok.

3

Madabhushi, S.P.G and Singh, S.K., (2000), "Integrity of landfill liners following earthquake loading", International Symposium on Physical Modelling and Testing in Environmental Geotechnics, NECER, LCPC Centre de Nantes, Bouguenais, France.

4

Singh, S, (1996), "Liquefaction characteristics of silts", Jnl. Geotechnical and Geological Engineering, Vol.14. doi:10.1007/BF00431231

5

Singh, S. and Sun, J, (1995), "Geotechnical considerations in the seismic response evaluation of municipal solid waste landfills", Earthquake design and performance of solid waste landfills, ASCE, San Diego, California, USA.

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