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Seismic Response and Stability Analysis of Landfills

 

Seismic Response of a Composite Landfill Cover (Case study) 

The Olympic View Sanitary Landfill (OVSL) is located approximately 15 km southwest of Port Orchard, Washington. The site started receiving municipal solid waste and construction debris in the 1960s, transferring from an unlined waste disposal site to a modern lined sanitary landfill. The Phase I area of the landfill founded in the glacial moraine that underlies the entire site, and three areas lined with composite liners. Three types of composite landfill cover were used in the Phase I. Cover types A and B apply native soil mixed with bentonite as the low permeability soil layer beneath the geomembrane. Cover types C employs a needle-punched reinforced geosynthetic clay liner (GCL) as the low permeability soil layer instead of the native soil mixing with bentonite.

The reason why the OVSL landfill is chosen is that it is the first recorded case history of a composite cover system shaken by strong underground motions. On February 28th, 2001, a moment magnitude (Mw,) of 6.8, Nisqually earthquake happened. The main shock occurred at the interface of the Juan De Fuca and North American tectonic plates, approximately 52 km below the ground surface. According to The Pacific Northwest Seismograph network (2001), the OVSL site was approximately 39 km away from the earthquake epicenter (the point on the ground surface directly above the focus) and 65 km from the zone of energy release. There is a station called Kitsap County Moderate Risk Waste (KIMR), which is located at 1 km from the OVSL site. It is founded on “soft rock/dense soil”, defining as Site Class C by the NEHRP (BSSC 1998) Site Classification system. Soft rock/dense soil is defined as a site with a shear wave velocity over the upper 30 m of between 360 and 720 m/s or an average standard penetration test (SPT) below count (N60) in excess of 50. The NEHRP Site Classification is made upon geologic maps for Kitsap County, where the KIMR station and the OVSL are located. During the Nisqually earthquake, the KIMR station recorded acceleration time histories in three orthogonal directions. The recorded PHGA values in the north-south (KIMR-NS) and east-west (KIMR-EW) directions were 0.15 g and 0.16 g. Based on the current stability analysis of earth structures and seismic design landfill, the vertical components of ground motions were not necessary for the seismic analysis of the landfill cover.

The landfilling operations were in progress at the OVSL site when the Nisqually earthquake happening. Reports from the operators at the site working on the ground surface indicated that they were immediately alarmed by the earthquake. However, operators working on the solid waste fill reported that they barely noticed that the ground was shaking. After 5 days of the earthquake, a formal reconnaissance team arrived at the OVSL site. Neither the landfill crews nor the team found any evidence of seismically including permanent lateral displacement of the composite landfill cover. However, they discovered that many landfill gas risers had moved laterally relative to the cover by up to approximately 30 mm. After two and half years of the Nisqually earthquake, a spectral analysis of surface waves (SASW) geophysical survey was conducted at OVSL site. The results of the site-specific SASW measurements conducted on top of each of the landfill areas covered by a composite cover are shown in Figure. 2.3. The results indicate that the shear wave velocity of the OVSL site is stress dependent, as it is greater at depth than the near surface in all three profiles. 

2.3

Figure 2.3 – Results of SASW measurements at OVSL Site

Four different methods were applied to analysis the seismic effects on the composite landfill cover of the OVSL site. Method 1 and 2 were evaluated using two simple chart solutions outlined in the United States Environmental Protection Agency (EPA) guidance document on seismic design of solid waste landfills (Richardson et al. 1995). Method 3 is a more rigorous screening procedure proposed by Bray et al. (1998). Method 4 is a conventional decoupled equivalent-linear site response/Newmark-type (Newmark 1965) permanent seismic deformation analysis. Method 4 is close to the displacement analysis on the 1-D seismic analysis of liner system. The results of four methods are shown in the following Table 2.1

Table 2.1 – Summary of Seismic Response and Deformation Analysis

7

Method 1 and 2 are the most conservative because they still considered satisfactory performance, which leads the results from method 1 and 2 higher than other two methods So, they may not be as conservative as they were for the OVSL site since they overestimated the response of the OVSL landfill. In addition, Method 3 values still significantly greater than the method 4 values and is still too conservative. More computational effort is required in the Method 3 than other methods. Hence, method 4 is more accurate in the case. Based on method 4, the range of maximum transient displacement of 35 to 55mm is consistent with the observed relative movement between the cover and landfill gas risers of up to 30 mm.

 

Geosynthetics and Composite Landfill Cover Closing Remarks

Both geosynthetics and composite landfill cover are very commonly used in the modern landfill. At the site like OVSL landfill, it is important to not only perform seismic design procedures, such as slope stability or liquefaction,but know what performances of geosynthetics and landfill cover are expected under certain shear strength and period. So, engineering judgments are involved to select appropriate methods to analysis geosynthetics and landfill cover under seismic condition. 

 

 

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