Shake, rattle & hold: landfill stability in seismic regions

Landfill forensic analysis, the examination of landfill durability following seismic events, has taken root in the last two decades, rising from the rubble of California’s San Fernando, Loma Prieta and Northridge earthquakes. Although complex in some respects, the emerging research can aid design engineers, geomembrane manufacturers and government regulators in estimating landfill slope stability under static (normal) and seismic conditions.

Earthquakes pose a number of threats to landfills, including the potential for tom liners and covers as well as leachate and gas leaks. And, while California is the state most readily associated with quakes, 40 percent of the United States is actually at risk for significant seismic events. (See “Seismic Impact Zones” on page 42.)

For example, New York state has experienced 17 events since 1840, while Maine has earthquakes measuring 3.0 to 4.0 on the Richter Scale every three or four years. Pennsylvania’s most recent earthquake was in 1984.

The nation’s most powerful earthquakes occurred near Anchorage, Alaska (1964), and in New Madrid, Mo. (181 1). The Alaskan quake, measuring 9.2 and accompanied by a tsunami, was still less destructive than the Missouri jolt which, at 7.9, killed hundreds of people and caused stream banks to cave in, nearby towns to disappear and the Mississippi River to reverse itself between two islands.

Recognizing the geographic scope of risk, the Environmental Protection Agency (EPA), under Subtitle D, requires designers of all municipal solid waste landfills to consider the impact of seismic conditions on landfill slopes.

Northridge Lessons

H. Bolton Seed, considered by many to be the father of geotechnic earthquake engineering, conducted earthquake research at the University of California, Berkeley, in the 1970s and 1980s by studying earthen dams and liquefaction. Since then, landfill-related research has grown, but last year’s Northridge earthquake gave engineers their first opportunity to survey post-quake landfills built to Subtitle D standards.

A team of experts from Berkeley and GeoSyntec Consultants, Huntington Beach, Calif., examined 22 landfills within 44 miles of the Northridge epicenter. None of the locations sustained major damage as a result of the earthquake, and, of the seven Subtitle D sites, minor damage was discovered at only one.

At Chiquita Canyon Landfill, 16 miles from the epicenter, engineers found a 10- to 15-foot tear in a geomembrane located above the trash line. The reason for the damage, which was easily repaired, has not yet been determined.

Closer to the epicenter, Lopez Canyon Landfill remained intact, sustaining no damage to its bottom composite liner or the 1:1 side slopes containing geomembrane and geosynthetic clay liner. No evidence of either transient or permanent displacement between the interred waste and liner system was noted.

Until a more complete database of Subtitle D landfills is accumulated, Edward Kavazanjian, a Geosyntec consultant, urges designers to be conservative in planning landfills in seismic impact zones. He recommends that liners should perform to spec with a 300-foot-high waste mass under a peak ground acceleration of 0.69 gravity pull (g) during a 6.5-magnitude earthquake. (At the time of the Northridge quake, the Lopez site was filled 100 feet deep, and peak acceleration was 0.44 g.)

Lopez represents the strongest shaking of a Subtitle D landfill, and its performance is especially relevant to east coast landfill designers, says Kavazanjian. Eastern peak ground acceleration is not expected to exceed 0.40g in the next 250 years, he notes; as long as the waste load is 100 feet or less, landfill performance should parallel the Lopez model.

Other observations made at the Lopez site relate to base isolation, the ability of geomembranes to move against another material without friction. Engineers noted from markings in the dirt and dust that the Lopez geomembrane had moved 6 inches during the earthquake. They concluded from their survey that, when placed beneath equipment and low-rise structures, geomembranes could be a cost-effective means of earthquake protection.

Fundamentals Of Stability

A sesmic impact zone, as defined by Subtitle D, is an area with a 10 percent or greater probability that the peak horizontal acceleration of bedrock will exceed 0.1 g within 250 years. Estimated peak horizontal bedrock accelerations for different regions according to state and county are available from the U.S. Geological Survey.

EPA engineering requirements are general for demonstrating the predicted stability of a Subtitle D landfill during an earthquake. Briefly, Subtitle D mandates that new municipal solid waste landfills and lateral expansions not usually be sited within 20Q feet of a fault that has exhibited movement during the last 1 1,000 years.

Additionally, EPA requires engineers, when locating a landfill in a seismic impact zone, to demonstrate that a landfill’s slopes, liners, leachate collection and gas control systems and final cover are designed to resist the estimated greatest potential horizontal acceleration at the site.

To bridge the gap for engineers who are not familiar with static and seismic slope analysis, the University of Wisconsin (UW) offers training seminars to assist in standardizing the new field. Timothy Stark, assistant professor of civil engineering at the University of Illinois and a recent instructor at the seminars, emphasizes that the key parameter to slope stability (both static and seismic) is interface strength where geomembranes, geogrids, textiles and clay liners lie against one another.

Interface strength is site-specific depending on the applied normal stress and the water content, density, chemical resistance and texture of the various layers. Other slope stability parameters concern the steepness of the slope, the height of the waste mass, pore pressures in the soil and waste, local loading conditions and the potential for settlement.

According to Stark, there are many methods for calculating the safety of proposed interface conditions. Software designed for stability calculations includes UTEXAS3 & GRAPHIC3, BISTAT, GSLOPE, PC STABLE5M, SLOPE/W and STABR.

Studies that attempt to understand seismic conditions by modifying static, normal analyses are not widely accepted, Stark says. In these cases, engineering calculations offset the assumed rotation by employing a seismic coefficient, which is dependent on the magnitude of the earthquake, to more closely predict whether movement at a specific magnitude would cause significant landfill effects.

At a January presentation of the UW seminars, the software SHAKE, available from the National Information Service for Earthquake Engineering at Berkeley, was demonstrated to predict acceleration at the bottom and at the top of a specific landfill under design. It is assumed for the analysis that the waste itself behaves as a soil.

Calculating the anticipated acceleration at specific depths, particularly at the depths of the bottom liner and the final cover, allows engineers to change their landfill design in line with EPA and state regulations. It also gives engineers a strong clue whether leachate collection pipes, gas vents or surface structures could survive an earthquake at a given magnitude.

In addition to software models, engineers can use a more comprehensive method of slope analysis, using liquefaction to predict landfill deformation. This method was first proposed by Neven Matasovic of Geosyntec as an extension of early liquefaction calculations, and it requires consideration of soil and water loss, water table depth and characteristics of the waste to predict the strength of the cover design.

Differing Regulations

Unlike Subtitle D, EPA’s Subtitle C for hazardous waste landfills does not establish seismic criteria for design and construction. As a result, Subtitle D, which sets minimum standards for hazardous waste sites, has become a de facto standard for many Subtitle C facilities.

“You can put Subtitle D waste in a Subtittle C landfill,” says Kavazanjian, “so it is logical to infer hazardous waste sites should meet Subtitle D standards.”

In California, regulations exist to ensure that hazardous waste sites are designed to withstand a maximum earthquake without damage to the foundation or leachate, drainage, erosion and gas controls. These regulations differ from Subtitle D, which specifies only that a municipal solid waste landfill resist maximum horizontal bedrock acceleration.

The language of the California law is important, says Kavazanjian, because the quake with the largest peak acceleration is not necessarily the most damaging. For example, an earthquake measuring 5.0 with one pulse might occur directly underneath a landfill site and cause little damage, while, 100 miles away, an event registering 8.0 with half the peak acceleration could create many cycles of motion and greater disruption.

Kavazanjian believes the high peak acceleration standards of EPA’s D regulations are meant to cover any earthquake eventuality, since the probability of significant activity is relatively low, often 10 percent within 250 years for seismic impact zones. However, he adds, prudence dictates that engineers consider both the maximum earthquake and maximum peak acceleration standards during landfill design.

Future Resources

Historically, the effects of peak acceleration on landfills have not been easily sampled and quantified. In an effort to gather more precise seismic data, a few landfills in California are being equipped for exact measurements during an earthquake.

So far, only the Operating Industries Inc. Landfill in Monterey, Calif., has been outfitted, and it has experienced only a few small tremors plus one intermediate shake at a distance. Shortly, two sites in the Los Angeles area will install seismic instrumentation, and Kavazanjian expects other facilities to follow suit.

Records from these landfills, together with the data that continues to surface from Northridge and other recent earthquakes, will be used to form a cumulative database and aid municipalities nationwide in the study of landfill slope stability under earthquake duress.

Anne Magnuson is a free-lance writer living in Clifton Park, N.Y.