SYMPOSIA PAPER Published: 01 January 1990
STP25312S

Stability Consideration of Vertical Landfill Expansions

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It should be expected that vertical expansions of ash residue and municipal solid waste landfills in the future will be required to demonstrate an acceptable level of structural stability in addition to meeting other federal and state regulations. In particular, the liner systems of the vertical piggyback expansions should not suffer damage due to settlement of the underlying existing waste material or due to side slope instability.

In May 1987, prior to the development of any New York state or federal regulations addressing the issue of structural stability relative to vertical piggyback expansions, separate ash residue and municipal solid waste vertical expansions were designed by Malcolm Pirnie, Inc. for the Town of Islip, New York. A fundamental design consideration was the evaluation of long-term piggyback structural stability. Construction of both piggyback expansions began in September 1987, with substantial completion in August 1988.

The design of both piggybacks evaluated structural stability in the following modes: (1) failure between the various natural and geosynthetic materials comprising the piggyback liner system, (2) deep seated circular failure down through the underlying existing waste material and subgrade and (3) subgrade reinforcement with polyethylene geogrids. Malcolm Pirnie's analyses indicated that several geotechnical parameters were of critical concern for the stability of the Town of Islip's vertical piggyback expansions: (1) bottom liner and existing side slope grades, (2) interface friction angles, (3) type and orientation of the geosynthetics relative to grade, (4) depth of leachate in the drainage layer above the piggyback liner system, and (5) utilization of geogrids for subgrade reinforcement.

The stability analyses which assessed design conditions (1) through (4) utilized the computer program STABL, written at Purdue University for the general solution of slope stability problems. Calculation of factor of safety values against incipient instability of the slope was performed for the sliding block and deep-seated circular failure analysis, using the Modified Bishop's and Simplified Janbu methods, respectively. The results of the structural analyses indicated that a combination of geogrid reinforcement and appropriate design would yield vertical piggyback expansions which would have acceptable structural stabilities over their operational and post-closure lifetimes.

Author Information

Tieman, GE
Druback, GW
Davis, KA
Weidner, CH
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Details
Developed by Committee: D18
Pages: 285–302
DOI: 10.1520/STP25312S
ISBN-EB: 978-0-8031-5130-7
ISBN-13: 978-0-8031-1285-8