Important First Steps in Landfill Post Closure

Original Landfill Grading Plan: This document is often times the only reliable way of establishing landfill bottom elevations. It is critical for landfills originally constructed by excavation of a pit or open chamber. This is so because rapid changes in the landfill's bottom elevation generally represent boundary conditions where the depth of refuse, and hence future settlement, changes rapidly. Such areas are particularly difficult to locate through field investigations.

Sequence of Landfill Operations: This information can occasionally be obtained from written logs and operation reports. More often it is available only through interviews with landfill operation personnel. It is important for landfills operated over a long period of time because decomposition, hence settlement, is time-dependent. There are landfills that are filled in horizontal layers or filled in cells over long, disconnected periods of time. Similarly, the original sequence of landfill operations may indicate differences in the composition of landfill waste, and hence differences in decomposition and settlement characteristics in different areas of the landfill. While sequencing Existing Topography/Utilities and Landfill Gas Collection Systems: Subsurface utilities such as gas collection systems, where present, will either need to be relocated or incorporated into the final design. Existing topographic surveys with subsurface infrastructure either should be obtained or prepared prior to geotechnical investigations to accurately establish bench elevations at boring locations. In this way, the barrier layer and refuse masses logged in the borings can be accurately defined by vertical elevation.

Previous Uses and Topography: Landfill sites commonly serve as reclamation and storage sites after closure and prior to final post-closure development. These and similar past uses can have the effect of surcharging portions of the landfill site and altering anticipated settlement patterns. Sources of information on previous uses include historic aerial photos, topographic maps, city use permits, and, most important, testimony of landfill owners and operators.

Collecting Empirical Data Settlement Characteristics

With historical documents in hand, the site civil engineer should begin collecting empirical data settlement data for the landfill site. This can be done by setting settlement monitoring points (generally iron pipes), recording the initial elevation of the settlement monitoring points and re-recording over time. In general, settlement monitoring points should have a higher density in boundary areas where settlement characteristics are anticipated to change rapidly (i.e., locations of large differential settlements). This includes boundaries of the landfill site where refuse depths change rapidly and areas that have undergone previous surcharging. For locations where settlement is anticipated to be relatively uniform, a grid spacing of approximately 150 to 200 feet should be adequate. This leads to a monitoring density of to 2 points/acre, plus the additional monitoring points defining boundary conditions. After initially establishing the monitoring point's elevation, intermittent readings should be recorded every two to four months for no less than one and preferably two years. Only such extensive readings can produce the data sufficient to predict future rates of settlement and prepare a settlement-rate contour maps of

1. depth to the barrier layer and to refuse,
2. anticipated settlement, and.

The previous article represents material extracted from the Waste Expo '97 (Atlanta, GA) conference paper, "Commercial Post Closure Development," by Max A. Keech of Brian Kangas Foulk, Redwood City, CA

Edited by Paul Lisk