Date of Award

Spring 2005

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Civil Engineering - (Ph.D.)

Department

Civil and Environmental Engineering

First Advisor

Jay N. Meegoda

Second Advisor

J. P. A. Hettiaratchi

Third Advisor

Raj P. Khera

Fourth Advisor

Hsin Neng Hsieh

Fifth Advisor

John Tavantzis

Sixth Advisor

Issa S. Oweis

Seventh Advisor

Salah el- Haggar

Abstract

Prediction of landfill gas generation and settlements are of concerns in design and maintenance of biocell landfills. Accurate settlement prediction is essential for design of piping systems used for the delivery of re-circulated leachate and recovery of landfill gas. Landfill settlement is the result of change in overburden stresses and biodegradation of waste. Biodegradation-induced settlement results from the re-arrangement of waste skeleton in response to the decomposition of waste mass.

Current practice of landfill settlement modeling is predominantly empirical, thus most of the available techniques make no attempt to simulate the real mechanisms of waste settlement. Traditionally compressibility index is defined similar to that of clays, to explain the general settlement behavior of waste. Although a landfill is an interacting multiphase medium there is limited research to explain landfill gas generation and dissipation and moisture distribution as integral parts of the process of landfill settlements.

This dissertation describes a model which couples settlements of a biocell landfill with the generation and dissipation of landfill gases and distribution of moisture. The major mechanisms of waste settlement were identified as mechanical compression and biodegradation-induced strain. Mechanical compression was modeled with the help of laboratory simulations. To model the biodegradation-induced settlement, it was assumed that waste degradation obeys the first order reaction kinetics. The mass balance of the landfill gas was used to link settlement with gas pressure. The Richards equation was used to simulate the distribution of moisture.

A computer program was written to numerically predict the settlements, gas pressure and volumetric moisture content in a biocell landfill using landfill geometry and waste properties. In the absence of a complete set of data, settlement and gas pressure components of the model were validated using data from two different landfills.

The model was then used to predict the settlement behavior of The City of Calgary Biocell Landfill. The model predicted higher strain values, when moisture as well as gas pressure were incorporated in to the simulation. Therefore, it was concluded that modeling settlement without taking gas pressure and moisture in to account, could underestimate the total settlement. The model was capable of predicting landfill density, and the density values predicted for twenty five years matched with those reported in literature.

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