ABSTRACT:

Subsidence due to groundwater withdrawal in the Latrobe Valley, Australia, has been simulated using a one-dimensional finite difference model of nonlinear consolidation. The model uses transient changes of fluid pressure within the more permeable zones of the aquifer system(s) as input and calculates the resulting time-delayed compression and/or expansion of both the permeable sands and the more compressible but more slowly draining lenses (aquitards) and separators (semi-confining beds) as output. The standard output of a groundwater model (namely, pressure changes within the permeable zones) serves as input for the subsidence (consolidation) model at any site of interest. Examples of the predictive technique for the Latrobe Valley are presented.

RESUME:

On a simule, en employant un modèle monodimensionnel de differences finies de consolidation non lineaire, un effondrement dû au retrait de l'eau souterraine dans la vallee Latrobe en Australie. Le modèle emploie en tant que donnees d'entree les changements transitoires de pression hydraulique à l'interieur des zones plus permeables du système d'aquifère et calcule comme sortie la compression et/ou la dilatation retardee(s) des sables permeables ainsi que celIe des inclusions plus compressibles mais qui s'assèchent plus lentement (aquitards) et des separateuses (couches semi-impermeables) qui en resultent. Les donnees de sortie standard d'un modèle de l'eau souterraine (c.-a-d. les changements de pression à l'interieur des zones permeables) servent de donnees d'entree pour le modèle d'effondrement (de consolidation) à toute localite consideree. On presente des examples de la technique de prediction appliquee à la vallee Latrobe.

ZUSAMMENFASSUNG:

Durch Grundwasserzurueckziehung verursachte Senkung im Latrobetal (Australien) ist unter Verwendung eines eindimensionellen Differenzenmodells nichtlinearer Verdichtung nachgeahmt worden. Beim Modell werden vorubergehende Veranderungen des Fluessigkeitsdruckes innerhalb der durchlassigeren Zonen des Grundwasserleitersystems als Eingabedaten verwendet und die sich daraus ergebende, verzogerte Verdichtung bzw. Ausdehnung sowohl der durchlassigen Sande als der leichter zusammendrueckbaren, aber sich langsamer entwassernden Einschluesse (Aquitarden) sowie Trennschichten (palburdurchlassiger Schichten) als Ausgabe berechnet. Die Normalausgabedaten eines Grundwassermodells (d.h.Druckveranderungen innerhalb der durchlassigen Zonen) dienen als Eingabedaten des Senkungs-(Verdichtungs)-Modells bei jeglichem in Frage kommenden Ort. Beispiele der im Latrobetal verwendeten Vorausberechnungstechnik werden vorgestellt.

1 INTRODUCTION

Open pit coal mining in the Latrobe Valley, Australia, by the State Electricity Commission of Victoria (SECV), requires depressurizing two regional confined aquifer systems in order to minimize potential heave, fracturing and flooding of the floor of the mine workings. Depressurizing the aquifer systems has in turn caused regional subsidence. In order to predict this subsidence, the mechanism of groundwater flow has been coupled through computer simulation to the mechanics of rock mass deformation. The most compressible units in the Latrobe Valley are coal and clay which not only form lenticular interbeds within each aquifer system but also serve as regional semi-confining beds that separate one heterogeneous aquifer system from the other. These compressible units have low permeabilities and hence at any site of interest there occurs a time lag between the imposed stress due to observed drawdown and the resulting compression and subsidence. The input to the subsidence model is the fluctuating drawdown within the aquifers and the output is the partly nonrecoverable compressional response of the heterogeneous porous material. This paper presents the strategy used to model land subsidence due to groundwater withdrawal in the Latrobe Valley as well as some examples of the prediction effort. The subsidence model COMPAC (a one-dimensional finite difference code) is calibrated at specific sites using both measured changes in hydraulic head within the aquifers and the resulting observed subsidence as functions of time. An independent calibration of a regional groundwater model of the Latrobe Valley was carried out concurrently (Evans, 1983, 1986). The groundwater model is used to predict future drawdowns in response to anticipated scenarios for pumpage. These predicted drawdowns are then used to estimate future subsidence at key locations as a function of time. Subsidence contours can then be drawn between these key locations for any future time of interest. One output of the subsidence model is an estimate of residual subsidence which results from the fact that in the field the compressing beds almost never reach an equilibrium stress distribution. Time constants are in terms of decades to centuries. The model COMPAC essentually uses nonlinear consolidation theory. It distinguishes between recoverable and non-recoverable compressibilities (Helm, 1975). Strain hardening of each compressibility can be introduced as well as porosity-dependent permeability values (Helm, 1976).

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