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FAULTS IN CLAY : THEIR DETECTION AND PROPERTIES

Objetivo

FAULTS OCCUR IN MOST MIXED SEDIMENTARY ENVIRONMENTS BUT THEIR EFFECTS ON REGIONAL GROUNDWATER FLOW PATTERNS ARE POORLY UNDERSTOOD. THE HYDROGEOLOGICAL SIGNIFICANCE OF FAULTED CLAY LAYERS IS OF PARTICULAR RELEVANCE WHERE MUDROCKS ARE POTENTIAL HOST FORMATIONS FOR RADIOACTIVE WASTE REPOSITORIES.

IN CORPORATION WITH ISMES OF ITALY TWO FAULTS THROUGH CLAY LAYERS WILL BE STUDIED (ONE SITE IN THE UK AND ONE IN ITALY). THE PROJECT HAS THREE OBJECTIVES :

1) TO DEVELOP SUITABLE GEOPHYSICAL TECHNIQUES TO DETECT WATER BEARING FAULTS IN CLAYS. THESE TECHNIQUES WILL AIM TO DIFFERENTIATE BETWEEN HYDRAUTICALLY ACTIVE FAULTS AND THOSE WHICH ARE EITHER SEALED OR NON-WATER BEARING.

2) TO MEASURE THE HYDROGEOLOGICAL PROPERTIES OF FAULTS IN SEQUENCES OF MUDROCKS AND AQUIFERS. THIS WILL BE ACHIEVED BY MEASURING THE HYDRAULIC AND CHEMICAL PROPERTIES OF THE FAULT DIRECTLY AND BY MEASURING THE EFFECT OF THE FAULT ON THE UNDERLYING AQUIFERS.

3) TO DEFINE SUITABLE TECHNIQUES FOR USE IN SITE INVESTIGATIONS AND METHODS FOR ASSIMILATING FAULTED BOUNDARIES INTO FLOW AND TRANSPORT MODELS IN CLAYS AND MIXED SEDIMENTS.
Measurements of groundwater head and hydraulic conductivity have been made around the Down Ampney Fault in an array of purpose drilled and completed piezometers and boreholes. The Down Ampney Fault consists of a northerly dipping (70 degree) normal fault, showing a northerly displacement of 50 m. The formations tested at Down Ampney are all of Jurassic age. The Great Oolite Group and Cornbrash aquifers are interbedded with a number of mudstones, silty mudstones and thin sandstones which comprise the aquitard sequence of Oxford Clay, Kellaways Beds, Forest Marble Clay and Fullers' Earth.

Artesian conditions were anticipated in the aquifers at Down Ampney due to its low relief in relation to the main areas of aquifer recharge. However, the measured groundwater heads in the aquifers were considerably below ground level and can only have been caused by groundwater abstraction from the Great Oolite Group aquifer in nearby wells.

The groundwater heads in the mudrocks are not in equilibrium with the aquifers and appear to show heads which are transient from the prepumping flow regime.

Analysis of pulse tests performed in the clays gave values of hydraulic conductivity ranging from 5e-12 to 2e-8 m/s, comparable with similar formations elsewhere. Although showing some overlay with the unfaulted clays the hydraulic conductivity in the fault zone appears to be enhanced with respect to the adjacent unfaulted clays by up to one or two orders of magnitude. Slug tests and constant rate abstraction tests were performed in the Great Oolite Group and Cornbrash aquifers. The results from some tests showed evidence for the no flow boundary effect of the fault.

Numerical modelling of the effects of groundwater abstraction from nearby wells on the site using the finite element code FEMWATER suggested that both changes in fault geometry and varying fault properties could be invoked to explain adequately the measured head profiles through the Down Ampney Fault. The modelling used the traditional assumptions of Darcian flow, although it is recognized that coupled flow processes may be of great significance in clay formations.
B1. DESK STUDY TO EVALUATE A NUMBER OF POTENTIAL UK STUDY SITE; SELECTION OF TWO PRELIMINARY SITES.
B2. INITIAL GEOLOGICAL AND GEOPHYSICAL INVESTIGATIONS OF THE PRELIMINARY UK SITES; SELECTION OF THE FINAL STUDY SITE.
B3. DEVELOPMENT OF GEOPHYSICAL TECHNIQUES FOR FAULT IDENTIFICATION.
B4. DETAILED GEOPHYSICAL SURVEY OF THE STUDY SITE.
B5. BOREHOLE DRILLING.
B6. WIRELINE GEOPHYSICAL LOGGING OF THE BOREHOLES.
B7. HYDROGEOLOGICAL TESTING OF THE BOREHOLES.
B8. SYNTHESIS OF THE RESULTS TO EVALUATE THE SIGNIFICANCE OF THE FAULT.

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Régimen de financiación

CSC - Cost-sharing contracts

Coordinador

Natural Environment Research Council
Aportación de la UE
Sin datos
Dirección

NG12 5GG Nottingham
Reino Unido

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Coste total
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Participantes (1)