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Fracturation of the Boom Clay around the underground Excavation(s)

Fracturation of the Boom Clay around the underground Excavation(s). J.Mertens. Overview. Test Drift Excavation of 2nd shaft, starting room Two cored borings Excavation of the assembly room Discussion. I. Test Drift (end 80’s). Discontinuities with slickensides / mirror surface

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Fracturation of the Boom Clay around the underground Excavation(s)

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  1. Fracturation of the Boom Clay around the underground Excavation(s) J.Mertens

  2. Overview • Test Drift • Excavation of 2nd shaft, starting room • Two cored borings • Excavation of the assembly room • Discussion

  3. I. Test Drift (end 80’s) • Discontinuities with slickensides / mirror surface • Orientation on an average perpendicular to the tunnel axis orientation. • Low angles (30°-40°)

  4. II. Excavation of the second shaft and starting rooms Excavation of second shaft

  5. Enlargement of the foundation

  6. Southern starting room

  7. 35° N 35° S Large fall out in southern starting room

  8. III. Two cored borings

  9. Boring 2000-11 (vertical section)

  10. Boring 2000-11 • Natural discontinuities • Formed as a result of the excavation of the second shaft • Formed as a result of the drilling process itself

  11. Boring 2001-2

  12. Boring 2001-2 (vertical section)

  13. Conclusions • Significant increase with depth of the fracturation is entirely the result of the drilling process and the present stress field. • The direction of two fractures correspond more or less to a circular pattern. (dip: S) • No indication for the presence of natural discontinuities was encountered in the cores. Expectations: Presence of Slickensided, planes, mostly dipping S, in first 6m. The planes would lie more or less circular around the 2nd shaft

  14. IV. Excavation of the assembly room Sesam.....

  15. Encountered discontinuities • Large slickensided planes (> 5 m²) • Small slickensided planes (< 1 m²) • Large tension fractures (> 1 m²) • Small tension fractures (< 1 m²)

  16. Large slickensided planes

  17. Large slickensided planes • Large planes ( > 5 m²) containing slickensides • Opened till 2 cm • Orientation EW on an average • Inclination 35° on an average. Steeper when deeper • Spacing 70 cm on an average • Oxidation present on several planes • Specific curved cross section on front • Splitting, ondulating • Observations more or less fit in model

  18. Small slickensided planes

  19. Small slickensided planes • Small planes ( < 1m²) containing slickensides • Some pieces of clay have slickensides all around them • A lot of times, clear relation between movement of blocks • More frequent in upper part of front

  20. Large tension fractures 2 m depth !

  21. Large tension fractures • Clear plumose structures • Seem to start at different nuclei • Irregular plane • Orientation EW on an average • Low dip angles (20°-30°) towards N • Create crossing planes together with large slickensided surfaces. Responsible for fall outs • More frequent in upper 3th part of excavation • Likely the result of decompression and gravitation. • Opened till 2 cm

  22. Small tension fractures

  23. Small tension fractures • Clear plumose structures • Complete morphology of “hard rock” • Only in upper part of excavation • On an average: // with initial front. • Opened till 2 cm • Similar to dryer parts of clay pits • Likely the result of dessication of the clay

  24. Overview Fracturation in the form of: • Large slickensided planes, dipping south. Whole front. Origin: prob. excavation 2nd shaft. Decompression. • Small slickensided surfaces, mostly upper part front. Origin: prob. excavation starting room and assembly room. Decompression, gravity, movement. • Large tension joints,dipping north, in upper 3th of front. Origin: prob. excavation starting room. Decompression, gravity, tension (dessication). • Small tension joints, upper part excavation. Origin: prob. time after excavation starting room. Dessication (decompression).

  25. Conclusions / Expectations / Discussion. Fracturation can be / is major problem. • Shear fractures: in TD, boreholes, 2nd shaft.  Large slip planes expected to form in connecting gallery. Unavoidable ? SELFRAC ! • Keeping decompression / convergence to minimum might prevent intensive fracturation • Avoiding loss of porefluid due to dessication is of utmost importance ! Sealing ? Long Term ?

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