a euphemism for problematic geo-engineers?

Transcription

1 Geoteknikdagen Problematic soils - a euphemism for problematic geo-engineers? Jørgen S. Steeenfelt COWI 1

2 Outline - introduction Problematic soils? - does the term make sense? -what are the clues? - global/local validity Is it a euphemism? - soul searching question for all Case histories - Rødsand 2 Offshore Wind Farm - Ruwais Sulphur Extension Closing remarks 2

3 Problematic soils? 3rd Int. Conf. Problematic Soils, Adelaide 2010 Peat and organic soils Volcanic soils Expansive soils Collapsible soils Carbonate rich soils Lateritic soils Tailings Deep fills Dispersive soils Unsaturated soils Paper aspects Identification of problem soils Test methods Engineering geological aspects Avoiding distress to infrastructure Case studies 3

4 How about closer to home? Danish Code of Practice - only indirect reference (GC 3) Tertiary clays Palaeogene clays Chalk with cavities Soils with incr. k with depth below GWT 4

5 How about closer to home? DS 415 (Geotechnical Category 3) Multi-storey basements Structures with large differential water pressures Temporary or permanent GW-lowering involving serious risk of damage to structures or soil strata Fills and pavements subjected to significant dynamic loading Machine foundations subject to significant dynamic exitation Structures with extreme cyclic loading Larger bridges and tunnels 5

6 Postulate: problematic = failure to recognize or understand Local, comparable experience Importance of adequate and timely ground investigations Difficulties in specifying, testing and interpreting lab. and field tests Confusion of measured, derived, characteristic & design values Geological set-up and stratification Application of proper theoretical framework Abuse of numerical tools (garbage in garbage out) 6

7 Euphemism? Problematic soils? No! Challenging soils Yes! Avnstrøm (DGF's 40th anniversary): Øresund soil conditions - straight forward; no work for geo-engineers! 7

8 Challenge is our middle namenot problematic 8

9 Case histories Rødsand 2 Offshore Wind Farm 9

10 Location of Rødsand 2 offshore wind farm 10

11 Project with 90 wind turbines (2.3 MW) Client: E.ON with Grontmij Carl Bro as consultant Contractor: Aarsleff-Bilfinger Berger JV with COWI as designer Certifying body: DNV 11

12 Ground conditions: predominantly glacial deposits, clay till/sand till Extent of site assessment data 183 CPTs tests - depth 2.9 ± 2.4 m; 86 outside footprint 37 vibrocores 80 geotechnical boreholes - majority to 7 m depth 200 UU tests - in lieu of field vane testing 19 triaxial CU tests supplementary soil investigations by ABJV - geotechnical boreholes with SPT 12

13 Foundation in Clay till/sand till business as usual? CPT tests very shallow Stop on R f (<0.8 MPa) -6 c u [kpa] Design Basis: c u = 200 (250) kpa below CPT/borehole data Need for confirmation Correlation of SPT - s u Drained strength parameters from CU triax -8 Level [m] Foundation level Row 2 13

14 Comparison of UU and CU Undrained shear strength (kpa) Depth below seabed (m)

15 Foundation in Clay till/sand till Triaxial CU-tests t (kpa) 500 Individual tests vary tr ' = 24.4 to 39.5 c' = 23 to 116 kpa Plot minimum and maximum failure points Agree on design values MIT stress s (kpa) Maximum failure Minimum failure Fi =33, c' =25

16 Foundation in Clay till/sand till business as usual? Steel, concrete, challenging soil = wind power in near future Foundations on chalk & Palaeogene deposits pending 16

17 Case histories Ruwais Sulphur extension Pile load testing 17

18 Ruwais Sulphur Expansion - Abu Dhabi Future structure Existing structure 18

19 Design Concrete Design Mooring Dolphins Precast shell with cast-in steel girders Shell positioned on top of the piles Filled with in-situ concrete -> Reduces the weight during construction 19

20 Pile Design Tubular steel piles - Ø1067 mm, 22 mm (Dolphins) - Ø 914 mm, 19 mm (Trestle & Pivot) Raking 1:4 Vertical capacity: Fleming et al. s = 0.41 sqrt(ucs/1 MPa) Horizontal capacity: p-y curves - sand acc. to API for superficial deposits - weak rock acc. to Reese (1977) 20

21 Quadrant beam and pivot foundation 21

22 Pile installation procedure Temporary casing driven to bedrock Coring rock socket Flush & clean Tremie concrete socket Place pile with spacers c d Fill annulus with sand Retract temp. casing Pour mass concrete 22

23 Pile installation and testing offshore Wirth drilling rig Loading platform Ø762 mm 23

24 Unconfined compressive strength of bedrock Soil parameters UCS (MPa) Level (m)

25 How do we trust the design assumptions? Comparable experience UCS [MPa] Bearing Capacity [MN] Depth [m] Design profile Depth below top of rock [m] Shaft adhesion Fleming Shaft adhesion Tomlinson Shaft adhesion AASHTO MN 25

26 Design application Dolphins FEM analysis Breasting Dolphins and Mooring Dolphins: FEM program Robot Three- dimensional space frame / plate model Governing forces in piles and capping beam Non-linear springs 26

27 Available borehole information N New facility N 27

28 Principle of load testing 28

29 Evaluation of load testing mob ult 4 mob 3 mob ult 5 4 Top rock socket Appendix A Pile head - elastic Appendix ult = 832 kpa ult = 3.5 mm (Design ult 615 kpa) Top of rock socket (mm) Tension load on trial pile (MN) 29

30 Evaluation of onshore load testing Mobilized shear stress (kpa) Depth below top of trial pile (m) 2L-14 2L-12 2L-8 1L-4 1L-2 30

31 Evaluation of onshore load testing 0.6 Creep displacement (mm) Extensometer 1 Pile head 0.33 mm/lct 0.07 mm/lct Time (minutes) 31

32 Evaluation of onshore load testing Ultimate displacement ult (mm) 8 dult derived Average stress in L max. pile displ. Design UCS char. UCS average Ultimate shear stress ult (kpa) Conclusion Pile capacity >> design Pile system works Offshore tests confirm this Tension pull out not an issue Results forms back-bone data 32

33 Evaluation of 762 mm offshore pile capacity (MN) Ø762 mm net capacity > 7.2 MN Char. shear stress Design Onshore test 615 kpa 832 kpa Cone Pile diameter uplift Socket diam no Pile diameter uplift Socket diam

34 Keep challenges at bay! Problems challenges engineering solutions Soil - structure - water - environment interaction Have fun with your projects 34