Geosynthetic reinforcement of high-alkaline soils: Basics and two typical projects

Transcription

1 Geosynthetic reinforcement of high-alkaline soils: Basics and two typical projects Dr Dimiter Alexiew Huesker Synthetic GmbH, Gescher, Germany Graham J. Horgan Huesker Limited, Warrington, UK 1

2 Two main possibilities to improve the mechanical behaviour of cohesive soils to use them as fill material for construction purposes: chemical improvement or stabilization by cement / lime mechanical stabilization using appropriate reinforcing geosynthetics 2

3 Combining these techniques: synergetic effect by reinforcing lime/cement stabilized soils by appropriate geosynthetic reinforcement Compatibility, interaction (bond)??? 3

4 Geosynthetic reinforcement The geotechnical engineer s ideal geosynthetic reinforcement: High tensile modulus (tensile stiffness) short- and long-term Low propensity to creep (high long-term strength and minimum creep strain) High bond coefficient with the soil in both shear- and pull-out modes (short anchorage lengths, good interaction between reinforcement and soil) 4

5 Geosynthetic reinforcement Very high permeability (lowest hydraulic resistance and as a result, no increasing water pressure problems) Low damage during installation and soil compaction High chemical and biological resistance in all conceivable environments Low costs 5

6 Geosynthetic reinforcement Unfortunately, the ideal reinforcement does not exist yet. Nevertheless, geotechnical engineers have today the fortunate possibility to choose an optimal reinforcement always and for any case due to the wide range of materials available: wovens, geocomposites and geogrids made of different polymers. Optimum for stabilized soils PVA. 6

7 Geosynthetic reinforcement Short-term behavior depending on polymer & product PVA 7

8 Geosynthetic reinforcement Long-term behavior Creep (Isochrones) of FORTRAC M (PVA) 8

9 Geosynthetic reinforcement Chemical Resistance of PVA In Cement Slurry at 80 C 9

10 Geosynthetic reinforcement Interaction with soil 10

11 Geosynthetic reinforcement Interaction with soil It is important to gain knowledge of the interaction in the shear- and pull-out mode between soil and reinforcement. There is a lot of experience in this regard for non-cohesive fills, some experience with cohesive fills, but unfortunately there is a lack of experience with lime/cement-stabilized cohesive soils. 11

12 12 Coefficient of interaction shear strength geogrid / soil CI = shear strength soil / soil CI shear mode CI pull-out mode CI >= 1.0 no negative interface effects, perfect bond

13 Vertical fixity or load cell for support force Air pressure Soil Roller bearing Clamp unit A A H P H A Load plate Upper shear frame (floating/fixed) Clamp unit B Gear K, T s 200 mm K 500 mm Lower shear frame (on precision linear guidance) P P P P shear test friction test pullout test overlapping test tension test 13

14 Typical plastic clay + 6% cement or lime 14

15 Geogrid FORTRAC 750 M from PVA 15

16 Coefficient of interaction (CI) in shear mode FORTRAC M (PVA) / clay + lime FORTRAC M (PVA) / clay + cement FORTRAC M (PVA) / clay Shear stress [kn/m2] clay / clay Normal pressure [kn/m2] 16

17 Coefficient of interaction (CI) in shear mode FORTRAC M (PVA) / clay FORTRAC M (PVA) / clay + lime FORTRAC M (PVA) / clay + cement Coefficient of interaction [-] Normal pressure [kn/m2] 17

18 Coefficient of interaction (CI) in shear mode CI always > 1.0 de facto no interface, perfect bond Even better than in non-stabilized clay Nearly stress-independent Similar results in the pull-out mode! 18

19 19 Two projects with FORTRAC M (PVA) geogrids and cement-stabilized soils

20 Retaining Wall Unterkaka, Germany,

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24 Büchen, German Rail (DB), 2003 Extremely flat geogrid-reinforced embankment incl. cemented soil on mixed-in-place cemented columns Upgrade of Rail Link Hamburg-Berlin 24

25 Typical cross section 25

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27 13 APR 2005 Dr. D. Alexiew Eng. Dept. 27

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29 That s it, more or less... Questions are welcomed! Thank you! 29