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LESSONS FROM THE FAILURE OF FULL-SCALE MODELS AND RECENT GEOSYNTHETIC-REINFORCED SOIL RETAINING WALLS

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LESSONS FROM THE FAILURE OF FULL-SCALE MODELS AND RECENT GEOSYNTHETIC-REINFORCED SOIL RETAINING WALLS

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    1. LESSONS FROM THE FAILURE OF FULL-SCALE MODELS AND RECENT GEOSYNTHETIC-REINFORCED SOIL RETAINING WALLS F. Tatsuoka; Department of Civil Engineering, University of Tokyo   M. Tateyama; Railway Technical Research Institute   Y. Tamura ; Integrated Geotechnology Institute Ltd & H. Yamauchi; Penta-Ocean Construction Co.

    2. GRS-RWs having a full-height rigid facing constructed by the staged construction procedure - now supporting railway and highway embankments for a length more than 35 km;

    3. ABSTRACT 1) Geosynthetic-reinforced soil RWs having a full-height rigid facing have been constructed; - for a total wall length of more than 35 km in Japan; and - as permanent important railway and highway soil retaining structures.

    4. 2) Staged construction; the wall is first constructed with a help of gabions filled with crushed gravel; and

    6. BACKGROUND ·History of elevated railway and highway structures in Japan;

    7. GRS-RWs with a full-height rigid facing

    8. (continued) 3) Full-height rigid facing makes GRS-RWs; stable and rigid enough even with relatively short reinforcement; advantageous when reconstructing an existing gentle slope to a vertical wall.

    9. A typical latest project at Shinjuku, Tokyo   - reconstruction of an old bridge and associated relocation of two railway tracks for the busiest and most important rapid transits in Japan, Chuo and Yamanote Lines.

    10. Life in Japan (Shinjuku)

    11. - One of the most critical and challenging case histories, started 1995 and completed in the beginning of 2000.

    12. Why was GRS-RW having a full-height rigid facing selected ?   1) high cost-effectiveness; 2) sufficiently stable and stiff walls to support extremely important railways;

    13. Yodobashi site, Shinjuku, Tokyo

    14. RESEARCH TO DEVELOP THE GRS-RW SYSTEM - started early 1980's by; · small-scale static loading and shaking table tests in the laboratory; · numerical analysis by LEM & FEM; and

    15. FIELD TEST PROGRAM - Three soil types for the backfill; 1) On-site nearly saturated volcanic ash clay (Kanto loam); ·Chiba No. 1; · Chiba No. 2; · Kami-Onda; · Chiba No. 3; and · JR No. 2. 2) On-site nearly saturated highly weathered tuff ;- Nagano; 3) Sand- JR No. 1.

    16. - to examine whether vertical stable reinforced clay walls can be constructed;

    17. Large deformation already during construction and also for a long period after construction, particularly by heavy rainfalls.

    18. a) Rain water percolated through the backfill and accumulated in the bottom soil layer, a reduction in the soil suction and further an increase in the positive pore water pressure, making the soil very weak;

    19. Lessons from the failure-1:   1) It is very important for the wall stability to prevent a local soil failure in the soil immediately behind the wall face.

    20. Lessons from the failure-2:   3) A vertical spacing of 80 cm between geotextile layers is too large; i)   to effectively drain water from each clay layer and to maintain a high suction in the soil layer*: and ii) to effectively confine the clay backfill, in particular immediately behind the wrapped-around wall face.

    21. · to confirm the lessons from the behaviour of Chiba No. 1; and · to investigate the effects of gabions at the wall face on the wall stability.

    22. - To bring the walls to failure, 70 m3 of water (= 900 mm precipitation) was supplied from a pond on the crest for eight days in October 1985.

    23. Lessons from the failure: 1) Although gabions were filled with the clay backfill (i.e., Kanto loam), their use at the shoulder of each soil layer was very effective for; a good compaction of backfill; and confining the soil near the wall face, maintaining a high soil strength.

    24. 2) Major cause for the wall deformation by the artificial rainfall test; - the decrease in the suction; and - the increase in the positive pore water pressure.  

    25. 3) Three failure modes:

    26. - R & L: total deformation by the rainfall test. - Ra; deformation only in the last day of the rainfall test. - K; similar data for Kami-Onda embankment.

    27. -Three failure modes;

    28. 4)  Despite the use of so-called very extensible reinforcement (i.e., a non-woven geotextile), no failure plane and tension cracks in the reinforced zones, as with Chiba No. 1 test embankment.

    29. 5) Practically no creep deformation of the walls after the rainfall test in the second year (1985) : - due to effects of rainfall as preloading.

    30. 1) constructed in 1986 using the same types of clay backfill soil and non-woven geotextile as before.

    31. 2) to confirm the lessons from the previous three tests; by comparing the behaviours of; a) a wrapped-around wall (without gabions); b) a discrete concrete panel wall; and c) a wrapped-around wall (with gabions) covered with a 8 cm-thick shotcrete layer.

    32. Lessons from the failure:   Different behaviours of the three walls according to very different facing rigidities.

    33. 2) Facing of relatively small discrete panels; not rigid enough; and very difficult to compact soil immediately behind the facing and to achieve a good wall face alignment.

    34. 3) Wall constructed by staged construction; - behaved well; and - much better construction efficiency than with discrete panel facing.

    35. -constructed at Japan Railway (JR) Technical Research Institute in the beginning of 1988.

    36. Clay backfill (Kanto loam); wi= 120 - 130 %; Sr= 90 %; and ?d= 0.55 - 0.60 g/cm3. Three types of reinforcement; section a): a non-woven geotextile, as used for the other embankments; section b): a grid sandwiched between two gravel drainage layers; and section c): a composite consisting of non-woven/woven geotextiles.

    37. - A very good and similar performance of the three sections for a long duration, reconfirming that“the facing type could be much more important than the stiffness of reinforcement for the stability of reinforced soil retaining wall”.

    38. Nearly saturated highly weathered tuff – Nagano wall  -constructed in 1994 to reconfirm the function of full-height rigid facing; ·in conjunction of the construction of proto-type GRS-RWs for 1993 - 1994.

    39. Nearly saturated highly weathered tuff – Nagano wall a) a complete wall height of 2 m for a length of 2 km, supporting a yard for Shinkansen (bullet train); b) the first actual clay wall using a nearly saturated soft clay as a railway structure in Japan,; c) constructed on a thick very soft clay deposit; d) a large ground settlement of about 1 m by preloading before casting-in-place a rigid facing; and e) no pile foundation.

    40. JR No. 1 test embankment with sand backfill

    41. JR No. 1 test embankment with sand backfill

    42. Two types of facing; ·segment h; a discrete panel facing fixed to gabions filled with gravel.

    43. ·Long-term behaviours for about two years:   a) Segment h, having a discrete panel facing; - much larger deformation than that the other walls.

    44. Loading test of No. 1 to failure after very stable behaviour for about two years:

    45. FAILURE AND LESSONS - JR No. 1 embankment:  

    46. 2) f (L= 1.5m) vs. d (L= 2.0 m).  

    47. 3) Failure at the construction joint in the unreinforced facing controlled the yielding of the test wall segments f and d (No. 1 embankment).  

    48. 4) Two-wedge failure mode in wall segment h

    49. Loading test to failure of JR No. 2 clay wall after very stable behaviour for about two years:

    51. 2) When properly reinforced, a clay wall is not very weak, compararable with a sand wall !

    52. 3) Gabions as a buffer for the relative settlement between the rigid facing and the backfill soil;

    53. 4) Any clear failure plane in the reinforced zone !

    56. CONCLUDING REMARK-2   The construction of sufficiently stable and rigid clay walls as permanent important structures is quite feasible by reinforcing the backfill with a proper composite geotextile having a sufficiently high drainage function and a tensile rigidity and by using a full-height rigid facing.  

    57. CONCLUDING REMARK-2   The construction of sufficiently stable and rigid clay walls as permanent important structures is quite feasible by reinforcing the backfill with a proper composite geotextile having a sufficiently high drainage function and a tensile rigidity and by using a full-height rigid facing.  

    58. CONCLUDING REMARK-3   A number of prototype GRS-RWs have been constructed for a total wall length being about 35 km by the staged procedure for the last decade in Japan.  

    59. Thank you very much for your kind attentions !

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