Column-Supported Embankments: Past, Present, and Future

Transcription

1 Column-Supported Embankments: Past, Present, and Future Jie Han, Ph.D., PE, ASCE Fellow Professor The University of Kansas Column-Supported Embankments Also referred to as Pile-Supported Embankments Piled Embankments 1

2 Differential Settlement Approach Slab 2

3 Bump over Piled Culvert Courtesy of Gue, S.S. Conventional Pile-Supported Embankments s Embankment Large size Pile caps s 0 Inclined piles Firm soil or bedrock Vertical piles 3

4 Geosynthetic-Reinforced Column -Supported (GRCS) Embankments Geosynthetic-reinforced fill platform (also Load Transfer Platform) s 0 Embankment Geosynthetics Small size Pile caps s 0 columns Firm soil or bedrock Masada in Israel 4

5 Masada in Israel According to Josephus, the Siege of Masada by troops of the Roman Empire towards the end of the First Jewish Roman War ended in the mass suicide of 960 people. (Wikipedia) Masada Bathhouse in Israel Built between 37 and 31 BCE (before the Common Era) 5

6 Construction of Road over Peat in Holland 23 December 1935: How to construct a road in the Krimpenerwaard? Based on 6 CPTs, Keverling Buisman thinks: using a fascine mattress Keverling Buisman ( ) Courtesy of Suzanne van Eekelen Fascine Mattress fascine mattress: 80 cm thick reed Courtesy of Suzanne van Eekelen 6

7 Second Thought 1937: second thought after taking more CPTs: road on piles. Keverling Buisman wrote: to prevent complaints and unfavourable comments piled road would meet more appreciation Keverling Buisman ( ) Courtesy of Suzanne van Eekelen Pile-supported Embankment Concrete Sand Timber piles (upside down to get sufficient bearing capacity) Courtesy of Suzanne van Eekelen 7

8 Design Guideline for Piled Embankments Height of Coverage by pile caps (%) Embankment (m) Crushed stone fill Gravel fill 1.5 to to 70 > to to to to to to to to to to 4.0 >30 >40 Rathmayer, H. (1975), Piled embankment supported by single pile caps, Istanbul Conference on Soil Mechanics and Foundation, Istanbul, Turkey, Bridge Approach Support Piling Bridge Fill Geosynthetic Concrete Pile Structural Embankment Support Piles Soft Alluvium Transitional Embankment Support Piles First documented column-supported embankment with geosynthetic reinforcement Reid and Buchanan (1984) 8

9 Westway Terminal The first application of column-supported embankments (CSE) with geosynthetic reinforcement in the United States was in 1994 for the Westway Terminal in Philadelphia, PA. Courtesy of James Collin Westway Terminal Courtesy of James Collin 9

10 Westway Terminal Courtesy of James Collin Westway Terminal Courtesy of James Collin 10

11 Hewlett, W.J. and Randolph, M.F. (1988). Analysis of piled embankments. Ground Engineering. 21(3): British Standards Institution BS8006 (1995). Code of Practice for Strengthened/Reinforced Soils and Other Fills. London, U.K. 11

12 SCI Web of Science citations: 70, Google citations: 257 (by June 17, 2013) SCI Web of Science citations: 91, Google citations: 322 (by March 20, 2014) SCI Web of Science citations: 131, Google citations: 411 (by December 7, 2015) SCI Web of Science citations 146, Google citations: 428 (by April 20, 2016) s, H, J, E p, E s investigated Applications Bridge Fill Geosynthetic Geosynthetic New Existing Concrete Pile Structural Embankment Support Piles Reid and Buchanan (1984) Soft Alluvium Transitional Embankment Support Piles VCC Column Han & Akins (2002) Centerline Pavement Ringwall footing Storage tank Geosynthetics Subgrade Geosynthetic Vibro concrete column Soft organic silt & peat Soil-cement column Soft alluvium Medium dense sand and gravel layer Tsukada et al. (1993) ASCE G-I (1997) 12

13 Piles and Caps Geosynthetics Courtesy of Chris Dumas Design of Geosynthetic-reinforced Column-Supported Embankments Load Transfer Platform Design Fill Fill Geosynthetic Columns Columns Single-layer reinforcement Multi-layer reinforcement Column Foundation Design 13

14 Soil Arching, Stress Concentration and Tensioned Membrane Effect W H H cr p b T c Soil arching ratio s = p b /( H) Critical height H cr Modified from Han (1998) Contributions of Geosynthetics Tensile resistance: - Reduce lateral thrust on columns Tensile anchorage: - Stabilize slope Column Tensioned membrane effect: - Reduce differential settlement - Transfer load onto columns - Stabilize soil arch Stiffened platform or plate effect: - Include all the above contributions Reversely tensioned membrane effect: - Prevent soil yielding above columns Column 14

15 Development of Soil Arching H cr H cr No deformation Small deformation Large deformation Low Embankment H cr H cr No deformation Small deformation Large deformation High Embankment H cr and vs. Displacement H cr /H 1.0 Low embankment Partially-mobilized soil arch H cr /H High embankment Fully-mobilized soil arch Displacement 15

16 Courtesy of Huesker Courtesy of Huesker 16

17 Critical Height Vertical stress Equal settlement plane Equal stress H cr Equal stress s-a s Depth Soil Cap Settlement H cr Equal settlement H cr Depth Cap Soil Critical Height H cr Chen et al. (2016) 17

18 Critical Height Hewlett and Randolph (1988): H cr > 1.0 (s a) BS8006: H cr > 1.4 (s a) H cr Lab settlement data (Chen et al., 2007): H cr > (1.4 to 1.6) (s a) Equal settlement plane Equal stress s-a s Field earth pressure data (Chen et al., 2010): H cr > (1.1 to 1.5) (s a) Lab settlement earth pressure data (Xu et al., 2016): H cr > (1.1 to 1.5) (s a) Failure case: H 0.7(s-a) (Camp and Siegel, 2006) Possible Problems Courtesy of Gue, S.S. 18

19 Lab Study Filz et al. (2012) 19

20 Field Study Sloan (2011) Sloan (2011) 20

21 Sloan (2011) Critical Height H/d s /d 21

22 Modeling of Soil Arching q q p c p b K T z H p c p b a s BS8006 (1995) Adopted Terzaghi Hewlett and Randolph (1988) Soil arching ratio, = p b /( H+q) Modeling of Soil Arching =60 o 2-D 3-D Carlsson (1987) Miki (1997) 22

23 Multiple Soil Arching Model Zaeske and Kempfert (2002) Unit Cell Model h L z h e Top of embankment Equal settlement plane Outer column Cap Pile Inner column Soft soil Ground surface S o +W s (0)=S i +W p (0) W s (0) W p (0) S e Top of embankment Equal settlement plane P i F d z F P i +ΔP i Pile Soft soil Ground surface after settlement D p W p (L) W s (L) D i D o Chen et al. (2008) 23

24 Comparison of Soil Arching Ratio a/s BS8006 H/s Adopted Terzaghi (K T = 1) Adopted Terzaghi (K T = 0.5) Kempfert et al. Hewlett & Randolph Carlsson Modified from Filz & Smith (2005) Measured vs. Calculated Vertical Stresses Sloan (2011) 24

25 Comparison of Load Share Ratio Load share ratio = pile load/total load Chen et al. (2008) Concentric Soil Arching Model Van Eekelen (2015) 25

26 Concentric Soil Arching Model for Vertical Stress Distribution van Eekelen (2015) Vertical Stress Distribution Pile Han and Gabr (2002) 26

27 Stress Distribution Model Triangular Uniform Inverse triangular Findings: Inverse triangular stress distribution above geosynthetic reinforcement resulted in the shape of deformed geosynthetic matching the measured van Eekelen et al. (2012) Tensioned Membrane Theory T L = s -a T p b T AcpbL A c = relative coverage area of reinforcement (A c = 1 for 2D) BS8006 (1995, 2010) 27

28 3D Relative Coverage Area T a L = s-a s A c s a 1 2a Rogbeck et al. (1998) Tension in Single Reinforcement Columns 28

29 Tension in Single Geosynthetic Han and Gabr (2002) Liu et al. (2007) 2000 Chen at al. (2016) Strain ( ) Bhandari (2010) Displacement of trap door (mm); N = Numerical, E = Experimental 2; N 6; N 10; N 14; N 5; E 10; E 15; E Distance (mm) Tension in Multiple Reinforcements X-tension in the upper layer X-tension in the lower layer Huang et al. (2005) 29

30 Tension in Multi-layer System Tension in geogrid (kn/m) lower layer 30 upper layer Distance from the toe (m) c b d a Pile cap Geosynthetic layers Huang et al. (2005) Borgesn & Gonçalves (2016) confirmed this phenomenon. Effect of Foundation Soil Resistance Tension in reinforcement (kn/m) No contribution from foundation soil Partial support from foundation soil (soft clay) Height of embankment (m) Center to center spacing of columns, s(m) Jones et al. (1990) 30

31 Calculated vs. Measured Strains in Geosynthetic Reinforcement Concentric soil arching model + More foundation soil resistance Less foundation soil resistance van Eekelen et al. (2015) Design Method Mainly based on the research done by Smith (2005) and Filz and Smith (2006, 2007) Adapted Terzaghi Method to estimate vertical stress on top of geosynthetic Force equilibrium and deformation compatibility above and below geosynthetic 1D compression of pile and soil Spreadsheet GeoBridge required for calculations 31

32 DEM Modeling of Dynamic Behavior Embankment m Optional geogrid Pile cap m 0.3 m 0.9 m 0.3 m Numerical model of a GRCS embankment Total number of particles = 11,793 Bhandari and Han (2010) Cyclic Loading Stress concentration ratio Cycle Unreinforced Reinforced Tension (kn/m) No. of cycle Distance (m) Bhandari and Han (2010) 32

33 Cyclic Loading increase Chen et al. (2016) Future Research Settlement calculation Under dynamic loading (traffic & earthquake) Multiple geosynthetic reinforcement layers Different column type and stiffness effects Floating columns Column pattern Down drag force effect Stability analysis 33

34 Future Research Concluding Remarks There is a long history for the concept of columnsupported embankments. Column-supported embankments have been increasingly used and researched. Significant progresses have been made in soil arching theory, vertical stress distribution, and tensile strain distribution in geosynthetic reinforcement. Reliability of design methods has been improved. Critical height is an important parameter for design and field performance. Further research is still needed. 34

35 Thank You! 35