1. CT5806701 Reinforced Earth Structures
Problems and Controversial Issues
of MSE Structure Design
National Taiwan University of Science and Technology
Department of Construction Engineering
Professor Kuo-Hsin Yang

2. Problems in Earth Pressure Method
It is theoretically-based thus limited to relatively simple geometric structures. It is difficult to extrapolate to complex geometries, such as narrow walls and multi-tiered walls.
Limited to uniform granular soil; difficult to extrapolate to non-ideal reinforced fill soils.
Do not consider pore water pressure or seepage forces in the reinforced fill by assuming adequate drainage.
Does not determine global stability.
Downdrag at connections is not evaluated.
Unable to evaluate deformation of the wall structure or soil.

3. Problems in Earth Pressure Method
Problems of Assumptions in Earth Pressure Method
Accuracy of Earth Pressure Method to predict Tmax
Mobilization of soil shear strength along failure surface
Distribution of reinforcement forces with depth

4. Problems in Earth Pressure Method
Accuracy of Earth Pressure Method to Predict Tmax
Allen et al. (2003) investigated quantitatively the accuracy of reinforcement loads predicted by earth pressure theory using careful interpretation of a database of 30 well-monitored full-scale walls.
Allen, T.M.; Bathurst, Richard J.; Holtz, Robert D.; Walters, D.; Lee, Wei F. (2003) “A New Working Stress Method for Prediction of Reinforcement Loads in Geosynthetic Walls”, Canadian Geotechnical Journal, v 40, n 5, p

5. Problems in Earth Pressure Method
Accuracy of Earth Pressure Method to Predict Tmax
Tmax predicted by
Earth Pressure Method overestimates (2~3 times)
the measured Tmax
Comparison between the reinforcement loads in different GRS walls and those predicted using earth pressure theory (Allen et al. 2003)

6. Problems in Earth Pressure Method
Accuracy of Earth Pressure Method to Predict Tmax
Source of conservatism in current design methods

7. Problems in Earth Pressure Method
Mobilization of soil shear strength along failure surface
Current methods of analyzing the internal stability of GRS structures using Earth Pressure Method assume that the soil shear strength along the failure surface mobilizes equally and reaches peak shear strength simultaneously.
Soil reaches its peak shear strength (active failure)

8. Problems in Earth Pressure Method
Mobilization of soil shear strength along failure surface
Peak
Softening x
Residual x
Hardening
Constant Volume
Dilatancy
Compression
Soil Stress-Strain-Volume Relationships 8

9. Problems in Earth Pressure Method
Mobilization of soil shear strength along failure surface

10. Problems in Earth Pressure Method
Mobilization of soil shear strength along failure surface

11. Problems in Earth Pressure Method
Mobilization of soil shear strength along failure surface
The failure surface corresponds to the
maximum tensile loads at each reinforcement layer.

12. Mobilization of soil stress was non-uniform along the failure surface
Stress evolution along failure surface: (a) 20g (FS ≈1.35); (b) 30g (FS ≈1.2); (c) 32g; (d) 35g; (e) 40g (FS ≈1.1)

13. Problems in Earth Pressure Method
Distribution of reinforcement forces with depth
For Wall
Critical Points?
Depth
Maximum Reinforcement Tensile load, Tmax
EP method assumes Tmax linearly increases with sv’
Many measured data show Tmax is uniformly distributed (Trapezoid)

14. Problems in Earth Pressure Method
Distribution of reinforcement forces with depth
For Slope
Depth
Maximum Reinforcement Tensile load, Tmax
Linear increase of Tmax is used to develop design chart

15. Problems in Earth Pressure Method
Distribution of reinforcement forces with depth
Uniform under working stress conditions
Triangular under large soil strain conditions
Reinforcement Breakage initiates
Little Tmax is mobilized if foundation is good

16. Problems in Earth Pressure Method
Distribution of reinforcement forces with depth
Location of maximum Tmax associated to the maximum sv’

17. Controversial Issues in Design
Use of fpeak or fresidual in design
Use fpeak to utilize the peak shear strength for design
Use fresidual because GRS structures is a flexible system so can reach a deformation larger than soil failure strain.
Discontinuous design methods for walls and slopes
Use Earth Pressure Method for wall
Use Limit Equilibrium Method for slope
Unified approaches for wall and slope

18. Controversial Issues in Design
Use of fpeak or fresidual in design
Zornberg, J.G. (2002) “Peak versus Residual Shear Strength in Geosynthetic-Reinforced Soil Design.” Geosynthetics International, Vol. 9, No. 4, pp

19. Controversial Issues in Design
Use of fpeak or fresidual in design
Difference in fpeak
Same in fresidual
Stress-strain curves of
Monterey No. 30 sand
at different densities
and tested in triaxial compression

20. Controversial Issues in Design
Use of fpeak or fresidual in design
Conduct two centrifuge tests with different backfill densities
(different fpeak but same fresidual)

21. No Controversial Issues in Design Do physical (centrifuge) models
Use of fpeak or fresidual in design
Do physical (centrifuge) models
with the same residual shear strength
but different peak shear strength
fail at the same loading (g-level) No

22. Controversial Issues in Design
Use of fpeak or fresidual in design
Model with denser backfill density fail late
Peak shear strength governs the system stability of MSE structures.

23. Controversial Issues in Design
Use of fpeak or fresidual in design
Failure Surface for high density backfill
Failure Surface for high density backfill

24. Soil Shear Strength Limit Equilibrium
Controversial Issues in Design
Discontinuous design methods for walls and slopes
Wall (b>70o)
Slope (b<70o) f
Soil Shear Strength Limit Equilibrium
Earth Pressure Theory
Unified approaches for wall and slope ?

25. Controversial Issues in Design
Discontinuous design methods for walls and slopes
A current work is to examine the design of MSE walls using limit equilibrium methods
by comparing the limit equilibrium predictions
and experimental results
-loading at failure
-location of the failure surfaces