1. CT5806701 Reinforced Earth Structures
Design of Reinforced Soil Slopes
National Taiwan University of Science and Technology
Department of Construction Engineering
Professor Kuo-Hsin Yang

2. Design Step of Reinforced Soil Slopes
Site investigation
Lab tests
Establish project requirements
Determine engineering properties of in-situ soils
Determine engineering properties of reinforced fill
Establish reinforcement design parameters
Check unreinforced slope stability
Preliminary reinforced soil slope design
(design reinforcement layout and length by design charts)
Check reinforced slope internal stability
Check external stability
Check seismic stability
Detail design (auxiliary components)
Prepare plans and specifications
Stability analysis
Detail design

3. Limit Equilibrium Analysis of RSS
Advantages
Many experiences by practitioners
Simple input data and easy to use
Able to deal with pore water pressure
and complex geometry
Useful (but limited) design information
Modeling of reinforcement is capable

4. Limit Equilibrium Analysis of RSS
Disadvantages
Unable to deal with displacements
FEM Compatibility (Stress-Strain Relation)
Limited representation of interaction between dissimilar materials
Selection of material properties and safety factors based on state of practice insures acceptable performance

5. Design Objective For the given geometry, surcharge soil properties
desired minimum factors of safety,
find the required strength and layout of reinforcement to satisfy required stabilities (FSs).

6. Failure Modes Internal: cut thru all reinforcement layers Compound
External: Not cut thru any reinforcement layer

7. External Failure Modes

8. Limit Equilibrium Analysis
Design Procedure of
Limit Equilibrium Analysis

9. Establish Project Requirements And Determine Soil Properties
Surcharge
Design for reinforcement
Tult, L, Sv
GWT
or pore water pressure
Retained soil Properties
Height
Seismic Acceleration
Reinforced soil Properties
Slope angle
Foundation soil Properties
GTW

10. Required Factor of Safety
Internal Slope Stability:
Breakage: FS≥1.3
Pullout: FS≥1.5 (sand)
FS≥2.0 (clay)
Compound Stability: FS≥1.3
External Stability:
Sliding: FS≥1.3
Global (Deep Seated): FS≥1.3
Bearing Capacity: FS≥1.3
Seismic Stability: FS≥1.1

11. Establish Reinforcement Design Parameters
Determine reinforcement ultimate tensile strength and reduction factors Tult, RFc, RFD, RFID from Laboratory tests

12. Check Unreinforced Slope Stabilitytf
Use limit equilibrium program
L is length of failure surface

13. Preliminary Reinforced Slope Design
Determine Required
Reinforcement Force
Mobilized soil friction angle
STs=nTs
Equivalent
height
Schmertman (son) Chart (1987)
Schmertmann, G. R., Chouery-Curtis, V.E. Johnson, R.D., and Bonapart R. (1987), “Design Charts for Geogrid-Reinforced Soil Slopes”,
Proc., Geosynthetics’ 87 Conf., New Orleans, La.,

14. Preliminary Reinforced Slope Design
Determine Required
Reinforcement Force
Mobilized
Slope angle
Leshchinsky and Boedeker Chart (1989)-log spiral
Leshchinsky, D. and Boedeker, R.H., (1989) “Geosynthetic Reinforced Soil Structures”,
Journal of Geotechnical Engineering, ASCE, Vol. 115, No. 10, pp

15. Preliminary Reinforced Slope Design
Determine Required
Reinforcement Force
Developed=Mobilized
Jewell (1991)
Jewell, R. A., (1991) “Application of Revised Design Charts for Steep Reinforced Slopes”, Geotextiles and Geomembranes, Vol. 10, No. 3, pp

16. Preliminary Reinforced Slope Design
Determine
Reinforcement Length
LT: Top Length
LB: Bottom Length
Schmertman Chart (1987)

17. Determine Distribution of
Reinforcement Forces
H≤6m1 zone ; H>6m2 or 3 zone
For 3 zone
Ttop=1/6Ts-max
Tmiddle=1/3Ts-max
Tbottom=1/2Ts-max
For 2 zone
Ttop=1/4Ts-max
Tbottom=3/4Ts-max

18. Determine Reinforcement Force
For each zone
Required Reinforcement Force for Required FS

19. Check Reinforced Slope Internal Stability
Assume tangent: reinforcement can kink at failure
Assume horizontal
Use limit equilibrium program
Reinforcement loads increase to slope stability

20. Check Reinforced Slope Internal Stability
Method A:
Moment from reinforcement is used to reduce driving moment
FSR is only applied to soil

21. Check Reinforced Slope Internal Stability
Method B:
Moment from reinforcement is added to resisting moment
FSR is applied to soil and reinforcement

22. Check Reinforced Slope Internal Stability
Method A: FS =2.91
Method B: FS =1.72

23. Check Reinforced Slope Internal Stability

24. Check Reinforced Slope Internal Stability

25. Check FS against Pullout
Le>1m
Use the critical failure surface from limit equilibrium analysis to determine reinforcement embedment length Le

26. Check Reinforced Slope External Stability
Set a thin interface layer and input interface properties in limit equilibrium analysis.
Check sliding for each layer
(fix the location of failure surface)
3, FSsliding=1.3
Check global and compound by free searching
FS=1.3

27. Check Reinforced Slope Seismic Stability
Perform Seismic Limit Equilibrium Analyses
Allow Structural Deformation Kh=0.5A
Fsdynamic=1.1
very conservative

28. Primary and Secondary Reinforcement
Detail Design
Primary and Secondary Reinforcement

29. Primary and Secondary Reinforcement
Detail Design
Drainage System
Primary and Secondary Reinforcement

30. Case Example

31. Case Example Statement
A highway widening project requires the design of a 8m high geogrid-reinforced slope. The slope and backfill information are provided as follows. Conduct a preliminary design using the design charts proposed by Schmertmann et al. (1987).
Design the reinforcement force and length to satisfy the required FS for internal stability.
Reinforced Soil Slope:
H=8 m, b=45o, q=100kPa, Sv=0.5m
Backfill:
Use sand
g= 18 kN/m3, f=35o 31

32. Determine Required Reinforcement Force
Check Chart 0.13

33. Determine Reinforcement Forces
H>6m2 or 3 zone
Ttop=1/4Ts-max=41 kN/m
Tbottom=3/4Ts-max=123 kN/m
For each layer

34. Determine Reinforcement Length
LT/H’=0.4
LT=0.4*13.5=5.4m
LB/H=0.55
LB=0.55*13.5=7.4m LB LT

35. Limit Equilibrium Program-ReSSA

36. Slope Configuration and Surcharge

37. Reinforcement

38. Soil

39. Critical Failure Surface
Factor of Safety and
Critical Failure Surface
Increase the LT for
improve pullout resistance

40. FS Contour