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

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

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

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

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).

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

External Failure Modes

Limit Equilibrium Analysis Design Procedure of Limit Equilibrium Analysis

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

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

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

Check Unreinforced Slope Stability tf Use limit equilibrium program L is length of failure surface

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., 108-120.

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.1459-1478.

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. 203-233.

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

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

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

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

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

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

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

Check Reinforced Slope Internal Stability

Check Reinforced Slope Internal Stability

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

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

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

Primary and Secondary Reinforcement Detail Design Primary and Secondary Reinforcement

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

Case Example

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

Determine Required Reinforcement Force Check Chart 0.13

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

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

Limit Equilibrium Program-ReSSA

Slope Configuration and Surcharge

Reinforcement

Soil

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

FS Contour