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

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.

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

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 976-994

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)

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

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)

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

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

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

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.

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)

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)

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

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

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

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

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. 301-318.

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

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

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

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.

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

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 ?

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