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The Simplified Method as reported in AASHTO and Federal Highway Administration (FHWA) manuals has been demonstrated to give poor predictions of unfactored reinforcement loads and strains, especially for geosynthetic reinforced soil walls. The writers have proposed the K-stiffness Method to improve the load prediction accuracy for walls under working stress (operational) conditions. However, it has also been recognized in recent publications by the writers and others that further improvements to the K-stiffness Method are needed. Furthermore, acceptance of the K-stiffness Method has been hindered due to its perceived complexity and the use of the plane strain friction angle to quantify the strength of the reinforced soil. This paper takes a fresh look at both methods and uses lessons learned from the K-stiffness Method development to improve the accuracy of the AASHTO/FHWA Simplified Method. Key parameters introduced during the development of the K-stiffness Method are applied to the Simplified Method and updated to further improve load prediction accuracy. Additional wall case histories have been added to the database used for the original K-stiffness Method to calibrate the new model and to broaden its utility. An important improvement is a single model that allows for seamless load prediction across a range of walls constructed with relatively extensible geosynthetic reinforcement and inextensible steel reinforcement materials. The quantitative improvement of the new model (Simplified Stiffness Method) compared to the current AASHTO/FHWA Simplified Method is demonstrated through statistical analysis of load bias values (i.e., the ratio of measured to predicted reinforcement load). [ABSTRACT FROM AUTHOR]

The grade raising associated with the Third Runway Project at Seattle-Tacoma International Airport included construction of tall mechanically stabilized earth (MSE) walls, including the near-vertical, two-tier, 26-m North MSE wall and the near-vertical, four-tier, 46-m tall west MSE wall. Twenty reinforcement strips at critical wall cross sections were instrumented with over 500 strain gauges to monitor strains during and following construction. The reinforcement loads inferred from observed strains are of interest because of their great height and global reinforcement stiffness, which place these walls outside the range in height and stiffness used to calibrate commonly used design methods. This paper presents the development and distribution of reinforcement strains measured during and following the construction of these walls. The reinforcement stresses calculated using the original reinforcement load design methods and design friction angle agreed with those inferred from the measured strains. The accuracy of two standard-of-practice and two alternate design methods is evaluated by comparing the reinforcement loads inferred from measured strains to those calculated using the actual friction angle of the reinforced fill material. Advantages and limitations in these design methods are identified, and recommendations for the improvement of some of these methods are provided. [ABSTRACT FROM AUTHOR]

A response is from the authors to the reviewers of their article “Predicted Loads in Steel Reinforced Soil Walls Using the AASHTO Simplified Method” which is in Volume 135 of the February 2009 issue is presented.

The paper investigates the accuracy of the AASHTO simplified method by using load measurements reported in a large database of full-scale instrumented walls for bar mat, welded wire, and steel strip soil reinforced walls. The accuracy of the AASHTO simplified method is quantified by computing the mean and coefficient of variation of the ratio (bias) of measured loads under operational conditions to predicted loads. The paper shows that for steel strip walls, the AASHTO simplified method is reasonably accurate for granular backfill soils with friction angles less than 45°. For bar mat walls, the method is demonstrated to be slightly conservative. The simplified method underpredicts reinforcement loads at shallow overburden depths for steel strip walls with backfill friction angles greater than 45° due to compaction-related effects. It is concluded that these compaction-induced loads near the wall top do not contribute to internal instability due to pullout. [ABSTRACT FROM AUTHOR]

The paper investigates the accuracy of the coherent gravity method by using measurements reported in a large database of full-scale instrumented walls that was not available at the time the original method was developed. The new database includes data for bar mat, welded wire and steel strip soil reinforced walls. Measured reinforcement loads under operational conditions are compared with predicted values for bar mat and steel strip reinforced walls. The accuracy of the coherent gravity method as presented in the BS 8006 design standard is quantified by computing the mean and coefficient of variation of the ratio (bias) of measured to predicted loads. The paper shows that for steel strip walls the coherent gravity method is reasonably accurate for soils with friction angles less than 458. For granular soils with higher friction angles and bar mat walls, the current coherent gravity method is shown to be less accurate and, on average, non-conservative for design. Modifications to the method as currently described in BS 8006 are proposed to improve the accuracy of the method for bar mat reinforced soil walls and steel strip reinforced soil walls with high friction angle backfill soils. [ABSTRACT FROM AUTHOR]