Proper estimation of soil reinforcement loads and strains is key to accurate internal stability design of reinforced soil structures. Current design methodologies use limit equilibrium concepts to estimate reinforcement loads for internal stability design of geosynthetic and steel reinforced soil walls. For geosynthetic walls, however, it appears that these methods are excessively conservative based on the performance of geosynthetic walls to date. This paper presents a new method, called the K-stiffness method, that is shown to give more accurate estimates of reinforcement loads, thereby reducing reinforcement quantities and improving the economy of geosynthetic walls. The paper is focused on the new method as it applies to geosynthetic walls constructed with granular (noncohesive, relatively low silt content) backfill soils. A database of 11 full-scale geosynthetic walls was used to develop the new design methodology based on working stress principles. The method considers the stiffness of the various wall components and their influence on reinforcement loads. Results of simple statistical analyses show that the current American Association of State Highway and Transportation Officials (AASHTO) Simplified Method results in an average ratio of measured to predicted loads (bias) of 0.45, with a coefficient of variation (COV) of 91%, whereas the proposed method results in an average bias of 0.99 and a COV of 36%. A principle objective of the method is to design the wall reinforcement so that the soil within the wall backfill is prevented from reaching a state of failure, consistent with the notion of working stress conditions. This concept represents a new approach for internal stability design of geosynthetic-reinforced soil walls because prevention of soil failure as a limit state is considered in addition to the current practice of preventing reinforcement rupture.Key words: geosynthetics, reinforcement, walls, loads, strains, design, K-stiffness method.

The paper describes measurements taken from a series of four full-scale modular block walls that were constructed with reinforcement layers having different stiffness. The walls were 3.6 m high and were reinforced with two different polypropylene geogrid reinforcement materials, a polyester geogrid and a welded wire mesh. Each wall was constructed with the same modular block facing and reinforcement spacing of 0.6 m. The influence of compaction effort on wall displacements and horizontal toe load measurements at the end of construction was detectable in this investigation. These values were adjusted to account for the influence of different compaction methods on end-of-construction wall response. However, during subsequent surcharging the effects of initial compaction effort were erased. Reinforcement loads are computed from strain readings and results of in-isolation constant-load (creep) tests. Computed maximum reinforcement loads are compared with values predicted using the current AASHTO Simplified Method and the K-stiffness Method. The predicted magnitude and distribution of reinforcement loads are shown to be more accurate using the K-stiffness Method for polymeric reinforcement materials. For the relatively stiff welded wire mesh product, the measured reinforcement loads fell between values predicted using both methods.

No abstract

Measurements indicative of the internal behavior of full-scale geosynthetic-reinforced soil walls typically consist of reinforcement strains and overall deformations. The focus of this paper is the development of a methodology that can be used to convert measured reinforcement strains to load using properly selected reinforcement stiffness values. The loading of the geosynthetic in the field can be simulated in the laboratory using creep, relaxation, and constant-rate-of-strain tests. It was found that in-isolation creep stiffness data is sufficiently accurate to estimate reinforcement loads from strain measurements, at least for geogrids and most woven geotextiles. The approach is validated using data from carefully instrumented wall case histories in which reinforcement loads were measured directly and compared to loads estimated from measured reinforcement strain data.

The paper reviews geosynthetic reinforcement strain measurement techniques that have been reported in a database of well-documented case studies and more recent full-scale laboratory test walls. Interpretation of strain measurements, accuracy of readings, and advantages and disadvantages of different techniques are discussed. In general, properly calibrated strain gauges have proven useful to estimate reinforcement strains at low strain levels (0.02 to 2%). Extensometers are shown to be accurate at strains greater than 2% and to have marginal reliability at strains between 0.5 and 2%. A strategy to improve confidence with interpretation of strain readings is to use strain gauges and extensometers in the field and to adjust strain gauge calibration factors based on in situ measurements from both devices. Corrected reinforcement strains can be used together with appropriately selected reinforcement stiffness values to estimate reinforcement loads. Estimated loads can then be compared to predicted values using current and proposed design methods for the internal stability of geosynthetic-reinforced soil walls.