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.

A verified fast Lagrangian analysis of continua (FLAC) numerical model is used to investigate the influence of horizontal toe stiffness on the performance of reinforced soil segmental retaining walls under working stress (operational) conditions. Results of full-scale shear testing of the interface between the bottom of a typical modular block and concrete or crushed stone levelling pads are used to back-calculate toe stiffness values. The results of numerical simulations demonstrate that toe resistance at the base of a reinforced soil segmental retaining wall can generate a significant portion of the resistance to horizontal earth loads in these systems. This partially explains why reinforcement loads under working stress conditions are typically overestimated using current limit equilibrium-based design methods. Other parameters investigated are wall height, interface shear stiffness between blocks, wall facing batter, reinforcement stiffness, and reinforcement spacing. Computed reinforcement loads are compared with predicted loads using the empirical-based K-stiffness method. The K-stiffness method predictions are shown to better capture the qualitative trends in numerical results and be quantitatively more accurate compared with the AASHTO simplified method.

Reliability-based design concepts and their application to load and resistance factor design (LRFD or limit states design (LSD) in Canada) are well known, and their adoption in geotechnical engineering design is now recommended for many soil–structure interaction problems. Two important challenges for acceptance of LRFD for the design of reinforced soil walls are (i) a proper understanding of the calibration methods used to arrive at load and resistance factors, and (ii) the proper interpretation of the data required to carry out this process. This paper presents LRFD calibration principles and traces the steps required to arrive at load and resistance factors using closed-form solutions for one typical limit state, namely pullout of steel reinforcement elements in the anchorage zone of a reinforced soil wall. A unique feature of this paper is that measured load and resistance values from a database of case histories are used to develop the statistical parameters in the examples. The paper also addresses issues related to the influence of outliers in the datasets and possible dependencies between variables that can have an important influence on the results of calibration.

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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 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.

Installation damage can be expected to modify the mechanical properties of geosynthetics in reinforcement applications. The paper reviews data from controlled installation damage trials reported in the literature and other sources. Geosynthetic properties taken from the results of index tensile tests are reviewed and the peak strength, strain at failure, and modulus of control and damaged specimens compared. The results of this interpretation of test data are grouped according to reinforcement type and constituent polymer. The data reveals that there are critical levels of reinforcement damage described as percent of peak strength retained that identify the onset of degradation in modulus and/or strain at rupture for each reinforcement classification. The interpretation of the test data presented in the paper illustrates that the current interpretation of installation damage based on peak strength retained is conservative for some types of geosynthetic materials. For these materials, the percentage of modulus retained is a more rational quantitative measure of resistance to site installation damage.

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.