Jennifer Nicks

Secondary Settlement of Geosynthetic-Reinforced Soil Piers: Preliminary Results

Four geosynthetic-reinforced soil (GRS) piers, each built with different reinforcement strengths and aggregate backfill material, were constructed at the Federal Highway Administrations (FHWAs) Turner-Fairbank Highway Research Center in collaboration with the Long-Term Bridge Performance Program. The objective of the research is to assess the secondary deformation characteristics of GRS for load-bearing applications under service load conditions. The experiment consisted of axially loading the piers with 490 kN decommissioned prestressed concrete girders, resulting in an equivalent vertical applied stress of 200 kPa. The geometry of each GRS composite was 1.2 m square by 2.3 m in height, for an approximate base to height ratio of 0.5. The facing element for each GRS pier is a simple, split-faced concrete masonry unit (CMU) that is frictionally connected to the GRS composite at a nominal vertical reinforcement spacing of 0.2 m. The piers were instrumented to record reinforcement strain, earth pressures, and deformations. This paper will discuss the objectives of the experiment, explain the testing program, and share some of the vertical deformation results after almost four months in service. In addition, the results will be compared with the results of a long-term (since 1999) study on secondary settlement of a GRS abutment.

Large Diameter Triaxial Testing of AASHTO Open Graded Aggregates and the Effect of Relative Density on Strength

Open-graded aggregates (OGAs) are commonly used for the construction of roads and bridges due to their ease of construction, lower unit weight, and favorable drainage characteristics. However, their material characteristics are not well understood leading to the common practice of assigning conservative default strength parameters in the absence of testing or basing strength on conventional test methods with scalped samples using either 63-mm triaxial (TX) compression or direct shear tests. The practice of removal of the large grain sizes of OGAs may contribute to under representation of OGA strength. As part of a larger effort by the Federal Highway Administration to characterize OGAs, two of American Association of State Highway and Transportation Officials (AASHTO) M43-05 designated samples having small and large grain sizes were tested at two relative densities in a large-scale (152 mm diameter) TX device under consolidated drained conditions at confining stresses of 34, 69, 138, and 207 kPa. The effect of relative density on strength parameters was significant at all studied stress levels. The computed strength values even at worse case scenarios were relatively higher than the conservative default strength values. The implication is that higher strength values can be used in design. The quantitative information can also be used for improved constitutive modeling.

Mini-Pier Testing To Estimate Performance of Full-Scale Geosynthetic Reinforced Soil Bridge Abutments

The geosynthetic reinforced soil (GRS) performance test (PT), also called a mini-pier experiment, was developed by the Federal Highway Administration (FHWA) to evaluate the material strength properties of GRS composites built with a unique combination of reinforcement, compacted fill, and facing elements. The PT consists of constructing a 1.4-m square column of alternating layers of compacted granular fill and geosynthetic reinforcement with a facing element that is frictionally connected up to a height of 2 m, then axially loading the GRS mass while measuring deformation to monitor performance. The results can be directly used in the design of GRS abutments and integrated bridge systems. Considering that the geometry of the PT is square in plan, the equivalency of the results to a bridge application, which more resembles a plane strain condition, is evaluated and presented in this paper. The analysis indicates that the PT closely approximates the bearing resistance, or capacity, of a typical GRS abutment, and is a conservative estimate when predicting stiffness. These results indicate that the PT can be used as a design tool for GRS abutments at both the strength and service limit states.

Case Study: Condition Assessment of a 36-Year-Old Mechanically Stabilized Earth Wall in Virginia

AbstractIn 2012, a mechanically stabilized earth (MSE) wall was demolished in Virginia due to road realignment activities, providing an opportunity to assess the condition of the steel-bar mat rein...

Electronic Thickness Gauge Measurements on a 36-year old Steel Bar Mat Reinforced MSE Wall

During road realignment activities in 2012 on the I-495 Washington Capital Beltway, the Virginia Department of Transportation (VDOT) demolished a MSE wall originally built in 1976. The wall supported traffic on the interstate bridge ramp and was therefore salted each winter as part of VDOTs standard winter maintenance program; the wall did not have any observable performance issues. Demolition provided an opportunity for researchers at the Federal Highway Administration (FHWA) to investigate the condition of the reinforcement in the 36- year old wall and determine any key factors affecting corrosion related to the design, construction, and maintenance practices. Steel and soil samples were collected from various locations behind the concrete panels along the length of the wall to assess the thickness of the remaining zinc coating on the bar mats. This thickness was analyzed as a function of wall elevation and distance from the bridge abutment face. This paper will describe the corrosion sampling and testing program and present preliminary conclusions on the results.

Composite properties from instrumented load tests on mini-piers reinforced with geotextiles

Four pairs of large-scale instrumented geosynthetic reinforced soil (GRS) square columns were load tested to study the effects of varying reinforcement strength to spacing ratio, to discern the lateral pressures during construction and during load testing, and to derive shear strength parameters of the GRS composite. Each pair was identical in every respect, except one was loaded with a dry-stacked concrete masonry unit (CMU) facing in place and the other without. Lateral pressures during construction were found to be small for the facing type used in this study. Also, based on the derived GRS composite shear strength parameters, it was found that (1) the GRS composite Mohr-Coulomb envelopes are not parallel to those for the unreinforced soil; (2) the reinforcement increased the composite cohesion compared to the unreinforced soil (cohesion increases with decreasing spacing and increasing reinforcement strength); (3) the composite friction angle is less than that of the unreinforced soil (friction angle increases with decreasing reinforcement strength and increasing spacing); (4) as the composite friction angle increases, the active lateral earth pressure coefficient decreases; and (5) the benefits of reinforcing a soil become increasingly significant as the reinforcement spacing decreases.

USE OF FULLY SOFTENED VERSUS PEAK STRENGTH TO PREDICT THE CAPACITY OF FOOTINGS ON GEOSYNTHETIC REINFORCED SOIL

A database of load tests performed on geosynthetic reinforced soil (GRS - reinforcement in this study is of the extensible variety) was developed using results of recent load tests performed on large scale GRS structures at the Federal Highway Administrations Turner Fairbank Highway Research Center as well as results from the literature. The measured capacities were compared to those predicted using the Wu and Pham (1) equation utilizing both the peak and fully softened soil shear strength parameters. It was found that the fully softened strengths yielded capacities that agreed better with the measured capacities. A rationale for this finding is that the robust reinforcement in a GRS strengthens the soil considerably causing the GRS to experience large strains prior to failure. Because the soil peak strengths are mobilized at relatively small displacements/strains even in large scale direct shear or triaxial tests compared to the GRS load tests, it is postulated that the fully softened values are more appropriate to estimate the GRS bearing capacity. A follow-on to this is that since large movements are required to fail say a GRS abutment, the design of GRS abutments will most likely be governed by the serviceability limit state rather than the ultimate limit state.

Large-Scale Direct Shear Testing of Common Open-Graded Aggregates

Manufactured open-graded aggregates are increasingly being used as backfill material for many geotechnical assets including retaining walls, bridge abutments, and foundations because of their free-draining and constructability advantages over traditional well graded materials. In design, many transportation agencies often use a default friction angle of 34 degrees, regardless of the material selected. To study the strength properties of open-graded aggregates, testing was performed on a large-scale direct shear device (LSDS) at the Federal Highway Administrations (FHWAs) Turner-Fairbank Highway Research Center. The LSDS device is 0.3 m x 0.3 m x 0.2 m and is capable of testing aggregates up to 30 mm. For this series of experiments, five commonly used open-graded aggregates are tested at four applied normal stresses, 34, 69, 138, and 207 kPa, at a shear rate of 0.4 mm/min and a gap size equal to the D85 of the material (i.e., the aggregate size where 85% of the sample is smaller). Results indicate that friction angles for these uncompacted open-graded aggregates are all greater than 45 degrees, well above the 34-degree default value.