Foundation Design for High Tension Cable Guardrails
Author: Zhu, Ling | Size: 3.53 MB | Format:PDF | Quality:Original preprint | Publisher: University of Nebraska, Lincoln | Year: 2010 | pages: 49
High tension cable guardrail is becoming increasing popular in median and roadside applications due to the promise of reduced deflections upon impact and reduced maintenance. As the performance of these systems is observed in service, there is a growing concern over the end anchorage foundation performance of current systems. Foundations for high tension systems must not only be capable of restraining the impact load of a vehicle but must also restrain the initial pretension on the cable system as well as temperature induced loads. While it may be acceptable for many roadside safety devices to require foundation repair after impact, foundation failure due to environmentally induced loads would be a serious maintenance problem. As initial tension and temperature induced loads can be greater than those loads applied during impact, this type of loading must be considered in foundation design. Foundation deflection can reduce cable tension, increasing deflection of the system during impact and letting the cables sag after impact. The soil conditions in which these foundations are placed vary significantly. This report considers the potential impact, tension, and temperature loads and develops a set of suggested foundation designs to accommodate a range of in situ soil conditions. These designs will vary significantly in different areas around the nation due to variations in both weather and in situ soil conditions. Deflection during full-scale crash tests may not accurately represent the foundation deflection that will be experienced in the field.
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This report and the accompanying software are part of efforts to improve the characterization and analysis of pile-stabilized slopes using one or two rows of driven piles. A combination of the limit equilibrium analysis and strain wedge (SW) model technique is employed to assess the stability of vulnerable slopes before and after using driven piles to improve the slope stability. This report focuses on the entry of input data, interpretation of the output results, and description of the employed technique. In addition to a comparison study with a full-scale load test, the finite element (FE) analysis using a general-purpose FE package, “PLAXIS,” is performed to verify the results. The characterization of lateral load induced by slipping mass of soils can be accomplished using the modified SW model technique. The SW model for laterally loaded pile behavior is a new predictive method (recommended as an alternative method by AASHTO [2007]) that relates the stress-strain behavior of soil in the developing three-dimensional passive wedge in front of the pile (denoted as the strain wedge) under lateral load to the one-dimensional beam-on-elastic foundation parameters. Two failure scenarios are employed in the developed computer program to include pile stabilization for 1) existing slip surface of failed slope and 2) potential failure surface. The two scenarios evaluate the distribution of the soil driving forces with the consideration of the soil flow-around failure, soil strength, and pile spacing. The developed procedure can also account for the external pile head lateral load and moment along with the driving force induced by the sliding mass of soil. The developed computer program is a design tool in which the designer can select an economic pile size to stabilize slopes. In addition to the external lateral loads applied at the pile head, the presented research work determines the mobilized driving force caused by sliding mass of soil that needs to be transferred via installed piles to stable soil layers below the slip surface. The side and front interaction between piles and sliding mass of soil is one of the main features of this project. The work presented also evaluates the appropriate pile spacing between the piles in the same pile row (wall) and the spacing between the pile rows. The computer program provides a flexible graphical user interface that facilitates entering data and analyzing/plotting the results. The finite element analysis (using PLAXIS) was used to investigate the results. A field test for pile-stabilized slope is used to validate the results obtained from the finite element analysis and the developed technique.
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Group Efficiencies of Grout-Tipped Drilled Shafts and Jet-Grouted Piles
Author: McVay, Michael | Size: 11.03 MB | Format:PDF | Quality:Original preprint | Publisher: University of Florida, Gainesville | Year: 2011 | pages: 206
In current practice, driven piles/drilled shafts are constructed in a group at 3D center to center (c/c) spacing to minimize group interference (i.e., group efficiency factor ~ 1) as well as reduce the cost of the reinforced concrete cap. Presently, group efficiency factors for a number of new foundation types, e.g., post-grouted drilled shafts and jet-grouted piles, are unknown. This research looked into the group interaction of post-grouted drilled shafts and jet-grouted piles at typical 3D spacing. Two sets of group tests for grout-tipped drilled shafts and jet-grouted piles were performed in the Florida Department of Transportation (FDOT) test chamber in medium dense sands. The experimental top down group testing revealed that the jet-grouted piles behaved as a block at 3D spacing, whereas postgrouted drilled shafts acted independently of one another, i.e., no group interaction. A group interaction factor, as well as an analytical approach for predicting load versus displacement for single or group of jet-grouted piles, is suggested. In the case of the grout-tipped drilled shafts, the grout pressure always resulted in an upward flow of grout alongside the shaft which increased both the side and tip areas. The increased areas and grout tip preload were the three factors identified with the increased shaft and group resistance. Due to uncertainty in estimating areas, a conservative design approach for assessing single and group capacities of post-grouted drilled shafts at 5% displacement is proposed. Finally, the use of stage grouting to assess the increase in shaft capacity (skin and tip) is suggested.
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Author: Helwany, Sam Panda, Ritu Titi, Hani | Size: 5.67 MB | Format:PDF | Quality:Original preprint | Publisher: University of Wisconsin, Milwaukee | Year: 2011 | pages: 127
The alternate post system offers benefits such as ease of construction, reduced construction time, and lower wall costs. While this system seems feasible, there are concerns regarding its performance, in particular the amount of bending in the post and the defection of the wall due to active earth pressures exerted by the retained soil. Other concerns include the potential damage to the plate during driving, control and accuracy of post alignment, and long term issues such as corrosion and soil creep. The objective of this research project is to assess the feasibility of the alternate post system. If the system is deemed feasible, the research team will develop design criteria for the alternate post system based on exposed wall heights, applied soil loads, post dimensions, and parameters of the retained soil and the foundation soil.
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Author: Pierson, Matthew | Size: 4.49 MB | Format:PDF | Quality:Original preprint | Publisher: University of Kansas, Lawrence | Year: 2011 | pages: 164
Mechanically stabilized earth (MSE) walls are recognized as a cost effective earth retention technology. In some cases structural foundations must pass through the reinforced fill due to the required footprint of the reinforced zone behind the facing. Limited information about the interaction between the structure and the MSE mass has been published, making efficient design difficult. It would be prohibitively expensive to construct and test all possible geometries or applications; therefore numerical modeling must be used to supplement physical data. This report contains a discussion of the analysis of physical test data and numerical modeling of an MSE test wall containing foundation elements. The test wall consists of an MSE wall with cast-in-place shafts contained within and solely supported by the reinforced fill. The finite difference numerical modeling program FLAC3D was used for analysis. A parametric study was conducted to determine how the various constituents of the physical wall as well as wall height affect wall-shaft behavior. Geogrid properties, particularly stiffness, were found to have the greatest influence on behavior. Wall height has a large influence on capacity at shaft movement of more than 2 inches. Analyses of the modeling results were used to create design recommendations for MSE walls with foundation elements.
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Drilled Shaft Bridge Foundation Design Parameters and Procedures for Bearing in SGC Soils
Author: Houston, William N Arizona State University, Tempe Harraz, Abdalla M Walsh, Kenneth D Houston, Sandra L Arizona State University, Tempe Perry, Courtland | Size: 2.11 MB | Format:PDF | Quality:Original preprint | Publisher: Arizona State University, Tempe | Year: 2011 | pages: 126
This report provides a simplified method to be used for evaluating the skin friction and tip resistance of axially loaded drilled shafts. A summary of literature and current practice was completed and then a comprehensive set of field and laboratory tests was performed. Several soil samples were collected from different sites from Arizona and surrounding states. Large scale direct shear apparatus was developed and used to determine the friction between soil and concrete. Finite element analyses were conducted on several prototype cases to determine effect of soil parameters such as dilation on the skin friction values. A step-by-step simplified approach was introduced to determine the skin and tip resistance of drilled shaft foundations in gravelly soils. An example application was presented to guide users in utilizing the simplified approach.
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Drilled shaft foundations are principally used to support many structures such as bridge piers, towers, buildings, transmission towers, and roadway cable barriers. This research focuses on the use of drilled shafts in the cable median barrier systems which play an important role in protecting people’s lives due to cross-over collisions on highways. During December 2006 to February 2007, several failures of 3-cable median barrier (TL-3) were observed in Kaufman County near Dallas without any traffic-related vehicular impacts. Preliminary investigation of failures showed that failed drilled shafts were located in high plasticity clay. Causes of failures are attributed to cold temperature induced shrinkage in the cables that increased in the tension in them, soil saturation due to long periods of rainfall and small sizes of drilled shafts used. Various sizes of drilled shafts were established and constructed in an environment similar to the one in which foundation distress was observed. Geotechnical sampling and laboratory testing were performed, and a new test setup for the application of an inclined tensile loading on drilled shafts was designed to simulate the loading under real field conditions. The capacities of different sizes of drilled shafts from field test were tested and measured under this setup. Once good simulation was obtained, the models are used for various foundation dimensions and various undrained shear strengths of soils which, in turn, provided results that are used in the development of foundation design charts. Additionally, construction guidelines and recommendation for periodic maintenance are provided in this report.
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Geosynthetic Reinforced Soil for Low-Volume Bridge Abutments
Author: Vennapusa, Pavana Iowa State University, Ames White, David Klaiber, Wayne Iowa State University, Ames Wang, Shiyun | Size: 15.32 MB | Format:PDF | Quality:Original preprint | Publisher: Iowa State University, Ames | Year: 2012 | pages: 132
This report presents a review of literature on geosynthetic reinforced soil (GRS) bridge abutments, and test results and analysis from two field demonstration projects (Bridge 1 and Bridge 2) conducted in Buchanan County, Iowa, to evaluate the feasibility and cost effectiveness of the use of GRS bridge abutments on low-volume roads (LVRs). The two projects included GRS abutment substructures and railroad flat car (RRFC) bridge superstructures. The construction costs varied from $43k to $49k, which was about 50 to 60% lower than the expected costs for building a conventional bridge. Settlement monitoring at both bridges indicated maximum settlements ≤1 in. and differential settlements ≤ 0.2 in transversely at each abutment, during the monitoring phase. Laboratory testing on GRS fill material, field testing, and in ground instrumentation, abutment settlement monitoring, and bridge live load (LL) testing were conducted on Bridge 2. Laboratory test results indicated that shear strength parameters and permanent deformation behavior of granular fill material improved when reinforced with geosynthetic, due to lateral restraint effect at the soil-geosynthetic interface. Bridge LL testing under static loads indicated maximum deflections close to 0.9 in and non-uniform deflections transversely across the bridge due to poor load transfer between RRFCs. The ratio of horizontal to vertical stresses in the GRS fill was low (< 0.25), indicating low lateral stress on the soil surrounding GRS fill material. Bearing capacity analysis at Bridge 2 indicated lower than recommended factor of safety (FS) values due to low ultimate reinforcement strength of the geosynthetic material used in this study and a relatively weak underlying foundation layer. Global stability analysis of the GRS abutment structure revealed a lower FS than recommended against sliding failure along the interface of the GRS fill material and the underlying weak foundation layer. Design and construction recommendations to help improve the stability and performance of the GRS abutment structures on future projects, and recommendations for future research are provided in this report.
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Unknown foundations affect about 9,000 bridges in Texas. For bridges over rivers, this creates a problem regarding scour decisions as the calculated scour depth cannot be compared to the foundation depth, and a very conservative costly approach must be taken. The objective was to develop a global approach, which will reduce significantly the level of uncertainty associated with unknown foundations. This approach was developed in two parts: a data mining and inference approach where no testing at the site was necessary, and a testing approach where new tests for unknown foundations were used. The data mining and inference task made use of existing data such as soil type, known foundations on neighboring bridges, design practice, and the age of the bridge to infer the type and length of unknown foundation elements. The testing task consisted of developing two geophysical techniques, resistivity and induced polarization imaging, to obtain a picture of the soil and foundation below the surface level or river bottom. The outcome was a global framework in which one of the approaches or any combination thereof, as well as the most useful current techniques (nondestructive testing methods if necessary), can be used to decrease dramatically the uncertainty associated with the unknown foundation. The inference process was trained by using bridges where the foundation was known and verified by comparison against case histories. The two testing techniques mentioned above were tested at the National Geotechnical Testing Site on Texas A&M’s Riverside campus and then against full-scale bridges selected in cooperation with TxDOT.
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Development of Variable LRFD Φ Factors for Deep Foundation Design Due to Site Variabi
Author: McVay, Michael C Klammler, Harald Faraone, Michael A Dase, Krishnarao Jenneisch, Chris | Size: 3.68 MB | Format:PDF | Quality:Original preprint | Publisher: University of Florida, Gainesville | Year: 2012 | pages: 134
The current design guidelines of Load and Resistance Factor Design (LRFD) specifies constant resistance factors (Φ) values for deep foundation design, based on analytical method selected and degree of redundancy of the pier. However, investigation of multiple sites in Florida reveals significant variability of soil/rock properties from site to site (coastal conditions) suggesting the introduction of variable Φ values based on reliability-based design approach. Building on previous work (BD545-76) a geostatistical (variograms) approach was developed to quantify the spatial uncertainty for site specific conditions. As a result, Φ values are evaluated due to both a site’s measured spatial uncertainty and error associated with a particular analytical method. This report summarizes subsequent efforts to further expand the applicability of the reliability design to the analytical models currently available in the FB-DEEP software program. For the geostatistical analysis, a simple yet robust graphical user interface (GUI) was developed, which considers two design scenarios: 1) conditioning to nearby boring data, and 2) unconditional mean site data. For either scenario the GUI generates thousands of potential data sets, which are evaluated by FB-DEEP to assess mean pile/shaft resistance and spatial uncertainty at a pier location. Spatial uncertainty is then combined with the design method error associated with the selected FB-DEEP model to assess Φ. For demonstration of the application of the GUI, standard penetration test (SPT) and laboratory strength data were collected from seven FDOT projects and subsequent Φ values were evaluated. The Φ values ranged from 0.3 to 0.7 depending upon amount of subsurface data, measure summary statistics, and degree of spatial correlation. The report concludes with recommendations (in situ measurements, load testing, etc.) on improving the computed Φ on a site-by-site basis.
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