User's Manual for GeogridBridge 3.0: A Guide to the Load Displacement Compatibility Method (LDC) for Design of Column-Supported Embankments

This introduction is a guide for operating the GeogridBridge 3.0 (GGB3) Microsoft Excel spreadsheet, which applies the Load Displacement Compatibility (LDC) method for the analysis and design of column-supported embankments. The LDC method implemented in GGB3 performs settlement analysis considering the vertical load transfer to the columns by soil arching within the embankment, tension developed in geosynthetic reinforcement included in the embankment, and negative skin friction between the settling soil and the columns. The LDC method assumes the embankment is of a broad lateral extent and thus the finite boundaries of a column-supported embankment are not considered. Settlement computed in GGB3 is owing to compression of columns and foundation soils above the neutral plane. The chapter is organized to guide the user through the spreadsheet's Main tab, where the user can define embankment characteristics, run the analysis, and view the results.

GGB3 is a completely new version of the GeogridBridge spreadsheet that applies the Load Displacement Compatibility (LDC) method for the analysis and design of column-supported embankments. GGB3 was developed from a blank Microsoft Excel macro-enabled workbook and shares no content from previous versions. This chapter presents a table that compares the features available to users of GGB3 to previous versions. The features include greater flexibility to specify material properties, stratigraphy, and the ability to run batches of analyses to permit Monte Carlo Simulations. GGB3 is organized into nine tabs: Main, Definition Sketches, Constr Analysis, Post-constr Analysis, Batch Output, Time rate of consol, MCS histograms, Load transfer plot, and Revision History. The units that are input into GGB3 must be consistent throughout the spreadsheet. The units for Length, Force, and Time are represented in the spreadsheet as L, F, and T, respectively.

Layers in GGB3 are specified top-down according to the post-construction embankment profile. The column is assumed to start at the base of the embankment layers and extend to the depth of the bottom soil layer. The LDC calculations are performed incrementally based on a discretization of the soil profile. GGB3 allows the profile to be discretized into a maximum of 2,000 increments. GGB3 interprets the top of the soil profile prior to embankment construction as coinciding with the elevation of the bottom of the embankment, including the Load Transfer Platform (LTP), also known as a Load Transfer Mat. GGB3 computes compression of sand and clay layers comprising the foundation and elastic compression of the column. GGB3 treats the bottom of the embankment, including the LTP, as the top of the soil profile prior to construction. GGB3 can accommodate both rectangular and triangular arrays of different center-to-center spacings.

GGB3 allows the analyst to apply a Monte Carlo Simulation (MCS) to consider the uncertainty of input parameter values. The spreadsheet has the capability of considering uncertainty within each of the soil profile parameters. If the user does not wish to run an MCS, the number of runs in batch N should be set to 1. Usually, a reliable MCS in GGB3 requires a minimum of 1,000 trials. GGB3 can run a MCS with a maximum of 10,000 trials. GGB3 defines the probability of serviceability failure as the percentage of MCS iterations that result in an output value that exceeds a prescribed maximum acceptable value. GGB3 considers parameter uncertainty using either a normal or log-normal distribution for the nondeterministic parameter values. The spreadsheet considers the parameter values input for a deterministic analysis to be the average parameter values for the probabilistic analysis.

The tension and strain in the combined reinforcement layers across the spans are defined in the Reinforcement section. In general, tension should be lower than the allowable tensile strength of the geosynthetic. A lower SRR means that there is less load felt by the foundation soil. The critical height is defined at the vertical distance from the base of the embankment to the top of the zone of arching. The neutral plane is the depth at which the column and the surrounding soil settle at the same rate. Above the neutral plane, the soils settles more than the columns. The maximum differential settlement at the base of the embankment is defined as the difference between the settlement between the location in a unit cell that is farthest away from an edge of a column or pile cap and the settlement at the column top.

This chapter presents a simple worked example of an embankment and defines the post-construction profile for the embankment. The chapter discusses how to define material properties for each layer of the embankment and how to determine surface loading conditions. The Bilinear Adapted Terzaghi (BAT) method is used in this example to model the mobilization of soil arching within the embankment. When the macro is run, GGB3 will automatically populate the “Mobilization of Arching” section with values in accordance with the BAT method. The modulus of arching soil should be set to the representative Young's modulus for the embankment material, which is evaluated to be 750,000 psf. In this example, the designer can consider uncertainty in unit weight, earth pressure coefficient, friction angle, compression ratio, and recompression ratio. Each parameter is assumed to vary independently according to a Normal distribution.

This chapter presents a complex worked example of the post-construction profile of an embankment and discusses the development of the post-construction embankment profile. It describes how to define material properties for the layers, including sand and clay, of the embankment, and shows how to determine surface loading conditions for the embankment. The Bilinear Adapted Terzaghi (BAT) method is used in this example to model the mobilization of soil arching within the embankment. When the macro is run, GGB3 will automatically populate the “Mobilization of Arching” section with values in accordance with the BAT method. The modulus of arching soil, Ea, should be set to the representative Young's modulus for the embankment material, which is evaluated to be 800,000 psf. In this example, the designer can consider uncertainty in unit weight, earth pressure coefficient, friction angle, cohesion, compression ratio, and recompression ratio.