Subgrade Improvement

Subgrade Improvement for Paved and Unpaved Surfaces Using Geogrids

By Stephen Archer, P.E.

October 2008

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Professional Development Series
Subgrade Improvement for Paved and Unpaved Surfaces Using Geogrids
By Stephen Archer, P.E.

T

he use of geogrid reinforcement is a common practice for engineers, owners, and contractors for buildingstructures over soft soil conditions. First introduced in the United States in the early 1980s, the unique characteristics and mechanisms specific to geogrids offer significant benefits compared with the following conventional construction practices: • excavation and replacement with select fill material, • thick structural (pavement) sections to account for weak subgrade soil conditions, •chemical stabilization or modification with calcium-based materials (i.e., cement, lime, fly ash), and • stabilization with woven or nonwoven geotextiles. A geogrid is defined as a geosynthetic material consisting of

connected parallel sets of tensile ribs with apertures of sufficient size to allow strike-through of surrounding soil, stone, or other geotechnical material (Koerner, 1998; see Figure1 on page PDH 4). Commercial geogrid products marketed and sold today include extruded punched-anddrawn geogrids, woven and coated geogrids, welded geogrids, and geogrid composites. Structural biaxial geogrids can be used to reinforce earth fill over soft ground and provide a stable subgrade under flexible and rigid pavements, unpaved roads, railroad track beds, industrial yards, equipment workplatforms, parking areas, and building foundations. Many small and full-scale studies have been performed to better understand how geosynthetics interact with fill materials to contrast their performance with unreinforced conditions in a variety of civil engineering

applications. This historical empirical data is the basis for development of a number of design methods to quantify the fillthickness required over a geosynthetic reinforcement element to achieve a minimum level of serviceability. As the use of geosynthetics in soft soil conditions has evolved during the last three decades, so has the number of design methodologies and criteria by which geosynthetics are evaluated. This article addresses the following two current design methods commonly used by engineers within the UnitedStates and abroad: The Giroud-Han Design Method (2004) and The U.S. Army Corps of Engineers Design Method (2003).

Geogrid reinforcement mechanisms
A subgrade soil beneath a paved or unpaved surface can fail under load in two ways: localized shear failure

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Learning Objectives
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Instructions
First, review the learning objectives below, then read the Professional Development Series article. Next, complete the quiz and submit your answers to the Professional Development Series sponsor.Submittal instructions are provided on the Reporting Form on page PDH 10. Your quiz answers will be graded by the Professional Development Series sponsor. If you answer at least 80 percent of the questions correctly, you will receive a certificate of completion from the Professional Development Series sponsor within 90 days and will be awarded 1.0 professional development hour (equivalent

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