Geosynthetics February/March 2022 - 15

Case Studies
Reinforcement over soft soils
with high-strength geogrids
By Marianna Ferrara
C
onstruction over soft foundation soils is usually associated with poor bearing capacity,
excessive differential settlements and lateral sliding instability. Furthermore, the
presence of poor ground conditions at a construction site is the most frequent cause of
long construction durations.
There are several alternatives that can be employed to achieve project objectives in
these conditions. Common primary functions of these alternatives are to increase shear
strength and bearing resistance, increase density and drainage, and provide lateral stability
(Schaefer et al. 2017). These alternatives include (1) bypassing the poor ground through
relocation of the project to a more suitable site or using a deep foundation; (2) removing
and replacing the unsuitable soils; (3) designing the planned structure to accommodate the
poor/marginal ground; or (4) modifying (improving) the existing soils, either in place or
by removal, treatment and replacement of the existing soils (ASCE 1978; Mitchell 1981).
Typical ground improvement techniques include prefabricated vertical drains and
accelerated consolidation, lightweight fills, deep compaction, aggregate columns, columnsupported
embankments, soil mixing, grouting, pavement support stabilization, and
reinforced soil structures (Schaefer et al. 2017). All these ground-improvement techniques
have inherent limitations on their applicability as related to the type of soil, depth, or
treatment and performed function. Long-duration ground improvement projects are
too costly and, in many instances, unacceptable for the current financial and political
approach to highway jobs.
In addition, many new highway projects involve widening of existing roads, or they
cross special soil deposits where traditional methods are not technically viable.
An alternative solution is to use a high-strength geosynthetic reinforcement placed
directly on top of the soft soil and build the structure on top of it. This type of geosynthetic
application is often referred to as basal reinforcement (Figure 1). The use of a geosynthetic
reinforcement under embankments or other types of structures constructed over soft soils
or localized weak areas (i.e., sinkholes) can be categorized into two different applications
based on the design target:
* Basal reinforcement alone or combined with vertical drains to control the stability of
the structure without controlling overall settlement
* Basal reinforcement combined with a foundation stabilization system, such as
rigid intrusions, to control the stability of the structure, but also to prevent or
limit the settlements
Marianna Ferrara, M.S., is GSY
business development manager
North America for Officine
Maccaferri S.p.A. in Zola
Predosa, Bologna, Italy.
All figures courtesy of the author.
www.GeosyntheticsMagazine.com
15

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Geosynthetics February/March 2022

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