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The Application of a Novel Design Approach for Construction over soft soils: The
Hybrid Undrained-Drained model
L'application d'une nouvelle méthode de conception pour des constructions sur sols mous: le
modèle hybride non drainé - drainés
Espinoza D.
Geosyntec Consultants, Columbia, Maryland, USA
Li C.
Geosyntec Consultants, Columbia, Maryland, USA
ABSTRACT: The number of case histories of construction of high mechanically stabilized earth (MSE) berms over soft foundations,
such as dredge disposal sites, is limited for obvious reasons: it is difficult to build without risking foundation instability. The purpose
of this paper is to describe the challenges of designing and building embankments over extremely soft foundations using innovative
design and construction techniques. The case history site for this paper is located in Wilmington, Delaware, where a 2400-m long,
21-m high MSE berm was completed in July 2010. The completion of this 1.5-million-cubic-meter berm represents a significant
engineering achievement considering the size of the embankment and the 30-m deep layer of very soft soils (undrained shear strength
as low as 10 kPa) over which the berm was constructed. The project was recently selected by the American Society of Civil
Engineering among the five finalists for the 2012 Outstanding Civil Engineering Achievement Award.
RÉSUMÉ : Le nombre d'études de cas de construction de digues en terre compactée (MSE) sur des couches de fondations molles,
comme le sont les sites de dépôts de dragage, est limité pour des raisons évidentes : il est difficile de construire sans risquer
l'instabilité de la fondation. Le but de cet article est de décrire les défis de la conception et de la construction sur des fondations
extrêmement molles en utilisant une conception innovante et des techniques de construction appropriées. Le site de cette étude de cas
se trouve à Wilmington, dans le Delaware, où sur 2400 m de long une digue de 21 m de haut a été achevée en Juillet 2010. La
réalisation de cette digue d’1,5 millions de mètres cubes représente une réalisation technique importante compte tenu de la taille du
remblai et la couche de 30 m d’épaisseur des sols très mous (résistance au cisaillement non drainée voisine de 10 kPa) sur laquelle
l’ouvrage a été construit. Le projet a été récemment choisi par la Société américaine de génie civil parmi les cinq finalistes pour le
prix d'excellence 2012 des réalisations de génie civil.).
1 INTRODUCTION
This paper presents a case history of a reinforced earth structure
constructed over extremely soft soils. The design and
construction techniques developed for this project are applicable
to many of the dredge management, levees, and other waterfront
earthen structures.
Figure 1. Site Location Plan
The site where this case study was developed is the Cherry
Island Landfill (CIL), located in Wilmington, Delaware, which
was constructed over an area that was partly reclaimed from the
Delaware River in the early 1900s (see Figure 1) and that had
been used for many years as a dredged material disposal site for
the U.S Army Corps of Engineers (USACE). Under the dredge
layer lies an alluvium deposit with similar geotechnical
characteristics as the overlying dredge. As a result, the
subsurface at the site consist of unconsolidated, very soft, low
permeability and extremely compressible materials with
undrained shear strengths as low as 10 kPa and thickness
ranging from 18 to 30m. Under the alluvial deposit lies the
Columbia Formation (a 12m to 15m thick deposit of medium-
to-coarse, medium-dense-to-dense sand).Because of the dredge
layer’s thicknessand permeability, it was initially estimated that
it would take over 30 years for any excess pore pressures to
dissipate.
To meet the growing demands for waste disposal in the
Wilmington area, an additional 17 million cubic meters (mcm)
of waste disposal capacity (i.e., 20 plus years capacity at the
disposal rates at the time of design) was estimated. Because of
the subsurface conditions, the additional airspace required to
satisfy the needs of the state could not be obtained by increasing
either the landfill sideslopes (8Horizontal:1Vertical in the
original landfill layout) or the landfill height without
compromising the overall foundation stability.Because the site
is located at the confluence of the Delaware and Christina
Rivers (See Figure 1), the potential for a horizontal expansion
was limited;hence, the main alternative to obtain additional
capacity at this facility was to expand it vertically.In order to
obtain the required capacity, the only option available that
would provide the additional disposal capacity was to build a
2400-m long and 21-m high mechanically stabilized earth
(MSE) berm around the facility and place waste behind this
structure. The preliminary feasibility study indicated that, in
order to build the 21-m high MSE berm, the foundation shear
strength needed to be improved from 10 kPa to 160 kPa as a
minimum. The preliminary conceptual solution for achieving
this strength gain was to use deep soil mixing (DSM), a
technique that consists of mixing soil with cement. Because of
the depth, length, and width of the soft soils that needed to be
improved, the volume of soil that needed to be treated was
approximately 2.5 mcy. At the time the construction of the soil
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