1975
Computed and observed ground movements during top-down construction in
Chicago
Mouvements de terrains calculés et observés en construction descendante à Chicago
Finno R.J., Arboleda L., Kern K.
Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL. USA
Kim T.
Korea Railroad Research Institute, Uiwang, Korea
Sarabia F.
AECOM, Vernon Hills, IL, USA
ABSTRACT: Two detailed case studies of deep excavations in Chicago made with top down techniques are presented. The
importance of considering all aspects of the construction process when estimating ground movements is emphasized. Detailed
construction records were maintained at both sites. Inclinometers located within the walls, close to the walls and 7 m from the wall
provided lateral movements throughout construction. Ground surface settlements were obtained by optical survey of several hundred
observation points at each project. In addition, one of the projects included 88 strain gages installed in the floor slabs to measure time
dependent responses of the concrete slabs used as lateral support for more than four years. The movements are presented in relation to
construction activities and causes of incremental movements are identified. The lateral movements that arose from cycles of
excavation and bracing accounted for approximately one-quarter and one-half the total movements at the two sites. Of these, field
performance data and results of numerical simulations showed that approximately 40% of the movements arose from the time-
dependent responses of the concrete floor slabs.
RÉSUMÉ : Deux études de cas détaillées d’excavation profondes réalisées à Chicago en construction descendante sont présentées.
L’accent est mis sur l’importance de prendre en considération tous les aspects du procédé de construction. Des registres de
construction détaillés ont été maintenus sur les deux projets. Des inclinomètres situés dans les murs, à proximité des murs et à 7m du
mur ont mesuré les déplacements horizontaux tout au long de la construction. Les affaissements de surface ont été observés à l’aide
d’un relevé topographique optique comprenant plusieurs centaines de points pour chaque projet. De plus, l’un des projets comprenait
88 jauges de déformations installées dans les dalles de plancher pour mesurer les réponses dans le temps des dalles de béton utilisées
comme supports latéraux pendant plus de quatre ans. Les mouvements sont mis en relation avec les activités de construction et les
causes des mouvements progressifs sont identifiées. Les mouvements latéraux causés par les cycles d’excavation et de
contreventement ont représenté environ la moitié et le quart des mouvements totaux sur les deux projets. Pour ceux-ci, les résultats
des tests sur le terrain et des simulations numériques ont montré qu’environ un tiers des mouvements était causé par les réponses dans
le temps des dalles de planchers en béton.
KEYWORDS: Excavation, top-down support, ground movements, clays, time-dependent concrete response
1 INTRODUCTION
Use of top down construction has increased as more
developers have seen the benefit of taking the excavation
portion of a project off the critical path. Top down methods use
permanent walls and flooring systems as temporary support and
thus the support systems are very stiff. Yet, there are conflicting
data concerning whether resulting movements are smaller than
those associated with bottom-up methods. For example, Long
(2001) observed no discernible difference in the performance
of internally supported, anchored, or top-down systems based
on examination of 296 excavation case studies. Kung (2009)
reported results of 26 excavations made through Taipei silty
clay which showed the maximum lateral wall deflection induced
by the top-down methods were 1.3 times larger than that
induced by bottom up methods. These observations are
surprising given that the floor slabs are in theory much stiffer
than either cross-lot braces or ground anchors, and that it is not
possible to overexcavate the soil – i.e., make a deeper cut than
planned so that the support system is essentially more flexible
than planned in design - during construction since one must cast
each floor on the ground.
This paper summarizes two case studies of deep excavations
in Chicago made with top down techniques. Detailed
construction records were maintained at both the Block 37 and
One Museum Park West (OMPW) projects. Performance
during construction is illustrated with results of inclinometers
located close to the walls and optical surveys of points on the
ground adjacent to the excavations. In addition, one of the
projects included 88 strain gages installed in the floor slabs to
measure for four years the time-dependent responses of the
concrete slabs used as lateral support. The movements are
presented in relation to construction activities at both sites and
causes of incremental movements are identified. It is shown
that the movements that occurred during cycles of excavation
and bracing are small, and about 40% of these movements are
attributable to the time-dependent responses of the concrete
slabs. The importance of considering all aspects of the
construction process when evaluating movements is
emphasized.
2 SUBSURFACE CONDITIONS
The subsurface conditions at the two sites are summarized in
Figure 1 which shows the natural water contents and undrained
shear strengths found from results of site investigations at each
location. The surficial layer is an urban fill material consisting
of sandy soils and construction debris. Underlying the fill
material is a sequence of glacially deposited clays. The first
two layers are soft to medium stiff clays which are very similar
mineralogically,
but
exhibit
different
geotechnical
characteristics due to the type of glacial deposition. The
Blodgett stratum underlies the urban fill and was deposited in a
