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Design prediction of the strengthened foundation base deformation by field tests
data
La prèvision de calcul des déformations de la base des fondements reportès à partir des
recherches prises en nature
Gotman A., Gotman N.
BashNIIstroy, Ufa, Russia
ABSTRACT: The paper presents the solution of the complicated practical geotechnical problem of the skeleton structure foundation
strengthening. The strengthening was done due to change of the spatial arrangement of a building and essential load increase. The
experience of a foundation strengthening with jet grouted piles is described based on results of the base deformations monitoring. The
main design principles of the foundations under strengthening are given. The results of the deformation design prediction based on jet
grouted piles test and the base deformation measuring are presented.
RÉSUMÉ : Dans cet article on présente la solution d’un problème géotechnique pratique compliqué du renforcement des souches
d’un bâtiment en carcasse construit sur les sols de fondation dangereux à cause du karst, en raison du changement lors de la
construction de la conception de plan et de volume du bâtiment et de l’accroissement signifiant de charge. L’expérience est décrite du
renforcement des souches par des papillons d’injection de forage sur la base des résultats du monitoring des déformations des sols de
fondation. Les résultats sont présentés des pronostiques prévisionnelles des sols de fondation des souches renforcées à la base des
essais des papillons d’injection de forage et de la mesure des déformations du sol de fondation.
KEYWORDS: foundation strengthening, settlements, pile vertical load test
1 INTRODUCTION
Design prediction of the strengthened foundation base
deformation by field tests data was executed for the new
shopping centre located in Ufa (Russia). The 500x250m
shopping center was designed as a skeleton one-storey building
with column spacing 16x8 m. The building construction was
started in April, 2007, then restarted in June, 2009 and finished
in 2010. Since May, 2008 till August, 2009 the construction at
site has been not performed. In 2007…2008 the foundations and
the most part of the bearing structures have been constructed.
During construction time, the building part was changed (by
investor’s demand). The column spacing was increased (8x16m
to 16x16m or 12x16m) and foundation loads to the moment of
construction stoppage 1,2…1,5 times increased the design ones.
After construction restarting, other changes of building frame
design have taken place. At the significant area the number of
stories and floor loads have been increased. As a result, all these
changes provoked 30…70% increase of the foundation loads
and the further foundation strengthening.
2 ENGINEERING-GEOLOGICAL CONDITIONS
Under the foundation base stiff clays, tough and soft loams
occur underlain with water saturated medium coarse sand and
gravel at the depth of 8…10 m. (table 3). Maximum predicted
ground water level is 2 m under the foundation base. The
building site applies to the third category of stability about karst
deformations and is divided into sections according to extent of
their risk in accordance with Russian Codes. At this site, areas
are located that are classified according to their karst risk as
potentially not dangerous and potentially dangerous (fig.1) with
the probable design diameter of karst hole 7 m.
Due to site severe engineering-geological conditions
characterized with lack of homogeneity and karst risk, the
following foundations were designed: post- and strip
foundations on the bed; piled foundations with the in-situ raft
(pile groups); strip foundations on the bed and piled foundations
with the strip in-situ raft reinforced considering a karst hole
formation.
Severe engineering-geological conditions of site demanded
foundation settlement observation and expert investigation of
construction.
3 TECHNICAL EVALUATION OF THE STRUCTURAL
CONCEPT ON FOUNDATION STRENGTHENING
When design working out and the way of the foundations
strengthening selecting, the following was taken into attention.
1. To the moment of the strengthening design development,
the building was 1,5 years. In axes 1…15’ the bearing structures
were completely constructed and at the rest part of the area
foundations, columns and floors of the ground floor were built.
2. According to monitoring results (table 1), it was stated
that to the moment of strengthening design development
(August 2009), the settlements on the whole were stabilized.
The settlements of the column foundations with the loads
exceeding the design ones already in the process of construction
were 6…10cm; the settlements of the rest foundations were
3…4 cm. The settlements data were used to evaluate the
coefficients of subgrade reaction of strip- and post foundations
bases that demanded strengthening (fig.2, table 2).
Taking into account that the significant part of the structures
was constructed and more than half of the base loads have
already been transferred, when selecting the method of the
foundation strengthening the minimum digging out and
dismantling (drilling, cutting, etc.) of the existing foundation
should have been provided. Method of strengthening with jet
grouted piles was selected with loads transfer from the building
through the connection of the reinforced concrete column with
the in-situ raft rested on piles.
