947
Critical State Modelling of Soil-Structure Interface for Advanced Design
Modélisation à l'état critique d'interface sol-structure pour la conception avancée
Sarma D., Sarma M.D
LM-IGS, M-ISSMGE, Independent Consultant (Southern Africa), Gaborone, Botswana
ABSTRACT: Information on the influence of impregnation of cementitious slurry at the soil-structure interface of bored cast in-situ
foundation is inadequate in available literatures. Moreover, influence of such impregnation on negating the detrimental effects of
smear zone, formed by construction tools, surrounding the borehole is also unknown. In classical foundation engineering, influences
of smear and impregnation are neither considered as dependent functions in determining contributory or negative shaft resistances,
nor in shaft and base resistance interaction. This ignorance contributes empiricism in bearing capacity evaluation recognising it as one
of the possible causes of variation of field performance with respect to prediction. Solution to these problems has been explored
through field and simulated laboratory studies of smear and impregnation, developing new device and technique. Further, an approach
to interface modelling of soil-structure is presented considering impregnation.
RÉSUMÉ : L'information disponible dans la littérature sur l'influence de l'imprégnation de coulis de ciment à l'interface sol-structure
de fondations coulées en place est inadéquate. En outre, l'influence de l'imprégnation sur la négation des effets néfastes de la zone de
souillure, dus aux outils de construction, entourant le trou de forage est également inconnue. Dans les travaux de fondation classiques,
les influences des souillures et l'imprégnation ne sont jamais considérées comme des paramètres liés dans la détermination du
frottement négatif ou positif, ni dans la résistance de pointe. Cette ignorance contribue à l'empirisme en cours dans la détermination de
la capacité portante et peut être l'une des causes possibles de variation des performances sur le terrain par rapport à la prédiction. Une
solution à ces problèmes a été explorée in situ et en laboratoire en développant un nouveau appareillage et une nouvelle technique. En
outre, une approche de modélisation des interfaces de sols structure est présentée en prenant en compte l'imprégnation.
KEYWORDS: Impregnation, smear zone, soil-structure interface, effective diameter, shaft resistance.
1 INTRODUCTION
Barring the situations where a permanent casing is left in the
borehole, in all other bored cast in-situ deep foundations, fresh
concrete comes directly in contact with the ground. During the
process of concreting, cementitious slurry from the body of the
unset concrete of the cast in-situ deep foundation starts
impregnating and upon setting, strengthens the surrounding soil
within the impregnation depth. Physical evidence of
surrounding cement-impregnated soil becoming a part of the
foundation shaft, stated by many authors, was reconfirmed
(Sarma, 1992). In such a case it is apparent that the adhered soil
shall behave as an integral part of the foundation, increasing its
effective diameter. Other researchers (Berezantzev, 1965;
Sowers, 1979) also found evidences of such a phenomenon,
investigation on which is, however, limited. Such impregnation
may increase negative shaft resistance causing serious treat to
both friction and end bearing foundations, positive contribution
of which, may enhance shaft resistance for all types of cast in-
situ foundations. Consideration of such phenomenon in design
can contribute immensely in economic aspects. Therefore,
extent and effect of such impregnation into the surrounding soil
have been studied in this paper for wide variety of soils. These
are aimed at ascertaining the soil-structure interface strength
mobilisation for bored cast in-situ foundations considering
smear zone, which is formed by construction equipments.
Necessary equipment for field simulation was designed and
developed (Sarma, 2000) for impregnation study and eventually
for simulation modelling. Investigations through microscopic
and staining techniques were carried out for insights into the
effects of smear and impregnation with associated effect of
shrinkage. Ignorance of this complex soil-structure interface
interaction contributed empiricism in bearing capacity
evaluation witnessing uncertainty in field performance.
2 FORMATION OF SMEAR ZONE
The process of installation of cast in-situ deep foundation
disturbs the sub-soil formation surrounding the borehole with
respect to its virgin state. The mechanical process involved in
the boring changes physical characteristics weakening the
structure of soil to reduce its shear strength within the zone of
influence of the boring tools. During the progress of boring, soil
in the upper portion of borehole squeeze inward due to the loss
of lateral support and their shearing during withdrawal and
reinsertion of boring-tool further disturbs the structure of clays
or in case of sand reducing its density despite stabilisation under
drilling mud. This process repeatedly continues during each
insertion and withdrawal of the boring tool. Further, the surging
effect that occurs during repeated withdrawal of boring
equipment further loosens the soil. The process of boring also
changes the particle orientation at the interface. Thus a smear or
distortion zone is formed circumscribing the borehole. The
extent of smear zone is thus function of the sensitivity or
relative density of the surrounding soil, magnitude of vibration
and disturbances caused by the boring equipments, properties of
the soil responsible for propagation of velocity wave for the
vibration and disturbances caused during boring etc.
3 DEPTH OF SMEAR ZONE AND INTENSITY
OF SMEAR
Meyerhoff G.G (1976) reported that depth of smear surrounding
bored pile might exist up to 1 inch (25 mm). Besides that, there
is not much information in the literatures about the depth of
smear zone associated with cast in-situ deep foundations. This is
perhaps due to the fact that it is rather a problem primarily
associated with workmanship for installation of such foundation
along with a host of secondary factors and thus any effort for
rational analysis of the problem bristles with difficulties. The
depth of smear or distortion zone depends on the type of boring
equipments, method of boring and more importantly on the
nature and status of the soil. Effort has been made to fulfil parts
