Some Issues On The Bearing Behavior Of Existing Loaded Foundation Piles Subjected To Creating Underground Storey

The bearing behavior of existing piles and underpinning piles would be changed by excavation beneath existing buildings.At first,the settlement behavior of existing single pile and pile groups is analyzed by the case of Zhejiang Hotel Expansion Project.As the excavation depth increased,the constraint applied by soil to pile was degraded.This may lead to buckling instability of the pile foundation.Thus,the buckling stability of the existing single pile and underpinning pile is analyzed by the cases of Zhejiang Hotel Expansion Project and No.3 subsection of the Ganshuixiang Construction Project.On this basis,the influence of different constraints applied at the head of pile on the buckling stability is studied.At last,the influence of excavation beneath existing building on the settlement behavior of superstructure,bearing behavior of underpinning pile and new structure pillar are analyzed by the field data and finite element model of No.3 subsection of the Ganshuixiang Construction Project.The main work and conclusions are summarized as follows:(1)The mechanical model and governing equation of pile at the condition of excavation beneath existing building are eastablished.The load transfer behaviors of pile shaft and pile tip are simulated by the skin friction softening model and hyperbolic model,respcetively.The result shows that the skin friction is much more sensitive to the excavation depth than its end resistance,and the settlement at the head of pile increases with the increase of excavation depth.The interaction between existing pile and underpinning pile is taken into account with the assumption that the sectional dimensions of the existing pile and underpinning pile are the same.The effect of settlement behvaior of pile groups under excavation is analyzed by the mixed method.The result shows that,the head settlement of the central pile among square arrangement increases with an increase in excavation depth;under the same excavation depth,the settlements at the head of the corner pile,the side pile and the central pile are the maximum,the medium and the minimum,respcetively;in addition,the different arrangement has a certain influence on the settlement behavior of the central pile;the influence of excavation depth on the settlement behavior of central pile among the circle,square and rhombus arrangement is greatest,middle and least,respectively;(2)Based on the loading test of the underpinning pile loacted at the No.3 subsection of the Ganshuixiang Construction Project,the settlement at the head of the underpinning is 1.61 mm,when the counter force supplied with the reaction frame reaches ultimate value;the residual settlement is 1.22 mm after the unloading stage;the skin friction softening with in upper soil layer ocurred when the load applied at the pile head exceeded 150 kN.Combined with the other field data,the axial force of the underpinning pile increases with increasing excavation depth,while it decreases with increasing embedded depth;the skin friction is slowly mobilized from top to tip;the end resistance is zero during the whole construction project;the settlement increases slowly with the time,but increases rapidly during the process of pile cutting;the axial force of structure pillar increases rapidly with the number of the cutting piles,particularly the No.3 pile is cut;the effect of pile cutting on the performance of neighboring pillar could be negligible;(3)Based on the cases of the Zhejiang Hotel Expansion Project and the No.3 subsection of the Ganshuixiang Construction Project,the total potential energy of the pile-soil system of the rock-socketed pile and non-socketed pile is established by the energy method.The expression of critical buckling capacity is deduced by minimum potential energy principle.Then,the influence of excavation depth and constraints applied at the head of the pile are analyzed.It shows that the critical buckling capacity converges with the half-wave number;different constraints have some certain effect on convegence rate,the convergence rate of fixed pile head is fastest,the hinge takes second place and the fixed with sway is slowest;the skin friction and self-weight of the non-socketed pile impose little influence on the buckling load of pile shaft;the critical buckling capacity of the non-socketed pile increases with an increase in proportional coefficient m,but the influence of proportional coefficient m could be negligible when the excavation depth exceeds 4.0 m;the critical buckling capacity decreases rapidly with increasing excavation depth;constraint conditions at pile head affect critical buckling capacity,the ratio of critical buckling capacity of fixed pile head is largest,fixed with sway takes the second place and the hinge is smallest at the same excavation depth;the ratio of critical buckling capacity of fixed pile head decreases slowly with increasing excavation depth,however,the hinge and fixed with sway at pile head decrease rapidly;(4)The three dimensional finite element model of the 7C cap is established by the Plaxis 3D finite element software.Then,influence of excavation beneath existing building on the settlement and the bearing behaviors of underpinning pile and new structure pillar is analyzed.It shows that,the influence of excavation on the cap settlement before foundation underpinning is much more sensible than the underpinning pile is installed;the casting concrete of bottom slab has some certain influence on the cap settlement,after the excavation depth reaches design depth;while the effect of foundation underpinning and casting the concrete of structure pillar on the cap settlement could be negligible.The different cutting sequences have some influence on the axial force of structure pillar.