The Influence Of Pit-in-pit Excavation To The Horizontal Bearing Capability Of Engineering Piles In Soft Clays

In recent years,the urban construction of our country has been in full swing,the complex superstructure and the large urban underground complex are increasing.The emergence of these new forms of construction led to a "deep,big,tight,and close" foundation.Pile foundation is one of the most widely used forms in the construction,especially in the building being built on soft soil foundation,it has an obvious advantage in controlling of the settlement and uniform settlement of upper structure and supporting the upper load.As for the engineering piles in the bottom of foundation pit,because of engineering piles construction prior to foundation pit excavation,together with the engineering properties of low strength and easy to rheological of soft soil,the excavating of foundation pit will have an impact on the work performance of engineering piles.The impact of multiple excavation unloading will make the horizontal bearing capacity more complicated when faced with more complex "pit-in-pit" excavation.Traditional pit in pit,like elevator shaft,has a small physical dimension,and the excavation effect on the existing pile foundation was not been concern because of the slightly influence.But as the inner pits' size is bigger and bigger,this problem is more and more outstanding,become the urgent topic.Relying on Su Zhou Cultural Exhibition Center project,this paper,begin with SuZhou Grand Theatre block field test,based on the parameters and constitutive model verified by the comparison of field test results and the results of numerical simulation.A three-dimensional finite element model of pit-in-pit was been established,the internal force and deformation response regularity and the horizontal bearing capacity loss of existing engineering piles in the region between the inner pit and the out one under the pit-in-pit excavation condition was been studied,Here are the results and conclusions:(1)Ground field horizontal static loading test,internal force and deformation response monitoring of existing test pile when excavated to the bottom of out pit and the inner one has been carried out on the fluvial facies soft soil layer of Taihu Lake.SuZhou Grand Theatre pit three dimensional finite element model was been established and the field test was been simulated,the horizontal load-bearing behavior and the internal force and deformation response of single pile under the influence of the excavation of inside pit and outside pit was been studied on by contrast the field test results and the numerical simulation results.The results show that the load transfer depth is slightly deeper and lateral soil resistance is smaller than the general soil layer due to the weak soil quality of the fluvial facies soft soil layer of Taihu Lake.(2)Based on the soil parameters and constitutive model had been proven by the contrast of the field test results and the numerical simulation results,three dimensional pit-in-pit finite element model was been established,the additional influence of inside pit excavation on the horizontal displacement of the underground continuous wall and the surface settlement behind retaining wall was been analyzed through the contrasting and analyzing of the experimental method estimated results and numerical simulation results;the horizontal bearing properties of piles which adjacent to the outer retaining wall and located the symmetric axes of the outer retaining wall under the cumulative impact of inside pit and outside pit excavation was been studied,the regularity of horizontal displacement and bending moment distribution of piles under the cumulative impact of inside and outside pit excavation was been acquired;the horizontal bearing properties of piles which adjacent to the side wall of outside pit and located the symmetric axes of the outer retaining wall under the additional influence of the inside pit excavation was been studied,the regular of horizontal displacement and bending moment distribution of piles under the additional influence of inside pit excavation was been acquired.It turned out that the internal force and deformation response of piles in the area between the outside pit and the inside pit was the result of coupled effect of outside pit and inside pit excavation,the primary factor of internal force and deformation of piles in different position is different:the internal force and deformation of piles close to the outer retaining wall is influenced mainly by the outside pit excavation,however,the internal force and deformation of piles close to the inner retaining wall is affected by the inside pit excavation more obviously;the proportion of two factors(outside pit excavation and inside pit excavation)which effected the internal force and deformation of other piles vary as the difference of their position.(3)Internal force and deformation response regularity of piles affected by the pit-in-pit excavation under the condition of different inner pit physical dimension was been studied.First,two conditions of the plane area of inside pit remain constant while the depth of the inside pit changed and the depth of the inside pit remain constant while the plane area of inside pit changed were been discussed respectively.The internal force and deformation regularity of piles close neighbor the outer retaining wall and the inner retaining wall affected by pit-in-pit excavation under the condition of the plane area(the depth)of inside pit remain constant while the depth(the plane area)of the inside pit changed was been acquired;based on this,combining different plane area and depth of inside pit(20 kind of conditions),horizontal displacement of pile head y0 and the maximum bending moment of pile Mmax were been chosen as the check index to study the response regularity of internal force and deformation of piles when the physical dimension of inside pit changed.The results indicated that,the internal force and deformation of existing piles in the area between the outside pit and the inside pit showed much more sensitive with the inside pit plane area change than the inside pit digging depth change.(4)The losing condition of the horizontal bearing capacity of existing piles in the area between the outside pit and inside pit caused by pit-in-pit excavation was been studied quantificationally.Double control standards of horizontal displacement of pile head control standard and the maximum bending moment of pile control standard were been recommend,according to the?Code for Design of Concrete Structures?and field test data the failure criterion of the horizontal displacement of pile head ysO and the failure criterion of the maximum bending moment of pile Mmax were been given respectively,based on this,loss ratio of horizontal bearing capacity ? was been introduced to represent the loss condition of horizontal bearing capacity of piles after the pit-in-pit excavation quantificationally;loss ratio of horizontal bearing capacity of piles ?y and ?m were been computed from the point of horizontal displacement of pile head and the maximum bending moment of pile,this two loss ratio's change rules of different position piles under the influence of pit-in-pit excavation were been studied;the distribution condition of horizontal bearing capacity loss ratio of piles in the research area were been counted and analyzed.The result shows that,the horizontal bearing capacity of pile close neighbor the outer retaining wall loss most,and of all the piles close neighbor the outer retaining wall,the piles close to the middle part of long and short sides of retaining wall loss most,some of them even had lost eficacy;With keeping away from the outer retaining wall and closing to the main affected zone of inner retaining wall,the horizontal bearing capacity loss level of piles presented a decreased firstly then increased change rules.(5)The engineering measures for reducing the horizontal bearing capacity losing of the existing piles in the area between the outer retaining wall and the inner retaining wall were given.Starting from changing the supporting structure parameter,foundation soil parameter and pile foundation parameter(other influence factors for the piles internal force and deformation besides the physical dimension of pit-in-pit),the loss condition of horizontal bearing capacity of piles under the influence of pit-in-pit excavation was been discussed.Based on this,The engineering measures for reducing the horizontal bearing capacity losing of the existing piles in the area between the outer retaining wall and the inner retaining wall were given,the result shows that:strengthening the bottom soil of pit can not only control the horizontal displacement of the outer retaining wall and the surface settlement behind the outer retaining wall effectively,but also reduce the loss of horizontal bearing capacity of piles dramatically;the horizontal bearing capacity loss of piles close neighbor the inner retaining wall can be decreased effectively by increasing the thickness of underground continuous wall properly and setting up support for the inner retaining wall reasonably.