Experimental Research On The Strength And Influencing Factors Of The Frozen Wall Of Foundation Pit

With continuously social development, various kinds of the undergroundconstruction, the deep foundation pit of high buildings and the city subway are beingbuilt, which raises an outstanding problem to the support of deep foundation and deepfoundation pit. Based on these constructions, the frozen wall of deep foundation pit,as an effective support way, is being paid more and more attention to because of itshigh strength, no pollution and great sealing.The frozen wall of foundation pit refers that making use of the artificiallyrefrigeration technology to freeze the soil surrounding the foundation pit, the soilwater is frozen to solid and improves the soil's strength and rigidity, which builds acontinuous and close frozen curtain wall surrounding the expected excavating site.This wall maintains the pit's stability and cuts the connection between groundwaterand the pit inside until the permanent buildings are built, which provides a safe, dryand convenient construction environment and minimizes the influence on surroundingbuildings.In the supporting process of foundation pit, the frozen wall resists the pressure ofsoil and water by itself. The key to design frozen wall is to control its strength andstability. Therefore, the wall needs to be thick and its thickness should meet therequirement of strength. Because the soil's individual characteristics and the wall'stemperature differences, the frozen soil's strength is not homogeneous. Therefore, it isnecessary to study the influencing factors and regularity of the frozen wall's strengthto meet the requirement of wall's strength and protect the supporting construction.Aiming at the fact that the frozen soil's strength of foundation pit changes with thefreezing time, the paper based on theoretical analysis builds the connection betweenthe strength and thickness of frozen soil. Based on the experimental study on influencing factors of frozen soil's strength, its basic changing regularity is studied.Constructing theoretical model to analyze the temperature field during freezing andapplying the ANSYS, the finite element analysis, the distributing regularity of thetemperature field is analyzed. Through the indoor experimental study on frozen wall,the temperature's distributing regularity during the freezing process is verified. Thefrozen system is controlled by designing the temperature controlling equipment,which improves the freezing efficiency and provides the basis for the frozen wall'sdesigning and construction of the foundation pit. The following are conclusions indetail.First, Through the experiment of frozen soil's strength with different water ratiosand freezing temperatures, when the water content is between13%and27%, and thetemperature is between-1oC~-26oC, the general trend is that the homotaxial's pressivestrength of frozen soil gradually increases as the temperature falls; when thetemperature is-20oC, the pressive strength decreases; when the temperaturecontinuously falls, the pressive strength increases again. When the water content isbetween14%and25%, and the temperature is between0oC~-20oC, the tensilestrength of frozen soil tend to increase as the temperature falls; when the temperatureis-20oC, the tensile strength reaches its peak value; when the temperature is below-20oC, the tensile strength decreases as the temperature falls; when the freezingtemperature is fixed, the tensile strength decreases as the water content increases.Second, according to the persistent boundless line-heat-inflow mode, themonotube's freezing mode is built and the analytic solution is offered. The position offreezing facies-change surface is s(t)=2δast.Third, through the ANSYS of temperature field simulation, the temperature fielddistributions of different monotube's calibers are analyzed when the ektexine'stemperature is constant. It is thought that the freezing pipe's caliber has littletemperature influence on freezing pipes with same distance. When the freezing pipesare distributed in single row tube, the freezing speed clearly falls as the freezing spaceincreases; when the single row freezing pipes are frozen, their temperature field is not in circular distribution but in irregular ellipse distribution. When the freezing pipesare distributed in double row tube, the quincunx distribution of freezing holes, whichis compared with the parallel distribution, has slower freezing speed and in the sametime the center temperature of holes in quincunx distribution is lower than that inparallel distribution.Forth, with AT89C52as a core, a set of intelligent temperature control system isdesigned, which can automatically record and store the temperature data and controlthe refrigerating fluid driving pump. The temperature control system applies themicrocomputer technique to the flow control of driving pump, which solves theproblem of controlling the refrigerating fluid driving pump. Thus, the freezingtemperature in the wall can be known at any time and according to this temperature,the refrigerating fluid recycle pump is opened and closed, which control thesoil-freezing temperature of the foundation pit's frozen wall and decrease unnecessaryenergy consumption during the freezing.Fifth, through the simulation experiment of freezing system, when the watercontent of silty clay are10%,15%,20%,25%,27%, the radial distances (10cm,20cm,30cm) in5cm,25cm,50cm of the freezing depth, and the freezing time is prolonged,the changing law of freezing temperature is analyzed. When freezing time is fixed, thefreezing temperature of foundation pit's freezing inner wall falls considerably as itincreases towards parallel direction; meanwhile, with the increase of water ratio, thefreezing temperature falls and freezing range extends. Applying ANSYS to simulatechanging of the temperature field, it is reached that the simulating result is similar tothe experimental result and they closely tally with each other.Sixth, through the simulating experiment of freezing system, it is reached that thefreezing temperature changes little in the vertical direction of frozen wall. Therefore,when designing the experiment, it can be ignored that the temperature changing in thevertical direction of frozen wall is able to influence the frozen wall's strength, whichmeans that only the influence of temperature changes in the radical direction isconsidered.