Numerical Simulation And Stability Analysis Of Composite Soil-Nailing Of Xiamen Yinjuxiang Building Foundation Pit

With the utilization and development of high-rise buildings andunderground space, there are more and much deep foundation pit projects inpractical engineering. The engineering scale tends to be larger and thetechnique difficulty tends to be more serious. Traditional design methods offoundation pit bracing can not meet the need of today's development for itsinherent defect. Composite soil nailing is a new,flexible,widely appliedgeotechnical integrated supporting structure , which inherits and develops thetraditional soil nailing technique, this technique overcomes much defect oftraditional soil nailing, so it has a broad developing prospects. However, theresearch of theoretical analysis and engineering practice on composite soilnailing is not ripe and perfect at present, and then the research of its rolemechanisms and stability analysis is still on the primary stage.This paper simulates the whole procedure of the staged excavation andretaining on composite soil nailing by use of theoretical analysis andnumerical simulation, taking an engineering case as an example. and thendiscusses role mechanisms,variable laws of displacement of slide slope andstress distribution by numerical result with comparison ordinary soil nailing tocomposite soil nailing. Contents are mainly showed as follows:(1)Summarizing the formation, development, present research as well asthe main patterns and prospects of composite soil nailing, And analyzing thedeficiency of present research on its technique.(2)Discussing the basic structures, the role mechanisms of composite soilnailing, and analyzing separately the role of cement mixing pile, the role ofpre-backing micro pile, the role of pre-stressed anchor in composite soilnailing.(3)The destabilizing patterns of composite soil nailing have beendetermined and the superimposed method of composite soil nailing stabilityanalysis has been put forward, and giving the safety factor of stability analysisof cement mixing pile, stability analysis of pre-backing micro pile, stabilityanalysis of pre-stressed anchor.(4)Studied the engineering case taken pre-backing micro pile andpre-stressed anchor as examples of composite soil nailing by numericalsimulation, and then discussing the role mechanisms,variable laws ofdisplacement of slide slope and variable laws of stress distribution bynumerical result which comparing ordinary soil nailing to composite soilnailing.This paper studies the composite soil nailing, composed of pre-backingmicro pile and pre-stressed anchor, which was taken at YinJuXiang deepfoundation pit bracing project. By taking a real case, base on and an idealMohr-Coulomb model was selected as a soil constitutive law, a numericalanalysis with FLAC(Fast Lagrangain Analysis of Continua) program wascarried out. Through the comparing analysis on the two support structures,some significant conclusions are remarked as follows:(1)Distributions of horizontal displacement of composite soil nailingfoundation pit with depth are curve in shape, which is as similar as those oftraditional soil nailing while the excavation and bracing of foundation pit isdeep, and its incremental tendency increases firstly then decreases graduallywith depth. Reinforcement parts of composite soil nailing structure play aimportant role with the depth increasing gradually, which restrain effectivelythe development of horizontal displacement. However deformationaccumulation always takes place at the latter stages of excavation and bracingat foundation pit slope by traditional soil nailing. The maximum occurs at theunderneath middle of foundation pit.(2) Distributions of vertical displacement of composite soil nailingfoundation pit along with distance to foundation pit slope are curve in shape.To common soil nailing, its settlement increases firstly then decreasesgradually with distance to slope, the settlement maximum value of whichoccurs at 0.25H to slope;To composite soil nailing, its settlement decreasesgradually with distance to slope, while the settlement maximum of whichoccurs at the top. Settlement increment occurs with each stage of excavation,the position of which takes on a tendency away from the slope, and the slopetop heaves firstly then decreases gradually with excavation.(3)Soil nail in composite soil nailing structure is affected by pullingwork. The internal force distributions of soil nail is uneven, maximum stressof which occurs at the joint part of anchor and face layer, and the stressdecreases nonlinearly along with the length of the soil nail;the axial force ofpre-stressed anchor is constant force and the axial force of anchor groutingbody fades nonlinearly. The stress of soil nail and anchor has a graduallyincreased tendency with every stage of excavation.(4) Pre-stressed anchor in composite soil nailing can restrain effectivelythe development of soil deformation. Pre-stressed anchor set at the top canrestrain effectively slope horizontal displacement which decreases with theincrement of pre-press value, many lines pre-stressed anchor can restraineffectively slope horizontal displacement when the depth of foundation pit isdeeper and the deformation is restricted strictly, and better effect of bracing isacquired.(5)Pre-backing micro pile in composite soil nailing can maintain thetemporary stability of slope soil in process of excavation and bracing, andwhich reduces discharging degree of slope soil, restricts the development ofslope displacement and yield plasticity at the zone reinforced by pre-backingpile, and restrains the further development of soil lateral deformation.Pre-backing micro pile combined with pre-stressed anchor can restraineffectively slope horizontal displacement in process of excavation and bracing.(6)Stress distribution level is lower at the zone reinforced by soil nailand pre-pressed anchor as which bearing most of the load, so soil bears letterstress, and they perform the role of diffuse and transfer, and better intensivestate of stress in soil. Shear-stress aggregates badly at base of slope and soil isat the state of yield plasticity, so taking slope toe seriously and refinedmonitoring, overexcavation prohibited, bracing in time in process ofexcavation and bracing.