Study On Key Techniques And Bearing Mechanism Of Section-variable Reinforced Jet-grouting&Mixing Cement-soil Pile

Section-variable reinforced jet-grouting&mixing cement-soil pile, abbreviatedas SRC pile or SRCP, boasts both technical and economical advantages of jet-groutingand mixing technique. Its strength and bearing capacity can be greatly enhanced byvarying its section and inserting some stiff core-materials, and hence the pile has abroad application perspective in compressive and anti-uplift engineering projects. Keytechniques of SRC pile including construction devices and construction processeswere developed, compressive and anti-uplift bearing mechanisms of the pile werestudied by means of in-situ tests, in-lab modeling tests and numerical simulations, etc.,and furthermore, a calculation method of bearing capacity was put forth by taking intoconsideration the coordination of displacements. Finally, two typical engineeringcases were introduced, which were constructed and calculated with the constructiontechniques and design methods of the paper.Firstly, a set of devices and processes were developed for the construction ofSRC piles. In designing the construction devices, the idea of modular design wasapplied, so that they could be used both for SRC pile construction and for otherconstruction purposes, and hence “one instrument, multiple purposes”. Thejet-grouting technique facilitates the realization of variable section with relativelysimple construction devices and processes. The combination of jet-grouting techniqueand mixing technique can bring into full play all the technical and material advantagesof cement-soil pile and reinforced pile. With the combined technique, a larger pilediameter can be obtained in a relatively economical way and the drilling capabilitycan be greatly enhanced. Moreover, the jet-grouting construction technique combingrod spraying and blade spraying with a relatively small quantity of returning cementpaste has a wider adaptability in various strata, and hence a construction technique ofenergy saving and environmental protection.Secondly, modeling tests were carried out in laboratory, and results showed that for a variable-section pile with one enlarging plate or two enlarging plates which werewide apart, the soil mass under the plates was compressed, while the soil mass uponthe plates tended to collapse in a pear shape. When the plates were close to each other,the soil mass between them integrated with them. The settlement curve ofuniform-section pile was similar with that of the variable-section pile; however, theside friction (including the pile side friction and the plate bottom resistance) of thelatter was much higher than pile end resistance, regardless of the quantity of plates orinterval between them. The position of the enlarging plate has a certain influence onthe bearing capacity of the variable-section pile; but the interval between the platesinfluenced the bearing capacity in an insignificant way. For the variable-section pilewith three enlarging plates, its bearing capacity was a bit higher than that with twoplates. All in all, the bearing capacity of variable-section pile has been greatlyenhanced to the degree of being not less than the uniform-section pile of the samediameter as the enlarging plate. Distribution of axial stress along variable-section pileis different from that of uniform-section pile. With the increasing of load level, theaxial stress under the plates decreases sharply due to the bearing capacity of theplates.The bearing characteristics of the SRCP were studied with the numericalsimulation method. For the uniform-section pile, the plastic damage zone was focuson the pile end; but for the pile with the enlarging plate, the displacement field and theplastic damage zone changed significantly, its load-bearing characteristics weresimilar with the multi-pivot end bearing-friction pile. The enlarging plate beared mostof the load, became a stress concentration area instead of the pile end. The pile-endbearing capacity played a noticeable lag, and the plastic damage zone changed tofocus on the enlarging plate and expanded with the increasing of the load. When twoplates were close to each other, the stress field around the upper and lower platesappeared to superposition; but when the interval between the plates increased to acertain degree, around two plates formed stress concentration area and plastic damagezone respectively. But the plastic damage zone around the upper plate was larger thanthat around the lower plate; a small plastic damage zone would also form around the pile end. The enlarging plate was set much more upper, the sooner it started to play itsrole, the later the pile end started to play its role accordingly.Thirdly, in-situ pull-out tests and numerical simulations were carried out andresults showed that the improvement in the interface between pile and soilsignificantly influenced the ultimate anti-uplift bearing capacity. With the function ofthe pile taken into consideration, hereinafter the pile is addressed as anti-floatinganchor in small-scaled anti-uplift engineering projects, or anchor in short. With thevariable-section technique, the side friction was greatly enhanced and thedisplacement under loads was significantly decreased, compared with conventionalmud protection technique or anchor-soil interface improvement technique. When theload level was low, the anchor-soil interface improvement technique insignificantlyconfined the elastic displacement as well as plastic displacement; however, even inthis case, the application of variable section could decrease both the elasticdisplacement and the plastic displacement. Compared with the conventional mudprotection technique, the eigenvalue (characteristic value) of cohesion strengthbetween pile (anchor) and soil was enhanced by3.0times when the anchor-soilinterface improvement technique was applied and by3.5times when the variablesection technique was applied, while the unit cost increased only by about20%and30%respectively, and the total cost was estimated to decrease over50%. Moreover,with SRCP technique, the construction efficiency could be enhanced to10~20timesthat with the conventional mud protection technique. The higher the strength ofanchor-soil interface is and the larger the diameter of the enlarging plates are, thehigher the up-lift resistance will be and the smaller the displacement will becorrespondingly.Fourthly, a method of calculating the eigenvalue of bearing capacity was putforth by introducing different efficiency coefficients of failure side friction and failureend resistance of pile, and calculation formulae were established for calculating theeigenvalues of bearing capacity of the uniform-section pile as well as thevariable-section pile. On basis of analyzing the failure mode of variable-section pile,procedures were proposed for designing compressive SRC pile and anti-uplift SRC pile respectively.Finally, two typical engineering cases in anti-floating projects were introduced.Engineering practices showed that the application of SRCP method could achievegreat social and economical results in solving anti-floating problems undercomplicated geological conditions.