Study On Bearing Mechanism Of Squeeze And Expansion Cast-in-Place Pile And Its Application In Power Engineering Projects

Pile foundation has a long history of application. It is widely used in allkinds of industrial and civil construction due to its excellent performance ofload bearing capacity, and easy quality control. However, work cost of thepile foundation is relatively higher, and therefore, technical experts andengineers have made a lot of researches on its form, and they have createdso many special form piles of bottom expansion type and reduction type sothat the unit volume can exert bigger load bearing potential. The squeezeand expansion cast-in-place pile is a special pile of variable cross sectionoriginating from common straight-hole cast-in-place pile. Since 1990swhen it was borne, the pile is widely used in engineering constructionprojects due to its remarkable technical and economic performance. Thepower project integrates a lot of funds, and the technical features of the pilealso provide higher requirements for foundation of building structure. Inrecent years, thermal power generation projects tend go grow withgenerator set of big capacity (with capacity of single unit being 600MW orhigher), high parameters (super critical or super-super critical, circulatingfluid bed) and availability of environment protection (desulphurization,electronic dust removal and air cooling technologies, etc.). Now, the maingeneration equipment has been developed into fixed type matchingproducts, and the space for optimization of the construction structurebecome less and less. However, bigger space really exists for reducingexpenses of base treatment, and reasonable optimization ofbase/foundation type will be directly related with the amount of investmentper Kwh. Since it was borne, the squeeze and expansion pile has beenadopted in power construction project from the initial trial use in small powerplants and auxiliary building structures into more extensive application inlarge-scale power plants and main building structures, from which valuableexperience has been accumulated in the aspects of design, constructionand test. However, the technology is restricted for further popularization tosome extent due to limited theoretic study of the squeeze and expansionpile, especially the insufficient understanding of the bearing mechanism ofits supporting tray. The Temporary Technical Specifications for SupportingTray Cast-in-Place Pile Used for Thermal Power Plant, which is a standardof power industry issued in 2000, has widened the application of thesqueeze and expansion pile technology since it was borne, but it has to berevised since some contents of the specifications mentioned above can nolonger fully satisfy the production requirements with continuousaccumulation of work experience, and continuous development of squeezeand expansion tray technology and the equipment. This thesissummarizes the work experience about application of squeeze andexpansion pile in power construction project completed in recent years, andcarries out further study on bearing mechanism of squeeze and expansioncast-in-place pile used in hard clay base, of which, the study achievementswill produce certain practical significance for further revision to thespecifications and engineering construction in the future.This thesis carries out overall systematic study and summarization ofthe squeeze and expansion cast-in-place pile concerning its generation,development and research theory. In combination with technical featuresof the power construction projects, it forwards technical principles andimplementation procedure for application of the squeeze and expansioncast-in-place pile in the power construction projects. In consideration ofhard clay base conditions, it uses large scale test of primary body, testresearch of soil in original conditions, and Cambridge models, etc toestablish mathematic models of joint action from pile and earth. Throughsimulation of finite element values, it carries out deeper study on loadtransfer mechanism of the squeeze and expansion pile. Further, itforwards the bearing load calculation formula for squeezing and expansionof the pile into the hard clay base. Simultaneously, it makes analysis andevaluation on technical and economic effect of the squeeze and expansioncast-in-place pile being used in large scale power construction projects, ofwhich the main work contents and study achievements are as follow:1)By means of reference to tremendous data and information aboutthe squeeze and expansion cast-in-place pile, this thesis reviews itsorigination, development and research theory systematically, and fullymasters the research situations and development trend in the field;itsummarizes the technical principles and process implementationprocedures for application of the squeeze and expansion cast-in-place pilein power construction projects, and expounds the practical significance oflarge scale primary body test on study, application and theoreticdevelopment of the squeeze and expansion cast-in-place pile.2) The large scale primary body test is adopted to study the loadtransfer conditions of the pile squeezed and expanded into the hard clayearth. Then, analysis is made between the squeeze and expansion pile ofdifferent number of trays and ordinary cast-in-place pile before achievingthe following conclusions: Compared with ordinary cast-in-place pile, due tosetting of body supporting tray, the substantial change has happen on theload bearing performance of the squeeze and expansion pile. Theobvious sudden changes have happen on the supporting tray to the axialforce of pile body in depth direction, and the supporting tray can beexecuted under smaller load action, and the settlement of the squeeze andexpansion pile is less than that of the ordinary cast-in-place pile under thesame load. The rise of bearing capacity of the squeeze and expansionpile will be decided by degree of execution of the resistance force of thesupporting tray, of which the action is played accordingly, but notsimultaneously.3)Based on analysis of various body structure relation models anddestruction rules of the soil, it uses the Cambridge elastic model, Mises rule,and Mole-Coulomb rule as the numerical simulation model to simulate thestress and strain path for loading conditions of the squeeze and expansionpile, and through laboratory test of hard clay soil in primary conditions(including routine physic/mechanical test, 3-axis shearing test, and equaldirection solidification test, etc.), it determines the parameters for bodystructural models, and considers the influence from early solidificationstress factors of the hard clay layer. The model can properly simulate thedeformation features of hard clay under loading conditions, and reflect theshearing expansion performance of soil body, and therefore, the simulationachieves satisfactory result.4)The Cambridge mechanical body structural model of elastic-plasticsoil is used to simulate the whole process of load application for primarybody test, and the finite element method is used to analyze the axial forcetransfer process of pile body in the aspect of stress, deformation of adjacentsoil body around the pile and force development and load distribution oneach supporting tray so as to study the load bearing mechanism of thesqueeze and expansion cast-in-place pile. The following main results areachieved:Stress may be concentrated on soil body adjacent to end corner andlower slope of the supporting tray when the vertical load is applied onthe squeeze and expansion cast-in-place pile, while the soil bodyadjacent to its upper slope may loosen or fall collapse, some pilebodies may be separated fro the soil body so as to lead insufficient playof friction force of the pile body within certain range at upper part of thesupporting tray, and therefore, the horizontal and vertical stress of soilbody will decrease. According to calculation result, the side frictionreduction caused by the hollow side in upper part of the supporting traywill fluctuate within a range of 2～4d (d means diameter of the mainpile);with increase of stress in soil body adjacent to the lower slope ofthe supporting tray will fluctuate within a range of 4～6d.Performance of pile and soil under different vertical load action: Undermajor vertical load action of soil adjacent to the pile, stresscentralization phenomenon of the soil body looks more obvious, andthe plastic zone exists from nothing, to the soil body developed within arange of 1～2d. Earlier development is found on the plastic zone atthe lower supporting tray and pile bottom than the upper supporting tray,and its range of influence is relatively wider.Numerical simulation further clarifies the special features of bearingconditions of the squeezing and expanding tray type pile. At earlystage of loading, both side friction resistance and end resistance of thepile obviously participate in the load action. With increase ofsettlement on crest of pile, both side friction resistance and endresistance will increase accordingly, and crest load sharing proportionremains unchanged. Apply the load further, the side resistance willgradually come to its utmost limit, and its crest load sharing proportionwill decrease, while the end resistance will continuously increase.Almost all of the increment of crest load will be borne by the endresistance, and its crest load sharing proportion increases with obviousincrease of settlement, which shows obvious development of plasticzone of the base soil of the pile. When the crest load increment ismade big enough, the settlement of pile crest will increase sharply, andthe end resistance will gradually reach the utmost limit, and crest loadsharing proportion by the end resistance almost remain unchanged.Resistance changing trend of all supporting trays: Supporting trays withhigher depth can provide higher end resistance at the same settlement.Therefore, if conditions allow, all supporting trays should be set into thesoil as deep as possible so as to use bigger gravity stress from thedepth to promote the end resistance provided by all supporting trays soas to promote the utmost bearing force of the entire squeeze andexpansion pile. Meanwhile, resistance of tray will grow with increaseof crest settlement. When settlement on the upper supporting traybecomes bigger, the end resistance will grow vigorously. This isbecause that the soil body surrounding the supporting tray exceeds itsearlier solidification stress at the moment, it comes into the plastichardening stage, and therefore the strength goes up. For 2-tray or3-tray piles, the end resistance may reach 80% and 95% of the totalend resistance, respectively. This shows that the end resistance ofthe squeeze and expansion pile is mainly supplied by all supportingtrays.5)Through analysis on calculation result of existing various bearingforces, and considering the infinite elements analysis result, this thesisforwards the correction parameter values for resistance of the supportingtry in the existing power industrial rules, submits the formula for calculationof tray resistance by means of soil strength indicators of hard clay base,and enriches the theory on bearing force of the squeeze and expansionpile.6)The achievement of this thesis is used in the design of Baorixilepower plant, which makes good technical and economic profits. Analysisand evaluation have been also made on application effect of squeeze andexpansion piles used in several large-scale power construction projectswith different base soil conditions (clay, silt and sandy soil), and resultshows that this technology can fit for diversified base conditions, and theeconomic profits are remarkable. It also accumulates many ripe designand construction experience, which are helpful for popularization andapplication of the squeeze and expansion cast-in-place pile.