Pile Partner (Variable Rigidity Pile) Optimized Research On Direct And Indirect Foundations

The Chinese invention patent "hoop of pile head&pile with hoop (200710160966.1):s commonly known as Pile Partner (or Partner Pile), which is closed ring hoop set around pile head up and down, the inner diameter of the pile head is larger than the outer diameter the hoop, hoops and piles are separated; piles and hoops of piles head work together by force transmission of soil and cushion, combining to piles with hoops. Because of its attributes of simple and flexible adjustment for pile top stiffness, its academic name should be "Variable Rigidity Pile". In order to deeply understand the action mechanism and the bearing properties of partner pile, pilot study has been carried out on the technology, and the main research results, innovation and conclusions include:(1) To weaken or enhance some parts of the community of pile and soil on purpose. can change the way of their collaboration, and make the characters of load and settlement to the concerted or predetermined direction.(2) It’s not rational to differentiate foundation type on relative deep or shallow. while the expressions of "direct foundation" or "indirect foundation" to classify foundation type seems to be more reasonable, and the traditional expressions of "deep foundation" and "shallow foundation" could only refers to the relative depth of the foundation. (3) The same meaning or specific explainion of "settlement exchange with capacity" is the "virtuous cycle" of foundation capability, which is:"loads increase→compact foundation soil→foundation soil improved→more load can be undertaken—further consolidate foundation soil→foundation soil improved more→......," whereas, almost all the current study have paid no or little adequate consideration on the contribution of subsidiary stress acted on the sliding surface which could resist shear sliding; Pile partner can reduce the risk of occurring the whole shearing destruction of the direct foundation.(4) The foundation with partner pile accord with the very realistic hypothesis of "Circular sliding and downward shearing" of rigid body displacement, the slip line is a semicircle, which diameter equai to the width of the foundation slab, and the centroid point of the foundation slab is the centre of the circle, based on the Coulomb and Moore strength theory, and the Flamant Additional Atress Solution, an analytical solution the resistance moment about the centroid point of the soil shear resistance on the assumed slip line could be deduced, which additional stress and the soil weight have been taken into consideration; meanwhile, take all tilting moments into an equivalent eccentric, and then get hold of a completely new method to evaluate foundation bearing capacity, which can be called the method of equivalent eccentric; According to the method of equivalent eccentric, the foundation bearing capacity is not only be determined by the characters of soil, the width and depth of the foundation, but also the equivalent eccentric, which is mostly influenced by the structure. smaller equivalent eccentric get larger bearing capacity and vice versa; Preliminary comparisons show that the Terzaghi’s ultimate bearing capacity is corresponding to about△F/B0.154, and the standard value is corresponding to about△F/B=0.188, not considering the earthquake and other horizontal load; Based on the method of equivalent eccentric, analyse "stop sinking" and "stop turning", and the optimized arrangement of piles.(5) The disadvantages of indirect foundation have been discussed; Put forward the concept of individual safety coefficient to explain and question the results of the conventional variable stiffness leveling optimal design, that is "outside weak and inside strong", for if foundation settlement uniform is only but one control parameter, and adjusting pile bottom stiffness is the single means of variable stiffness leveling optimization design for indirect foundation, as a sesult,"outside weak and inside strong", which is less rational (compare with legs of a table or chair), and the cost of the result is the lower of individual safely coefficient; However, pile partner has the ability of adjusting pile lop stiffness, so pile can be put evenly according to the load above, or even strengthen edge piles, increase the ability of resistance whole overturned, and its easy to realize the leveling of variable stiffness by adjusting the distance between pile top and foundation slab, such as side piles and comer piles more distance and center piles less.(6) Pile partner has almost all the advantages of similar technology, and and shock isolation, and serves the double purpose of delivering level force to soil and cutting off level force to pile, meanwhile, pile partner increase the stiffness of the foundation slab; On the application of the rigid pile composite foundation, attention shall be paid lo the basement well pit and destructing isolation, and the modulus of cushion influencing isolation, which are adverse factors and dangerous to the pile of low level bending and shear capacity, in addition, the conventional rigid pile composite foundation also has serveral shortcomings such as reaction force concentrated ("forced to average"), high cost of foundation base, and the "flow compensation" leading to the loss of the cushion.(7) Finite element calculation show that the ultimate bearing capacity of foundation increase greatly according to convergence criteria, it is necessary to relax the judge criteria according to static load test; Field tests confirm that partner play a good role to share load for pile, and improve the reaction force of soil between piles.(8) Put forward an integration bearing capacity calculation formulas of composite foundation and composite piled foundation, so suggest the composite foundation technique standard (drafts) should be amended as: if there is only cushion, the rigid concrete pile composite foundation must be cushion type pile, but if there are reliable measures to ensure that the pile and soil work together in succession, soil or cushion between pile head and bottom of foundation won’t have overall shear failure or other sliding model destroy, the concrete pile in the rigid pile composite foundation should be good at pile end, and tip of pile should put on the good soil; The overall bearing capacity safety coefficient of pile partner is deduced, which show that the safety degree of all foundations with pile partner arc within the scope of reasonable greater than2; suggest that different seismic fortification level buildings use different safety coefficient; suggest that the moderate uneven settlement can be treated as external load to test engineering construction quality, and the engineering design and construction quality level can be measured by the "resist uneven settlement index"; Put forward the concept of "minimum ratio of pile"; Partner pile has the settlement characteristic of "stop sinking", the main settlement is caused by compression of the upper foundation soil, so although the influence depth of situ static load test is shallow, the test result should be closed to the real fanial settlement, and can predict settlement directly, furthermore, put forward a practical method of "overall tilting" for variable stiffness leveling "concept design"; Put forward an optimal design to deal with a foundation pile defects accident case by pile partner, with reduction the depth of cap and cancel flag leveling course; and Put forward the other optimal design,"pile sister" plan, making direct foundation withstand pulled force.(9) when there’s few friction between pile caps and foundation soil, or the modulus of foundation soil around pile shaft are small enough, or combinations thereof, pile foundation with low cap is not a "typical pile foundation with low cap", and should be treated as "pile foundation with non-typical high cap" Keeping the construction of the pile foundation form with only extra pile partner is the "amend" improvement, and finite element model calculations show that: partner is a reliable medium that help cap deliver lateral load to soil. reducing the stress and displacement of a pile greatly even there’s few friction between pile caps and foundation soil, and also ease the stress and displacement when the modulus of foundation soil are small, so pile partner should be widely used for pile foundation with low cap; The horizontal bearing character of pile foundation with low cap depends on the horizontal load capacity of foundation soil under the cap or around the pile cssentially.As long as the pile break away from foundation slab to some distance, the traditional pile foundation will change from indirect to direct foundation, and realize the "revolutionary" improvement, the finite element numerical simulation show the exponentially reducing of stress of a pile; pillow is not necessary and can be cancelled in composite foundation when pile partner is applied; the separation between pile and foundation slab encourage parter to play a stronger roie, and the internal forces of pile partners are large and complicated.