2 Volumes Final Proceedings - Washington 1984.pdf - IARC Research

21cmc.-0 100 200 300Anchoring length, cmFIGURE 5 Resistant force of anchor-arm vs anchoringlength.directly proportional to the freezing area of thepile, Le. to the length of the pile, provided thediameter remains unchanged.In 1980, the Northwest Institute of the ChineseAcademy of Railway Sciences made a series oftests on the resistance force of anchors in thepermafrost region of the Qinghai-Xizang Plateau.Reinforced concrete anchors of 10 cm in diameterwere adopted with burial depths of 2-4 m. Testresults showed that the relationship between theresistant force and the length of the anchor tendedto be nonlinear as the anchor reached a certainvalue (Figure 5) (Ding, 1981a). That is to say,there is no direct ratio between the resistantforce of the anchor and its length.The specific bearing capacity of the anchoris determined by gradual damage and the rewantadfreezing strength. The tests showed that underexternal load the distribution of shear stressalong the freezing boundary surface of the anchorwas uneven, attenuating exponentially with increasingdepth. The destruction of adfreezingstrength, therefore, does not take place simultaneouslyalong the whole boundary, but occurs at acertain point in the boundary where shear strengthhas attained a critical state. The maximum shearstress then moves downward, bringing the freezingstrength of the neighboring part to a criticalstate. This gradual destruction does not stop untilthe anchor loses its stability.However, a remnant adfreezing strength willact on the freezing boundary of the anchor even ifthe adfreezing strength as a whole is destroyed.Continually increasing the length of the anchorcan only serve to make further use of the rewantadfreezing strength, which is why the resistantforce increases slowly, presenting a clearly nonlinearproperty.Generally, the nonlinearity, as mentionedabove, occurs in the relationship between bearingcapacity and pile length if the length-to-diamzterratio of a single pile in permafrost reaches acertain value. Therefore, a revision factor ofpile length should be taken into account in theformula for computing the bearing capacity of asingle pile (Ding, 1981b).Considering these bearing characteristics, weare sure that there rmst be an optimum length fora single pile in permafrost where the average adfreezingstrength will be at the maximum. %reover,it will be the most reasonable, both economicallyand technically.The elastic foundation beam method is stillused to calculate the horizontal load of a singlepile in permafrost. After comparing results calculatedon the basis of the assumed distributionpattern of ground coefficients with the test data,we found the distribution pattern achieved by the"value K method" was most similar to the testdata; in other words, ground coefficients abovethe first elastic zero point are distributed inthe form of a concave parabola and below it thedistribution pattern with a constant is in relativeconformance with the reality of piles in permafrost(Cheng et al., 1981).REFREEZING GROUND SOIL SURROUNDING PILESThe time taken to refreeze the soil surroundingthe pile is of great importance to the designand construction of the pile. It is controlled bysuch factors as the construction method, the typeand dimension of the pile, the construction Beason,the temperature of the permafrost, and BOforth. 'Ihe refreezing tiues of ground soil surroundingdriven, inserted, and poured reinforcedconcrete piles were measured in the permafrostareas of the Qinghai-Xizang Plateau. The resultsindicated that heat introduced by driven piles wasthe least, that is, refreezing occurred in theshortest period of time, from 5-11 days after thepile had been set. The ground temperature wasdisturbed the most by heat introduced by mixed materialsalong with the heat of liquefaction ofconcrete; as a result, refreezing took as long as30-60 days. The time taken for refreezing theground soil surrounding the inserted pile lay betweenthe above two, and lasted from 6 to 15 days(NICARS, 1977).In the permafrost region of the Great Xing'anMountains, winter is usually chosen as the constructionseason to speed up the refreezing rateof the soil around poured piles. In this case,care must be taken to prevent the concrete fromfreezing and to guarantee the necessary strengthof the concrete.In the 19708, along with the use of pouredpiles for highway bridges built in permafrost regions,the study of negative-temperature concretewas initiated. To reduce the heat effect of concrete,and increase its freeze-resistant durabilityand mildness and its hardening rate at law temperature,the researchers devised a way to addchemicals to the concrete. Different types andamounts of chemicals are added according to theconstruction season, the permafrost temperature,and other design specifications so the concretewould meet the needs mentioned. Trial houseand bridge construction projects in the GreatXing'an Mountains and the Qinghai-Xizang Plateauhave proved that the strength of concrete withchemical additives can attain or surpass the designgrade regardless of the dimension of thepoured pile (RCI, 1979; Beilongjiang Microtherm