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

23PROTECTING PILE FOUNDATIONS FROM FROST-HEAVEThe design of the burial depth of deep foundationsin permafrost is often restricted by therequirements of frost-heaving stability. Frequently,piles mst be buried more than 2.5 tiesdeeper than the frost table to prevent pulling byfrost-heaving. Even so, in sow cases, deformationor destruction occurs because the foundationlacks the strength to resist the pulling.There are two ways to reduce the tangentialfrost-heaving pressure acting on foundation surfaces.One way is to reduce the foundation material'ssusceptibility to water, to weaken ice cementingbetween the frost and the foundation; theother is to improve the soil body around the foundationto reduce frost heave.A number of tests to reduce the tangentialfrost-heaving force on pile foundations have beenconducted in the permafrost areas of the Qinghai-Xizang Plateau. In the tests, residuum and anionsurface active agents were used to treat foundationscomprehensively to met the defined requirerents.The process was carried out in this way:First, the foundation surface was daubed with athin layer of residuum to change its hydraulicproperty, then certain parts of the soil surroundingpiles were treated with active agents to reducefrost heave. This method reduced 95% or soof tangential frost-heaving pressure. After acouple of freeze-thaw cycles, no clear rise intangential frost-heaPlng pressure was observed,and the stability has remained sound (Ding et al.,1982). This achieveraent was obtained in both laboratoryand field model tests. Its effectivenessand durability remain to be verified in practicalprojects.Problem still exist in design, construction,and use of deep foundations in permafrost regionsthat need further investigation and resolution:1. Providing exact adfreezing strength is thekey to correctly designing pile foundations: Bytests, we see that there is a peak value and aremnant value of adfreering strength. The averageadfreezing strength used to design pile foundationsrelates not only to the constituents ofsoil, water content, ground temperature, and pilematerial, tut also to the length and diamter ofthe pile. Of these, Khe first four factors influencepeak and remnant adfreezing strength. Therefore,studying the strengths and making use ofthem along with the length and diameter of thepile to compute average freezing strength willgive accurate parameters for computing adfreezingstrength.2. The rational distribution pattern of frostground coefficients and the values of ground coefficientsunder different frost conditions.3. The relationship between the bearingstrength at the pile end and the adfreezingstrength of the pile circumference: Although variousrthods of computing the bearing capacity ofpiles proposed in the past took into account thereaction force at the pile end, the action raechanismwhen both the pile end and the circumferenceare under load at the sa= time has not been fullyillustrated.4. The reasonable interval between piles, andtests and computions of pile groups.5. The reasonable burial depth of buried-pilefoundations: Buried-pile foundations are conparativelypopular in China owing to their simple construction,the easy treatmnt to prevent frostheave,and the low cost. It has been observed inearlier projects, however, that the annual variationof ground temperature for shallowly buriedpile foundations is considerable, yielding frostcreep as well as frost heave, which tends to causedeformation of the foundation. To prevent or reducedeformation, an allowable maximum temperaturevariation in the frost beneath such foundationsseem to be inevitable.6. Loading tests on pile foundations in permafrost:The step-by-step loading method currentlyused has some disadvantages, such as long testingtime and considerable variations of boundaryconditions. In addition, the criterion taken fordeformation stability (0.3-0.5 m/24 hr) lackssufficient scientific support. Therefore, a methodthat not only reflects the varying characteristicsof frost but also shortens testing ti= willbe the goal of further study.REFERENCESChen Xiaobai, Tong Boliang, Ding Jinkang, and ZhouChanqing, 1979, Thirty years' study and engineeringpractice on frost in China: Journalof Glaciology and Cryopedology, V. 1, no. 2.Cheng Zhohuai, Le Guoliang, Zhao Xisheng, and WangHuaqirig, 1981, Study on vertical and horizontalloading of pile foundation in permfrostarea: Northwest Institute of China Academyof Railway Sciences.Cui Chenghan, and Zhou Laijong, 1981, Study of reactionforce against frost heave: Third Surveyand Design Institute of China RailwayMinistry.Ding Jinkang, 1981a, Study on the long-term resistantforce of anchor-arm in permafrost area:Northwest Institute of China Academy of RailwaySciences.Ding Jinkang, 1981b, Study and test of severalfrost-mechanical problems.Ding Jinkang, 1983, Outdoor experimental researchon tangential frost-heaving force, Proceedingsof the Second National Conference onPermafrost, Lanzhou (1981): Lanehou, GansuPeople's Publishing House, p. 251-256.Ding Jinkang, Zhang Luxin, Ye Cheng, Yan Jingshanand Yu Changhua, 1982, Physical and chemicalmthods for decreasing the force of tangentialfrost-heave, Proceedings of the Symposiumon Glaciology and Cryopedology held bythe Geographical Society of China (Cryopedology),Lanzhou (1978): Beijing, Science PublishingHouse, p. 204-206.FSDI (First Survey and Design Institute of ChinaRailway Ministry), 1978, Report of study onhouse building in permafrost area of Qinghai--Xirang Plateau: Northwest Institute ofChina Academy of Railway Sciences.He Changgen, 1981, Test and study on poured-innegative temperature concrete in permafrost:Third Survey and Design Institute of ChinaRailway Ministry.