第55次Rankine讲座提出的一个现象和一个悖论

岳中琦

前天晚上，我在香港参加了第55次Rankine讲座的复讲。讲座人是挪威岩土工程研究院前院长Suzanne Lacasse博士。正式讲座是在今年3月17日在位于伦敦的帝国理工大学举行的。她的讲座题目是她的讲座题目是《岩土工程实践中的灾难、风险和可靠度》（Hazard, Risk and Reliability in GeotechnicalPractice）。

Rankine讲座是英国岩土协会组织，以英国皇家学会院士William John Macquorn Rankine（1820年7月5日生 – 1872年12月24日离）冠名的每年一次学术讲座。Rankine院士是早期土力学学者，提出了因土体重力引起的挡土墙横向土压力理论和计算公式。Rankine讲座起始于1961年，由哈佛大学的A. Casagrande教授担任首次讲座人。五十多年来，每次Rankine讲座都是一次世界土力学和岩土工程的盛会。

在回顾和分析了不少土石坝、土堤和斜坡灾难性破坏后，Lacasse博士提出了一个现象和一个悖论。它们值得关注！

这一个现象是：岩土破坏常常因综合因素产生。一个重复出现的因素是，施工没有紧跟原始设计，或者，实际使用了原来设计没有包括的设计变更。英文原文是：Failures usually occurdue to a combination of factors. One recurring factor is the construction not  following the original script, or changes in design underway that were not included in the original design.

这一个岩土工程界的悖论是：即使存在重要的不确定因素时候，时间和经费限制和价格竞争把工程场地勘察和岩土设计费用都降低到最小值。但是在另一方面，一旦一个大型破坏发生了，似乎会有无限度的经费来做场地勘察（实验室和现场），分析和专家咨询。英文原文是：A paradox of our profession:Time and budget constraints and competition on price bring down the budget forsite investigations and design to a minimum, even when there are importantuncertainties. When a major failure occurs, there seems to be unlimited budgetfor site investigations (lab and in situ), analyses and expert advice (Mount  Polley review report: an impressive quantity of tests and analyses in 5 months).

Lacasse博士给出了图1和图2中的两个事例。 

从图1和图2的两个岩土工程破坏案例可看到，对按照现代岩土力学理论和稳定性安全设计方法而设计的土石坝、路堤和斜坡，进行微小改变、改动，就极有可能会造成巨大的岩土体失稳破坏！

在破坏发生后，人们又花费大量的人力和物力，来调查、分析和研究这些岩土体破坏的原因。又回到了上述岩土工程界的悖论。

我不禁要问，这到底为什么呢？！

我认为，产生这一个现象和这一个悖论的根本原因是，现在土力学理论的压剪局限性和扩大化使用的灾难性后果。在完全压剪状态下，土体的强度随着压应力的增大而增大。因此，岩土勘察和设计利用了这些人为增大的土体强度，首先造成了岩土勘察、设计和施工费用，与其它工程相比，相当低。近百年的不断积累，人们早已习惯了这种做法，和低费用。这种扩大化使用是“颠覆性错误”，它导致了现在岩土工程产品，因微小拉张应力和位移而发生的破坏和灾难不断出现。在破坏发生后，人们又要在压剪土力学理论框架下，来论证破坏原因。这必然要做大量、各种测量和分析。从而，调查费用极高，且得到的造成破坏的原因相当一致：综合因素造成了岩土体破坏。

相关详细、具体论述，请参见科学网博文[1-5]和学术论文[6-7]。

参考文献

[1] 岳中琦，2014，香港山坡地滑坡风险管治-人的作用，http://blog.sciencenet.cn/blog-240687-781243.html

[2] 岳中琦，2014，经典土力学理论的根本局限性和灾难性后果，http://blog.sciencenet.cn/blog-240687-836715.html

[3] 岳中琦，2014，全球岩土工程领域共同挑战性问题的经典土力学理论根源，http://blog.sciencenet.cn/blog-240687-838492.html

[4] 岳中琦，2014，突围范式Paradigm，走岩土力学与工程强国之路， http://blog.sciencenet.cn/blog-240687-851224.html

[5] 岳中琦，2015，造成山体滑坡灾难的两个潜伏癌细胞，http://blog.sciencenet.cn/blog-240687-935416.html

[6] 岳中琦，徐前，2014, 现今斜坡工程安全设计理论的根本缺陷与灾难后果，《岩土工程学报》，36(9):1601-1906

[7] 岳中琦，2015，全球岩土工程领域共同挑战性问题的土力学理论根源，《岩土工程学报》，37(S2): 11-15

2015年11月26日21点正写成于香港大学办公室

附件

第55次Rankine讲座摘要

More and more, societyrequires to know the risk which people, property and the environment areexposed to. The role of the geotechnical engineering profession shouldincreasingly be reducing exposure to threats, reducing risk and protectingpeople. The objective of the 55th Rankine Lecture is to convince you that youcan implement, with benefit, concepts of hazard, risk and reliability to assistyou in design, decision-making and engineering recommendations. After an overviewof the basic concepts, the lecture discusses the advances of hazard, risk andreliability in geotechnical engineering, and explains several "reallife" case studies. In these examples,  specific engineering questions had to beanswered, and risk and reliability applications provided insight for informeddecision-making. The factor of safety remains the main indicator of safety inpractice, and its significance and that of key parameters used for design, e.g.the characteristic strength, are discussed. The examples presented are takenfrom a wide realm of geotechnical problems, including the selection of soilproperties, the mapping of hazard and risk, as well as hazard and riskassessment associated with slope stability, dams, offshore installations andcode calibration. The contributions of risk assessment and management togeotechnical engineering, the strengths and drawbacks of the approach andissues such as tolerable and acceptable risk, perception of risk and cascadinghazards are discussed. The lecture shows how interaction with other disciplinesis part of providing a soundly engineered solution. The geotechnical engineer’srole is not solely to provide judgment on selection of parameters, methods ofcalculations and resulting safety, but also to take an active part in theevaluation of hazard, vulnerability and risk.

Suzanne Lacasse博士简介

Dr Suzanne Lacasse is Canadian and lives in Norway. She isTechnical Director at the Norwegian Geotechnical Institute (NGI). She wasManaging Director of NGI from 1991 to 2011. Dr Lacasse first focused her workon laboratory techniques, in-situ investigation methods and soil behaviourmodelling studies. She then worked on foundation engineering and design, slopestability and the development of calculation procedures for offshorestructures. She concentrated on combining mathematical and numerical analyseswith practical geotechnical engineering design methods. She then developed andapplied probability, reliability and risk concepts to assist in foundationdesign and decision-making. She gave invited keynote lectures in over 30countries, and she is author or co-author of 300 papers. She gave ASCE's 37thTerzaghi Lecture on Offshore Geotechnics (2001) and ISSMGE's 8th TerzaghiOration on Slope Stability (2013). Dr Lacasse received many awards, includingPhDs Honoris Causa from the University of Dundee and from the NorwegianUniversity of Science and Technology. She was elected Foreign Member of the USNational Academy of Engineers (2001) for "enlightened direction of NGI andadvancements in foundation engineering for offshore structures". Shereceived the Legget Award of the Canadian Geotechnical Society and the first EffectiveTeaching Award in Civil Engineering at MIT. She is member of the Royal Societyof Canada, the Canadian Academy of Engineers, l'Académie des SciencesTechnologies (France), and the Academy of Engineering and Sciences and theAcademy of Sciences and Letters in Norway. She is Honorary Professor at ZhejiangUniversity in China.