Acceptable probability of excessive deformation for deep urban excavations

Ehsan Momeni, Mehdi Poormoosavian, Ali Fakher

In the proceedings of: GeoOttawa 2017: 70th Canadian Geotechnical Conference; 12th joint with IAH-CNC

Session: Innovative Foundation Systems

ABSTRACT: Deformations more than acceptable values areusuallydefined as excessive deformations. In this paper,areview wasconducted on the proposed criteria for the acceptable excavation-induced ground deformations.Anumber ofdeep urbanexcavation projectswerealsostudied.For each case study, thereported probabilityof excessive deformation(PED),determinedwith the aid of random set-finite element method (RS-FEM), is compared with anacceptable probability ofexcessive deformation (APED) of 10-1which is suggested in thispaper.Based on the real situation of the excavationprojects,findings indicatethat the suggested value of APED is a reasonable value and the RS-FEM isafeasible tool inestimating the PEDs of deep excavations.R•SUM•Les d†formations sup†rieures aux valeurs acceptables sont g†n†ralement d†finies comme des d†formations excessives.Dans cet article, un examen a †t† effectu† sur les crit‡res propos†s pour les d†formations du sol induites par desexcavations acceptables. Un certain nombre de projets de fouilles urbaines profondes ont †galement †t† †tudi†s. Pourchaque †tude de cas, la probabilit† de d†formation excessive (PED) rapport†e, d†termin†e … l'aide d'une m†thodeal†atoire d†finie par †l†ments finis (RS-FEM), est compar†e … une probabilit† acceptable de d†formation excessive(APED) de 10-1qui est Sugg†r† dans cet article. Sur la base de la situation r†elle des projets d'excavation, les r†sultatsindiquent que la valeur propos†e de l'APED est unevaleur raisonnable et le RS-FEM est un outil r†alisable pour estimerles PED des fouilles profondes1INTRODUCTIONThe protection of adjacent utilities during and afterexcavations is of prime importance.Owing to high cost ofland,lack of sufficient spacesfor urban development andthe regulation requirements (e.g.providing car park),therate of underground constructionsin metropolitansisincreasing rapidly. In urban excavation,as expected,theprobability of havingbuildings within azoneinfluenced byexcavationsis relatively high. Therefore, assessment ofbuilding damage is often required in order to identify theextent of influence of excavation on adjacent buildings.Sometimes, an excessiveamount of excavation-induceddeformationdamages the surrounding buildings while thedeepexcavationis not collapsed.In other words,thelikelihood of serviceability failure is high.Implementing theconcept ofthe acceptableprobability ofexcessivedeformation(APED)instead ofacceptableprobability offailure which is relativelylower (to be discussed later) canlead to a better risk management and decision making.However, to have a better understanding ofacceptableortargetprobability of excessivedeformation, one should firstunderstand the concept ofexcavation-induced grounddeformationwhich can damage buildings adjacent toexcavations as well as the suggested limitations or criteriafor minimizing the possible catastrophic consequences ofthe aforementioned displacements. Hence, an attempt hasbeen made to gain insight into recommendations forestimating the maximum wall deflection(horizontaldeformation)and settlement behind the wall due to deepexcavations as well as the proposed criteria for damageassessments of buildings due to excavation-induceddisplacement. Nevertheless, it should be mentioned thatapart from the review,the focus of the study is on theAPED.Probability ofexcessivedeformations(PED)can bedetermined using non-deterministictechniques like finiteelement-based random set theory (Nasekhian andSchweiger,2011).Details on the aforementioned techniques are beyondthe scopeof thispaperand can be found elsewheree.g.Kendall (1974).For deep excavation problemswherecompiling related data is difficult, implementation ofrandom set theorycoupled with finite element method (RS-FEM)for estimating the probabilities of failure or excessivedeformation is of advantage.Therefore, this paper, apartfrom recommendingan acceptableprobability ofexcessivedeformation,discusses the aforementioned probabilitiesobtained using RS-FEMfor a number ofdeep excavationprojects.2EXCAVATION PROJECTSSTUDIEDSinceRS-FEMis implementedfor estimating theprobabilitiesof collapse (Pf) andexcessive deformationofthe deep excavation projects considered in this study, abrief discussion on RStheoryis presented in the followinglines.Random set theoryis a mathematical approachthatworks well with imprecisely specified distributions,unknown dependencies, inconsistency in the quality ofinput data andaleatory andepistemic uncertainties.(Ferson 2004).Taking soil parameters in the form ofintervals isthe main advantageof RStheoryamong othernon-deterministic methods.

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