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Model uncertainty of design tools to analyze block stability
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
2009 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Block failure is one of the most common failure modes in tunnels. Design tools have some simplifications and, therefore, they also have some model uncertainties. The purpose of this licentiate thesis is to assess the model uncertainty for different design tools in order to estimate block stability.

Different approaches of kinematic limit equilibrium (KLE) including conventional KLE, limited joint length, limited joint length and stress field consideration and probabilistic KLE were compared to that of DFN-DEM. In this approach, the results of the calibrated DFN-DEM with field mapping were considered to be of true value. The results show that the conventional KLE is overdesign due to it’s over simplification. By considering fracture length and stress field, the volume of predicted unstable blocks is reduced. The probabilistic approach of KLE by considering finite joint length and stress field predicts the volume of unstable blocks to be lower than DFN-DEM approach. Therefore there is a great model uncertainty of our standard design tools for block stability analysis.

The assumption made in this study is that the results from DEM were considered to have a true value; the results from analytical solution based on joint relaxation process were compared to those of DEM in a different condition of depth, K0, apical and friction angle, Kn and Ks value, and ratio of Kn/Ks. The comparison shows that for shallow depth with K0 less than 1, analytical solution leads to an overestimation of block stability. The analytical solution predicts that the block is stable, while the analyses from numerical solution show the block is unstable. The analyses show that by increasing K0, accuracy of analytical solution also increases. Moreover, for the cases with close value of friction angle to semi-apical angle, the use of analytical solution is not recommended. As the ratio of Kn/Ks increases, the accuracy of analytical solution decreases. Increasing the angle ratio (ratio between semi-apical angle to friction angle) is one source of increasing uncertainty in the model. The analytical solution is very uncertain in cases with a low value of K0, and a high value of stiffness ratio and angle ratio. On the other hand, the analytical solution is more certain in conditions with a high value of K0 and a low value of stiffness ratio and angle ratio. According to current information (K0, angle ratio, stiffness ratio), one can determine the value of model uncertainty by using the diagrams presented in Chapter 6 of the thesis. The analyses show that by having more information about the key parameters, the model uncertainty could be identified more precisely. However, having more information means spending more money, and this increase in cost must be compared to the cost of failure or delay in the project or overdesign.

 

 

Abstract [sv]

Blockutfall är en av de vanligaste brottformerna i tunnlar. Dimensioneringsverktyg har förenklingar och därför har de viss modelosäkerhet. Syftet med licentiat avhandlingen är att bedöma modelosäkerhet för olika dimensioneringsverktyg för att uppskatta blockstabilitet.

I Olika metoder av KLE inkluderad konventional KLE, begränsad spricklängd och insitu spänning och sannolikhetsbasserad KLE är jämförda med DFN-DEM. I den här metoden kalibreras DFN-DEM med fältkartläggning som är betraktad som sanna värden. Resultat visar att konventionell KLE ger starkt konservative resultat. Genom att betrakta spricklängden och spänningsfältet, så minskar volymen på uppskattade instabila block. Den sannolikhetsbaserade metoden för KLE genom att betrakta finit spricklängd, och spänningsfältet förutser att volymen av de instabila blocken är mindre än de som bedöms med DFN-DEM metoden. Det finns mycket osäkerhet i vår standard dimensioneringsverktyg att uppskatta block stabilitet.

Antagande som gjorts i den studien är att resultatet från DEM är betraktade som sanna värden och resultaten från analytiska lösningar baserad på sprickavlastning är jämförda med resultatet från DEM. Jämförelse visar att för grunda djup och med K0 mindre än 1, den analytiska lösningen leder till en överestimering av blockstabiliteten. Den analytiska lösningen förutsäger att blocket är stabilt medan analys av den numeriska lösningen visar att blocket är instabilt. Analysen visar attgenom en ökning av K0 så ökar tillförlitligheten av den analytiska lösningen. Det visar sig att även att för fall med friktionsvinkel nära semitoppvinkeln så kan den analytiska lösningen inte rekommenderas. Vidare leder en ökning av förhållandet Kn/Ks till att tillförlitligheten av den analytiska lösningen minskar. En ökning av vinkelförhållandet mellan semitoppvinkeln och friktionsvinkeln är källa till en ökning av osäkerhet i modellen. En analytisk lösning är mer osäker i fall av lågt värde på K0 och högt värde på styvhetsförhållandet och vinkelnförhållandet. Å andra sidan, så är den analytiska lösningen mer säker i fall av högt värde på K0, och lågt värde på vinkel förhållandet och styvhetsförhållandet.

Vid given information (K0, styvhetsvärde och vinkel förhållande) kan man bestämma värdet på modellosäkerheten genom att använda diagrammen i avhandlingen. Analysen visar att vid ökad information om nyckelparametrarna, så kan modelosäkerheten identifieras mer exakt. Hur som helst så betyder mer tillgång till information att mer pengar måste satsas och denna kostnad  måste gemföras med kostnader för blockinstabilitet eller överdesign.

 

 

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , xi, 169 p.
Series
Trita-JOB. LIC, ISSN 1650-951X ; 2014
National Category
Civil Engineering
Identifiers
URN: urn:nbn:se:kth:diva-10580OAI: oai:DiVA.org:kth-10580DiVA: diva2:219724
Presentation
2009-06-08, V3, KTH, Teknikringen 72, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2009-05-28 Created: 2009-05-28 Last updated: 2012-01-14Bibliographically approved
List of papers
1. Design consideration of large caverns by using advanced drilling equipment
Open this publication in new window or tab >>Design consideration of large caverns by using advanced drilling equipment
2007 (English)In: Felsbau, ISSN 0174-6979, Vol. 25, no 5, 131-136 p.Article in journal (Refereed) Published
National Category
Civil Engineering
Identifiers
urn:nbn:se:kth:diva-25091 (URN)2-s2.0-35448996070 (Scopus ID)
Note

QC 20101008

Available from: 2010-10-08 Created: 2010-10-08 Last updated: 2016-05-11Bibliographically approved
2. Some aspects on model uncertainty in the calculation of block stability using Kinematics Limit Equilibrium
Open this publication in new window or tab >>Some aspects on model uncertainty in the calculation of block stability using Kinematics Limit Equilibrium
2008 (English)In: 42nd U.S. Rock Mechanics - 2nd U.S.-Canada Rock Mechanics Symposium, American Rock Mechanics Association , 2008Conference paper, Published paper (Refereed)
Abstract [en]

Kinematics Limit Equilibrium (KLE) is one of the most commonly used approaches for evaluating block stability around underground openings. Model uncertainty in using (probabilistic) KLE has been assessed by comparing block stability results with the results of the Discrete Fracture Network-Distinct Element Method (DFN-DEM) approach. The KLE analysis has been performed based on Monte Carlo random generation for fracture orientations and lengths (probabilistic KLE). The results show that the probabilistic KLE underestimates the unstable block volume. The calculated mean value of unstable block volumes using probabilistic KLE is much smaller than DFN-DEM simulation results, which provide a closer representation of reality.

Place, publisher, year, edition, pages
American Rock Mechanics Association, 2008
Keyword
DEM Simulation, Discrete fracture network, Distinct element methods, Fracture orientations, Limit equilibrium, Mean values, Model uncertainties, MONTE CARLO, Random generation, Stability results, Underground opening
National Category
Civil Engineering
Identifiers
urn:nbn:se:kth:diva-25094 (URN)2-s2.0-69549129644 (Scopus ID)
Conference
42nd U.S. Rock Mechanics - 2nd U.S.-Canada Rock Mechanics Symposium 2008; San Francisco, CA; 29 June 2008 through 2 July 2008
Note
QC 20101008Available from: 2010-10-08 Created: 2010-10-08 Last updated: 2012-03-22Bibliographically approved
3. Some Aspects of Model Uncertainties of Block Stability Estimation
Open this publication in new window or tab >>Some Aspects of Model Uncertainties of Block Stability Estimation
2008 (English)In: ARMS 2008, Teheran, 2008, 675-681 p.Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
Teheran: , 2008
National Category
Civil Engineering
Identifiers
urn:nbn:se:kth:diva-25096 (URN)
Conference
5th Asian Rock Mechanics Symposium
Note
QC 20101008Available from: 2010-10-08 Created: 2010-10-08 Last updated: 2012-01-14Bibliographically approved

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