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Design consideration of large caverns by using advanced drilling equipment
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
KTH, School of Architecture and the Built Environment (ABE), Land and Water Resources Engineering (moved 20130630).
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.
2007 (English)In: Felsbau, ISSN 0174-6979, Vol. 25, no 5, 131-136 p.Article in journal (Refereed) Published
Place, publisher, year, edition, pages
2007. Vol. 25, no 5, 131-136 p.
National Category
Civil Engineering
Identifiers
URN: urn:nbn:se:kth:diva-25091Scopus ID: 2-s2.0-35448996070OAI: oai:DiVA.org:kth-25091DiVA: diva2:355741
Note

QC 20101008

Available from: 2010-10-08 Created: 2010-10-08 Last updated: 2016-05-11Bibliographically approved
In thesis
1. Model uncertainty of design tools to analyze block stability
Open this publication in new window or tab >>Model uncertainty of design tools to analyze block stability
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:nbn:se:kth:diva-10580 (URN)
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
2. Block stability analysis using deterministic and probabilistic methods
Open this publication in new window or tab >>Block stability analysis using deterministic and probabilistic methods
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents a discussion of design tools for analysing block stability around a tunnel. First, it was determined that joint length and field stress have a significant influence on estimating block stability. The results of calculations using methods based on kinematic limit equilibrium (KLE) were compared with the results of filtered DFN-DEM, which are closer to reality. The comparison shows that none of the KLE approaches– conventional, limited joint length, limited joint length with stress and probabilistic KLE – could provide results similar to DFN-DEM. This is due to KLE’s unrealistic assumptions in estimating either volume or clamping forces.

A simple mechanism for estimating clamping forces such as continuum mechanics or the solution proposed by Crawford-Bray leads to an overestimation of clamping forces, and thus unsafe design. The results of such approaches were compared to those of DEM, and it was determined that these simple mechanisms ignore a key stage of relaxation of clamping forces due to joint existence. The amount of relaxation is a function of many parameters, such as stiffness of the joint and surrounding rock, the joint friction angle and the block half-apical angle.

Based on a conceptual model, the key stage was considered in a new analytical solution for symmetric blocks, and the amount of joint relaxation was quantified. The results of the new analytical solution compared to those of DEM and the model uncertainty of the new solution were quantified.

Further numerical investigations based on local and regional stress models were performed to study initial clamping forces. Numerical analyses reveal that local stresses, which are a product of regional stress and joint stiffness, govern block stability. Models with a block assembly show that the clamping forces in a block assembly are equal to the clamping forces in a regional stress model. Therefore, considering a single block in massive rock results in lower clamping forces and thus safer design compared to a block assembly in the same condition of in-situ stress and properties.

Furthermore, a sensitivity analysis was conducted to determine which is  the most important parameter by assessing sensitivity factors and studying the applicability of the partial coefficient method for designing block stability.

It was determined that the governing parameter is the dispersion of the half-apical angle. For a dip angle with a high dispersion, partial factors become very large and the design value for clamping forces is close to zero. This suggests that in cases with a high dispersion of the half-apical angle, the clamping forces could be ignored in a stability analysis, unlike in cases with a lower dispersion. The costs of gathering more information about the joint dip angle could be compared to the costs of overdesign. The use of partial factors is uncertain, at least without dividing the problem into sub-classes. The application of partial factors is possible in some circumstances but not always, and a FORM analysis is preferable.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. xiv, 72 p.
Series
Trita-JOB PHD, ISSN 1650-9501 ; 1016
Keyword
Block stability analysis, Model uncertainty, Joint relaxation, Partial factor, Sensitivity analysis
National Category
Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-49447 (URN)
Public defence
2011-12-14, D3, Lindstedtsvägen 5, Entreplan, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20111201Available from: 2011-12-01 Created: 2011-11-28 Last updated: 2012-03-22Bibliographically approved

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