Change search
ReferencesLink to record
Permanent link

Direct link
Application of partial factors to block stability analysis
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), Civil and Architectural Engineering, Soil and Rock Mechanics.
(English)In: Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards, ISSN 1749-9518Article in journal (Other academic) Submitted
National Category
Earth and Related Environmental Sciences
URN: urn:nbn:se:kth:diva-50067OAI: diva2:460961
QS 2011Available from: 2011-12-01 Created: 2011-12-01 Last updated: 2012-01-14Bibliographically approved
In thesis
1. 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.
Trita-JOB PHD, ISSN 1650-9501 ; 1016
Block stability analysis, Model uncertainty, Joint relaxation, Partial factor, Sensitivity analysis
National Category
Geotechnical Engineering
urn:nbn:se:kth:diva-49447 (URN)
Public defence
2011-12-14, D3, Lindstedtsvägen 5, Entreplan, KTH, Stockholm, 10:00 (English)
QC 20111201Available from: 2011-12-01 Created: 2011-11-28 Last updated: 2012-03-22Bibliographically approved

Open Access in DiVA

No full text

Search in DiVA

By author/editor
Bagheri, MehdiStille, Håkan
By organisation
Soil and Rock Mechanics
In the same journal
Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards
Earth and Related Environmental Sciences

Search outside of DiVA

GoogleGoogle Scholar
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

Total: 81 hits
ReferencesLink to record
Permanent link

Direct link