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Analyses of shotcrete stress states due to varying lining thickness and irregular rock surfaces
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.ORCID iD: 0000-0001-8375-581X
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Shotcrete is sprayed concrete applied pneumatically under high pressure and was invented in the beginning of the 1900's. This new technique decreased the construction time and since steel fibres were introduced in the shotcrete during the 1970's, shotcrete has been the primary support method for tunnels.

Tunnels excavated with the drill and blast method creates a highly irregular rock surface which results in a shotcrete lining with varying thickness. The structural behaviour as well as the loads acting on the shotcrete lining depends on the interaction between the shotcrete, rock and rock bolts. There are several parameters influencing this interaction, e.g. bond strength, the stiffness of the rock and thickness of the shotcrete. All of these parameters are difficult to predict accurately which makes the structural design of the lining to a complex problem.

This thesis present the first part of a research project with the long-term goal to improve the understanding of the structural behaviour of the shotcrete lining. To achieve this, numerical modelling have been used to study the build up of stresses and cracking of shotcrete when subjected to restrained loading caused by e.g. temperature differences and drying shrinkage. The response in the lining when subjected to a gravity load from a block has also been studied. The model is capable of describing the non-linear deformation behaviour of both plain and fibre reinforced shotcrete and uses presented in situ variations in thickness to more accurately account for the effects of expected variations in thickness. The thesis discuss and demonstrate the effect of important loads that acts on the shotcrete lining and how the irregular geometry of the rock surface in combination with the varying thickness of the shotcrete affect the development of stresses in the lining. It is also discussed how a full or partial bond failure affect the structural capacity of a shotcrete lining.  

 

Abstract [sv]

Sprutbetong är betong som appliceras pneumatiskt under högt tryckt, en metod utvecklad i början av 1900-talet. Kort därefter gjordes de första försöken att använda sprutbetong som bergförstärkning. Den här nya tekniken minskade produktionstiden och när stålfibrer introduceras under 1970-talet kunde det tunga arbetet med att placera armering minimeras. Sedan dess har sprutbetong blivit den preliminära förstärkningsmetoden, särskilt för tunnlar i hårt berg där tunna lager av sprutbetong ibland kan användas som den enda förstärkningsåtgärden. 

Tunnlar byggs normalt genom metoden "borrning-sprängning" vilket leder till att bergytan där sprutbetongen appliceras få r en oregelbunden form. Under sprutning är det svårt att fastställa den exakta tjockleken och sprutbetongen har därmed en oregelbunden tjocklek. Beroende på in situ förhållanden kan oarmerad eller fiberarmerad sprutbetong i kombination med bergbultar användas för att förstärka berget. Det strukturella beteendet och lasterna som påverkar förstärkningen beror på interaktionen mellan sprutbetong, berg och bergbultar. Denna samverkan styrs av flera parametrar som t ex; vidhäftningshållfastheten, bergets styvhet och tjockleken hos sprutbetongen. Dessa parametrar är svåra att förutsäga vilket gör dimensionering av en sprutbetongförstärkningen till ett komplext problem.

Den här uppsatsen presenterar den första delen av ett forskningsprojekt med det långsiktiga målet att öka förståelsen för det strukturella beteendet hos en sprutbetongförstärkning. För att uppnå detta har numerisk modellering använts för att studera spänningsuppbyggnaden och uppsprickningen av sprutbetong som utsätts för förhindrade rörelser orsakade av temperaturförändringar eller uttorkningskrympning. Sprutbetongens beteende när den utsätts för en blocklast har också studerats. En numeriskt modell för att analysera spänningarna i sprutbetong som tar hänsyn till tidsberoende materialegenskaper har använts. Modellen kan beskriva det icke-linjära deformationsbeteendet av oarmerad samt fiberarmerad sprutbetong och använder sig av presenterad fältdata för att beskriva de förväntade tjockleksvariationerna. Uppsatsen disskuterar och demonstrerar effekten av viktiga laster som verkar på sprutbetongförstärkningen och hur bergets oregelbundna yta i kombination med sprutbetongens varierande tjocklek påverkar spänningsuppbyggnaden i förstärkningen. Det diskuteras också hur ett fullständigt eller partiellt vidhäftningsbrott på verkar sprutbetongförstärkningens bärförmåga.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. , 75 p.
Series
TRITA-BKN. Bulletin, ISSN 1103-4270 ; 150
Keyword [en]
Shotcrete, cracking, shrinkage, block load, varying thickness, numerical simulations, fibre reinforcement
National Category
Infrastructure Engineering
Research subject
Civil and Architectural Engineering
Identifiers
URN: urn:nbn:se:kth:diva-205311ISBN: 978-91-7729-368-2 (print)OAI: oai:DiVA.org:kth-205311DiVA: diva2:1088502
Presentation
2017-05-17, B21, Brinellvägen 23, Stockholm, 13:00 (English)
Opponent
Supervisors
Funder
Rock Engineering Research Foundation (BeFo)
Note

QC 20170418

Available from: 2017-04-18 Created: 2017-04-12 Last updated: 2017-04-20Bibliographically approved
List of papers
1. Numerical simulations of restrained shrinkage cracking in glass fibre reinforced shotcrete slabs
Open this publication in new window or tab >>Numerical simulations of restrained shrinkage cracking in glass fibre reinforced shotcrete slabs
2017 (English)In: Advances in Civil Engineering / Hindawi, ISSN 1687-8086, E-ISSN 1687-8094Article in journal (Refereed) Accepted
Abstract [en]

Modern tunnels in hard rock are usually constructed by drill and blast with the rock reinforced by shotcrete (sprayed concrete) in combination with rock bolts. The irregular rock surface and the projection method of shotcrete leads to a tunnel lining of varying thickness with unevenly distributed stresses that affect the risk of cracking during shrinkage of the young and hardening material. Depending on water conditions, shotcrete is either sprayed directly onto the rock surface or over a drainage system, creating a fully restrained or an end-restrained structural system. In this paper, a method for non-linear numerical simulations has been demonstrated, for the study of differences in stress build up and cracking behaviour of restrained shotcrete slabs subjected to shrinkage. Special focus was given to the effects of the irregular shape and varying thickness of the shotcrete. The effects of glass fibre reinforcement and bond were implemented in the study by changing the fracture energy in bending and in the interaction between shotcrete and the substrate. The study verifies that an end-restrained shotcrete slab is prone to shrinkage induced cracking, and shows the importance of a continuous bond to avoid wide shrinkage cracks when shotcrete is sprayed directly onto the rock. 

Place, publisher, year, edition, pages
Hindawi Publishing Corporation, 2017
Keyword
Shotcrete, Cracking, Varying thickness, Shrinkage
National Category
Infrastructure Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-205302 (URN)10.1155/2017/8987626 (DOI)000400618200001 ()
Funder
Rock Engineering Research Foundation (BeFo)
Note

QC 20170418

Available from: 2017-04-12 Created: 2017-04-12 Last updated: 2017-05-30Bibliographically approved
2. Investigation of non-linear drying shrinkage for end-restrained shotcrete with varying thickness
Open this publication in new window or tab >>Investigation of non-linear drying shrinkage for end-restrained shotcrete with varying thickness
(English)In: Magazine of Concrete Research, ISSN 0024-9831, E-ISSN 1751-763XArticle in journal (Refereed) Submitted
Abstract [en]

Tunnels in hard, jointed rock is commonly reinforced with shotcrete (sprayed concrete) applied directly on the irregular rock surface. The thickness for such lining can be as low as 50 mm which result in a fast drying. The resulting shrinkage of the restrained lining is a well known phenomena that causes cracking. Installation of some drainage system also results in an end-restrained shotcrete lining which is more prone to shrinkage cracking. The drying process is a complex problem that depends on multiple factors such as cement content, porosity and conditions of ambient air, i.e. temperature, relative humidity and wind speed. In this paper, two numerical models capable of capturing the structural effects of drying shrinkage was compared. Results shows that inclusion of non-linear drying shrinkage is important to accurately describe crack initiation in a end-restrained shotcrete slab. Best fit to experimental data was obtained when the rate of drying was described as a non-linear decreasing function.

Keyword
Drying shrinkage, cracking, shotcrete, varying thickness, rock support
National Category
Infrastructure Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-205309 (URN)
Funder
Rock Engineering Research Foundation (BeFo)
Note

QC 20170418

Available from: 2017-04-12 Created: 2017-04-12 Last updated: 2017-04-19Bibliographically approved
3. Shrinkage cracking of thin irregular shotcrete shells using multiphysics models
Open this publication in new window or tab >>Shrinkage cracking of thin irregular shotcrete shells using multiphysics models
2016 (English)In: 9th International Conference on Fracture Mechanics of Concrete and Concrete Structures / [ed] John E. Bolander, Eric N. Landis, Victor E. Saouma, 2016Conference paper, Published paper (Refereed)
Abstract [en]

Shotcrete (sprayed concrete) is commonly used to support tunnels in good quality hard rock. Including a drainage system often results in end-restrained sections of shotcrete, which have created problems with shrinkage induced cracking. In this paper a multi-physical material model with coupled behaviour between thermal actions, moisture transportation and mechanical strain has been used to model and describe the complex behaviour and effects of shrinkage of such a structure. The model was first calibrated against a free shrinkage test and then used to simulate an experimental set-up for testing of end-restrained shrinkage. The first results lead to a need of tuning of the parameters controlling the drying of the shotcrete to accurately describe the experimental results. This tuning could be an indication that the shrinkage behaviour differs between a restrained and an un-restrained sample. However, further research about possible changes in the pore structure as well as more detailed measurements of the early shrinkage behaviour is needed before any such conclusions can be drawn.

Keyword
Cracking, Fibre Reinforced Shotcrete, Multi-physics, Shrinkage
National Category
Infrastructure Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-186228 (URN)
Conference
FraMCoS-9, May 29-June 1 2016
Note

QC 20160826

Available from: 2016-05-06 Created: 2016-05-06 Last updated: 2017-04-18Bibliographically approved
4. On failure probability in thin irregular shotcrete shells
Open this publication in new window or tab >>On failure probability in thin irregular shotcrete shells
2017 (English)In: Proceedings of the World Tunnel Congress 2017, 2017Conference paper, Abstract (Refereed)
Abstract [en]

Tunnels through hard jointed rock are commonly reinforced with a combination of fibre reinforced shotcrete (sprayed concrete), FRS, and rock bolts. The design of such reinforcement is a complex task. First, the interaction between rock bolts, FRS and rock should be considered. Secondly, a natural variation in important parameters such as thickness of the shotcrete, fracture energy, and bond strength between shotcrete and rock exists. In this paper, a numerical framework for non-linear analyses of FRS suitable for Monte Carlo simulations is presented. As a case study, a 2D FE-model of a bolted shotcrete lining subjected to load from a pushing block was used to perform a sensitivity analysis for the variation in thickness. Results indicate that an irregular shotcrete thickness highly affects the failure load but has a smaller impact on ductility.

Keyword
Shotcrete, failure, fibre reinforcement, block load, varying thickness
National Category
Infrastructure Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-205310 (URN)
Conference
ITA-AITES World Tunnel Congress,Bergen Norway,9.-15. June 2017
Funder
Rock Engineering Research Foundation (BeFo)
Note

QC 20170418

Available from: 2017-04-12 Created: 2017-04-12 Last updated: 2017-04-19Bibliographically approved

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