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Verification of failure mechanisms and design philosophy for a bolt-anchored and fibre-reinforced shotcrete lining
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.ORCID iD: 0000-0001-8375-581X
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.ORCID iD: 0000-0003-2594-4107
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.ORCID iD: 0000-0003-3586-8988
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.ORCID iD: 0000-0001-8336-1247
(English)In: Engineering Failure Analysis, ISSN 1350-6307, E-ISSN 1873-1961Article in journal (Refereed) Submitted
Abstract [en]

Falling or sliding of loose blocks is one of the most common failure modes in a rock tunnel. For tunnels in hard and jointed rock, fibre-reinforced shotcrete (sprayed concrete) in combination with rock bolts is one of the most commonly used supports to prevent such failures. The structural behaviour, and especially the failure, of this type of rock support, is complex and involves several failure mechanisms; such as cracking of the shotcrete and interface failure along the shotcrete-rock, bolt-grout and rock-grout interface. Therefore, rock supports are normally designed using analytical solutions based on the independent failure modes. However, these failure modes are derived based on experimental testing and the assumption that no interaction between the failure modes occur. This assumption has not been verified. Therefore, this paper presents a numerical model capable of simulating the failure of a bolt-anchored and fibre-reinforced shotcrete lining. The model includes bond failure between shotcrete and rock, cracking of the shotcrete and pull-out failure of rock bolts. The structural behaviour for each failure mode and the complete structure have been verified against experiments from the literature. This shows that the model is capable of simulating the different phases of failure, and show good agreement with results from full-scale experimental tests from the literature. Furthermore, results from the numerical simulation confirms that the design of the shotcrete lining can be based on individual failure mechanisms. Moreover, it was shown that a design based on the residual strength of the fibre-reinforced shotcrete is conservative compared to a design based on the bond strength.

Keywords [en]
design of rock support, failure mechanisms of shotcrete, material models
National Category
Infrastructure Engineering
Research subject
Civil and Architectural Engineering, Concrete Structures
Identifiers
URN: urn:nbn:se:kth:diva-273009OAI: oai:DiVA.org:kth-273009DiVA, id: diva2:1428886
Funder
Rock Engineering Research Foundation (BeFo), 379
Note

QC 20200511

Available from: 2020-05-07 Created: 2020-05-07 Last updated: 2020-05-12Bibliographically approved
In thesis
1. Structural behaviour of shotcrete in hard rock tunnels
Open this publication in new window or tab >>Structural behaviour of shotcrete in hard rock tunnels
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Tunnels in hard and jointed rock are normally excavated in an arch shape to enable the rock mass to support its weight. Since the beginning of the 1980's, fibre reinforced shotcrete (FRS) in combination with rock bolts have been the dominating support method for hard rock tunnels. This type of rock support is a complex composite structure in which the structural behaviour depends on interaction between shotcrete, rock and bolts. The design is commonly based on a rock mass classification system in combination with analytical solutions or finite element (FE) modelling. However, the in-situ variations of important properties of the shotcrete are normally neglected.

The aim of this thesis is to describe and explain how the variations in shotcrete thickness and bond strength affect the structural behaviour and capacity for a shotcrete lining. Especially, the influence of local variations in shotcrete thickness and bond strength has been studied in detail. For this purpose, a numerical framework capable of simulating bond failure, cracking of FRS and pull-out failure of grouted rock bolts have been developed. Moreover, in-situ data for shotcrete thickness and bond strength have been collected and analysed to characterize  the variations in important shotcrete parameters.

The results in this thesis show that when shotcrete is subjected to shrinkage, local variations in shotcrete thickness affects the crack pattern. However, the number and width of the cracks are similar to the case with uniform thickness. Most importantly, a pattern of fine and narrow cracks develops in unreinforced shotcrete subjected to shrinkage when a continuous bond to the rock exists. When shotcrete is subjected to the load from a loose block, the force is transferred to the surrounding rock through bond stresses distributed over a narrow band. Simulations have shown that the structural capacity, with respect to bond failure, depends on the shotcrete thickness. Moreover, a strong linear correlation was found between the mean value of the bond strength and shotcrete thickness around the perimeter of the block and the structural capacity. Local weak areas, i.e. with low bond strength or thickness, may exist around the perimeter without having a significant effect on the structural capacity. Design of bolt-anchored shotcrete linings is based on failure modes previously derived from experimental testing. This thesis has contributed to an increased understanding of the failure mechanisms of the lining and has confirmed that the design can be based on individual failure mechanisms.

Abstract [sv]

Sedan borjan av 1980-talet har stalfiberarmerad sprutbetong i kombination med bergbultar varit den dominerande bergforstarkningen for tunnlar i hart berg. Den har typen av forstarkning ar en komplex samverkanskonstruktion vars strukturella beteende styrs av interaktionen mellan sprutbetong, berg och bult. Dimensioner­ingen baseras vanligtvis pa ett klassificeringssystem for bergmassan i kombination med analytiska losningar eller modeller baserade pa finita elementmetoden. I dessa fall bortser man oftast fran de i fa.It forekommande variationerna hos sprut­betongens viktiga egenskaper. Syftet med denna avhandling ar forklara och beskriva hur variationerna i sprut­betongens tjocklek och vidhaftning paverkar <let strukturella beteendet och bar­formagan hos bergforstarkningen. Framforallt har lokala variationer i sprutbeton­gens tjocklek och vidhaftning studerats. For att genomfora detta har ett numeriskt ramverk utvecklats som kan simulera uppsprickning av fiberarmerad sprutbetong, vidhaftningsbrott och utdrag av injekterade bergbultar. Dessutom har faltdata samlats in och analyserats for att karaktarisera fordelningen av viktiga sprutbe­tongegenskaper. Resultaten i den har avhandlingen visar att lokala variationer i sprutbetongens tjocklek paverkar sprickmonstret nar sprutbetongen krymper. Antalet sprickor och <less vidd ar liknande dem som uppstar nar tjockleken ar jamn. En viktig slutsats ar att ett manga sprickor med liten sprickvidd uppstar nar oarmerad sprutbetong med kontinuerlig vidhaftning till berget krymper. Nar sprutbetongen utsatts for lasten fran ett lost bergblock overfors lasten till den omkringliggande bergmas­san langs ett tunt band. Numeriska simuleringar har visat att barformagan med avseende pa vidhaftningsbrott beror pa sprutbetongens tjocklek. Dessutom visade simuleringarna att <let finns ett starkt linjart samband mellan medelvardet for sprutbetongens tjocklek och vidhaftningshallfasthet langs block­ets periferi och <less barformaga. Lokala ytor med liten tjocklek eller vidhaft­ningshallfasthet kan finnas runt periferin utan att paverka barformagan. Dimen­sioneringen av bultforankrad sprutbetong ar baserad pa brottmoder framtagna utifran experiment. Den har avhandlingen har bidragit med en okad forstaelse kring dessa brottmoder och visat att dimensioneringen bar baseras pa individuella brottmoder.

Place, publisher, year, edition, pages
Stockholm: Kungliga Tekniska högskolan, 2020. p. 78
Series
TRITA-ABE-DLT ; 209
Keywords
shotcrete, structural behaviour, material models, rock support, bond strength, fibre-reinforcement
National Category
Infrastructure Engineering
Research subject
Civil and Architectural Engineering, Concrete Structures
Identifiers
urn:nbn:se:kth:diva-273010 (URN)978-91-7873-498-6 (ISBN)
Public defence
2020-06-04, Registrera dig här: https://kth-se.zoom.us/webinar/register/WN_Bp8dzpcbQMKNNOGyj51R2g, Du som saknar dator/datorvana kan kontakta thoyra@kth.se för information / Use the e-mail address if you need technical assistance, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Rock Engineering Research Foundation (BeFo), 379
Note

QC 20200513

Available from: 2020-05-13 Created: 2020-05-12 Last updated: 2020-05-13Bibliographically approved

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Sjölander, AndreasHellgren, RikardMalm, RichardAnsell, Anders

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