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Structural behaviour of shotcrete in hard rock tunnels
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Concrete Structures.ORCID iD: 0000-0001-8375-581X
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 [en]
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: urn:nbn:se:kth:diva-273010ISBN: 978-91-7873-498-6 (print)OAI: oai:DiVA.org:kth-273010DiVA, id: diva2:1429657
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
List of papers
1. Probability distributions of shotcrete parameters for reliability-based analyses of rock tunnel support
Open this publication in new window or tab >>Probability distributions of shotcrete parameters for reliability-based analyses of rock tunnel support
Show others...
2019 (English)In: Tunnelling and Underground Space Technology, ISSN 0886-7798, E-ISSN 1878-4364, Vol. 87, p. 15-26Article in journal (Refereed) Published
Abstract [en]

A common support measure for underground excavations in jointed rock masses to support loose blocks is to apply a thin shotcrete layer to the periphery of the excavation and systematically install rockbolts into the surrounding rock mass. In this support system, large blocks are carried by the rockbolts and small blocks are carried by the thin shotcrete layer. To verify the shotcrete layer's load-bearing capacity and to stringently account for the large uncertainties incorporated in the variables involved in determining its capacity, analytical calculations in combination with reliability-based methods can be used. However, a lack of knowledge exists regarding the magnitude and uncertainty of shotcrete characteristics (thickness, adhesion, flexural tensile strength, residual flexural tensile strength, and compressive strength), making it difficult to apply reliability-based methods. A statistical quantification of these characteristics is therefore important to facilitate reliability-based methods in design and verification of shotcrete support. In this paper, we illustrate how shotcrete support against small loose blocks can be viewed as a correlated conditional structural system and how this system can be analyzed using reliability-based methods. In addition, we present a unique amount of data for the aforementioned variables, which are all incorporated in the design and verification of a shotcrete layer's ability to sustain loads from small loose blocks. Based on the presented data, we statistically quantify and propose suitable probability distributions for each variable. Lastly, we illustrate how the proposed probability distributions can be used in the design process to calculate the probability of exceeding the shotcrete's load-bearing capacity. Both the probabilistic quantification and the defined correlated conditional structural system along with the illustrative calculation example are followed by a discussion of their implications.

Place, publisher, year, edition, pages
Elsevier, 2019
National Category
Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-246439 (URN)10.1016/j.tust.2019.02.002 (DOI)000462421100002 ()2-s2.0-85061199710 (Scopus ID)
Note

QC 20190329

Available from: 2019-03-29 Created: 2019-03-29 Last updated: 2020-05-12Bibliographically approved
2. 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 1687-8094, no 8987626Article in journal (Refereed) Published
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
Keywords
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 ()2-s2.0-85019613854 (Scopus ID)
Funder
Rock Engineering Research Foundation (BeFo)
Note

QC 20170418

Available from: 2017-04-12 Created: 2017-04-12 Last updated: 2020-05-12Bibliographically approved
3. Investigation of non-linear drying shrinkage for end-restrained shotcrete of varying thickness
Open this publication in new window or tab >>Investigation of non-linear drying shrinkage for end-restrained shotcrete of varying thickness
2018 (English)In: Magazine of Concrete Research, ISSN 0024-9831, E-ISSN 1751-763X, Vol. 70, no 6, p. 271-279Article in journal (Refereed) Published
Abstract [en]

Tunnels in hard, jointed rock are commonly reinforced with shotcrete (sprayed concrete) applied directly on the irregular rock surface. The thickness for such linings can be as small as 50 mm, which result in a fast drying. The resulting shrinkage of the restrained lining is a well-known phenomenon, which leads to cracking. The installation of drainage systems also results in an end-restrained shotcrete lining that is more prone to shrinkage cracking. The drying process is a complex problem that depends on multiple factors such as cement content, porosity and ambient air conditions (i.e. temperature, relative humidity and wind speed). Two numerical models capable of capturing the structural effects of drying shrinkage were compared in this study. It was found that inclusion of non-linear drying shrinkage is important for accurately describing crack initiation in an end-restrained shotcrete slab. The best fit to the experimental data was obtained when the rate of drying was described as a non-linear decreasing function.

Place, publisher, year, edition, pages
ICE Publishing, 2018
Keywords
cracks & cracking, shrinkage, slabs & plates
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-223781 (URN)10.1680/jmacr.17.00171 (DOI)000425130400001 ()2-s2.0-85042038519 (Scopus ID)
Funder
Rock Engineering Research Foundation (BeFo)
Note

QC 20180307

Available from: 2018-03-07 Created: 2018-03-07 Last updated: 2020-05-12Bibliographically approved
4. Variations in rock support capacity due to local variations in bond strength and shotcrete thickness
Open this publication in new window or tab >>Variations in rock support capacity due to local variations in bond strength and shotcrete thickness
(English)In: Tunnelling and Underground Space Technology, ISSN 0886-7798, E-ISSN 1878-4364Article in journal (Refereed) Submitted
Abstract [en]

Fibre reinforced shotcrete (sprayed concrete) is often used to support tunnels in hard rock. The shotcrete should be designed to carry the load from a loose block, which is assumed to be transferred to the surrounding rock mass over a narrow band. It is commonly accepted that the width of this band is constant, and independent of the shotcrete thickness. In this paper, numerical simulations are used to show that the width of this band and the load capacity is strongly correlated to the mean shotcrete thickness around the perimeter of the block. Furthermore, it is also shown that a strong linear correlation exists between the mean bond strength and the rock support capacity. This indicates that, under certain conditions, local areas with low thickness and bond strength can exist around the block perimeter without having any significant effect on the rock support capacity.

Keywords
shotcrete, bond strength, local variations, structural capacity
National Category
Infrastructure Engineering
Research subject
Civil and Architectural Engineering, Concrete Structures
Identifiers
urn:nbn:se:kth:diva-273008 (URN)
Funder
Rock Engineering Research Foundation (BeFo), 379
Note

QC 20200511

Available from: 2020-05-05 Created: 2020-05-05 Last updated: 2020-05-12Bibliographically approved
5. Verification of failure mechanisms and design philosophy for a bolt-anchored and fibre-reinforced shotcrete lining
Open this publication in new window or tab >>Verification of failure mechanisms and design philosophy for a bolt-anchored and fibre-reinforced shotcrete lining
(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
design of rock support, failure mechanisms of shotcrete, material models
National Category
Infrastructure Engineering
Research subject
Civil and Architectural Engineering, Concrete Structures
Identifiers
urn:nbn:se:kth:diva-273009 (URN)
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

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