Tvångskrafter, rörelsefogar och minimiarmering - Vad blir effekten av nya generationen av Eurokoder?
(2025) In 0349-4969 VBKM01 20251Division of Structural Engineering
- Abstract
- Tunnel structures play an important role in Sweden’s infrastructure. At shallow depths, a common construction method is the so-called “Cut-and-Cover” tunnel, where the tunnel placement is excavated, cast in concrete on-site, and then backfilled. Such tunnels are typically subjected to constant water pressure, which yields high demands on crack control.
Cracking in elongated concrete structures often results from restraint effects. These arise when imposed deformations, for example due to temperature variations or shrinkage, are prevented by surrounding soil or adjacent structural elements. If the resulting stresses exceed the tensile strength of the concrete, cracking occurs. Since restraint forces increase with the length of the... (More) - Tunnel structures play an important role in Sweden’s infrastructure. At shallow depths, a common construction method is the so-called “Cut-and-Cover” tunnel, where the tunnel placement is excavated, cast in concrete on-site, and then backfilled. Such tunnels are typically subjected to constant water pressure, which yields high demands on crack control.
Cracking in elongated concrete structures often results from restraint effects. These arise when imposed deformations, for example due to temperature variations or shrinkage, are prevented by surrounding soil or adjacent structural elements. If the resulting stresses exceed the tensile strength of the concrete, cracking occurs. Since restraint forces increase with the length of the structure, tunnels are commonly divided into shorter segments, referred to as monoliths, with expansion joints in between to allow for controlled movement.
The aim of this thesis has been to investigate the possible monolith length in Cut-and-Cover tunnels with respect to crack width limitations, as specified in the new generation of Eurocode 2 (SS-EN-1992-1-1:2023). In the previous version of the code, monolith lengths could be determined using simplified rules of thumb, which have now been replaced by a requirement for calculation-based verification that includes restraint effects.
The thesis begins with a literature review followed by hand calculations based on the new Eurocode 2 and the Swedish Transport Administration’s infrastructure regulations. The literature review focuses primarily on friction between the tunnel structure and the backfill of crushed aggregate, as this is not extensively covered in current design standards. Two main methods for reducing friction were identified: the use of a leveling layer of concrete above the friction soil, and the application of a sliding layer consisting of plastic sheeting combined with a thin layer of sand. The reviewed experimental studies indicate potential for reducing the coefficient of friction, although all tests were performed at a much smaller scale than actual tunnel structures.
The analytical part included a parametric study where the wall height position, friction coefficient, reinforcement quantity, and bar diameter were varied. The results indicate that for the studied geometry and concrete quality, longer monoliths are possible compared to the previous rule of thumb. The minimum reinforcement requirement has also been lowered in the new generation of Eurocode 2. Furthermore, the results show that crack width decreases over the wall height, and that an increased amount of reinforcement allows for longer monoliths. However, larger bar diameters result in wider cracks, suggesting that a higher number of thinner bars or multiple reinforcement layers is preferable.
Friction was found to be the most critical factor affecting cracking, and reduction of the friction led to significant improvements.
In conclusion, the study shows that the previous rule of thumb regarding monolith length was conservative and that longer segments are possible if crack control is performed in accordance with the new Eurocode, especially in combination with optimized reinforcement strategies and friction-reducing measures. (Less) - Popular Abstract (Swedish)
- Nya byggnormer – smartare tunnlar?
Cut-and-Cover-tunnlar är vanliga vid grund anläggningsnivå i svensk infrastruktur. Dessa betongkonstruktioner utsätts för konstant vattentryck, vilket ställer krav på sprickkontroll. Sprickor uppstår ofta på grund av tvångslaster, exempelvis från temperaturvariationer och krympning, när dessa deformationer hindras av omgivningen. För att begränsa sprickbildning delas tunneln i kortare segment, så kallade monoliter, med rörelsefogar.
Figur 1 Cut-and-cover tunnel
Syftet med examensarbetet är att undersöka möjlig monolitlängd utifrån sprickviddskrav enligt andra generationen av Eurokod 2. Den nya normen kräver verifiering genom beräkning, till skillnad från tidigare tumregler.
Arbetet omfattar en... (More) - Nya byggnormer – smartare tunnlar?
Cut-and-Cover-tunnlar är vanliga vid grund anläggningsnivå i svensk infrastruktur. Dessa betongkonstruktioner utsätts för konstant vattentryck, vilket ställer krav på sprickkontroll. Sprickor uppstår ofta på grund av tvångslaster, exempelvis från temperaturvariationer och krympning, när dessa deformationer hindras av omgivningen. För att begränsa sprickbildning delas tunneln i kortare segment, så kallade monoliter, med rörelsefogar.
Figur 1 Cut-and-cover tunnel
Syftet med examensarbetet är att undersöka möjlig monolitlängd utifrån sprickviddskrav enligt andra generationen av Eurokod 2. Den nya normen kräver verifiering genom beräkning, till skillnad från tidigare tumregler.
Arbetet omfattar en litteraturstudie och handberäkningar enligt Eurokod 2 (2023) och Trafikverkets regelverk. Fokus ligger särskilt på friktion mellan betong och återfyllning, ett område som normen inte behandlar i detalj. Två metoder för friktionsminskning identifierades: avjämningslager i grovbetong och glidskikt av plastfolie med sand. Båda har visat god potential i experimentella försök.
En parameterstudie genomfördes där vägghöjd, friktion samt armeringsmängd- och diameter varierades. Resultaten visar att längre monoliter än tidigare tumregler är möjliga. Den nya normen medger även lägre minimiarmering. Ökad armeringsmängd ger bättre sprickkontroll, medan grövre stänger leder till större sprickor, och fler och tunnare stänger är därför att föredra. Friktionen visade sig vara den mest styrande faktorn. (Less)
Please use this url to cite or link to this publication:
http://lup.lub.lu.se/student-papers/record/9209889
- author
- Henningsen, Love LU
- supervisor
- organization
- alternative title
- Restraint Forces, Movement Joints, and Minimum Reinforcement – What Is the Impact of the New Generation of Eurocodes?
- course
- VBKM01 20251
- year
- 2025
- type
- H3 - Professional qualifications (4 Years - )
- subject
- keywords
- Rörelsefogar, minimiarmering, tvångskrafter, sprickvidd, Eurokod 2, cut-and-cover tunnlar, friktion
- publication/series
- 0349-4969
- report number
- TVBK-5306
- other publication id
- LUTVDG/TVBK/25/5306
- language
- Swedish
- additional info
- Examinator: Ivar Björnsson
- id
- 9209889
- date added to LUP
- 2025-08-18 11:43:58
- date last changed
- 2025-08-18 11:43:58
@misc{9209889,
abstract = {{Tunnel structures play an important role in Sweden’s infrastructure. At shallow depths, a common construction method is the so-called “Cut-and-Cover” tunnel, where the tunnel placement is excavated, cast in concrete on-site, and then backfilled. Such tunnels are typically subjected to constant water pressure, which yields high demands on crack control.
Cracking in elongated concrete structures often results from restraint effects. These arise when imposed deformations, for example due to temperature variations or shrinkage, are prevented by surrounding soil or adjacent structural elements. If the resulting stresses exceed the tensile strength of the concrete, cracking occurs. Since restraint forces increase with the length of the structure, tunnels are commonly divided into shorter segments, referred to as monoliths, with expansion joints in between to allow for controlled movement.
The aim of this thesis has been to investigate the possible monolith length in Cut-and-Cover tunnels with respect to crack width limitations, as specified in the new generation of Eurocode 2 (SS-EN-1992-1-1:2023). In the previous version of the code, monolith lengths could be determined using simplified rules of thumb, which have now been replaced by a requirement for calculation-based verification that includes restraint effects.
The thesis begins with a literature review followed by hand calculations based on the new Eurocode 2 and the Swedish Transport Administration’s infrastructure regulations. The literature review focuses primarily on friction between the tunnel structure and the backfill of crushed aggregate, as this is not extensively covered in current design standards. Two main methods for reducing friction were identified: the use of a leveling layer of concrete above the friction soil, and the application of a sliding layer consisting of plastic sheeting combined with a thin layer of sand. The reviewed experimental studies indicate potential for reducing the coefficient of friction, although all tests were performed at a much smaller scale than actual tunnel structures.
The analytical part included a parametric study where the wall height position, friction coefficient, reinforcement quantity, and bar diameter were varied. The results indicate that for the studied geometry and concrete quality, longer monoliths are possible compared to the previous rule of thumb. The minimum reinforcement requirement has also been lowered in the new generation of Eurocode 2. Furthermore, the results show that crack width decreases over the wall height, and that an increased amount of reinforcement allows for longer monoliths. However, larger bar diameters result in wider cracks, suggesting that a higher number of thinner bars or multiple reinforcement layers is preferable.
Friction was found to be the most critical factor affecting cracking, and reduction of the friction led to significant improvements.
In conclusion, the study shows that the previous rule of thumb regarding monolith length was conservative and that longer segments are possible if crack control is performed in accordance with the new Eurocode, especially in combination with optimized reinforcement strategies and friction-reducing measures.}},
author = {{Henningsen, Love}},
language = {{swe}},
note = {{Student Paper}},
series = {{0349-4969}},
title = {{Tvångskrafter, rörelsefogar och minimiarmering - Vad blir effekten av nya generationen av Eurokoder?}},
year = {{2025}},
}