Ralf Winterberg - Soporte en Suelo con Shotcrete Reforzado con Macro Fibras

Ground Support with Macro Fibre Reinforced Shotcrete – Presentation

Introduction Macro synthetic fiber reinforced concrete (MSFRC) or shotcrete (MSFRS) • Is a viable cost-efficient alternative or addition to steel reinforcement in tunnel linings • Used in both mining and civil tunnels • Standard reinforcement of shotcrete linings in Australian mines • Used for temporary (initial) and permanent linings • Used in shotcrete, cast-in-place final linings and segmental linings

Three Ground Categories • Soft Ground • Blocky Rock • Hard Rock

How does shotcrete support ground?

• The mechanism of support varies with the type of ground being supported • Shotcrete locks the surface of hard ground together and prevents it unravelling during relaxation • The effectiveness of this process depends on bond, strength development, and toughness • In soft ground, shotcrete generally acts as an arch that resists load through compression

Soft Ground Tunnelling Characteristics: • Soil or weak rock that generally behaves as a single mass • Typically 0 – 10 MPa • Stand up time of few hours Key Behavioural Mechanism: • Plastic yielding • Failure of the ground around the tunnel đ&#x;Ąş Shotcrete needs to be sprayed as soon after excavation as possible to provide immediate support.

Sequential excavation method (Image from Norma Consulting)

Shotcrete is Part of a Support System Four Golden Rules to get the most benefit out of your shotcrete: 1. Shotcrete is not a stand alone support system 2. Shotcrete should always be sprayed first 3. Bolts should always be installed after the shotcrete has been sprayed.

4. Shotcrete must be reinforced

Fibre Reinforced Concrete (FRC) Toughness and post-cracking behaviour • The addition of fibres substantially improves the concrete characteristics, transforming its behaviour from elastic-fragile (brittle) to elastic-plastic (ductile), making it suitable for structural applications.

Bending Capacity Reinforced concrete (R/C): Fc

x d

h

Fs

M

Fibre reinforced concrete (FRC):

Fc

x h

FF

M fRes

Influence of Fibre on Structural Properties • Elastic Stage: Marginal • Post Cracking: Significant (toughness!)

• Fibers bridging the opening crack – resistance under tensile stress • Failure mechanism of unreinforced concrete changes from brittle to elasto-plastic (tough/ductile) • Energy Absorption / Toughness used as performance criteria for shotcrete linings • Fibers are capable of replacing classical rebar/mesh

The Role of Fibre in Shotcrete • Ductility - to allow the composite material to carry flexural load beyond the flexural capacity of the shotcrete itself.

• Durability - to allow the composite material to continue providing support in the post cracked state • Do-ability - to provide an integrated reinforcement system which can automatically conform to irregular ground profiles.

The Role of Fibre in Shotcrete • Reductions in Rebound - removal of fixed reinforcement reduces rebound potential • Reduced Logistic Imposition - to reduce delivery, process, and application inputs leading to economies in cycle times and costs • Passive Fire Protection - under exposure to fire loading some fibres are able to melt and provide capillary egress for expanding vapours

Soft Ground Lining: Shotcrete Arch • Still interaction between rock mass and shotcrete lining • Shotcrete seals off the rock mass to avoid deterioration • Shotcrete lining provides an arching effect • Lining stiffer and thicker compared to hard rock tunneling • Static system: Shotcrete arch, hyperstatic • Typical test: Beam test as basis for stress-strain relationship of fiber reinforced shotcrete (tension side)

Design with N-M – Interaction Diagram • Basic design values are obtained from standard beam tests, e.g. ASTM C 1609 or EN 14651 • Setup of idealized stress-strain relationship with basic design values • Use of stress block on tension side • Cross-sectional equilibrium iterations yield moment capacity for given thrust • Factored design load couples to remain within capacity envelope

Final Lining Design • Structural design approach very similar to initial lining in soft ground: • N-M interaction diagram • Finite Element Method (FEM) analysis • Different, long term load cases compared to initial linings: Water pressure, settlements, near construction, seismic, ...

• Static system: Arch (shotcrete lining, cast-in-place lining) • Long term behavior and in-service performance are critical • Fibres typically partly replace rebar and improve cracking behavior.

Full replacement possible, if only minor or moderate eccentricity (e = M/N)

Long Term Behaviour of Tunnel Linings • Durability: Improved cracking behavior & crack width control đ&#x;Ąş reduced permeability đ&#x;Ąş less corrosion in combination with steel reinforcement Macro synthetic fibers are corrosion free • In-service performance: No embrittlement đ&#x;Ąş macro synthetic fibers do not “breakâ€? even if concrete hardens with age Arches resist load through compression Creep under tensile stress under further research (RILEM TC261, Asquapro)

Macro Fibre R&D Combined Bending-Thrust Performance of MSFRC - More details in the afternoon workshop -

N-M interaction FRC beams, no rebars, 7 kg/m3 BC48. N = 0, 2, 4 and 6 MPa

N=0

N = 2 MPa

Almost ideal elastic-plastic behavior from 2 MPa thrust

N-M interaction Hardening behaviour from 4 MPa until 6 mm crack width

N = 4 MPa

N = 6 MPa

0.03 radians times 200 mm thickness means 6 mm wide crack

N-M numerical modelling

2 MPa compressive stress

4 MPa compressive stress

N-M interaction Conclusions: • Standard beam tests (ASTM C 1609, EN 14651) significantly underestimate the FRC performance where compression is present • MSF significantly improve the bending capacity of a section under compression • Confinement provided by axial stress yields significant increase in ductility • Presence of thrust yields hardening behavior of the FRC section • No need for a strain-hardening FRC behaviour in tunnel linings

Macro Fibre R&D Seismic Performance of MSFRC - More details in the afternoon workshop -

Load Pattern

Lateral displacement was progressively increased up to 18% drift using ACI 374 guideline.

Performance of seismically designed Section

Plain concrete cross-section with stirrups at 50 mm spacing showed outstanding performance as expected.

This set of three specimens was the benchmark against which all other specimens were measured.

Plain Concrete Performance

Even for 100 mm stirrup spacing, concrete broke up and fell out leaving bars to buckle.

Influence of macro synthetic fibres

When 6 kg of BC54 was included in concrete, performance was greatly enhanced, but bars ruptured in tension for several specimens thereby reducing performance at large drift.

FRC Performance

6 kg of BC54 is very effective at holding concrete together even for 500 mm stirrup spacing.

Reference Projects

Reference projects – NATM Devils Slide Tunnel, USA

Reference projects – NATM Caldecott 4th Bore, USA

Reference projects – Permanent SCL A3 Hindhead Road Tunnel, UK

Reference projects – Permanent SCL North Strathfield Rail Underpass, AUS

Conclusions 20 Years with macro synthetic FRS: • Fibre Reinforced Shotcrete has revolutionized safety in underground excavation • Macro synthetic fibres have out-performed all alternatives when unbiased engineering assessment is applied to ground control using shotcrete • In the long term, macro-synthetic fibres are the only form of reinforcement that both retains capacity as the concrete ages and remains completely free of corrosion.

• Lining convergence and surface subsidence are most effectively controlled by the design of the tunnel and excavation sequence, not by the lining material (FRS).

• The demonstrated performance of macro-synthetic FRS makes it the most attractive available means of stabilizing ground both in the short and long term.