Conceptos generales de diseño para el soporte de suelos con hormigón proyectado con fibra

Support with Macro Fibre Reinforced Shotcrete – Workshop

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

Rock Support Objective The primary objective of support practice is to mobilise and conserve the inherent strength of the rock mass so that it becomes self supporting

Rock support Stabilize the rock mass by use of support elements Support system provides 3 primary functions: • Reinforcement • Holding • Retention A combination such as bolts and shotcrete will achieve all 3 functions

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  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

Rock Support with FRS • Shotcrete arches collect and transfer load to rock bolts • Shotcrete seals off exposed rock surface and avoids deterioration of structural properties • Failure modes are either block failure or weak rock zone • Static system: plane, fixed a the circumference; statically undetermined • Typical test: EFNARC (EN 14488-5) or Round Determinate Panel test

Simplified Design with modified Barton Chart

Rock Mass Quality determines: • FRS toughness performance levels • FRS layer thickness • Bolting patterns • Rock bolt diameters & lengths Fibers typically replace mesh in initial linings

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)

Parameters for Design • • • • • • • • •

Ground composition (excavation material) Overburden Ground loadings, water pressure Dimensions and geometry of tunnel/gallery Excavation sequence and duration N-M couples at critical sections and chainages Lining thickness (stiffness attracts loads) One pass or two pass lining Aim for compression arching, minimise bending

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

FEA Design – ATENA

Juhasz K.P., Nagy, L. and Winterberg, R. (2015). “Full-round Numerical Analysis of Traditional Steel Bar and Macro Synthetic Fibre reinforced Concrete Segments for the Shanghai Metro Extension”, Proceedings of the World Tunnelling Congress 2015, Dubrovnik, Croatia, 22–28 May 2015

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  reduced permeability  less corrosion in combination with steel reinforcement Macro synthetic fibers are corrosion free • In-service performance: No embrittlement  macro synthetic fibers do not “break” 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

Structural Design of MSFRC Do we need a strain-hardening material? • No, there is no general requirement for hardening behavior according to MC2010. • Section 7.7.2: In the case of linear elements, without conventional reinforcement, subjected to axial tension with small eccentricity (i.e. without compressive stresses in the section), in addition to the limitations provided by Eqs. (7.7-1) and (7.7-2), FRC should have a hardening behaviour in tension (see clause 5.6.1, Fig. 5.6-2).

Post-crack hardening (deflection hardening)

N-M interaction

N-M test setup

2.0m long beam columns, 200 × 200 mm2 cross section Pivot separation (i.e. effective length) is 2524 mm, load spacing 500 mm Shotcrete mix f’c = 32 MPa, testing @ 56 days (UCS = 49 MPa)

N-M interaction

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 N-M interaction, FEA modelling of the tests

JKP Static

N-M numerical modelling N-M interaction, FEA modelling of the tests

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

Seismic Performance of MSFRC

Modelling a curved tunnel lining

The distributed loading imparted on a curved tunnel segment was replaced with point loading of a straight section of Reinforced Concrete with either plain concrete or FRC used in 30 test specimens.

Seismic Test Setup

Test apparatus consisted of two servo-hydraulic actuators applying axial and lateral load to a beam-column.

Reverse cyclic loading regime complied with ACI 374 requirements for simulated seismic loading.

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.

Section with stirrups at 500 mm

Plain concrete specimen with stirrups at 500 mm spacing performed poorly, with large amounts of concrete falling out due to extensive cracking.

Plain Concrete Performance

Absence of stirrups causes concrete to break up and fall out quickly for 500 mm spacing.

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.

Summary Performance of Macro Synthetic Fibre Reinforced Tunnel Linings • Process: Easy to mix, pump and apply (shotcrete) Reduced wear and tear on pumps and slicklines Easier, faster and safer to handle compared to classical reinforcement • Design: Hard rock tunneling & mining: Barton Chart / Energy absorption capacity Soft ground initial & final lining: N-M-interaction diagram and/or FEA Segmental lining: N-M-interaction diagram and/or FEA • Structural Performance: Elasto-plastic behaviour  post cracking bearing capacity / toughness Corrosion free  more freedom in section design Reliable performance with aging – absence of both embrittlement and corrosion

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.