First use of MSFR segments in the USA 25 Apr 2019

The first use of a macro synthetic fibre reinforced segmental lining in the USA is been installed in the Blacksnake Creek stormwater runoff tunnel in St Joseph, Missouri. The 2km long x 2.74m i.d. segmental lined tunnel is currently under construction by US contractor Super Excavators Inc using a Lovsuns EPBM. The tunnel is the main element of the stormwater separation improvement project as part of the city’s combined sewer overflow long term control plan to improve water quality in the Missouri River, as mandated by the US Federal Government Clean Water Act.

MSF reinforced segments for the Blacksnake Creek CSO project

The use of macro synthetic fibre as the only reinforcement in the segmental lining follows the application of macro synthetic fibre reinforcement (MSFR) for the cast-in-place final lining of the starter and end sections of the Euclid Creek CSO tunnel in Ohio in 2015 for the Northeast Ohio Regional Sewer District⁽¹⁾. This application was followed by a series of laboratory trials and field tests for applying MSFR concrete to the production of segments⁽²⁾. It was based on the successful results of these tests and trials that an MSFR segmental lining was adopted for the Blacksnake Creek tunnel.

Designed by project designer Black & Veatch, the one-pass segmental lining is reinforced with BarChip 54 macro synthetic fibres as an alternative to steel fibre or traditional rebar reinforcement.

MSF being dosed at 7kg/m³ into the concrete mix

Steel fibre reinforcement was anticipated during project design but availability of steel fibres were not readily available to comply with the buy America iron and steel provisions in the contract. An alternative was needed and segment manufacturer CSI Tunnel Systems proposed using BarChip synthetic fibres. Synthetic fibers were permitted providing the segments met the contract compressive strength specifications of at least 41.4 MPa, a minimum flexural tensile strength at 28 days of at least 4.62 MPa, and the minimum post-crack residual flexural tensile strength of at least 3.17 MPa.

The use of synthetic fibre reinforcement provided additional benefits in reducing material costs and eliminating the risk of corrosion, which in turn increases the service life of the lining and reduces future infrastructure maintenance costs.

The segmental lining is composed of six bolted and gasketed MSFR trapezoidal segments per 2.74m i.d. ring. The universal rings of 1.2m long have a taper of 12.7mm to accommodate a turning radius of 300m. The segments are being produced by CSI Tunnel Systems at its plant in Macedonia, Ohio, and trucked to site.

The 60 degrees segmentation of the rings accommodates the fibre reinforced concrete only solution as the aspect ratio of the segments is limited. In an interview with Ralf Winterberg of BarChip, it was explained that the segment aspect ratio is 8.1 for the 190.5mm thick x 2.743m i.d. ring of six trapezoidal segments, which, said Winterberg, is well below the acknowledged limit of 10 to ensure segment robustness for temporary load cases and as per the ITAtech 2016 guidelines for precast FRC segments.

Test sample of an MSFR cast concrete beam

The MSFR segments are made using class C40/50 (f’c 48 MPa) concrete at 28 days age, with a striking strength of 14 MPa after 4.5 hours of steam curing. A dosage of 7kg/m³ of BarChip 54 MSF was sufficient to exceed the residual strength specification of 3.2 MPa at l/150, as per the ASTM C1609 standard.

The design approach adopted for the MSFR segmental lining in ultimate limit state (ULS) is the use of normal force-bending moment interaction diagrams or moment-thrust capacity limit curves. The factored design load couples acting on the section must remain within the N–M envelope (Fig 1). The MSF material properties are herein derived from the beam tests, which are eventually used as the basis to determine the stress-strain relationship of the concrete on the tension side. The idealized stress-strain diagram enables setting up the capacity limit curves, which are obtained by equilibrium iterations on the given cross-section.

EPBM being prepared for launch

Cooperating with the contractor’s design team, L-7 Services of Denver, the Technical Department of BarChip Inc (formerly known as EPC) conducted a feasibility study for the use of macro synthetic fibres as the sole reinforcement for the lining. The structural design for ULS also comprised a simulation of 1% ground loss.

All checks for temporary load cases as well as the checks for serviceability limit states (SLS) have been performed with finite element analysis. These included numerical analysis of the jacking forces on the segmental lining during TBM forward thrust (Fig 2).

The TBM was launched on umbilicals at the bottom of a 15m diameter secant pile shaft in late 2018. During the launch phase, the TBM pushed off a half-ring cradle of the lining segments. By mid March 2019, the tunnel drive was 610m or 30% complete with about 500 rings of MSFR rings installed and the TBM advancing at rates of 12m to 14m during the single shift per production day.

Half-ring installation for TBM launch

The initial drive installation has proven that the MSFR segments are robust enough to withstand the temporary load cases experienced during transportation, hoisting, handling and installation. The half-ring sections used in the shaft for the launching process were also inspected after the initial push and no damage was noted to the segments.

The overall performance of the MSFR segments is reported to have outperformed expectations by the contractor and have proven to be robust during the transportation, handling, TBM launching, installation and TBM shove processes. The project will build further confidence in macro synthetic fibre reinforced segmental linings.

More details about the application of the MSFR segmental lining on the project are to be provided in technical papers at the 2019 WTC World Tunnel Congress in Naples, Italy, in May and at the 2019 RETC conference in Chicago, USA, in June.