Annual backfill with MAI^® 440GE equipment 03 Oct 2019

Invert segment ready for annual gap backfilling

The Tulfes-Pfons construction lot of the Brenner Base transalpine railway project near Innsbruck, Austria, with the first 15km of the project exploratory tunnel was completed in July 2019. "Our contract section, with Strabag as the Technical Leader, encompasses about 40km of excavation, with some 12.5km (25km in total) of twin NATM drill+blast main running tunnels and about 15km of the exploratory tunnel completed using an 8m diameter open gripper TBM supplied by Herrenknecht, working south from the Ahrental access adit,” explained Sebastian Grüllich, TBM Construction Manager of the Strabag/Salini-Impregilo consortium. “The excavated volume amounts to about 2.3 million m³ of rock and some 430,000m³ of structural concrete has been processed for the inner lining of the main rail tunnels."

• MAI^® International GmbH
• Contact: Hannes Papousek, CEO
• Email: hpp@mai.at
• Website: www.mai.at

Invert segment backfilling

In the exploratory tunnel, invert segments are installed some 30m behind the TBM cutterhead and backfilled with a dry mortar pre-mixed at the factory. Once the backfill concept was adopted by the joint venture, the MAI^® 440GE mixing pump from the Austrian manufacturer MAI^® International, as a leader in design and manufacture of grouting pumps, was the obvious choice. "The design of the mortar mixing pump sets new standards regarding process safety, and the dual mixing system constant quality and consistency of the backfill mortar,” said Hannes Papousek, Managing Director of MAI^® International. “Complete documentation of the most important operating data of the pump has also been agreed with the client, including pressure, grouting quantity, injected volume and water flow. For quality assurance, the water-cement value of the installed grout can be verified retroactively. This is accomplished with the MAI® LOG data logging unit, which records the corresponding data every 10 seconds."

Dual mixing system of the MAI^® 440GE provides a mortar of constant quality and consistency

Operation of the equipment is self-explanatory so that anyone can operate it following a short period of instruction. As a result, a single person is capable of placing and backfilling the invert segments and the robust design reduces maintenance to a minimum. “We place great importance on ensuring that all wear parts can be replaced in a speedy and straightforward manner,” said Papousek. “For us that represents an important design feature. The compact mixing pump and the data logging equipment on the Tulfes-Pfons exploratory tunnel TBM were assembled directly below the dry mortar container in October 2015 and since then injected approximately 7,000 tonne of mortar.”

Grüllich described the importance of the control and mixing process of the invert segments backfill: "About 8m behind the invert segment placing and backfilling area, the drive units of the TBM trailing backup gantries run on the backfilled invert segments, the weight of gantry amounting to about 75 tonne. Consequently, complete and secure backfilling of the annular gap is imperative to safeguard the load capacity and immediate resilience of the backfilled invert segments. The necessary consistency of the backfill mortar is guaranteed by separating the mixer and the pump and the automatic grouting cycle without visual monitoring on the part of the operator”.

The invert segments are backfilled, without pressure injection, via the two lateral joints between the segment and rock surface using a hose of about 10m long. The mixing pump deployed must guarantee a constant mortar quality and consistency at all times good and provide good flowability to provide the bond between the invert segment and the rock as soon as possible.

Annual backfilling without pressure

The mixing pump parameters are set via a control station located a few metres behind the TBM, allowing adjustment at the point where the invert concrete segments are installed. Even under the most rigorous site conditions, the mortar can be backfilled with guaranteed precision. Optimal results are attained and, at the same time, the MAI^® data logging unit provides verification for the client. The real-time data logging system sets new standards for displaying and logging operating parameters including the quantity of water used for documenting the water:cement ratio, as well as grout flow pressure, volume, throughput and the time needed for placing mortar and grouting.

The Austrian company MAI^® International GmbH develops, manufactures and markets mixing pumps for the full spectrum of grouting technology. “MAI^® stands for man assisting innovations,” said Papousek. “In other words, we are united by the desire to create something that is novel to make people`s lives easier. The individual is our main concern.”