In Poland, engineers used a 3,500-ton concrete cradle to launch a giant boring machine beneath Łódź; the 2.9-mile tunnel will carry high-speed trains under the city without tearing up its streets

On the southwest edge of Łódź, Poland, four specialist tunneling crews from seven European countries spent nearly three weeks working around the clock to build a curved concrete cradle at the bottom of the Retkinia launch chamber. The structure is designed to support and correctly position a roughly 3,500-ton tunnel boring machine (TBM) as it begins cutting beneath the city.

The cradle was completed on August 18, and the first TBM components are already being assembled on it, as per an Autonocion report. Once underground, the machine will cut a 2.9-mile tunnel for Poland's high-speed rail network, allowing trains to pass beneath Łódź rather than requiring a cut-and-cover route through the city.

The concrete cradle has to get the alignment right

The TBM must leave the launch chamber at the correct angle and alignment. At the start of a tunnel drive, the machine has little behind it to help stabilize or guide it. That makes the cradle's position particularly important, as even small deviations could affect the machine's trajectory during the early part of the bore. The concrete was therefore poured continuously for nearly 20 days without a break.

The launch chamber is a major construction project itself

The Retkinia launch chamber is roughly 590 feet long and up to 75 feet deep, with diaphragm walls extending to about 143 feet. Construction required the removal of around 110,000 cubic yards of soil, nearly 39,000 cubic yards of concrete and about 3,300 tons of steel. Around 100 workers were on site at peak construction.

The TBM is being assembled underground from sections roughly 62 feet long and weighing about 235 tons. A 500-ton gantry crane is handling the components. The 460-ton cutterhead is being assembled above ground before being lowered into the chamber in a single lift.

The crane had to pass a 550-ton test

Before the cutterhead lift, crews tested the gantry crane in early July using water-filled ballast bags totaling about 550 tons. That represented 110% of the crane's rated capacity.

The test formed part of the technical acceptance process for the lifting equipment and was inspected by Poland's Office of Technical Inspection. The TBM cannot be transported to the site fully assembled, so it must be put together where its underground work will begin.

The new tunnel will pass through a city with existing rail tunnels

The new tunnel will have a diameter of about 46 feet and carry two high-speed railway tracks in a single tube. The project is particularly challenging because Łódź already has railway tunnels beneath the city.

PKP PLK has been building a cross-city rail tunnel since the late 2010s, with existing tunnels reaching depths of around 79 feet. The new high-speed bore will therefore pass through ground containing existing underground infrastructure. The construction plan includes diaphragm walls, jet grouting and load-transfer structures to limit settlement.

At the historic Łódź Cultural Centre, crews installed about 1,400 micropiles and jet-grouting columns to stabilize the soil beneath the building. Steel bracing was also added to its northern wing.

At the other end of the route, a receiving chamber near Łódź Fabryczna station has been built with five-foot-thick diaphragm walls. The structure is about 72 by 98 feet and was excavated 85 feet deep.

The tunnel avoids a cut-and-cover route through central Łódź

The tunnel is part of railway line No. 85, the high-speed "Y" corridor connecting Warsaw and the planned new airport with Łódź before splitting toward Wrocław and Poznań, as per the Autonocion report.

Trains are planned to reach speeds of up to 217 mph on the open sections of the route. Inside the tunnel, the design speed is 100 mph.

Using a TBM allows the railway to pass beneath the city while limiting its impact on existing buildings and urban infrastructure.

Excavation is scheduled to begin in 2027

The TBM is scheduled to begin excavation in the first quarter of 2027 and is expected to reach the Fabryczna receiving chamber during 2028. Fit-out, systems integration and safety testing are then expected to continue through 2029.

The railway section forms part of the wider CPK railway and airport program, which has been estimated at more than 150 billion złoty, according to the Autonocion report.