Project Report - Part 1: The Diver
Our slitting line in Menziken (AG) has reached the end of its life cycle. To continue meeting our customers’ evolving expectations, we are investing in a new, state-of-the-art line at our Sennwald (SG) site. The installation requires a sloping pit with a usable depth of 15 metres. This page offers updates on the construction challenges and the project's progress.
Slitting aluminium and steel coil is one of our core competencies. However, our 33-year-old system in Menziken (AG) no longer meets today’s standards; outdated technology, safety concerns and long changeover times have become critical limitations. Additionally, sourcing spare parts has become increasingly difficult. Due to spatial constraints in Menziken, we are investing in a new high-performance slitting line at our Sennwald site.
Video Report
Slitting aluminium coils down to 20 mm widths
Our new state-of-the-art slitting line is capable of processing aluminium coils up to 1,300 mm in width and between 0.2 mm and 3.0 mm in thickness. The minimum slit width is just 20 mm. The new system needs a sloping pit to ensure uniform recoiling of the individual strip sections, which naturally vary in length. At this stage, the hall where the system will be installed remains empty...
Groundbreaking ceremony: Excavation begins
The groundbreaking ceremony was held in November 2023, though it was going to need more than just a spade. Instead, an 80-tonne cable excavator rolled into the previously empty hall, joined by a heavy wheel loader and an 18-tonne wheeled excavator. Outside, three large tanks were on standby, each filled with a total of 235,000 litres of a bentonite-water slurry, which serves as a support fluid during the construction of the diaphragm walls. The pit comprises ten diaphragm wall segments. Typically, constructing a single wall section takes just under three days. Excavation soon ran into complications in the bedrock.
Vibrations throughout the hall and surrounding area
We encountered solid rock, so we had to reconfigure the cable excavator: Instead of using the 12-tonne grab bucket, we deployed a 15-tonne drop hammer, repeatedly driving it into the rock base. The force of the impact caused noticeable vibrations throughout the hall and surrounding area. Eventually, the bedrock fractured enough for removal using the grab bucket. When moving the reinforcement in the pit, the next challenge presented itself.
Segmented reinforcement cages were required
The pit reaches a depth of 19 metres, while the hall's interior height is only 10 metres. This made it impossible to lower full-length reinforcement cages in a single piece. Our solution: We lowered the first reinforcement cage so that it protruded slightly from the pit. The second segment was then placed above it and the two were welded together in situ.
Diaphragm walls: Concreting within bentonite slurry
Once a reinforcement cage was positioned in the pit, we concreted the corresponding diaphragm wall section in the betonite slurry, from bottom to top, using a tremie pipe, through which the specially formulate concrete was poured. The process was repeated until all elements were completed. Despite a few days' delay due to the rocky terrain, we were able to complete the ten slotted walls, forming a decagon. In total, 300 m³ of excavation material was removed, and 1,000 tonnes of concrete were used for the slotted walls.
450 m³ of material removed from the pit
We proceeded to excavate the pit within the decagon: removing approximately 450 m³ of material! Of the 19 m depth of the pit, 15 m will ultimately serve as the sloping pit for the operation of the system. The internal diameter of the pit measures 5.4 m.
If the base slab is not watertight
With the slotted walls completed and the pit excavated, we encountered the next challenge: the pit was still filled... with water! Why? This was due to the groundwater level being 2 m below the hall floor. This meant we had to carry out the subsequent construction steps underwater. Initially, we cast the underwater base slab, forming the bottom closure of the pit. This base slab comprises a 2-metre-thick concrete plate. We constructed the actual concrete floor slab on top of it.
Divers must work 'blind’
As the pit was still nearly full to the top with water, we hired two divers to assist with the reinforcement work. The floor slab lacked reinforcement connections to the wall after casting, which the divers addressed by drilling and placing the reinforcement underwater. This task was particularly challenging as the divers had to work 'blind’ - the murky pit water limited visibility to just 10 cm!
A dive could only last 1.5 hours at most
While one diver worked underwater, the other monitored from the pit's edge, alternating roles accordingly. Each dive was limited to a maximum of 1.5 hours due to workload and fatigue. To combat cold and moisture, divers wore three layers of gloves, relying solely on touch due to the zero visibility conditions.
The pit is dry
Fortunately, the diving operations concluded successfully by mid-December 2023. The water was subsequently pumped out and the pit was dried out. We then sealed the pit using an injection process. In the days that followed, we installed a pump shaft, ladder, emergency ventilation and lighting within the pit to facilitate future maintenance work. Thanks to the lighting, we would later be able to observe how deep the aluminium loop extended into the pit when the machine was operational.
