Road builders are concreting a disused tunnel under Prosta Street. Over 80 meters long corridor will be flooded with over 900 cubic meters of mineral mixture. The works are carried out due to the poor technical condition of the facility. In the past, it was probably used to transport the press, but currently it is not possible to restore it to use.
The tunnel is located under Prosta at the intersection with Wronia. It was probably created in the 50s or 60s of the last century. At that time, the plot on the southern side of Prosta Street was adjacent to the back of the printing plant of Dom Słowa Polskiego, and at the intersection with Wronia there was a back office for a forwarding company dealing with the distribution of newspapers around Poland. Road builders therefore assume that the tunnel was used to transport press between these facilities. “This is evidenced by the fact that through the longest part of the tunnel, crossing Prosta Street, there is a track that could be used for a trolley driven by a chain driven by a special actuator” – explains the Municipal Roads Authority.
The underground facility is over 84 meters long in total and is just over two meters high. The whole is divided into three parts: the main chamber, the main corridor and the side corridor. The central part of the tunnel, i.e. the main chamber, measures 11.9 m x 6.26 m (length and width). It is located on the western side of the northern inlet of Wronia to Prosta. A side corridor leads from the chamber towards the west. It is 2.25 m wide and 19.1 m long. The width of the main corridor is 3.45 m.
As described by road builders, there are reports that once the tunnel was even about 200 meters long. They do not rule out that this could be true, because the main corridor is now 65 meters long and ends with a wall in the place where a new building has now grown. “Placing the wall and ‘cutting’ the tunnel violated its structure – the wall is not tight and groundwater penetrates through it” – says ZDM.
The ceiling is leaking, stalactites have formed on the ceiling
ZDM has recently assessed the condition of the tunnel. He turned out to be evil. “Stalactites formed on the ceiling of the main chamber, which are the result of water leaks through the ceiling. The zones of expansion joints are also leaky, which is why the walls of the building are heavily damp. The main corridor looks the worst, especially the lintel in the zone of the entrance to this corridor. corrosion losses of the concrete and exposure of the corroded main reinforcement, the lintel was supported with ‘stamps’, i.e. special metal tubes.
Tunnel repair was out of the question. Road workers explain that it has not been used for years and the costs of such an undertaking are very high. In addition, bricking up its corridor on the south side meant that it could not be used as intended. It is also not possible to convert it into an underground passage, because the tunnel towards the south goes deeper and deeper.
The building was assessed by the Poviat Building Supervision Inspector and ordered to be concreted. This is necessary due to the safety of the Prosta roadway and the tram tracks running along it. The condition of the tunnel ceilings could cause the roadway to crack and collapse.
They will pour 900 cubic meters of concrete
Work on concreting started in the night from Wednesday to Thursday. First, it was necessary to pump out the water accumulated in the southern part of the main corridor. The contractor also reinforced two walls in the main chamber with steel profiles, which were used to wall up the niches to the underground rooms of the forwarding facilities.
Road builders describe that concrete was then pumped in. In total, the space of the facility will be filled with about 900 cubic meters of a special mineral mix made of aggregate, cement binder, mineral additives and chemical admixtures. First, it goes to the main corridor. It is a material specially adapted to fill voids such as disused channels or reservoirs. “The mixture is self-compacting, which means that after filling a given space (object) with it, it does not require vibration before it solidifies. After hardening, it has the properties of compacted soil, foundation or lean concrete. The advantage is that such a mixture eliminates the access of moisture and oxygen” – they enumerate.
The schedule assumes that concreting will be carried out every other night. This is because the layers of the mixture are stacked on top of each other, and each of them needs several hours to set. “To ensure that the entire process is carried out in accordance with technological requirements, during the works we pump concrete into the tunnel within 90 minutes of production. Concrete trucks arrive at the site almost one after another – five transports arrive within an hour. The flow of mix from trucks to the tunnel is about 30-40 cubic meters per hour. Completion of works is planned for the night from Saturday to Sunday, from 17 to 18 December.
Main photo source: ZDM