I’ve written about Arsenalna (Ukrainian: Арсенальна) station on the Kyiv Metro in Ukraine before – at 105.5 metres below ground, it takes two flights of escalators to reach the surface, passing through an underground vestibule at the halfway point. But how did they build such an elaborate chamber so deep underground?
The two escalator tunnels are 55.8 metres and 46.6 metres long respectively.
And meet at the intermediate vestibule 22.6 metres in diameter and 18.7 metres tall.
Due to local ground conditions, it was decided to build the concrete vestibule on the surface.
And then lower it to the design depth.
By slowly excavating the ground beneath it, and letting it sink down.
Олег Тоцкий describing the process:
Due to difficult hydrogeological conditions, about 400 wells (1) were drilled along the vestibule contour before starting work, and soil was frozen through them.
The intermediate vestibule (2) was built of monolithic reinforced concrete and had a bell-shaped shape without a bottom. The lower edge of the “bell” was made in the form of a knife, which, under the weight of the vestibule, crashed into the ground as it was pulled out. Thus, the vestibule, under its own weight, fell down as the soil underneath was excavated.
Almost the entire area of the soil was developed by an excavator (3), and only along the edges of the “bell” under the knives, the soil was removed manually.
With the help of a bucket (4), through a pit (5) previously passed through the center of the vestibule, the soil fell down, where through the transport adit (6) with the help of a belt conveyor (7) it was fed to the skip hoist (8) and then went up through the shaft.
As the vestibule descended, the space above the dome was covered with soil, and due to the increase in external pressure on the dome, the rate of lowering of the vestibule also increased.
Less than 4 months after the start of the descent, the vestibule reached the design depth (9) and rested with knives against the pre-cast foundation concrete ring (10).
After that, the bottom of the vestibule was concreted and the construction of the lower inclined passage began.
Because freezing was also used for its construction; guides for freezing wells were pre-laid in the body of the intermediate vestibule.
After lowering the vestibule, a drilling rig was dragged inside, with which wells were drilled along the lower slope, then a freezing was carried out and a lower slope was built from the intermediate vestibule.
In the diagram, we also see the adjoining of the upper inclined passage (11) to the intermediate vestibule.
I’ve seen caissons used to excavate bridge piers underwater, but using them for underground structures is ingenious!
Footnote:Zoloti Vorota station
Zoloti Vorota station also has an underground vestibule.
And was built in a similar way:
Since its entrance is located on a hillside, two separate escalator tunnels had to be created, connected by an underground station vestibule. The upper tunnel is 35 m (115 ft) long, while the lower tunnel is 56 m (184 ft) long.
The vestibule was first completed at ground level, and then lowered to its final location underground. The vestibule is a 20 m (66 ft) tall monolithic dome with an approximately similar diameter.
The installation began in November 1987 and was completed by 1988, lowered at a pace of half a meter a day. Since the construction took place in water-saturated soils, over 200 frozen wells had to be formed so it could be lowered to its proper location. In addition, over 250 m2 (300 sq yd) of rock had to be removed to make way for the vestibule. After it was finally installed, construction work began on the lower escalator tunnel.