During underground construction in areas with a long history, unexpected archaeological discoveries are frequently encountered. How to continue underground construction while protecting archaeological finds has become a common challenge in the underground engineering field.

During the construction of the Sintrivani metro station in Thessaloniki, Greece, after the diaphragm walls were completed, important historical building remains were discovered inside the station. In this issue, let us take you through how this project responded to such a situation!

Project Background    

Sintrivani Metro Station is located in Thessaloniki, the second most populous city in Greece. The Thessaloniki Metro is being built in an urban environment surrounded by high-rise buildings and containing many ancient architectural remains, making it a highly challenging project: the allowable ground surface settlement limits are very small, and the soil conditions along the route are poor with complex hydrogeological structures.

Construction of the Thessaloniki Metro

Sintrivani Metro Station is 76.60 m long and 19.75 m wide. The station consists of a four-level underground structure extending 27.20 m below the surface, constructed using the top-down method. Pre-construction site investigations indicated that the station location contains a layer of artificial deposits of varying thickness, estimated at 6 m (this layer is also referred to as the archaeological layer).

Station structure

Archaeological discovery

After the construction team completed the diaphragm walls and pile walls (used for excavation of the station and the passageway, respectively), it was found that the artificial deposits (archaeological layer) were far thicker than 6 m, reaching a maximum depth of 11 m.

The construction team then contacted the archaeological authorities and conducted an archaeological investigation together. Ultimately, it was found that in the southwest part of the station, inside the diaphragm wall, there were a large early Christian church and an ancient cemetery dating to approximately the late fourth century AD. The main structure of the church lies about 5 m below the ground surface.

Aerial view of the basilica remains inside the station

Further archaeological investigation revealed that the church was quite large, and the station's diaphragm wall had cut the church into two areas. The main nave of the church lies within the station's diaphragm wall, and some intact mosaic floor tiles are preserved inside the nave.

Byzantine (Eastern Roman Empire) style mosaic floor tiles discovered in the station

Relationship between the church and the station location (black portion indicates the church)

Considering the significance of the mosaic floor tiles for the study of Byzantine (Eastern Roman Empire) history, the Greek government decided to relocate them to a museum; at the same time, archaeological investigation of the church remains needed to continue, and a protected zone for the ancient structure was to be established.

To this end, the construction team needed to continue building the station without disturbing the church remains or their original foundation.

Countermeasures

At this point, the shield tunneling had already been launched, so the station could not be redesigned from scratch. The designers could therefore only modify and optimize the existing design to protect the church remains as much as possible while carrying out normal construction.

Since the diaphragm walls had already been built, the retrofit scheme was subject to many constraints. Considering that the foundation of the church remains was not deep, the designers concluded that the best solution was to create a base support beneath the church remains, and this base support itself could bear directly on the existing diaphragm walls.

This design required horizontal excavation and concrete placement beneath the foundation of the church remains. Ultimately, the contractor selected a solution using six intersecting microtunnels to form the base support.

Design structure

These six microtunnels have an average diameter of 2.5 m and were excavated using the New Austrian Tunnelling Method starting from the northwest ventilation shaft. To provide the space needed to excavate the six microtunnels, the ventilation shaft was deepened by one level and enlarged to the necessary width.

Microtunnel assembly

The construction sequence for the six microtunnels was as follows: first, microtunnels No. 1, 3, and 5 were constructed (see figure above, hereinafter referred to as "primary tunnels"). The primary tunnels were supported with steel-mesh-reinforced shotcrete lining, forming a closed ring structure capable of carrying loads and reducing disturbance to the ground surface. After excavation was completed, the construction team first built reinforced concrete cross beams inside the "primary tunnels."

When excavating the "secondary tunnels" (2, 4, and 6 in the figure above), the shotcrete shell of the primary tunnels was partially removed, and the cross beams inside the "primary tunnels" were used to support the shotcrete shell of the "secondary tunnels."

After all six microtunnels were completed, they were backfilled with concrete to form the base support for the church remains.

Interior view of a "secondary tunnel"

Although the microtunnels have small cross-sections, they are all located in artificial deposits, and combined with groundwater issues, there were certain construction risks. To detect loose soil layers in a timely manner and to dewater when necessary, the construction team drilled six horizontal probe holes. Each probe hole was located at the top of a microtunnel, with a diameter of 132 mm. After use, a 32 mm diameter rebar was inserted and grouted.

New station structure after completion of the base support beneath the church remains

With the help of the base support formed by the six microtunnels, the station was able to complete top-down construction without affecting the church remains. According to monitoring data, although the diaphragm walls carried higher loads than in the original design, the settlement values of the diaphragm walls remained within the allowable range.