Connection of Lines 8 and 10 of the Madrid Metro

The work included a 1.610 meter long tunnel, the Gregorio Marañón station, two 7x3,50 m ventilation shafts, a 3,50x4,00 m pumping shaft, the interchange with Metro Line 7 as well as the superstructure and all the necessary installations for the correct operation of the line.
Given the characteristics of the area, with intense construction and an incredible density of affected services, the use of tunnel boring machines was impossible. For this reason, the solution adopted was mixed with several methods depending on the needs.
From pk 0,000 to 0,730: the tunnel was made in the mine with a section of 7,80 m between gables and 5,50 m clearance. This section included the section between Alonso Martínez and General Martínez Campos street and had four attack fronts: the two located at the start and end of the section and the Rubén Darío ventilation shaft from which progress was made in both directions.
From pk 0,730 to 1,211 the tunnel could not be made in the mine 

From pk 0,730 to 1,211 it was not possible to make the tunnel in the mine due to the low coverage, so the excavation was carried out in a cut-and-cover tunnel between reinforced concrete screens. The Gregorio Marañón station was included in this section.

From pk1,211 to 1,560 it was carried out using the traditional Madrid method under Paseo de la Castellana from two attack fronts: the Bretón de los Herreros ramp and the Nuevos Ministerios telescope. From pk 1,560 to 1,614, the union telescope with Nuevos Ministerios was carried out between screens and in the open sky.

The layout was imposed by the existing situation at the start and end stations of the section. On the old Line 8, the connection was made from the 170-meter-long double-track tunnel that exists behind the Nuevos Ministerios station and located under the Paseo de la Castellana. At the Alonso Martínez station, the tunnel extended into a cul-de-sac for more than 300 meters parallel to Calle Almagro with a final curve to the left.

The point that conditioned the layout in plan was the intersection of the Castellana. The RENFE Atocha-Chamartín tunnel located at a depth of 10 meters and, 12 meters below, a collector of the Canal de Isabel II in service were located there. There were the options of crossing between the two or going under them.

On the other hand, the future intersection with Line 7 fixed the layout, both in plan and elevation, at the height of the Gregorio Marañón station. Since it was desired to keep the station close to the surface, the solution adopted was to pass between the two collectors as far as possible from Gregorio Marañón. In this way, the future crossing of the tunnel and the collector by Line 7 would take place in the vicinity of this station.

The resulting layout starts from the Alonso Martínez station in a straight line until it reaches the Glorieta de Rubén Darío where it curves to the left to run under Calle Miguel Ángel. The tunnel turns left again when it reaches La Castellana, running parallel to the RENFE tunnel and the Canal collector. The passage to the opposite side of the Castellana was made at the height of the Plaza de San Juan de la Cruz through a curve to the right that, followed by a countercurve, reaches its destination in the extension of the Nuevos Ministerios tunnel.
The plan layout was designed for type 5000 rolling stock. This material implies minimum plan radii of 300 m, exceptionally allowing 250 m.
The longitudinal profile was determined by the needs stated above and which set the plan layout. The exit of the Alonso Martínez station is flat, however, two hundred meters later an exceptional maximum ramp of 4% begins, which allowed the placement of the platforms of the Gregorio Marañón station as close as possible to the surface. Once past this station, the maximum slope is forced again to allow the passage of the Castellana between the RENFE tunnel and the collector. After it begins a series of gentle ramps and chained slopes of less than 2,25% that lead to the tunnel to its link with the existing one at the exit of Nuevos Ministerios.
The terrain crossed by the trace is made up of alluvial surface fillings with thicknesses of 1 to 4 meters. The next layer is made up of crumb sands and has a variable depth of 12 to 25 meters. The lower part of the substrate is fundamentally coarse. Along the Paseo de la Castellana there is a sandy alluvial soil 2 to 4 meters thick.

Water is present throughout the entire trace, with the water table varying between 7 meters below the tunnel grade and 15 meters above it. This inconvenience was mitigated due to the low permeability of the land.
There were also some aquifers hanging over the coarse sands, mainly in the first part of the section, which made excavation difficult in some sections.

In addition to the many difficulties involved in digging a tunnel under the central almond of the city, the two main construction challenges stemmed from the need to avoid damage to the existing structures.
The first infrastructure affected was an electrical substation located in the Glorieta de Rubén Darío between PK 0+446 to 0+483. The solution consisted in the application of compensation injections of seats through perforations made from the surface.
The second was the crossing with the RENFE tunnel between PK 1+340 and 1+420, which required a much more elaborate solution. The first step was to carry out topographical surveys at the Nuevos Ministerios and Alonso Martínez stations, as well as at the RENFE tunnel.

The next step was the execution of a waterproof slab reinforced with metal tubes to install tubes-to-sleeves between the RENFE tunnel and the tunnel to be built. From this slab the area would be injected later. The gables of the RENFE tunnel were also consolidated to improve its stability. Finally, the possible cavities located in the vicinity of the collector that ran under the tunnel to be built were filled with mortar injections.
All these precautions allowed the work to be carried out without incident and prevented any damage to the infrastructure in the area.