Ayesa is a global provider of technology and engineering services with more than 55 years of experience and headquarters in Seville, Spain. The company has participated in the design and engineering of more than 480km of tunnel construction projects, with a focus on metro projects.
Some of Ayesa’s recent tunnel projects include: Lines 2 and 4 of Lima metro, in Peru; and, in Spain, Line 5 of the Madrid metro, Line 8 of Barcelona metro, and Lines 1 and 2 of Malaga metro. This article looks at an aspect of some design and construction work using the ‘The Belgian Method’, adapted for local conditions and practices, on parts of Madrid metro.
The adaption gave rise to ‘The Madrid Traditional Method’, that under certain conditions could be applied elsewhere, including in London Clay.
MADRID METRO
The Madrid metro began operating in October 1919. To date it consists of 12 lines, 294km of tracks, 302 stations. In its centennial year, in 2019 it transported a total of 680 million journeys (pre-Covid figures).
In its beginning, Madrid metro started with the construction of Line 1, which was 3.5km long and had eight stations, connecting Cuatro Caminos, in the North, with Sol, in the city centre. Construction of the original Line 1 included a distance of 1.5km by the Belgian Method, and also 2km of cut & cover.
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By GlobalDataChamberí, one of the original stations of the Madrid metro network, was decommissioned in 1966 due to network upgrades (larger trains and stations were needed) and then reopened as a museum in 2008. The station was restored, and it is now possible to visit its original architectural features, such as the handmade tiles advertisements (see Figure 1), which would advertise local businesses in the area surrounding the station.
The first description of the Belgian Method is found in the construction of Line 1 in 1917.
The Belgian Method consists of using a curved top heading, which is constructed using temporary works (shoring) to support the excavation of the top heading, as shown in Figure 2.
In recent tunnelling upgrade works to the original Line 1, it was observed that originally bricks were used for the top heading line, stone masonry for the walls, and unreinforced concrete for the invert slab.
This early use of the Belgian Method was affected by the World War I, which caused a shortage of materials, so that concrete was only used for the invert slab. Later applications of the Belgian Method in the metro included use of unreinforced concrete for the entire cross-section profile.
As will be discussed later, the exact origins of the nomenclature of the Belgian Method may be lost in history. However, the technique was further developed over more than 100 years in Madrid’s soils and is currently known as the Madrid Traditional Method (MTM).
From its humble beginning with Line 1, the Madrid metro rapidly expanded to become a network of almost 300 km of tunnels, covering 12 districts of the capital. The expansion of the network was particularly busy between 1995 and 2017, when several Tunnelling Boring Machines (TBMs) were used for the longer tunnels (several kilometres), and the MTM was reserved for shorter tunnels and galleries (hundreds of metres).
Gran Vía station is another of the original stations of Line 1. This station was upgraded in 2021, while maintaining the operation of Lines 1 and 5. The upgrade works included step-free access and several technological updates: LED lighting; information screens; larger and modern gates; and the latest generation ticket machines. This is the first 4.0 (4th Industrial Revolution) station in Spain, offering advanced design and features, with large display screens, contactless payment, and improved user interface.
The upgraded station also pays tribute to the original 1919 entrance pavilion by building a granite replica of the original structure, conceived by Antonio Palacios (the architect mastermind of Madrid’s first metro lines).
MADRID TRADITIONAL METHOD (MTM)
According to the literature (‘La construcción del Metro de Madrid y la M-30’, Manuel Melis Maynar, 2012), the first reference to the Belgian Method comes from the construction of the Godarville Tunnel between 1827 and 1841 for the Charleroi Canal, in Belgium.
However, some recent references claim that there is not enough evidence that the method was brought from Belgium, and that it might have been used by Belgian engineers working on Line 1, back in 1919, and subsequently referenced them, and so was named after them.
Nevertheless, after more than 100 years of development of this method in the ground conditions of Madrid, this ‘Belgian Method’ is now well-established as a tunnelling technique and as such its name has become known as the Madrid Traditional Method (MTM).
The MTM consists of a sequential excavation with the following main steps (see Figure 3):
1. Miners excavate a small pilot gallery and support it with timber planks, waler steel beams and timber struts. The short gallery is 2.5m long and is used to identify ground conditions ahead of the main excavation
2. Lateral widening and shoring of the top heading are then undertaken, with five pocket excavations at each side with typical round lengths of 2.5m
3. Formwork installation and casting of the unreinforced concrete crown is performed (typically 3 sets of 2.5m long formworks are used)
4. Repetition of steps 1 to 3, up to 7 round lengths of the top heading
5. Excavation of the walls with excavators and casting of the unreinforced concrete side walls are then undertaken (in a staggered way and maintaining a maximum of 7 round lengths to the top heading face)
6. Construction of the unreinforced concrete invert slab is the final step to close the tunnel section There are many examples of the application of this method in Spain, and also in some tunnelling projects abroad that were designed and built by Spanish companies, such as in Quito metro, in Ecuador.
The experience of constructing tunnels with this method in the soils of Madrid has demonstrated the safety of the technique. The sequence of works follows a series of steps perfected and improved over the years by the experience of Madrid’s miners and engineers, and by studying the lessons learned from previous projects. Although this construction process requires specialised labour, it is a very versatile method that allows for a high degree of adaptation to the constraints of an urban environment.
This methodology is based on the principle of carrying out small excavations (less than 5m2), which greatly limit the open excavation face, to guarantee its stability during construction. As previously mentioned, the excavation begins with a small pilot gallery that is gradually shored and widened to form the top heading of the tunnel, always with a temporary support for each widening stage. In this way, the open face for each excavation plane is reduced and, therefore, the stability conditions are more favourable.
This advance gallery also acts as a pilot tunnel to identify any contingency or uncertainty about the state of the soil, thus allowing the adaptation of the construction procedure to respond to these singularities. The contingencies can include ‘toolbox’ items, such as reducing the round length or installing timber planks at the open face. In more challenging ground conditions, the face is also supported with timber planks, but these are not normally necessary.
Advance lengths of 1.25m or 2.5m are normally used, depending on the ground conditions, with shoring installed systematically. There are exceptions, linked to singular sections, either due to geotechnical conditions or to the need to adapt the construction methodology in sections with special needs, such as at junctions.
After the construction of the invert slab, the back of the concrete liner in contact with ground is filled with grout. The purpose of this process is to fill any gaps that may remain at the top heading extrados between the concrete, the timber shoring, and the excavation profile, and to reduce water ingress through the joints between the pours. The injection pressure must be limited to avoid excessive loads on the lining. The grouting pressure is usually limited to 1 bar.
Finally, MTM allows the creation of the Madrid metro typical cross section as shown in Figure 4, which has an internal section of approximately 8m wide by 6m high. This is a double-track tunnel with emergency and maintenance walkways on each side and an internal area of 45m2.
Typical MTM cross-section for Madrid metro
The typical cross-section uses a cast in-place unreinforced concrete lining with a design excavation volume of 72 m3/lm and concrete liner of 28 m3/lm. The concrete liner thickness ranges from 600mm to 1400mm and relies on the mobilisation of the hoop force through the section. They key for designing an unreinforced concrete liner is that there is little eccentricity between the mobilised thrust region and the centroid line of the section.
MODELLING THE MTM (3D SSI)
To design the MTM for Line 5, we decided to model the complete sequence in PLAXIS 3D for the Soil-Structure Interaction (SSI) model.
The three-dimensional Soil-Structure Interaction (3D SSI) model developed in PLAXIS 3D comprises the dimensions of 40m by 60m in plan, by 31m deep. The model has over 162,000 elements and the tunnel liner is modelled as volumetric elements with dummy plates at its centroids, to facilitate the extraction of internal forces for structural design.
To model the complex top heading pilot and sideways pocket excavations, six phases were used for each of the 2.5m top heading round lengths. We decided to model 16 round lengths to remove boundary condition effects from the central part of the model, thus the model represents a 40m long excavation (16 x 2.5m = 40m).
The overburden of the tunnel is 12.8m from the surface to the top heading extrados, which is approximately 1.4 times the tunnel excavation diameter.
Considering the delay in wall and invert slab constructions of seven round lengths from the face (7 x 2.5m = 17.5m), the total model has 115 phases, as shown in Figure 5.
Phases 1 to 6 correspond to the sideways excavations within a top heading excavation. Phase 42 corresponds to the last phase of advancing the top heading without advancing the walls construction, while phases 60 and 98 are examples of phases in which both walls and invert slab, together with the top heading, advance.
Note that these images show the volumetric elements used for modelling the unreinforced concrete liner and the structural elements modelled used for this 3D SSI model, such as: the waler steel beams, timber planks, and timber struts to model the temporary works that support the ground prior to the installation of the concrete permanent liner.
The Tunnel Designer feature in PLAXIS was used to create the large number of phases of this 3D model, for which seven tunnel parts were defined to optimise the phases generation.
The tunnel liner was modelled as volumetric elements and the ground model is mainly comprised of local overconsolidated clays, known as Peñuelas.
Peñuelas
These are overconsolidated clay deposits. They consist of silty clays, partially cemented by carbonates, green and brown in colour. In this group, there are small tabular levels of limestones and marl limestones, with a marl limestone, with a variable degree of silicification.
In terms of geotechnical parameters for this 3D SSI model, these were assumed as: c’= 50 kPa, Φ’= 28°, E50= 200 MPa and Eur= 400 MPa.
Regarding the properties of the models, it should be noted that the soil was assumed to be dry, based on the results of the geotechnical campaign. Therefore, a drained type of calculation has been carried out. On the other hand, the Hardening Soil constitutive model was used for the stress/strain behaviour of the ground.
The shoring elements were also modelled in the 3D SSI model. The timber planks were modelled as elastoplastic plates, the longitudinal waler steel beams and timber struts were modelled as beam elements. The use of elasto-plastic plates made it possible to limit the load taken by the timber planks and to accurately estimate the load transferred to the concrete lining.
The results of this model show that due to the pocket excavations, shoring and competent ground, the ground displacements mobilised due to MTM are low, for example the resulting surface settlements were calculated to be less than 8mm.
CONCLUDING REMARKS
There are different tunnelling methods used locally around the world, such as the Madrid Traditional Method (MTM). Madrid has a long history and experience in the use of this method, and the technique leads to low ground surface displacements.
The use of 3D SSI models allows further optimisation of section, sequence, and reinforcement without compromising safety.
It is our view that the method can be transferred to London Clay or similar soils with reasonable skill and care by design and construction teams.
