MADRID TRADITIONAL METHOD | BTSYM


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 elasto- plastic 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.


MADRID TRADITIONAL METHOD


Advantages ● Local experience with the method ● Adapts to different geometries and sizes ● Well-trained local workforce ● Does not usually use reinforcement ● Rapid mobilisation and demobilisation ● Uses simple and accessible equipment


Disadvantages ● Not internationally known by the industry ● Slow advance rates ● Intensive in labour ● Uses a large amount of concrete ● Difficult to use in permeable materials ● Limited space for the use of large machinery


ACKNOWLEDGEMENTS The speakers would like to express their gratitude to Madrid Metro for kindly providing photos for this presentation. We also thank BTSYM and AETOSYM for their support


and invitation to deliver the presentation. The recorded presentation of the presentation is available on the BTS YouTube channel.


See pp42 for Questions & Answers April 2023 | 23


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