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The construction of the Sea Outfall of Mompás (Guipúzcoa) was in 2000, at the request of the Northern Hydrographic Confederation, to solve the problems of wastewater discharges on the coast. The execution of the project was developed by the Joint Venture formed by Dragados y Construcciones S.A., Altunia y Uria S.A and Van Oord ACZ B.V. subcontracting the excavation of the wave breaking zone by means of jacking pipe technique with closed shield tunnel boring machine AVN to the company Europea de Hincas Teledirección S.A.U - EUROHINCA.


Sea Outfall for treated water discharge of Mompás.



Northen Hydrographic Confederation/Aguas del Añarbe, for the Environmental Ministry.

Main Contractor

Joint Venture: Altuna y Uria S.A., Dragados y Construcciones S.A. y Van Oord ACZ B.V.

Microtunnel Contractor

Eurohinca (Europea de hincas teledirigidas S.A.U.)


Construction of the Mompás sea outfall began in 1999 and was completed and put into operation on 20 July 2001.


Mompás, Gipuzcoa, España.



The Mompás Sea Outfall was designed to collect wastewater from the area of San Sebastián and Pasajes, so that once treated in the Loyola treatment plant, they were expelled into the sea through the sea outfall far from the coastline. Previously, discharges were carried out through the Urumea and Sagües tunnels, flowing at the foot of a cliff near the beaches of Zurriola and La Concha as shown in images 2 and 3.


The outfall intended to discharge the water from the Loyola treatment plant through a pipe of 2 m in diameter, at 1,200 m from the coast and at a depth of 45 m with a flow of up to 6 m³/s.


Image 2: San Sebastian wastewater discharge into Cantabrian Sea before the sea outfall construction.


Image 3: Wasterwater tide at Mompás before the sea outfall construction.


Image 4: Cliff after the construction and commissioning of the treatment plant and the sea outfall.


The submarine outfall was built with a combination of construction techniques:

  • The first phase wave breaking zone (the first 440 m), through the method of jacking pipe with closed shield tunnel boring machine type AVN.

  • A second phase, which links with the previous one by anchoring steel pipes, 900 m in total length and 2 m in diameter and includes the diffuser section of 200 m in length, formed by eight diffusers of variable diameter. The depth of discharge of the diffusers varies between -43.00 and -46.00 m.


The first phase was executed by tunnel boring machine by the jacking pipe method, in its day, a milestone for being the first sea outfall to be executed in the Cantabrian Sea and one of the first sea outfall executed with closed shield tunnel boring machine worldwide.


It is important to mention the hydrostatic load that the TBM had to support, since it finished the excavation at a depth of 38.32 meters below sea level. Considering the tidal difference of ±1.6 m and the wave height up to 4 m, the water pressure that the TBM had to withstand was up to 44 m or 4.4 bars of pressure.


For this reason, a project specific Herrenknecht AVN TBM was designed and built to excavate with a dynamic hydrostatic pressure up to 5 bar and a static pressure up to 7 bar. The standard TBMs manufactured by Herrenknecht for the execution of jacking pipe microtunnels have a capacity to work with a hydrostatic pressure up to 3 bar. This was a milestone in the design and construction of this type of TBM and the jacking pipes.


Image 5: Jobsite during project execution.

Due to the type of geology (diaclased sandstone) and the great length of the drive (440 m in 2000 was considered a long drive), the TBM was equipped with an airlock for the entrance into the excavation chamber in hyperbaric mode to guarantee the change of cutting tools.


Two stops of excavation were made to access the excavation chamber and to be able to check the discs and the cutting wheel in general.


The first inspection was made at 80 m. Checking that the TBM was in a solid rocky area and that there were no water leaks in the front and, therefore, it was possible to access the excavation chamber without using the lock. In this inspection it was possible to verify that both the cutting discs and the general condition of the cutting wheel were correct and no repair or change of discs had to be made.


The second inspection took place at 350 m and although the discs were in good condition, they already had some wear and it was decided to change them to ensure the completion of the section without having to access the excavation front again for inspection.


To access the excavation chamber at this second inspection, the TBM and the front had to be pressurized at a pressure of 2.5 bar, using the lock of the TBM with hyperbaric chamber. Prior to pressurization and to ensure the stability of the excavation front, bentonite was injected at a pressure of 3 bar. In addition, to guarantee the stability of the pressurization, a seal was made around the shield of the TBM (between the shields and the rock) by means of an expanded polyethylene ring of high density and adhesion.


Image 6: Pipe jacking inside view and first intermediate jacking station.

Due to the layout of the project and its characteristics, pipe jacking in a straight line with a continuous slope, it was decided to use the traditional laser guidance system. During the realization of the microtunnel, constant topographic controls were made every 50 m to check the guidance system.


During this project, a novel gyroscopic guidance system in the testing phase installed in the TBM was also used, which was in the process of research and development that would lead to the gyroscopic guidance systems used today.


Another critical point of the project was the construction of the launching shaft, since the difference in height in the cliffs of the area was approximately 60 m. The upper elevation of the well was 58 m above sea level, while the lower level of the well was 10 m below sea level.


It was necessary to build a launching shaft of 68 m of depth and a circular section of internal diameter of 4 m, through which the tunnel boring machine, the jacking frame and all the reinforced concrete pipes for the pipe jacking would be lowered. At the bottom of the shaft a cavern was dug for the installation of the jacking frame and TBM equipment. At the bottom of the shaft a cavern was dug for the installation of the jacking frame and TBM equipment. This cavern was adjacent to the Urumea and Sagües tunnel where the waters from the Loyola treatment plant flowed directly into the cliff as showed in images 2 and 3. Once the new sea outfall (tunnel and anchored pipe) was completed, the connection was made in this cavern with the previous tunnel, thus achieving the discharge of the treated water at 1,200 m from the coast and at a height of less than -40 m.


Laura Céspedes y Marc Martí


Image 7: Gyroscope installed in TBM over ELS system.



440 m

Inner diameter

2000 mm

Outter diameter

2400 mm


Negative 7.08%


Diaclased sandstones with medium resistance 750 Kg/cm²

Starting level

-5 m

Final level

-34,40 m

Work pressures

Up to 4 bars


Hidroescudo AVN Herrenknecht

Intermediate Jacking Stations



Hinca de Tuberías de Hormigón Armado: Emisario Submarino del Mompas - Construmatica 


Pozo Emisario Submarino de Mompás Donostia – Tunelan – Obras subterráneas


Regreso al mar - Aguas del Añarbe | Ciclo del agua ( 

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