The project Captacion de Agua de Mar de San Pedro del Pinatar - a quatum jump in microtunnelling

Microtunneling is a remote controlled procedure for the drivage of pipes with clear bores of more than 1,200 mm. In this procedure, drivage pipes are driven from a start shaft up to an end shaft by means of a pressing station. The soil is mined by a hydraulically driven boring head. At the boring head a special lift medium is used to reduce the friction forces and to support the soil.

The project Captacion De Agua de Mar de San Pedro Del Pinatar
The project on the Spanish shore included the construction of two pipelines to a seawater demineralisation plant. In the future 24 million cubic meters of sea water shall be supplied to the plant. The drilling project started in September 2005. The European Union supported the project by financing about 85 % of the construction sum. The total construction sum of the network amounted to 83 m €. The company DRAGADOS/FCC – SPAIN – was in charge of planning the ambitious project; the undertaking company was Sonntag Iberica. Dieter Bücher/Firma Dieter Bücherl Spezialtiefbau Amberg managed the construction from planning to finishing. Already in the planning phase the company BAROID IDP EUROPE had been entrusted with building a mud programme by which drivage in these dimensions would become feasible. Here, the main point was the annulus lubrication.

A start shaft was erected on the shore with a distance of 15 m to the sea. From this shaft two tunnels were driven. Tunnel I named Santa Anna was driven 625 m to the landside towards the dimineralisation plant. Tunnel II named Santa Maria was shifted 1,146 m under the Mediterranean Sea.

About the technical equipment
The automatic full-face cutter Herrenknecht AVN was used which had been modified for this particular application. The press capacity of the plant amounted to 1,200 tons. Here, an automatic annulus lubrication was very essential for this project. A separation plant equipped with mud screens and hydrocyclones provided by the company Schauenburg allowed a fast and clean preparation up to a cut point of 40,000 mm. The MAB performed 400 cubic meters per hour. The company Gollwitzer supplied the ferro-concrete drivage pipes (inside: DN 2000, outside: DN 2400). Intermediate jacking station provided by the company Gollwitzer were inserted along the drift.

Geology
According to the geological experts’ reports cross beddings of very different geology were expected in the subsoil: coral beds, sea salt licks, sand, loam, and silt. Especially at the end of the drift the occurrence of some possibly disturbing contaminants such as calcium salts and magnesium salts should be assumed due to the low lapping.

Annulus lubrication
As a result of the inhomogeneous geological conditions and the contaminants to be expected it was necessary to develop an annulus lubrication that would resist all influences possible, especially the time factor. The fresh water for preparing the mixture of the lubrication suspension proved to be a first problem: the surface water which was taken from a little river was of insufficient quality when being used as the basis medium for an optimum annulus lubrication. The pH-value was 5.8; measurements of the German hardness revealed values of more than 25 degrees. Even the drinking water from the local network could not be used without pre-treatment.

The concept of lubrication
Soda Ash + TUNNELGEL PLUS + NO SAG + PAC L + DINOMUL 2 kg Soda Ash were used to make the water user-friendly: the pH-value was increased to 8.5. After the treatment also hardness had optimum values. TUNNELGEL PLUS was the selected bentonite. It had already proven to be the best drilling bentonite in many problematic HDD drillings. NO SAG was used in order to essentially increase the gel strengths of the suspension. Furthermore, NO SAG stabilizes the suspension in the drilling in brackish water or salt water. PAC L should gain control over the contaminants to be expected. Furthermore, DINOMUL was added – a fluid additive that had been developed to reduce the rotation torques and stresses in the drill pipe. By the lubrication of a product pipe DINOMUL also reduces the traction in the HDD procedure. In addition, DINOMUL improves the stability of the borehole by forming a compact thin filter cake and protects reactive formation clays. The product DINOMUL also serves for increasing the lubrication effect in waterbasic muds and counteracting a sticking of the drill string. Experience has shown that DINOMUL can reduce the frictional forces by up to 60 %. In the present Microtunneling project the necessary thrusting forces were lower than 0.1 tons/m2 of the lateral surface. For the whole tunnelling about 1 cubic meter suspension per meter was needed for pumping. The maximum thrusting forces amounted to 600 tons, three of the 15 intermediate jacking stations were used for releasing the casing string only.

This project was scientifically accompanied by the Freiberg University of Mining and Technology.

At the end of June, 2006, the ambitious project could be completed.