4. Limits to the use of the technique

4.1 Lengths

The maximum distances between two working shafts (drive lengths) are :

• 100 m for diameters between 400 mm and 800 mm ;

• 200 m for diameters > 800 mm with intermediate push stations.

Above a certain diameter (> 1,200 mm), there is no precise limit to the length of a drive. A few companies claim to have built drives longer than 500 m.

The maximum distance between two wells depends on :

• of the admissible thrust limit of the tubes (table 1...