The Cleuson-Dixence shaft, which is more than 4000m long, was mainly excavated using a tunnel boring machine (TBM); the lower part by drilling upwards at a gradient of 54-70%, the upper part by drilling downward for 700m with a gradient of 29%. The shaft intersected stratigraphical levels of poor quality, water-bearing and mylonitized in places.

The longitudinal profile of the upper section was modified during the engineering work to avoid dismantling the TBM (supplied by Robbins) on the central shelf, and to save time. In addition, to meet time limits, it was decided to start a new drilling operation using a second TBM (supplied by Lovat). These changes in the work schedule involved construction of a second horizontal shelf 120m long. This resulted in a geometrically complex longitudinal profile, and additional difficulty when the shaft casing sections were assembled and welded.

Access is achieved via five openings (see diagram on p20) and the structure was divided into four construction batches:

•F I, 1233m, between F4 and F5

•F II, 925m, between F5 and F6

•F III, 814m, between F6 and F7

•F IV, 1078m, between F7 and F8.

Casing conception

Because of the poor geological conditions it was impossible to rely with certainty on a limit value for the rock modulus of elasticity. Therefore, the steel casing had to resist the full internal pressure, and the concrete casing only provided protection against corrosion. The thickness of the casing was limited because:

•It is not feasible to roll plate more than 60-70mm thick.

•The time needed to execute a weld bead increases with the cube of its thickness.

•It becomes very difficult to meet quality requirements for construction welds.

Provision was made to interrupt the concrete casing at each opening to create a drainage zone.

The maximum internal pressure varied from 26.5bar (260mCE) at the top of the shaft structure to 211bar (2067mCE) at the bottom. The static pressure is 188bar at the bottom of the shaft. The maximum dynamic pressure reaches 210.7bar here, considering the combined effect of heavy surging in the surge tank and the water hammer on the pipe. In view of the extreme pressure conditions in the lower part of the shaft, high-performance steels were essential: 80% of the Cleuson-Dixence metal-lined shaft was thus constructed of S 890 QL steel, and 10% of S 690 QL steel. The rest was constructed of standard StE 355 grade steel.

In the lower sections (FIII and FIV), thick plate and higher grade steel were used. In the upper part, (FII and FI), the internal pressure diminishes, so a lower grade of steel (S 690 QL and P355NL1) was chosen. At the same time, the diameter of the casing sections was increased in order to minimise pressure drops.

Execution of steel and concrete casing

Both the longitudinal and circular workshop welds were of the submerged arc type. On site, pre-fabrication welds used the submerged arc welding method, while in-shaft construction welds were executed, from the inside, in a chamfer suitable for the welding method.

The constructors chose their in-shaft welding process. Two used a coated electrode, while the third used automatic and (partially) semi-automatic welding machines. Both methods had advantages and disadvantages. Welded joints were executed at a similar speed: although the automatic machines deposited more material per unit of time, the chamfers to be filled were of greater volume, and the processes were thus equally efficient. In general, the automatic machines operated satisfactorily when the casing was assembled in the horizontal plane but were more difficult to apply in the inclined shaft.

Automatic machine welds had more repairs than those executed using a coated electrode. One reason was that the magnitude of the magnetic field induced in the pipe caused the arc to fluctuate at the beginning of a weld. This was easier to control with manual welding, using the skill of the welder, than automatically.

Nearly 11,700t of steel was used, supplied by European and Japanese steel works.

Because of the extremely short working periods, all the pipes were painted in the workshop, the majority using automatic equipment. Zones adjacent to site joints were sandblasted and treated in situ when the assembly work was completed.

The entire casing was embedded with concrete — around 37,000t over the 4km pipeline. A special flow concrete was used, for the first time in Switzerland, which was poured via a plastic gullet (half a PVC pipe 30cm in diameter).

The formula for this concrete comprised 0/16 aggregate and sulphate-resistant cement with silica dust or fly ash added. The E/C ratio was 0.53-0.55. The inclusion of silica dust or fly ash improved the plasticity, uniformity and physical strength of the concrete — it retained its characteristics even after more than 1000m of flow in the sloping shaft. A flow rate of 20m3/hr was achieved without difficulty.

Executing the work

The shaft was divided into four work packages which could be awarded separately. All the work, including the pouring of the concrete, was awarded to the GSN consortium which included Giovanola at Monthey (leader), Sulzer-Hydro at Kriens, and GEC Alsthom Neyrpic at Grenoble.

The pipes were fabricated in 6m and 9m lengths in the workshop and largely assembled into sections 12m long in situ (except at the shaft bottom working site) before being lowered in the shaft.

The GSN consortium shared the fabrication and assembly tasks. The fabrication of pipes for the part of the shaft with thick walls (72-41mm) was shared between Sulzer-Hydro (Ravensburg) and ATB Caldereria (Brescia), acting as sub-contractor. The other three sections of the shaft were fabricated entirely by Giovanola. The GSN group divided the assembly work into three batches:

•The casing at both ends of the shaft (FIV and FI) was executed by Giovanola. FIV is 3m in diameter, made up of 130 pipes of 6m and 9m length with a gradient of 68%. The total weight of the casing is 4043t of S 890 QL steel in thicknesses 66-48mm. The bottom of the lower shaft casing is connected to the distributor casing. FI begins with a shaft with a gradient of 68%, and is connected to a tunnel with a gradient of 30%.

The diameter of the casing changes from 3.2m to 3.4m and reaches 3.8m downstream of the surge shaft at the connection to the casing of the headrace tunnel. This part of the casing is made of 118 6-12m pipes. The total weight of the casing is 2674t of S 890 QL, S 690 QL and StE 355 grade steel, 35-21mm thick.

•Both central batches (FII and FIII) were executed by GEC-Alsthom Neyrpic. FIII is 3200mm in diameter and is made up of 81 12m and 6m pipes. The total weight of the casing is 2450t of S 890 QL steel in thicknesses of 42-36mm. The shaft is connected to the first horizontal shelf via an elbow with a radius of 100m. FII begins with a tunnel with a gradient of 15% and continues via a curved tunnel with a radius of 1540m. It is connected to the second horizontal shelf via an elbow with a radius of 100m.

This part of the casing has a diameter of 3200mm and is made up of 85 6-12m pipes. The total weight of the casing is 2099t of S 890 QL steel, 36-23mm thick.

•The bottom of the shaft, including the bottom elbow and the section of pipe connected to the distributor, was executed by Sulzer-Hydro.

Giovanola and Sulzer-Hydro chose the coated electrode method for in-shaft welding. GEC-Alsthom Neyrpic opted for the automatic or semi-automatic machine method.

At the face of each section an assembling machine was designed, that could:

•Position and shape.

•Carry out manual or automatic welding.

•Grind welds using a swivel grinder.

•Check welds, using a carriage which can be lowered independently to a point 40m downstream of the construction unit.

•Provide access for construction personnel and welders, using a winch-operated cradle.

•Provide a 16kV or 5kV electrical supply.

Pressure tests on the penstock are currently being completed.