In the Freeport area of Hamburg harbour the sewage system was changed from 300 mm (1 ft) piping to 900 mm (3 ft). For this purpose 2 diamond core drilling openings of 1200 mm Ø (4 ft) became necessary in the bridge viaducts of the old Norderhochbrücke (Northern High Bridge). According to the tender drilling length of 5 metres (16.5 ft) were to be expected. According to calculations carried out by the contracted company, J. Draeger & Co. Hamburg, it soon became obvious that the drilling length would be more than 5 metres (16.5 ft). In the workshop of Messrs. J. Draeger & Co. an existing drilling unit was modified and enlarged.
For this drilling unit with an approximate weight of 3.5 tons a removable inclination unit was designed and built. The adjustable inclination range was 0 – 45 degrees, which was sufficient for the holes to be drilled at angles of 28 and 35 degrees respectively. The 4 lynnettes for the guidance of the drill crown within the drilling unit, which so far were equipped with sliding guides were substituted by rolls from the large wire sawing storage equipment. At the lower part which has to carry and guide the complete weight of the drill 4rolls at 2 pairs each were positioned on a swivel. The upper rolls had only a guiding function and therefore 1 roll each was sufficient.
The drill bit of 1200 mm Ø (4 ft), 300 cm (10 ft) long and a weight of about 800 kg was purchased from the stores of Messrs. Schützeichel and transported by a haulage company to Hamburg. After receipt the drill bit was immediately  assembled into the drilling unit and the retipping started. The bit was tipped with 88 segments of 20 x 10 x 10 mm (0.8 x 0.4 x 0.4 in)
For this precision job the workshop foreman needed a whole day. According to the information of the diamond segment manufacturer 1 cm³ contains about 1.1 cts of industrial diamond. Therefore the total caratage of this drill bit is about 176 cts.

After completion of all preparatory work a 25-t mobile crane and a small articulated truck was ordered. The total length of the articulated loading area required only a minimum length of 7 metres (23 ft). After loading the drilling unit, the hydraulic unit, drill bit extension tubes and other tools required to carry out the job, the convoy started its journey to the building site consisting of the mobile crane, the articulated lorry and the staff for the job.

After arrival at the site the mobile crane lifted the drilling unit into the prepared northern excavation in about 7 metres (23 ft) depth. In this excavation several heavy truck lifting units were used to position the drilling unit with the drill bit into the correct direction. After this positioning and other fixing jobs the drilling commenced.

The drill bit was operated with about 30 – 40 rpm. The feed speed was 2,5 cm/min (1 in/min). This corresponds to a cutting rate of 942 cm³/min (57.5 cu in/min). After drilling the total usable length of the bit, the core of about 3 m (10 ft) length was broken and removed. With pneumatic drills 50 mm Ø (2 in) holes of 1.1 metres (3.6 ft) length were drilled. Using 2 Darda units and their hydraulic explosion principle due to their wedge system the core could be broken up into removable pieces which then could be lifted out of the excavation hole by means of the portable crane.
After recovery of the core and cutting off of the 300 mm (12 in) thick steel tube the operators reassembled   the m into the hole. A prepared 3 m (10 ft) extension tube had then to be assembled. After checking that all bolts and fixings were tight the second shot could be fired. Initially also this second drilling step worked perfectly. But suddenly nothing worked anymore. The extension units which were by now part of the system had to be completely removed. In the actual drill bit was a piece of a core about 1.5 metres (5 ft) long and broken at an inclined angle.

The reason for this soon became obvious after the operators had climbed into the hole. The building had a crack. Due to this the total weight of the core rested so heavily on the drill bit, that even the strong hydraulic motor via a reducing planetary gearbox was no longer able to rotate. 

The unexpected operations, removal of the extension tubes, disassembly of the drill bit, recovery of the core, then putting everything together again took a complete day. The remainder of the job was running, however, smoothly and according to plan.
Due to the exact positioning of the drilling unit before the actual drilling started it was possible to execute the drilling hole dead on the point. After completion of the first drilling operation the unit was lifted with the specially ordered mobile crane from the first excavation hole into the second southerly hole.
Here everything started again right from the beginning. With this second hole it was possible to recover the one day which was lost at the first one and therefore complete the whole job within the required time frame.
After the successful completion of the entire job the local  building management appreciatively remarked that they did not expect that such diamond drilling jobs could be successfully executed.

J. Draeger