THE TUNNEL LINING - The Westerschelde Tunnel: Approaching Limits - page 114

was not desirable either, because the segments had to be as smooth as pos-
sible on the outside, for example to prevent wear and tear to the brush seal
during boring as much as possible. Initially, several people were constantly
busy with smoothing out the concrete and filling in small holes; a labour
intensive and therefore expensive business. The contractor therefore
decided to develop a machine which smoothed the concrete and further
compacted it in 15 minutes.
Quality assurance
Electronic data system (DRS)
In the production of 52,800 segments, keeping a quality assurance system
up to date on paper would take up an unmanageably large volume. The
contractor therefore decided to record most of the quality assurance in an
electronic data system (DRS).
Each employee at each workstation had a checklist of tasks which he had to
carry out before he could give the next production process the go-ahead. As
soon as the employee had finished these, he used a laser pen to scan, in
sequence, the bar code of the formwork, the future bar code of the segment,
the bar code of the activity and his personal bar code. This recorded that
person X at time Y had finished activity Z for segment A. This DRS system
also made it possible to make links between the reinforcement cages pro-
duced, times of casting and demoulding, and the first dimensional check.
The DRS therefore gave a complete picture of the coming about of a tunnel
segment. In addition, this system made it relatively easy to select specific
segments, for example, one of the segments which was damaged and
rejected during the building-in, could be replaced quickly.The DRS therefore
made a databank available in which the life story of each segment, including
specific quality data and identification numbers, was recorded.
Dimensional check
Particularly in view of the enormous jacking forces which the segments had
to be able to absorb during the tunnel construction process, very strict
requirements were set on the dimensions of the segments: with respect to
flatness, a tolerance requirement applied of 0.3 mm, while the angle toler-
ances were set at 0.04°. Deviations in the dimensions could all too quickly
result in undesirable concentrations of forces which could subsequently
result in damage.
It is self-evident therefore that the check on the dimensions of the segments
formed an essential part of the quality assurance. The dimensions of seg-
ments are usually checked manually with the aid of templates. An important
objection to this is that human errors can easily occur, certainly when enor-
mous numbers of segments are involved in which a high degree of accuracy
is required. Aside from that, taking measurements manually involves a lot
of time and money. The contractor therefore developed a perfectly operat-
ing 3-D photogrammetrical measuring system, with which it was possible
to determine the dimensions of the segments and the deviations from this
relatively quickly and with great accuracy.
Operation of the system
In order to measure a segment or formwork, adaptors were first fitted;
clamps with reflection stickers on them each with its own specific reflection.
Then reference standards were placed on and next to the segment. These
standards were also fitted with reflection stickers and were calibrated to the
dimensions. Step three consisted of photographing the segment from vari-
ous positions relative to the segment. About 40 photographs were taken in