Brook. These deformations can indicate increased pressures on the 
linings resulting in bulges, or the converse can occur where cavities are 
created behind the lining causing decreased pressures. The presence of 
deformations in the tunnel structure can indicate one of three conditions:- 
i) Ihe shape of the tunnel as built. 
ii) Changes in pressures during the life of the tunnel, whicn have 
since stabilized, or 
iii) Changes in pressures still occuring. 
The third situation is the one which is the cause for concern. The problem 
is to detect these deformations with sufficient accuracy and as cheaply as 
possible. 
Detection of profile deformations in brick lined tunnels is best done by 
recording the profiles at right angles to the axis. Visual inspection, 
even with powerful lamps does not reveal deformations, unless they are 
prominent, a fact which we have proved by the commercial operation of 
our system. 
Recording Sections 
Many techniques exist for recording tunnel sections, which vary from 
decimeter to sub millimeter accuracy. Generally, the more accurate 
the method, the more time is required to record the section, and the 
more costly the operation. Early and high accuracy methods tend to 
be by direct measurement whilst contemporary development has moved 
towards remote sensing. 
B.K.S. became involved in tunnel sectioning some six years ago. The 
original technique used was stersometric photogrammetry, being an 
extension and development of our main line of business - aerial 
photogrammetry. The technique involved a pair of Wild P32 cameras 
mounted on a 1.2 metres base bar with lighting provided by numerous 
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