3.3. 
Typically the engineer requires a cross-section (profile) 
to be measured every 2.5m along the tunnel. Each cross- 
section includes measurements at the 8 cardinal points. 
These may be referred to as the crown, two shoulders, 
two axes, two knees, and the invert (this may be omitted 
as it is likely to be obscured by debris). 
Traditional Wriggle Survey employs a rig or template 
which is precisely set at the designed centre of the 
tunnel at a known chainage. 
(a) Chainage is accurately set out along the walls of 
the tunnel, and marked where profiles are to be 
measured. 
(b) A purpose-made rig is erected across the tunnel. 
A pair of scales are adjusted so that the pivot 
point is set at the tunnel centre as designed. 
(c) A measuring rod, 3m long is set on each of the 
8 profile points, and the radius from the pivot 
point is read off directly. 
With practice, a team of 4 should be able to set up the 
rig and take a complete set of readings in about 10 
minutes. Additional time is required for setting out 
chainages in the tunnel, pre-calculating offsets for 
setting up the rig, tabulating the results and calcu- 
lating throw. Throw is the deviation of the center 
or the ends of the coaches from the track around bends. 
Drawbacks to the method include the following: 
(a) Two visits must be made to the tunnel to set out 
chainage and to observe the profile. 
(b) Setting up data must be camputed before carrying 
out the fieldwork. 
(c) Setting up the rig involves cumbersome equipment 
and a time-consuming procedure prior to the 
measurement. 
(d) Results must be adjusted after the tunnelling 
has broken through.