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MONITORING ONE OF EUROPE’S LARGEST RETAINING WALLS USING OBLIQUE AERIAL 
PHOTOGRAPHY 
D M Stirling, J H Chandler & J S Clark 
Engineering Surveying Research Centre, Department of Civil Engineering 
City University, London, United Kingdom 
ISPRS Commission V 
ABSTRACT: 
Oblique aerial photography acquired with a hand held Zeiss 300 mm UMK camera from a helicopter is being used for the 
long term monitoring of two large retaining walls currently under construction. The location of the site has dictated the 
use of a helicopter to obtain suitable convergent geometry so that high homogeneous precision can be obtained. Various 
cameras and emulsions have been tested in order to develop a system to satisfy the unique problems of the task. Preliminary 
results (using inner- constraint bundle methods and congruency tests), including detected movements and rotations of 
individual pre-cast concrete panels which comprise the walls are discussed. Methods for visualising the displacements 
and rotations are also considered. 
KEY WORDS: Deformation, Engineering, Hand held, Helicopter, Monitoring, Oblique, Presentation 
1. INTRODUCTION 
The ASS is a major trunk route in North Wales to the port 
of Holyhead on the island of Anglesey. Over the last 
twenty years most of the route has been converted to dual 
carriageway. One of the few remaining single 
carriageway sections is the short stretch between 
Penmaenmawr and Llanfairfechan. Work has now begun 
on providing a second carriageway at this point. This 
involves driving a 880 metre long tunnel through a granite 
headland. At each portal large retaining walls are being 
constructed. The wall at the west portal will be 220 metres 
long and 35 metres high thus making it one of the largest 
retaining walls in Europe when completed. These walls 
are being constructed from the top down and consist of 3 
metre by 2 metre pre-cast concrete panels anchored to the 
rock with two tensioned rock anchors per panel. 
The Ground Engineering Division of the Transport and 
Road Research Laboratory (TRRL) of the Department of 
Transport are undertaking a monitoring study of the 
behaviour of these retaining walls during construction and 
after completion. One of the aspects of this exercise is 
deformation monitoring. Based on previous monitoring 
work (Cooper and Clark, 1984) TRRL approached City 
University for advice on how monitoring of deformation 
of these structures could be achieved. 
2. PRELIMINARY WORK 
The situation of the site concerned indicated that 
photogrammetry provided the best and most efficient 
method of monitoring the walls. The site is on the side of 
a cliff above the sea with the existing road being 28 metres 
above a railway at the shoreline. Monitoring by 
traditional geodetic surveying methods would not yield 
sufficiently accurate and homogeneous results because of 
poor network geometry, as all sight lines would have to 
be observed up from the existing road level and would 
result in highly inclined lines of sight to points on the top 
half of the walls. For good redundancy an excessive 
number of instrument stations would be required. 
Additionally as monitoring was required during 
construction it would be almost impossible to sight to 
targets through all the construction plant and other 
obstructions that would be on site, particularly in the early 
stages of construction. Using photogrammetry it would 
be possible to get much better convergent geometry by 
using camera stations raised above the existing road level 
and most of the plant. One possibility investigated was to 
employ an elevating platform to raise the camera to the 
high positions. However this would have resulted in 
severe congestion to the existing road, which is a major 
trunk route, with the platform parked at the roadside with 
outriggers extended, especially during the construction 
phase. 
Low altitude vertical aerial photography has been used in 
the past for monitoring unstable hillsides (Fraser, 1983, 
Fraser and Gruendig, 1984). In this present study oblique 
aerial photography of the almost vertical walls seemed 
ideal and therefore it was decided to use a helicopter to 
obtain the required photography. This had the added 
advantage of rapid acquisition of all the required 
exposures per site. Helicopters with special mounts have 
been used to acquire vertical photography (Stanbridge, 
1987) but in this instance it was very quickly decided that 
the camera would be hand held for the oblique 
photography. Therefore with no requirement for 
specialised camera mounts not only could the cost be 
reduced but, more importantly, allowed much greater 
flexibility in chartering the most conveniently situated 
helicopter. 
In order to obtain a large enough image scale for accurate 
measurement it was decided to use a Zeiss Jena UMK 
30/1318 camera with 300 mm lens and glass plates. 
Consideration was given to using a film back but this 
would have proved too heavy to hold conveniently and the 
weight of the control box and provision of a power supply 
was also a problem. It was felt that with some practice 
glass plates could be changed quickly enough for the 
operation to run efficiently. 
It was also decided to hire a Linhof Aerotechnica 45 EL 
camera with a 250 mm lens for the first sortie to compare 
the operation of the two cameras in the helicopter and also 
their suitability for providing accurate results. 
One of the major tasks was the design of a suitable hand 
held mount for the UMK. The resultant design had to be 
strong enough to support the weight of the camera but 
enable pointing and plate-changing to be accomplished 
easily and quickly. The final design (Figures 1 and 2) 
utilised the hamess of a bus conductor's ticket machine 
attached to a metal plate from which a short aluminium