Surveying
technical
Surveying on a
major airstrip upgrade
by Clare van Zwieten, PositionIT
The rehabilitation of this airstrip has included the construction of 23 000 m3 of raft foundations, the largest
ever done in South Africa, in order to provide sufficient support for aircraft landing in the dolomite risk area in
which the airstrip is located.
A
R700-million major construction
project to rehabilitate an airstrip
outside Pretoria is well underway
(see Fig. 1). The project has been split
into two phases – rehabilitation of the
main runway and rehabilitation of the
taxiways and other airside pavements.
The airstrip currently handles mainly
large transport and commercial aircraft.
The upgrade of the runways, taxiways
and airside facilities to meet international
standards and specifications was
necessary to ensure that the airstrip was
capable of accommodating its planned
future aircraft operations. (Future traffic
will include the Airbus A380, the Airbus
A400M, the Boeing 747-4002 and
other large bodied heavy aircraft.) It
is intended that the upgrade will allow
the airstrip to continue operations for an
additional 30 years [1].
The redesign of the runways and
taxiways was undertaken by a joint
venture consortium consisting of
Ninham Shand, Africon International,
Ndodana Consulting Engineers and
VGI Consult. The design ensures that
the airstrip meets International Civil
Aviation Organisation standards, as well
as the stringent guidelines regarding
infrastructure provision on dolomite.
Measures have been incorporated into
the design to limit possible sinkhole
formation and soil instabilities within
the vicinity of the runway and critical
taxiways. In addition to the civil
upgrades the entire runway lighting
system will be upgraded [1].
The runway contractor’s joint venture is
made up of Mvela Phanda Construction,
Protech Khuthele (incorporating South
African Road Testing Services) and
Gerolemou Construction. This article will
focus primarily on the work allocated
to Protech Khuthele and the surveying
required to complete the work.
Phase one of the project consists of
22 Fig. 1: Aerial view of the airstrip taken in October 2008.
rehabilitation of the 3,4 km runway
(3,6 km if the runoff areas are taken
into consideration). This includes tie-in
points for the secondary runway (to be
built at a future date) that will eventually
cross the main runway. Phase two of the
project involves working on the taxiways
serving the main runways. Phase three
(still in the planning stages) will focus on
the upgrading of the secondary runway
and parking aprons for the aircraft.
At this stage phase one is virtually
complete. There are still three out of a
total of six bellmouths, to build and a bit
of sweeping and cleaning still to do. (A
bellmouth occurs when one road ties into
another road creating a radius section.)
Phase two is about a month-and-a-half
from completion.
Surveyors
Gary Freeman from South African
Road Testing Services, a surveying and
geotechnical company falling under the
Protech Khuthele umbrella, manages the
surveying on the project and handles the
quantities for the job as well (see Fig. 2).
Fig. 2: Gary Freeman (South Africa Road
Testing Services) and Jonathan Cornes
(Protech Khuthele).
He is assisted by Arthur Nkoko, the only
full-time surveyor on site.
Equipment
Model Maker and Road Maker software
were used on the project. The Topcon
ATG 6 automatic levels and Topcon
PositionIT -Aug/Sept 2009
SURVEYING
into the same survey control that the
design is based on.) The South African
Road Testing Services team checked
and confirmed the control points and
then put in additional points of their
own approximately every 100 m.
Dynamic compaction
Fig. 3: The runway after dynamic
compaction has taken place [1].
GTS-230 series total stations were used
to check the tolerances. The Topcon
mmGPS Machine Control System was
used for the layer works. The automatic
level and the total station were used in
conjunction with the FC 200 Controller
and Topsurv software. The mmGPS
system uses Pocket 3D and 3DMC
Software.
Design
The design was issued by the
consultants. Freeman took the
information off the drawings, and
captured the details into Model Maker
and Road Maker. In order to generate
design model and quantities, details
from the original ground level (OGL)
are required. Freeman did an OGL
as an independent check to establish
whether there was a discrepancy
between this and the previous OGL
done for the consultants in order to
identify any potential problems upfront.
The survey control was handed over
to the contractors by the consultants,
so there was no need for the survey
contractors to bring in their own
control. (It is always preferable to tie
Fig. 4: Rafts covered in reinforced steel.
PositionIT -Aug/Sept 2009
Rehabilitation of the main runway
has involved the rebuilding of the
runway. The old runway had completely
deteriorated to such a point that it
could not simply be resurfaced – the
underlying foundation layers had failed.
The remains of the old runway had to
be stripped off and undercut down to
the stable layer. Being a dolomite area,
there is a high risk of sinkholes forming
so dynamic compaction was applied
to the main runway and taxiways. The
dynamic compaction was done by GCD,
a company specialising in geotechnical
work.
The dynamic compaction involved
using a crane to drop a 12 ton weight
a distance of 12 m and letting it hit
the ground. This process was done
every 25 m2. The impact of the
weight landing creates stress waves
that cause compaction effects deep
down compressing anything that has
the potential to collapse and thereby
enabling the identification of sinkholes.
The cable used to hoist the weight, has
a weakened section, so if the weight
punches through into a big sinkhole,
the cable will snap to prevent the
weight from pulling the whole crane
down into the sinkhole. Fortunately
no sinkholes were discovered while
conducting the dynamic compression
testing on the main runway and
taxiways.
technical
The dynamic compaction left the
runway riddled with row upon row of
circular indents about 3 m wide which
were covered up with bulk fill when the
layer works began (see Fig. 3).
Specifications for a runway are
generally slightly higher than those for
roads. Layer works for the main runway
consist of a layer of bulk fill averaging
1,2 m in depth, followed up with seven
layers made up of roadbed, two layers
of selected, two layers of stabilised,
bitumen treated base, and asphalt. This
part of the project has seen close to a
million cubic metres of soil moved.
Raft foundations
Runways are usually built parallel
to the predominant wind direction.
Generally the wind tends to blow from
one direction and when it changes it
often blows in the opposite direction.
For this reason runways are designed
to allow planes to land from either end.
Because of the increased pressures
on landing zones and the area having
a high sinkhole risk, it was decided
to construct raft foundations along
the impact zones on either end of the
runway i.e. the first and last
500 m sections of the main runway.
Raft foundations have been constructed
in South Africa previously but never
over an area of this size.
Raft foundations are a means of
increasing the bearing capacity of
impact zones. They are designed to
distribute the huge impact caused by
landing aircraft over a wider area. This
was particularly necessary due to the
sinkhole risk factor.
The rafts were created by cutting
trenches into the soil 1 m deep leaving
Fig. 5: Raft trenches filled with rainwater.
23
SURVEYING
technical
Fig. 6: Machine controller and grader working in conjunction
with the reclaimer.
a waffle pattern of 2,5 x 2,5 m squares
across the two 500 m by 45 m landing
zone sections of the runway (see
Fig.4). There were a total of eight rafts
per landing section, with each raft
made up of 506 blocks. Reinforced
steel was then crisscrossed and set at
measured depths in the trenches as
stipulated by the specifications provided
by the consultants. The specifications
provided details on how many steel
reinforcing bars were required in each
trench, their diameter, their spacing
intervals and so forth. The trenches
were then filled with concrete.
As the work progressed the survey
team had to pick up on all the surface
finishing, the bottom of the trench
finishing and the widths to check
tolerances and overuse of concrete.
There were reasonably tight tolerances
on the raft foundations – width
20 mm, depth 40 mm. When checking
the tolerances the survey team used
Topcon ATG 6 automatic levels and the
Topcon GTS-230 series total stations.
Challenges
Most of the work on the raft
foundations took place between
October 2008 and February 2009, the
rainy season. The trenches flooded and
the trenches sometimes collapsed due
to the changing wet and dry cycles
(see Fig. 5). In addition, cleaning and
clearing the trenches was a difficult
process as the trenches could not be
accessed by hand. Clearing had to be
done using poles and small scoops. It
was a very labour intensive job and at
one stage there were
250 people working on the raft
foundations. Coordinating their
activities in the rain was extremely
challenging.
Gary Freeman from South Africa Road
Testing Services had to generate the
setting out data for the raft foundations
Fig. 8: Layer works at a later stage in the project.
24 Fig. 7: Processing of a cement stabilised layer.
which involved a lot of painstaking,
tedious work when you take into
account that there were 16 rafts in
total, each consisting of 506 blocks.
Freeman used Model Maker software
to generate all the grid points for the
rafts. His site surveyor, Arthur Nkoko
then had to set out these grid points.
Service tunnel
At km 2,2 on the runway a service
tunnel was constructed to provide access
to services such as communications and
electricity. Once the excavation had been
done, the concrete tunnel structure was
assembled into position and the void
was back filled and the layer works done
according to the layer specifications for
the rest of the runway.
Layer works via machine control
As the job progressed, the decision
was made to acquire a Topcon Machine
Control System – the first in South
Africa. The mmGPS system has a
Fig. 9: Plan view of the runway in Model Maker with the contours
plotted and all the necessary setting out detail.
PositionIT -Aug/Sept 2009
SURVEYING
technical
Conclusion
Fig. 10: Screen control in the grader cab.
1 arc second accuracy and its ability to
enable the faster cutting of levels and
to work in conjunction with a reclaimer,
has contributed to a faster pace on the
contract, improved accuracy, increased
productivity, as well as savings on diesel
(see Fig. 6).
Cutting layers the conventional way with
a good grader operator usually results
in 15 mm accuracy in contrast to the
Machine Control System which provides
accuracy of between 3 and
5 mm. The traditional approach to layer
works would usually involve the grader
stabilising 600 m3 a day, however with
the Topcon Machine Control System and
the reclaimer, the team is now stablising
over 3000 m3 a day. A week’s output is
now being accomplished in a day (see
Figs. 7 and 8).
Fig. 11: Base station for the Machine
Control System.
There is no need for the operator to get
out and do any checks.
If the Machine Control System had
not been brought in, the surveying
on this job would have been far more
challenging. First, four or five surveyors
would have been needed instead of one.
Secondly those surveyors would have
had to knock in profile poles at a grid of
10 x 10 m, which would have required
additional labour, and they would have
had to maintain the profile poles as well.
The huge number of survey poles on site
would have created restrictions for the
operators who would have had to keep a
constant watch out for the survey poles.
All of this would have involved extra
labour, material, time and cost.
The main runway, which is now on
average 1 m higher that the old runway,
is expected to be completed by 23
September 2009 and flights are due
to resume in October 2009. Further
contracts planned at this air strip include
the upgrade of the secondary runway,
the finalisation of the runway and
taxiway lighting and the construction
of the new Airbus A400M facility. The
new Airbus A400M facility is expected to
include the construction of a new flight
lane and hardstands, the construction
of appropriate hangars to house eight
Airbus A400M aircraft, including a
maintenance hangar and wash bay, and
new logistics buildings [1].
Acknowledgements
I would like to thank Gary Freeman
(Manager: South Africa Road Testing
Services) and Jonathan Cornes
(Contracts Director: Protech Khuthele)
for their assistance with this article.
References
[1] www.af.mil.za/NEWS/2008/068.htm
Contact Gary Freeman,
South African Road Testing Services,
Tel 083 645-5204, gary@pkh.co.za 
Without the system, it would have
been necessary to have four or five
full-time surveyors on site, whereas now
it is possible to operate with just one
full-time surveyor on site. Before the
Machine Control System was brought in,
the site was an expanse of survey poles.
With the machine control, the team
now only has two poles in place every
200 m and they are purely there for
independent checking.
In the four months that it has been in
operation, the Machine Control System
has paid itself off by increasing the
tolerance accuracy on the layer works.
For example, a 5 mm improvement
on accuracy has potentially saved the
contractors R1,4-million in excess use of
bitumen treated base.
The system is user friendly and easy to
set up. The design is generated in Model
Maker (see Fig. 9) and then uploaded
to the Machine Control System with the
GPS transmitting corrections. (A rotating
laser provides coverage of 300 m from
the machine and up to four lasers can be
set up on the system providing distance
coverage of up to 2,4 km and a vertical
height of 10 m per machine.) The grader
operator then points the grader in any
direction and the system in the cab
controls the grader blade (see Fig. 10).
PositionIT -Aug/Sept 2009
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