GATEWAY BOULEVARD FOUNTAIN
REPORT ON FAILED DELIVERY PIPEWORK
& PRELIMINARY SYSTEM REVIEW
Prepared by
S. Osborn Pr Eng
SKP Engineers cc
23 May 2006
File No: 1366/17
Gateway Boulevard Fountain – Report on Failed Pipework & Preliminary System Review
1.
SKP Engineers cc
TERMS OF REFERENCE
SKP Engineers cc were requested by Old Mutual Properties, Engineering Services
Manager (Mr G. Rowlands) to carry out a review of the Gateway Boulevard Fountain
following problems experienced during its operation. In particular SKP was requested to
comment on the proposal for carrying out ad-hoc repairs to the pipe system and
construction of an access structure after exposing an area of buried pipework which had
failed.
2.
SITE INSPECTION
A site inspection was undertaken on 22 May 2006 and certain areas were examined.
Discussions with Mr Rowlands indicated that over the past months the delivery pipelines
had significant leaks during the operation of the fountain. The pipes from Pumps 1 & 5
had the highest water loss, and the pipe from Pump 4 had failed. This failure point was
examined.
2.1
PIPE FAILURE AT EAST SIDE OF STRUCTURE
An underground pipe failure (delivery pipe from Pump 4) had been exposed on the east
side of the structure, and the following points noted:
•
•
•
•
•
•
2.2
The pipework is 250mm diameter uPVC with solvent welded pipe fittings. The pipe
wall thickness was measured and the pipe noted as Class 9 (i.e. maximum
allowable working pressure of 9 Bars).
The delivery pipe had a longitudinal crack from the end welded inside the Tee piece
to the lower elbow, of approximate length 1m.
The Tee piece side wall had burst and a large section had broken away.
The reinforced concrete surface bed slab had been cast bearing directly on the pipe,
and the resulting load had deflected the pipework downwards.
No thrust blocks had been provided.
The backfill material is Berea Red sand, and the ground at the base of the
excavation was saturated and heaving.
CHANNEL SURROUND & SURFACE BED
•
•
•
The open channel surround had a horizontal joint approximately half way up the side
wall. No waterproofing was evident, and it appeared that this joint could leak.
The stepped surface was finished with mosaic tiles. Residue on the tiles indicated
that the slab had cracked. In addition certain areas of tiles have been replaced,
possibly due to movement of the concrete surface bed.
The surface bed slab is founded partly above the pumpstation, and partly on ground
fill. The portion located above the pumpstation would be less prone to settlement,
since the pumpstation would provide a sound support. However the area located
away from the pumpstation would settle if the ground became saturated and lost its
bearing capacity. This is quite probable, since significant water loss has been
recorded during pumping operations.
Page 2 of 5
Gateway Boulevard Fountain – Report on Failed Pipework & Preliminary System Review
2.3
PUMPSTATION
•
•
•
•
2.4
SKP Engineers cc
No structural deficiencies were noted at the pumpstation.
A core had been drilled through the back concrete wall. The measured wall
thickness is 250mm (as per drawing), and there is a sand drain with Geofabric lining
behind the wall.
The mechanical equipment included 4 x 90kW and 2 x 45kW centrifugal pumps, with
electrical control panels and variable speed drives. The 4 main pumps are Normaflo
pumps 125-250 supplied by Howden Pumps (now Denorco Pty Ltd).
The suction pipe was the same size as the delivery pipe (all 250mm dia). (The
suction pipe should typically have a larger diameter to reduce pipe friction, and be
arranged to provide a minimum resistance to flow, limiting obstructions such as
bends and filters. In addition, it is good practice to allow 7 x pipe diameter straight
lengths directly upstream of the pump, to provide an undisturbed flow into the pump.
This has not been provided.)
DOCUMENTATION
The following information was presented to SKP Engineers: •
•
Copy of a layout drawing showing the structure and pipe layout.
CD of photographs of the pipe failure.
No pump operational manuals were available.
2.5
ADDITIONAL INFORMATION
2.5.1
Pipeline leakage
It was reported that over the past months the water loss during pumping had been
exorbitant, and attempts were made to identify the source of the loss. This included the
use of a camera inserted into the pipeline, which revealed that the pipe bends were not
located at the positions shown on the drawings, and also that the pipes were sagging. The
latter point indicates that the ground has possibly settled as a result of leaks in the
pipework.
2.5.2
Pump Impellor trimming
It was reported that the pump impellors had been trimmed in the past to reduce the flow /
pumping head as the high pressure pumps were cavitating. This has been partially
successful, and it was suggested that the removal of the filter at the pump inlet would
further reduce the caviation.
Page 3 of 5
Gateway Boulevard Fountain – Report on Failed Pipework & Preliminary System Review
3.
SYSTEM REVIEW
3.1
PIPEWORK
SKP Engineers cc
The operation of the system involves pumping into pipelines with solenoid actuated valves
at the discharge end. These valves provide immediate flow stoppage, and since the pipe
lengths are short the water hammer head pressures will be severe as little damping will
occur. Although the actual pump duty is not known, the pump curves for a range of
impellor sizes of the pump installed indicate that the probable pump duty is approximately
60m (6 Bars) head with a flow of 100 l/sec. Preliminary calculations indicate that an
additional water hammer head of 100m (10 bars) is likely at full flow, resulting in a
pressure of approximately 16 Bars near the discharge valve.
Since the section of pipe that is exposed is rated as Class 9, the applied pressure of 16
Bars would certainly have ruptured the pipe. It would be reasonable to assume that similar
ruptures have occurred elsewhere in the pipe system.
The fittings used on the pipe system are rated as Class 16, and should be able to
withstand the water hammer pressures. The side wall failure of the Tee piece described in
section 3.1 may be attributed to the additional stresses resulting from the pipe failure
within the fitting, since the failures are on the same line.
4.
CONCLUSIONS AND RECOMMENDATIONS
4.1
AD-HOC REMEDIAL WORKS
It has been proposed by the Engineering Services Manager that the pipework be repaired
and that a culvert type structure be built at the present point of pipe failure on the eastern
side, which would allow access for future pipe inspection and repair. Such an
arrangement could be effective if there was confidence that the remaining pipework is in a
sound condition.
However, it is suspected that the class of pipe installed is inadequate to resist the surge
pressures due to repeated opening & closing of the fountain valves. This assumption is
supported by the fact that significant water loss has been measured on the remaining
supply lines.
It is therefore considered that a localised repair would be inappropriate and short lived.
4.2
ALTERNATE REMEDIAL WORKS
The effectiveness of the musical fountain is particularly susceptible to the hydraulic design
of the system, which in turn is dependant on the pipework layout. In any alternate
arrangement the hydraulic design would need to be carefully calculated to ensure that the
original designers’ requirements were satisfied.
A number of alternate designs could be considered, incorporating the following principles:
•
•
•
Replacing the high pressure delivery pipework with alternate materials.
Replacing/ re-arranging the low pressure suction pipework.
Improving the access to pipework for maintenance purposes.
Page 4 of 5
Gateway Boulevard Fountain – Report on Failed Pipework & Preliminary System Review
•
SKP Engineers cc
Reconstructing the stepped surface slab.
In order to carry out these modifications, the fountain would need to be demolished and
re-built under strict supervision of the design Engineer.
4.3
ALTERNATE USE OF SITE
It is possible that in the light of the above and of the high operational cost of the facility,
that the Client may consider an alternate use of the high profile site.
S. OSBORN
_________________
S. Osborn Pr Eng
Page 5 of 5