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Sat 13 Nov 2010 12:00 AM

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Digging deep

Infrastructure works are some of the most technically demanding construction projects. CW visits a $2.2 billion Doha sewage mega project that has challenged designers and contractors. By Carlin Gerbich

Digging deep
The Screen Chamber is 20m in diameter and 40m deep. The massive pump station (inset) sends effluent on to the new STW.

There’s no doubt about it: Doha’s star is rising. With a booming economy
fuelled by the gas industry, the capital’s rapid expansion over recent years has
been both a formidable and impressive accomplishment.

With expansion comes pressure and, in Doha at least, the government
has embarked on an unprecedented level of infrastructure spending to help alleviate
the extra stress the country’s growing population is placing on basic services such
as roading and sewerage.

Over the next four years, Qatar will spend more than $140 billion
on infrastructure projects, dedicating 43% of its budget for that period on major
public works.

One of those major projects is the Doha North Sewerage Treatment
and Associated Works – a $2.2 billion project that includes a suite of contracts
to bolster the city’s sewerage treatment capabilities. Doha is currently served by two treatment works,
Doha West and Doha South: the latter built in the 1980s and which has had a number
of extensions and refurbishments.

As Kevin Richards, the Public Works Authority’s (Ashghal) Senior
Project Engineer for the Doha North scheme, explains: “The situation has developed
by the ever growing population in the country. Every time you finish one phase of
expansion, it’s like playing a catch-up game of football: the population growth
is such that you always need another expansion, another programme.”

“The city’s current expansion is in the north which, geographically-speaking,
is best served by the Doha West treatment plant. However, the plant is unable to
realistically handle the amount of pressure large developments like Lusail and the
Pearl Qatar would place on it, once they are fully developed,” he said.

The Doha North Project is designed to pick up flows from Lusail,
the Pearl catchments and Gharaffa, West
Bay area, as well as alleviate
some of the pressure placed on the existing Doha West Sewerage Treatment Works (STW)
catchment.

The project

The Doha North Sewerage Treatment and Associated Works project
consists of four construction packages, all started at different times and designed
to come on stream at around the same period.

At $1 billion, the STW is the biggest ticket item. Awarded to
Keppel Seghers Engineering Singapore, the 30-month contract covers the design, construction
and operation of new plant which has been designed to handle 245,000m3 of sewage
daily covering a population of around 900,000 during its first phase of development.
The treatment works will be expanded to cope with up to 345,000m3 of sewage at a
later stage.

Work began on the project in March 2008 and was due to be completed
in August this year, but it is unlikely to be completed before work on the upstream
$490 million pumping station (PS70) and pumping mains has been finished. The contract
for PS70 and the pumping mains was awarded to a joint venture led by Vinci Construction
Grands Projets, with Qatari Diar Vinci Construction and Entrepose Contracting. The
33-month contract is due for completion by December 20 2011, and Vinci says it is
determined to hit its deadline.

Construction of the gravity-fed Interceptor Sewers which feed
PS70 is another major component of the project. Work began on the $360 million project
in March 2008 by contractors Ultra Construction and Engineering, and includes 33km
of micro-tunnels of different diameters and 107 shafts, which pick up the gravity
flows, primarily from the southern part of the Doha north project. That contract is due for completion
in March 2011.

The fourth contract covers the construction of a return pumping
station and twin 1600 mm diameter mains that will send treated effluent from the
STW to an existing TSE ring main for irrigation. The project is running concurrently
with the construction of the STW and is being carried out by the Marubeni Corporation.

PS70 and the pumping mains

Though it is a part of the wider Doha North Sewerage Treatment
and Associated Works, PS70 and the triple rising pumping main is a significant project
in its own right. The project not only calls for the construction of several complicated
structures on a 200x200m site that must also blend in with the residential area
when completed, but also for a triple 1600mm pumping main to feed the new Doha
North STW, 25km away. It is, by far, the most technically demanding of the four
construction projects.

The pumping station consists of a screen chamber: a 40m deep,
20m diameter chamber which filters the incoming waste flow and before feeding it
to a Lifting Pumping Station which draws the wastewater to the surface and on to
a Forwarding Pumping Station where it is pumped to the STW via three 1600mm diameter
pipes.

As Richards explains: “When people talk about PS70, they think
of a Pumping Station. It is actually two pumping stations which are in one site.
The Lifting Pumping station is a circular pumping station split in to two, with
two separate wet wells and two separate dry wells. It’s basically like the wheel
of a car, with the outside, the tyre, being the wet well and the centre portion,
the hub, being the dry well where the pumps and the pump motors are housed.”

“You have got five delivery pumps and five discharge pipes from
each half of each lift pumping station wet well which then discharge at high level
to the Forwarding Pumping Station which then connects in to the discharge mains
which pumps to the treatment works at Doha North, out in the desert.

“It is a very confined site, so being able to fit in a screen
chamber, a lift pumping station, a forwarding pumping station, an odour control
building, admin building, everything. That was a task in itself,” Richards said.

Project manager Christian Tricoire of Vinci Construction Grands
Projets said: “It’s a tight project. The critical part of it is the pumping station
which consists of civil works, electrical and mechanical works and a lot of pipe
works. It is a small area to work, so the sequence of work and activity on site
has to be carefully looked at; so that was one of the keys for us.”

“The tough thing for us
was the excavation of the two shafts through hard rock. As soon as you touch the
ground in Qatar
you are straight into limestone. You have something like 40cm of sand with stone,
and after that it is hard rock, so we had to play with blasting activities and a
rock breaker,” he said.

“At the very beginning of the project we had 27 excavators with
rock breakers to start the excavation work, then we moved to explosives, then came
back to rock breakers,” Tricoire added.

Vinci extracted 12,630m3 of earthworks for the Screen Chamber
and 44,610m3 for the Lift Pumping Station shaft.

“Was it tougher than we thought? Not really. We knew we had geological
information given at the tender stage and lucky us, QDVC, was already working in
Doha two years before
on other projects, and we had the opportunity to look at their projects to see what
problems we could face.”

Another problem encountered was the high water table. Tricoire
says that Vinci hit groundwater just 8m down in to the excavation – giving them
more than 30m of ground water to deal with.

Vinci is dewatering the area by using seven draw down pumps which
discharge in to a small surface lagoon where it is then taken and pumped deep into
the ground, further away from the site. As Vinci dug and blasted its way in to the
rock, it treated the exposed surfaces with shotcrete to secure the face of the excavation,
before lining the surface with a geotextile waterproof membrane.

“We then started from the bottom up, to construct the walls of
the chambers. For the outer wall, we used a single face formwork from Doka, 1.2m
thick, with the formwork facing the excavation,” Tricoire said.

To anchor the shafts, 17 ‘toes’ had to be constructed to reinforce
the area to stop if from collapsing while workers poured the concrete for the toes.

But the overriding challenge on the site, and particularly work
in the two shafts, has been access. Everything used in the construction of the Screen
Chamber and Lifting Pumping Station has had to be lowered over the side, which,
Tricoire says, presents safety issues with workers below.

“We have a weekly toolbox meeting where we have a HSE officer
come on site and go through what is to happen, we have a safety induction and we
do have translators. We organise trials on site; we stage accident (scenarios) on
site, and show people what to do. Even if they can’t understand, they can physically
see what
to do.

“We do it for any new activities, most of the major activities.
We have what we call a pre construction meeting to remind every single person who
will be involved in these works who is going to manage, who is going to monitor
the crane, who is going to give the instruction as to the lifting operation, and
what are the dangers.”

Tricoire says that the project has not hit any major snags, though
it did begin its midday summer work ban a month earlier to accommodate Qatar’s hottest
summer on record.

“Wind is a fact of life when you are working in Qatar. In February,
March, April, when you are working in the desert, wind plus sand is not always a
pleasant way to work,” he explained.

The pipeline also presented logistical challenges because it
was built alongside a stretch of the new North Road. The triple rising main had to
be cut and laid through a 13m wide, 8.5m deep corridor before the road and bridge
abutments could be finished. At the same time, Kahramaa – Qatar’s General
Electricity and Water Corporation – was laying a major arterial water mains to replace
an existing one.

Richards said: “In some cases, we couldn’t lay our pipes because
the new pipelines being constructed under the road were in conflict with ours, so
we couldn’t decommission the existing water main before the North Road contractor had supplied the new
pipelines, so that the old pipelines could be taken out and our lines could be run
through it. It was an absolute headache.”

Tricoire says the project has been a challenging one for his
team – but also a technically interesting one.

“The sad thing for the pipeworks engineer is that most of his
work will never been seen because it is all underground,” he explained. “But for
the young engineers on site, I have told them to make sure they were involved in
as much as they can be, because this job has everything: complex building design,
micro tunnelling, big excavations – everything that you might run into in other
jobs in isolation, all here together on one site.”

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