THE STRATEGIC TUNNEL ENHANCEMENT PROGRAMME (STEP) consists of 41km of deep gravity tunnel sewer and 45km of link sewers taking sewerage flows out of the city of Abu Dhabi to a major new treatment plant outside Abu Dhabi.. The talk focused on the challenges of designing and constructing the deep tunnels and shafts, which start at a depth of 24m in the city and end at a depth of 80m with a diameter of 6m at the new treatment works.
A total of eight modern EPB TBMS were used for the tunnel construction with groundwater pressures of up to 8 bar. The tunnel was lined with a Corrosion Protection (CPL) secondary lining. The success of designing such a sewer tunnel to have a long service life in the severe exposure conditions of the Gulf illustrates how modern developments in service life design and tunnel design can be applied to such projects and the experience gained will be of relevance to upcoming major tunnel projects in the UK.
The project is based in the small most populated part of the Emirate of Abu Dhabi – primarily covering the main Island, the surrounding islands and the populated parts of the Mainland. ADSSC has three licenced activities; Collection & conveyance (with a large asset base); Wastewater treatment (signi_ cant by its volume) and; Recycled water & bio solids. The presentation was based on the collection and conveyance activities.
Over the past decades Abu Dhabi has sustained tremendous growth from a population of about 100,000 in 1970s to over 1.6 million in 2014. The massive expansion of the green areas together with lots of new catchment basins has turned out to be a real challenge for ADSSC. In 2007 a hydraulic modelling exercise presented in fact a scary picture with high risk of flooding and a need for a massive investment.
The on-time delivery of Capital investment projects has been a challenge for ADSSC. Generally delayed Capital Expenditure (CAPEX) projects end up requiring additional budget, increased O&M costs (due to delayed improvements) and reputational damage. To overcome such a scenario ADSSC took a look at their own organisation and then came up with an innovative delivery approach by engaging with the private sector in a tripartite setup to include consultants and contractors. ADSSC young staff members were seconded to the Programme consultants, who were co-located with the client under one roof. Two new forms of contracts were used: Openbook consultancy contracts and Design & Build construction contracts – a first in UAE. Apart from targeting successful investment delivery, ADSSC also had the strategic objective of building internal capacity through knowledge and skill transfer while delivering the Programme and to works towards a partnership-type approach within the tripartite set up. Three leaning frameworks were used; Organisational learning (Rashman et al, 2009); Internal Stickiness (Szulanski, 1996) and Tacit Knowledge (Nonaka et al. 1991).
In the initial stage of the Programme, ADSSC undertook an optioneering exercise where four technical options were considered; Do nothing; Upgrade critical infrastructure; Construct an offshore pipeline; Construct a tunnel sewer. Options were assessed using both quantitative and qualitative criteria. The deep gravity sewer tunnel option emerged as the best option and was subsequently developed further. The selected deep gravity sewer option had a number of benefits including; reduced potential for overflows, reduced capital and O&M costs, reduced odour potential, improved health and safety and improved aesthetics.
The deep gravity sewer tunnel was planned to be connected to a series of link sewers to gravitate strategic parts of the existing system to the deep tunnel system terminating at a new large pumping station. In parallel, 35 number of existing pumping stations were planned for decommissioning and gravitation through the Programme.
STEP is a deep gravity system to collect the used water in Abu Dhabi Island and the mainland. It includes 41km of deep bored tunnel (4m to 5.5m in diameter), 45km of link sewers (0.2m to 3.1m diameter) and 1 large pumping station (ultimate capacity 39m3/s). The used water will flow under gravity to a new treatments works at Al Wathba. The new system will accommodate an average used water flow of 1.7Mm3/day by 2030. The first construction contract was awarded 2009 and the entire Programme is scheduled to be finished in 2016.
Design challenges of the step T-02 anD T-03 lots
One of the critical aspects of the project was designing and constructing tunnels to have a long service life in the very aggressive environment of the Gulf. This could only be possible if all the parties acted as a team. The designer had to be on the site permanently.
The overall solution was defined by ADSSC and their Engineer CH2M. The design was a 40-80m deep segmental bored tunnel, T-02 280mm thick segments and T-03 350mm thick segments (due to deeper alignment). The secondary lining was a cast-in-situ lining with cast HDPE lining (2.5mm) inside to avoid microbiologically induced concrete corrosion (sulphuric acid attack) to take place. The HDPE worked in conjunction with the un-reinforced concrete sacrificial lining. The tunnels had a design life of 80 years – this presented the designer with a major challenge.
The Gulf has some of the most aggressive soil / groundwater conditions in the world. Groundwater contains 10-12 per cent chlorides 4 to 5 times that of seawater, extremely high suphate contents (up to 5000 mg/l) are present that are not found in Europe. Not forgetting the temperature of about 30 degrees Centigrade throughout the year.
A lot of construction in the Gulf has learnt the hard way – that unless durability aspects are taken seriously into consideration concrete will not last very long (no more than 5-10 years). Taking account of the high risk of corrosion – new ground was broken. If the segments were designed in the conventional manner with black steel rebar cages then cover of 80mm and more would have been required to meet durability requirements assuming a normal quality concrete. This large cover is practically not possible for a bored tunnel due to high risk of spalling during transportation, installation and erection within the tunnel. There was a possibility to use stainless steel mainly at the joints where there is the highest risk of chloride concentration. This would be too costly and a new solution was sought. The solution was the use of Steel Fibre Reinforced Concrete (SFRC).
Even in aggressive environments with high temperatures SFRC is much less sensitive to corrosion compared to traditional steel reinforcement. It can be argued that there are no concerns with SFRC it is only an aesthetical issue – and what does that matter in a sewage tunnel?
One of the reasons SFRC is more durable is that the fibres are "swimming" or "floating" in the concrete which means that they get an excellent interface between the fibres and the concrete with reduced voids in the contact zone between the steel fibre and concrete. With conventional reinforcement there is always a small void between rebar and concrete as the concrete shrinks during curing and this is where reinforcement corrosion starts. Hence SFRC can tolerate much higher chloride threshold values – 5-10 times more than conventional reinforcement. In addition, electrochemically the formation of anodes and cathodes is impeded as the fibres are minute. Only the fibres that poke out of the concrete surface will corrode with no deeper corrosion as the fibres are discontinuous. If internal fibres do corrode, the volume (rust) of a single fibre would be so small such that no tension in the concrete would be created – so no risk of spalling and cracking of concrete. So SFRC is the obvious choice for a long lasting sewage tunnel in the Middle East.
However, the structural design was not so easy – high bursting stresses were determined at the segment radial joints, up to 2 MPa which forced the use of some traditional rebar at such locations. . It was decided to place a limited amount of rebar along the radial joints. Four single bars connected with stirrups. . In addition these bars were difficult to hold in place with the traditional cage.
The durability design approach for traditional reinforced concrete (RC) was based on the service life approach as given in the fib Bulletin 34 "Model Code for Service Life Design", 2010. The same approach has nowadays been incorporated in the fib Model Code 2010 and ISO16204. The key tool of the fib-34 approach is a mathematical tool to determine the concrete cover and the quality of the concrete to make sure the concrete will last for 80 years. In case of chloride ingress the chloride migration coefficient determines the concrete quality. The chloride migration coefficient tells how fast the chlorides penetrate from the outside of the concrete to the rebar. The lower the value the denser the concrete and the slower the chloride migration happens. The coefficient is very much dependent on the binder content which needs to include OPC + FA + MS or GGBS if working in the Middle East. Portland cement alone is not durable enough. Concrete with OPC + GGBS has a coefficient five times less than OPC concrete.
For a concrete cover of 65mm, the maximum limit the designers could tolerate, a maximum chloride migration coefficient of 2.4 x 10-12m2/s was determined. Only a high-performance concrete of grade C50/60 with a triple blend of 50 per cent OPC + 20 per cent FA + 30 per cent GGBS could comply with that low value. The fibre class was F1.4/0.6 (following the German approach for steel fibre design) and 40kg/m3 was used. It was not easy to develop the right quality of the concrete mix and the "fine tuning" of the concrete mix took 1 year to complete.
Mechanical testing of the concrete mix was also undertaken. This was done using the four-point bending test to verify the specified F1.4/0.6 fibre class. Actual segments were broken to verify that the fibres could be evenly distributed throughout the segment thickness. The durability testing for chloride migration, developed in Sweden and followed the NT Build 492 method, was carried out in the laboratories in Abu Dhabi.
Cross checking was carried out in a laboratory in Denmark which gave comparable results. This test methodology was done in preference to the ASTM C 1202 method as it is a direct measure of chloride ingress and gives the input parameter for the fib Bulletin 34 modelling.
The concrete mix was also judged to be sustainable. A C02 reduction of more than 60 per cent was achieved compared to OPC concrete and traditional reinforcement as used in other bored tunnel projects such as the Copenhagen Metro or the Channel Tunnel between England and France. For once sustainability and durability are going in the same direction.
In summary; SFRC is an obvious choice for segmental linings in the Middle East which need a long service life, with the following advantages; More durable than traditional reinforcement, Avoid a mix of steel fibre and traditional rebar, damage to segments is reduced with less repair and minor rejection of segments, linings can be thinner and sustainability advantages. For shafts improved constructability, reduced time and materials and reduced maintenance.
The main consTruction aspects of step contracts T-02 and T-03
Contract T-02 runs outside the Abu Dhabi Island (i.e. outside the main city centre) and passes underneath a low density urbanized area. It comprises the following scope of works:
¦ Main gravity sewage 5m ID tunnel, 15.5km length with depth varying from 35m to 59m;
¦ Three 16m ID work shafts (WS5, WS6, WS7) with depths varying from 45m to 64m;
¦ One vortex structure at WS6;
¦ Three 6m ID access shafts (AS4, AS5, AS6) with depths varying from 38m to 51m;
¦ Three adits from access shafts to main tunnel.
Contract T-03 comprises the following scope of works;
¦ Main gravity sewage 5.5m ID tunnel, 9.7km length with depth varying from 59m to 81m;
¦ Two 17m ID work shafts (WS8, WS9) with depth varying from 74m to 86m to be converted into access shafts at later stage; Two 6m ID access shafts (AS7, AS8), with depths varying from 70m to 81m;
¦ Two adits from access shafts to main tunnel.
The main challenge to be faced by the contractor right from the project start was the tight construction programme:
¦ 3 Work Shafts to be excavated concurrently during the first year of the project;
¦ 4 out of 5 TBMs running at the same time (total of 25km bored over 21 months);
¦ 14 tunnel secondary lining fronts to advance concurrently during the finishing work stage (total of 25km over 19 months).
The considerable shafts depths rendered all the above more demanding. It was in fact the first time that depths up to 86m and water heads up to 84m were reached in Abu Dhabi, with a lack of case histories that could be used for guidance. This impacted on the selection of the construction methods, design of plant and equipment and logistics in general.
The overall ambient conditions represented another key construction challenge. The Abu Dhabi geology consists of a liner coastline with an alternation of sedimentary beds prone to karst phenomena. In addition, the hyper salinity of groundwater (Cl- up to 120,000mgl/l and S04 up to 5000mg/l on T-03) combined with the regional hot climate (temperatures regularly in excess of 40°C during summer months with humidity above 70 per cent) produces an extremely aggressive environment. Efficient and effective logistics was a critical aspect for meeting the construction programme, in particular the planned TBM production rates. Key site installations at each Work Shaft included amongst others:
¦ one electrical generation substation (electric power network not available);
¦ daily storage of 240m3 of sweet water (water network not available for the expected consumption rates);
¦ one chiller station to cool water/ air from ambient (in excess of 40°C during summer ) to TBM (26°C);
¦ a tandem of gantry cranes delivering the construction materials to and mucking out from shaft bottom; one vertical conveyor belt magazine at surface. ShaftS
The temporary lining of the Work and Access Shafts was typically formed from diaphragm walls in the upper portion of the shaft and sprayed concrete lining in the lower. The continuous and watertight diaphragm wall lining was installed from ground level down to a typical depth of 45m with panels excavated in primary and secondary sequence, using bentonite slurry as a stabilising fluid. A hydromill capable of operating in any soft or hard material with accurate verticality (1:800 circumferential, 1:400 radial) was used for such works.
TBM Launch chambers
The TBM launch chambers at each Work Shaft consisted of a 60m long headshunt plus a 40m backshunt. The chambers were designed following a review of the available S.I. (permeability data in particular) and the actual geology encountered during the shaft excavation.
The excavation clear inner section was sized based on the TBM cut diameter. Ground support was provided by wet shotcrete reinforced with steel fibres in combination with steel arches. Advancements were maintained shorter than 1m.
The tunnel headwalls were reinforced with fibre glass dowels.
TBM Tunnels
T02 tunnels presented a 5.50m ID lining formed from 5+1 280mm thick PCC trapezoidal segments sealed with EPDM gaskets. T03 tunnels had a 6.00m ID lining formed from 5+1 350mm thick PCC trapezoidal segments also sealed with EPDM gaskets. On both contracts the ring, of universal type, was 1.4m long and used guide rods. The segments were reinforced with a combination of steel fibres and traditional cages installed in correspondence of the radial joints. EPBMs were chosen on the basis of the following criteria:
¦¦Machine capable of operating in close mode as per Client’s requirements;
¦¦Clay content in the majority of the rock formations, which would be very difficult to handle in the separation treatment plant required by a Slurry Shield;
¦¦EPBM capability to cope with the expected ground conditions maintaining face stability;
¦¦Salini – Impregilo’s extensive experience in driving EPB machines.
Corrosion Protect ion
The Corrosion Protection Lining (CPL) for the main tunnel comprises a 2.5mm primary lining of HDPE Anchor Knob Sheet embedded into a 250mm secondary lining of un-reinforced cast in situ concrete. The same concept was previously used on the Singapore DTSS Project. The finished sewer presents an inner diameter of 5m (T- 02) and 5.5m (T-03) with the primary lining covering 320º of the upper internal circumference. The HDPE liner of each tunnel section presents different colours for traceability in case of repair. The CPL for each bored tunnel was constructed from two independent work faces, one travelling upstream and the other downstream. Both fronts started from the centre of the main tunnel, each of them using two shutters of 12m length for forming the vault. Similarly to the TBM drives, this phase of the project required lot of planning and logistics design with also considerable investments in plants and equipment. Typical weekly productions for a work front were in the range of 120m of tunnel lined.
Conclusions
The STEP deep gravity Tunnels project was deemed a major success. The new gravity sewer system will meet the Plan Abu Dhabi 2030 demand.
The tunnel was completed with savings and with an amicable closure – no disputes and with highly successful branding results.
The new initiatives of staff secondment, co-location with consultants, open-book consultancy contracts and design-build construction contracts were all deemed successful. Apart from facilitating knowledge transfer from the private sector to the seconded staff, the local market and construction industry skill base has also been enhanced because of successful delivery of massive construction projects.
Key enablers have been; proactive risk management (packaging of works, form of contract and contract administration), risk appointment (unforeseen physical conditions) and a partnership approach to contract administration.
