The El Nino Weather event impacts climates all around the world. In Manila in the Philippines, it has contributed to a 60% decline in rainfall that has seen the city gripped by a severe water shortage. Residents have suffered through lowered reservoirs and actual water cuts. But while the rumbling from the skies may have stopped, the trucks had already been rolling for two years. The Novaliches to Balara Aqueduct number four (NBAQ 4) project is underway.

NBAQ 4 is part of Manila Water’s improvement and expansion initiatives and is one of the largest and most important infrastructure projects undertaken by them to date. The project involves the construction of a new intake facility at the La Mesa reservoir, a 7.3km-long, 3.1m ID tunnelled aqueduct and an outlet facility at the Balara Water Treatment Plant. NBAQ 4 will deliver 1,000Ml of water to approximately seven million people in Manila.

There are three existing aqueducts that are buried steel pipes and are in a poor state of repair, so NBAQ 4 will replace these and also allow for their rehabilitation.

The PHP 5.3bn (USD 104M) design and build contract was awarded to a joint venture of CMC di Ravenna, First Balfour and Chun Wo Engineering in August 2017. GHD was engaged by the JV for both the tender design and detailed design. The tender evaluation was on an 80% technical and 20% cost basis and the winning bid was the highest bidder. The relatively low cost of the bid despite this was a result of the local labour costs.

The alignment involves three 80m radius bends, two at one end, one towards the other, as well as ground cover varied from 8-52m. It is located entirely under public right-of-way land (largely under Manila’s 16-lane Commonwealth Avenue) as private land is so difficult to acquire for works in the Philippines.

A 4m-diameter Herrenknecht EPBM is to be launched from a 40m deep construction access shaft and a 75m NATM tunnel in weak volcanic rock to allow for TBM operations. The TBM will dock in the intake structure built 150m offshore in the reservoir in a 16m-diameter, 30m-deep temporary cofferdam.

Geology

The tunnel is entirely within volcanics. This comprises tuffs and re-worked volcanics and some conglomerate. The project site is situated within the mainland section of the Southern Sierra Madre stratigraphic group and the alignment is within a pyrochalistic sequence of deposits belonging to the Guadelupe formation.

It is also very close, but does not pass through a seismic, reactive fault called the Marikina Valley Fault System, which yielded very high seismicity and presented some interesting design requirements.

Weak rock strata (0.4 to 4MPa) with soil-like strength properties characterise the region, mixed face conditions, permeability is highly variable, Paleosol (compacted soils containing organic materials), quarternary deposits (a mixture of clays and silts) and subvertical joints.

Ground investigation done by the team included 17 additional boreholes, onshore and offshore seismic surveys, downhole geophysics and instrumentation within the boreholes.

Hydraulic design

The hydraulic design was for 1,000 MLD at 10-20m gross head operating range although they think 1,300 MLD is theoretically possible at maximum head. The roughness heights of the segmental lining are between 0.4 and 1mm for new and aged surfaces (roughed). Computational flow dynamic modelling was used to confirm head losses that could be expected.

Construction access shaft

A temporary construction access shaft is 44m deep and 9m in diameter. It serves to decouple the outlet from the TBM tunnel operations. During the tender and preliminary design it was a sequentially excavated shaft lined with shotcrete and secured with rockbolts, however the nearest borehole at this point was 100m away. Following additional investigation, weaker rock and soil-like layers was discovered, with a soil layer just above the mined tunnel and extending 2m into the tunnel profile.

A value proposal from Bauer involving oversized secant piles with no internal ring beams was adopted for the actual build. GHD designed it in collaboration with Bauer and it called for 1,500mm diameter unreinforced piles, 1,200mm diameter structural piles, a verticality of 1:200 (twice the industry norm) and a structural thickness of 815mm at the pile toe. Concrete of 50MPa was chosen for the piles to maintain verticality. Bauer would check for verticality every 10m and if a pile was out of tolerance, the team would backfill and start again. This was necessary a number of times.

Mined Tunnel

The 75m-long mined tunnel, as opposed to the main TBM conveyance tunnel, connects the construction access shaft with the outlet shaft, however it is predominantly used for TBM logistics. Excavated from the construction access shaft, it consists of a fore shunt with a 4.5m clear-span profile to slide the TBM into, then a full-size 9m span for a 7m clearance envelope that can take three rigs to service the TBM. The back shunt is the same size.

The tunnel has very variable permeability of 4.7 E-09m/s and 9.2 E-07m/s and there are concerns that the water treatment plant will have leaks too, resulting in serious inflows. There are about 22m of hydrostatic head to deal with but it is designed as a drained tunnel, so there are drainage holes in the face to dewater the ground.

The tunnel was designed using a combination of Adeco and NATM. In the tender design, which was working under the impression of more competent ground, a steel rib and shotcrete lining was at the shaft breakout areas and just a bolted shotcrete support for the remainder of the tunnel.

With the change of ground conditions and the fact that it’s all the contractor’s risk, they ended up changing to a steel rib support with forepoles for the entire length. This was more than the JV tendered, so work was undertaken to develop some different support classes, hence the combination of design principles.

The majority of ground was expected to be Type B or C (see support classification table), however the contractor has since chosen to use the heavier Type D support throughout the tunnel.

TBM Tunnel

The TBM tunnel is lined with a precast segmental ring comprising six universal segments with a parallelogram key and counter key and four rhomboidal segments. Two types of ring were designed owing to the tight 80m radius curves at either end of the drive. The standard segments are 1,200mm wide with a ±10mm taper while the special segments are 800mm wide with a ±30mm taper. This was developed with Herrenknecht and VMT and there is a recovery radius of 70m possible.

During the curve the 12 TBM thrust rams are offset and there is a high eccentric thrust. While researching past projects, the team noticed that steel segments were common, but they wanted to avoid this.

Six Fama Superconnector dowels were used for the segments, these were 80kN capacity dowels and effectively the whole of the ten-ring system is acting to support the ring as the shield moves forward. Fama also supplied the cast-in gasket.

Outlet Shaft

At the end of the TBM tunnel there is the outlet shaft. This brings the water back to the surface and distributes it down the downstream distribution network, but if the water treatment plant needs to shutdown, the water goes up the 28m surge shaft. A bathtub at the bottom will contain any overspill (a 32m shaft is required to contain the full surge, but the team wanted to limit its height). Much as with the access shaft, the Bauer proposal for oversized secant piles was adopted to the outlet shaft, although this time with 880mm-diameter reinforced piles. The shaft depth was also reduced from 75 to 60m.

The piles were used as the foundation for the structure but it is actually quite narrow if you consider the stresses of a seismic event. It has a lot of benefits but a seismic event creates huge tension and compression in the piles. The national structural code of the Philippines (the NSCP) is actually inappropriate for intake and outfall structures as it is for the building industry and addresses high-rise rectangular towers. The shape modification factors for what is effectively a silo shape are onerous. Instead of this code, the team used USACE EM 1110-2-1806, 2104 and 2400. This is very highly developed for seismic design also, and the client was persuaded to adopt it.

The site-specific seismic assessment had a VS30 of about 300m/s with an operating basis earthquake has a return period of 144 years with a PGA of 0.47g. The outlet was classified as a critical structure because if it failed there would be uncontrolled water release from the reservoir. A seismic event can come from any direction and cause a high overturning load. The design of the outlet shaft had two zones in which the piles were debonded from the internal lining, separated by a bonded zone, to make the structure less rigid during a seismic event than the original design. This transfers loads down around the tunnel below.

Intake Structure

The water intake structure was designed to have an inner diameter of 16.5m and approximately 30m deep temporary cofferdam structure. The wall of the cofferdam is to be constructed using 54 clutched tubular steel piles (51 no. 914mm diameter piles and three 1,575mm diameter piles). At the location where the TBM is to be received, the larger 3 piles will be driven to elevation +57.0, while the remaining part of the pile is to be constructed as 1,493mm diameter glass fibre reinforced plastic (GFRP) concrete piles. Moreover, after each excavation stage a steel ring waler beam (for a total number of nine) is to be installed.

Due to the variable permeability of the surrounding rocks (some layers being connected hydraulically to the reservoir body of water) the basal heave and uplift pressures were deemed to be too high for the structure to resist without dewatering. As a result, 14 dewatering wells have been included for installation within the cofferdam. The contractor is currently assessing alternatives to optimise this temporary structure.

The Intake is a gravity based structure of reinforced concrete incorporating a docking station for the TBM at the end of its 7.3km journey. Once inside the intake, the shield of the TBM will be grouted through the specially designed tail skin and the contractor can remove the cutterhead via the intake tower. The remainder of the TBM can then be dismantled and removed back through the tunnel to the CAS. With this method, a secondary concrete pour completes the lining through the shield and forms the bell mouth within the tower.

Alternatively, if the contractor can get heavy lift equipment on the water, the TBM can be fully removed via the intake tower and the segmental lining extended into the tower.