Bauer Foundations Philippines, Inc. (BFPI) has been working closely with its client the NovaBala joint venture, composed of CMC di Ravenna (Italy), First Balfour Inc. (Philippines) and Chun Wo Construction Holdings Co., Ltd. (Hong Kong), for the past year on the NBAQ4 project, one of the largest water supply infrastructure projects undertaken by Manila Water.

The NBAQ4 project entails the construction of a fourth aqueduct from the La Mesa Reservoir to the Balara Treatment Plants (BTP) 1 and 2 to improve the reliability and security of the raw water transmission system. The 7.30km long and 3.10m diameter NBAQ4 is the first tunnel to be constructed with a tunnel boring machine in Metro Manila.

BFPI's first visit to the site was to work on the tunnel boring machine (TBM) launch shaft. Bauer‘s scope on the project was to construct and demolish a unique extended guide wall followed by the installation of 22 x 1,500mm diameter primary piles and 22 x 1,200mm diameter secondary piles, the construction of the capping beam and then the bulk excavation down 45m to the slab level forming a 9m inner diameter.

The piling works were completed with BG30 and BG14 drilling rigs. The BG14 was used to a depth of 40m and the BG30 was used to complete the piles. Extended double cut tools were used to drill the piles due to a high strength concrete (7,000psi or 50MPa) and 1/200 or 0.5 per cent pile verticality requirement, i.e. over 45m distance 240mm or approximately 10in allowable deviation.

Bauer implemented a local drilling inclination system (DIS) on all piles to measure the verticality of each pile and - if needed - reduce the deviation and inclination by reaming to remain within the design tolerance.

A smaller drill rig, a UBW 08, was utilised to grout behind the secondaries to minimise water ingress into the shaft during the excavation phase of the project.

The TBM launch shaft is part of the new aqueduct system that will be capable of delivering 1,000 million litres of water per day to the treatment plants, ensuring the reliability and security of its raw water.

After the successful completion of the access shaft, BFPI was awarded the outlet shaft piling works in December 2019 and work on that section started in January 2020.

The outlet shaft consisting of 40 secant pile walls, 20 nos. of primary piles with Ø1,500mm and 20 nos. of secondary piles with Ø880mm and a drill length up to 29m for all piles using BFPI's bigger drill rig, the BG40 for the main drilling works and an 80t crane to assist.

The outlet shaft posed more problems than the launch shaft due to the reduced area available to work in.

The sequencing of the piles and the works had to be done in a way that allowed BFPI to continue drilling even during the concreting of the open piles to allow it to finish on time.

The actual works were again carried out with only 0.5 per cent insignificant tolerance of each pile.

This time for the construction of the piles 880mm diameter (secondary), double wall casings were utilised and had been installed by BG drill rigs in segments (4m maximum length of each segment) until pile toe. While construction of the 1,500mm piles (primary), single wall casing with a maximum thickness of 20mm and with a length of 6m installed with the same drill rig through the guide wall.

During casing installation, verticality was checked again with the DIS, a tachymeter inclination survey of a drilling hole. For each measurement, the drilling operation has to be interrupted for a short period desired depth for every checking of DIS should establish as proposed by BFPI with a cylindrical, heavy, weight measurement body, suspended on the auxiliary rope and is lowered into the borehole.

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