INTRODUCTION

In Ecuador, to support wastewater management improvements in Guayaquil, a coastal city and the largest metropolis in the country, a key piece of underground infrastructure needed was a 4km-long force main discharge line. The tunnel runs between the new pumping station (EBAR La Pradera) and treatment plant (‘PTAR Las Esclusas’). Inspection wells were also required in the dense urban area, which sits upon complex geology in a tidal environment, and the region is seismically active.

A further challenge on the project for the turnkey contract team – a partnership of Bessac and Soletanche Bachy Cimas – was the tight schedule to complete the DN1900 wastewater force main discharge line, which would have an operational pressure of 5 bar.

In the bid phase, the partners proposed an alternative solution after optimisation studies, considering the equipment and skills available to the team, the locality, the programme requirements and in trying to seek to minimise disruption to city residents. The choices were to use two micro-tunnelling machines (MTBMs) owned by Bessac, longer pipe jacking drives, and having multiple launch and breakouts at shafts in the difficult geology with high groundwater, although fewer shafts than project planners had envisaged.

The arrangement had steel pipes embedded inside the RC pipes, and they would be jacked between shafts on the long drives along the route linking the pumping station to the new treatment plant. The pipe assembly was prefabricated off-site, manufactured locally by the joint venture team.

The design and build contract was awarded in January 2019 and took a total of 32 months to complete, in the middle of 2021.

GEOLOGY AND GEOGRAPHY

Soils

The wastewater project is located in the estuary of the Guayas River and generally follows the river. Geological conditions are composed mainly of fine to very fine unconsolidated soils with many alterations of clay sands and silts. Peat lenses of great plasticity are also present.

Hydrogeology

The groundwater level is, on average, equal to sea level. The level of the ‘natural’ terrain (TN) is between +2m and +4m/TN. With its location, the area sees a tidal range of approximately 4m, which causes the level of the water table to vary.

The adjacent tidal environment in the coastal city also meant that the pipeline tunnel had to be designed for such an aggressive medium as salt water, especially the design of the final underground structures, such as the concrete pipe, to ensure its long-term corrosion protection.

Safety, Environment and Engagement

Adapting to local conditions was also a key part of preparing to undertake the turnkey contract. Factors to consider included industrial safety, the natural environment and community engagement.

Industrial Safety

A key issue for any project is industrial safety standards. In addition, the standards of funders, and that of team members are important fundamentals to be met. To satisfy these requirements, it is always necessary to ensure there is effective training.

Even where risks have been preliminarily assessed and consequently minimised early on, through the design process, it is essential to train employees to ensure they have the necessary skills, competence and safety knowledge before getting entry to an industrial site.

Natural Environment

Several deliberate choices were made to minimise the project’s impact on the natural physical environment. Wherever possible, the partners gave preference to locally produced consumables, such as for the jacking pipes. Other consumables, such as oils and grease, were biodegradable. A separation treatment plant for the slurry tunnelling filtered the water prior to discharge.

Community Engagement

Reducing the negative impact of the project on local communities was imperative as well as implementing many actions to support economic development.

The social management plan included information and awareness campaigns, monthly follow-up meetings, analyses of social and economic impacts on businesses and the implementation of actions to reduce their impact. The communication also give an immediate response to any situations with problems.

The design solutions also worked well for the communities:

  • The possibility of having fewer shafts and longer pipejack drives would also mean fewer road closures/ diversions and fewer other direct impacts.
  • Choosing to link the project’s construction sites to the power grid would eliminate the use of generators and therefore limit the levels of noise and odors.
  • Choosing equipment with good sound insulation for the local sites would also help to minimise potential for disturbances in the 24/7 works programme.

PROJECT AND CONSTRUCTION PLANS

Initially, during procurement and as presented in the tender documentation, the concept for construction of the force main sewer was to have 10 shafts (five to be launching shafts, five retrieving shafts) and for a total of 10 micro-tunnel drives to be undertaken.

The partners’ efforts to optimise the design and execution of the works took in many factors, including height restrictions caused by overhead high voltage power lines, discovery of existing pipes and removal of wells in addition to the other points mentioned – consideration of power source, equipment, skills and experience, and available local resources and security.

Planning

The project team came up with the following key characteristics for the project plan:

  • Decrease the total length of the structure by proposing a deeper passage under existing deep foundations
  • Invest in higher performance opportunities, including having wider shafts and longer intermediate stations (700mm elongation), and use of longer thrust jacks and high-capacity separation treatment plant.
  • Mobilise two MTBMs (type AVN2000) held in stock to operate on the 24/7 shift schedule
  • Undertake inhouse manufacture of hybrid steel/ concrete jacking pipes to enable both better production and quality control

Shafts and Drives

The proposed design solution significantly reduced the number of individual structures to be excavated on the project and required fewer micro-tunnel drives: only five shafts were needed in the end, in total, and five tunnelling drives; there was also a different choice of pipe lining, which enabled single pass construction.

Given the fewer shafts, the micro-tunnel drives would be longer. However, by going deeper with the alignment, the force main tunnel could have its total length reduced by 100m compared to the original plan. Alternative proposals were allowed for during procurement in the Request for Proposals (RFP), and to have equipment stock also helps in developing price and schedule advantages, and flexibility in equipment modifications.

Four of the proposed MTBM drives were to be more than 750m long, some notably so. The partners’ alternative solution also envisaged an open trench for recovery and connection to the treatment plant.

In more detail, the five shafts and drives linking them were as follows:

Shafts

The shafts had had these features:

  • Shaft No P1: 9.90m diameter shaft with slab bottom at -14.80m/TN
  • Shaft No P3: 7.00m diameter with slab bottom at -15.80m/TN
  • Shaft No P4: Micro-tunnelling machine #1 launch shaft, slab bottom at -10.60m/TN
  • Shaft No P5: 7.00m diameter with slab bottom at -14.40m/TN
  • Shaft No P6: Micro-tunnelling machine #2 launch shaft, slab bottom at -20.60m/TN

MTMB Drives

Micro tunnel drives had the following characteristics:

  • Drive from Shaft P1 to Shaft P3: a 345m-long MTBM drive, which included a 450m-radius minimum curve and slope of -0.5%
  • Drive from Shaft P4 to Shaft P3: the MTBM drive was 1120m long with a minimum curve of 1100m radius and slope of -0.5%
  • Drive from Shaft P4 to Shaft P5: the length of the MTBM drive was 890m, minimum curve on the bore was 800m radius, and slope was -0.5%
  • Drive from Shaft P6 to Shaft P5: 750m-long with a minimum 450m radius curve and slope +1.0%
  • Drive from Shaft P6 toward the treatment plant reached an open trench, used for MTBM recovery and also connection of the pipeline to the plant. The drive was 860m long with a minimum curve radius of 1100m and variable slopes, from -1.0% to +6%, and a vertical curve of 1550m radius

Pipe Production

To comply with the construction schedule and operational needs, the partners chose a hybrid design of 1400No steel/concrete jacking pipes.

This one pass system was alternative to the conforming design that envisaged long jacking drives with fibre glass reinforced pipes, which would have thinner walls but, for the partners, this presented concern under high thrusts. Another option would have been a double pass lining operation but that would have been more expensive.

In the alternative, hybrid, design of the jacking pipes, as selected by the partners, the steel pipe had to be installed inside the RC pipe. Steel sheets were prepared abroad to avoid long and costly welding at the local plant. The sheets were rolled and welded, and the welds quality tested. Random samples of the rolled pipes were pressure-tested on a test bench.

Manufacture of the reinforce concrete pipes was strongly impacted by geological and meteorological conditions of the area. The concrete mix design and casting conditions had to satisfy these criteria:

  • Achieve early high strength of 15MPa after only 6 hours, while remaining within US ACI temperature criteria
  • For strength and durability in salt water, have the water/cement (W/C) ratio low, at W/C<0.4, and provide extra concrete cover to the steel rebar cage, of more than 6cm
  • Enable the casting temperature to be less than 33°C while ambient temperatures outdoors were up to 35°C. This was achieved with a chiller and cold-water irrigation of aggregates

As noted, the partners decided to manufacture the hybrid steel/concrete jacking pipes inhouse to control quality and production time, and the choice also ensured involvement in the local economy.

 The rebar cage for the RC pipe was welded with an imported MBK machine and local personnel trained in its use. For concrete casting, the steel liner was also inside the mould to be integral in the pipe manufacture. An interior concrete lining onto the steel liner completed the hybrid pipe.

The RC pipes would experience the jacking force and their ends butted together. The internal steel pipes were slightly shorter and the resulting gap at each join would have mortar infill. Then the final welding would seal the steel pipes into a continuous steel lining, capable to taking the operating pressure of the live wastewater system.

PIPE JACKING DRIVES

While the tunnel drives have complex geometry, in that their horizontal and vertical alignments have curves and counter-curves, the modified route had advantages of being shorter overall. Tunnelling could also benefit from the partners’ experience with long MTBM drives and having a large stock from which to adapt and modify equipment as necessary.

But the very low cohesive soils were a challenge for the reactions of micro-tunnelling machines as the terrain offered only limited lift.

GEOLOGICAL VARIABILITY CHALLENGES

Analysis of boreholes had revealed very low standard penetration test (SPT) values (about 3-5) in sections of the project, and this presented risk from the very loose soils present and their low load bearing capacities.

The complication for tunnelling was that the machine, under its own weight as its worked and pipe jacking was underway, could progressively sink slightly with consequent effects to the vertical alignment. The phenomenon is more of a problem for upward vertical curves.

Potential problems arising from this geological peculiarity were resolved by adding a central cutting tool to the MTBMs and how excavation operations were undertaken. The aim is to lighten the front of the machine and also have the operation of slurry tunnelling avoid clogging, to help upward steering of the front of the MTBM.

Additionally, confinement pressures had to be managed to avoid settlement and heave problems.

The geology meant guidance of the tunnel drives would be a challenge even for experienced pilots. A surveyor specialised in MTBM works took charge of the works, including the specificities of the gyroscope guidance system and surface monitoring.

Slurry Management

Also due to the geological variability, there was particular need for very effective and adaptable slurry flow management during tunnel excavation. Passage through sandy areas required an immediate switch of drilling fluid to a bentonitic fluid to confine the ground and avoid potential settlement and ground loss. Meeting clayey soils would make the density of the fluid increase quickly.

With 90% of content being fines (< 80 μm) in the clayey portions, the drilling fluid would be rapidly contaminated as the standard screening equipment (grit remover and/or desilters) would not be able to treat these types of fines. Therefore, the density of the fluid would rapidly increase, making it unusable.

A separation treatment plant was installed near the hybrid pipe manufacturing plant to gain the most benefits in construction logistics. The plant had a centrifuge giving capacity to treat up to 30m3/hour of slurry, sufficient to extract the ultra-fines in these treatment activities which were key on the critical path of the programme.

Up to 400m3 of sludge per day was treated and, to do so without slowing the tunnelling works, the common slurry treatment site for both MTBMs also had two lagoons (1000m3 and 500m3). The arrangement was able to cope with rapid changes of mud for tunnelling and minimised the amount of equipment needed to be located within the urban area. Treated water was reused during excavation or elsewhere on the project.

Lubrication

Behind the machines, the jacked pipes had lubricating fluid on their external surfaces, to reduce friction from the changing geology. Performing effective management of the fluid injection was also vital.

Strategies to Shaft Exits

In saturated sandy soils, small water leaks can present high risk of triggering erosion near weak zones or main-made structural voids. Then, the initial erosion can worsen with washout and bring risk of failure, possibly to reach the surface.

With the tunnelling works passing through saturated sandy layers to bring the MTBMs to the arrival shafts, and then to penetrate the walls, a risk of uncontrolled inflows would be present under the hydrostatic pressure of the water table. An exit methodology had to be adopted.

The principle was to control the pressure in the shaft such that the MTBM could safely enter with risk of erosion and instability. To do so, the approach was to have pressure inside the shaft to be equal to or slightly more than the hydrostatic pressure outside, coming from the water table.

On this project, options to provide the extra pressure in the shaft for the arrival of the MTBMs included:

  • Flood the shaft: fill the shaft with water to match the external hydrostatic pressure from the water table. The machine breaks out to be immersed in the shaft. Blocking injections are made from the tunnel to ensure water tightness, after which dewatering of the shaft enable the machine to be extracted clear of the tunnel and lifted from the shaft. But the whole cycle needs repeated for each follow-on module behind the front of the MTBM.
  • Use a steel reception bell: the MTBM exits into a metal enclosure filled with water and pressurised to match or be slightly more than the hydrostatic head of the water table. After the bell is loaded, the follow-on pipe immediately behind is injected around with polyurethane (PU) foam. As soon as the seal is guaranteed, the pressure in the bell is released using drain valves. The chamber can then be opened to lift out the first module, and the process repeated until the entire machine has been removed.
  • Exit seal installed by divers: Installation of a submerged outlet seal after the machine perforates the wall and enters the flooded shaft. Divers then place an exit seal on the machine. The seal is plated with a metal square to guarantee the future sealing during the exit. Therefore, the water level in the shaft can be lowered and the MTBM can be removed without having to refill the well.

Breakthrough options for consideration depend on team experience in the different methods, local availability of resources, and the scale and number of the – potentially repeat – operations. For this project, local availability of divers and the need for repeated, multiple retrieval operations led to the 3rd option being selected – the exit seal approach.

COMPLETING THE PIPE

Following completion of tunnelling and then grouting of the annulus around the hybrid pipes, and mortar patching for continuity of concrete protection over the sheet metal, the embedded steel pipes were welded together. In total, approximately 17,000 linear metres of welds were manually performed in confined spaces in this last major step of the construction work.

The water tightness of the discharge line was finally successfully tested at a pressure of 7.5 bar.