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Novel sewer pipe installation method in Wellington, NZ

By Nick Wharmby, manager engineering, Derek Bilby, operations manager and Damon Winter, contract supervisor, Brian Perry Civil.

Construction of a 98m long link sewer with an invert level at between 4.5m and 7.5m below ground level within a roadway is not uncommon, but this project in Island Bay, Wellington, New Zealand had some novel features.

Traditionally the 1,050mm diameter pipes would have been laid within a steel sheet piled cofferdam or similar supported excavation and backfiled. Pipe jacking and micro-tunnelling could also have been considered. The site plan and a summary of the ground conditions are given in Figure 1.

The sloping site, in a sensitive urban environment on the edge of Wellington Harbour, has numerous buried services. Ground conditions consist of dune and beach sands, coarse beach gravels, greywacke outcrops and high tidal groundwater. There is a history of pipe installation difficulties in the area. The combination of these issues effectively precluded use of conventional trenching/cofferdam options, while micro-tunnelling would have required importing a specialist machine for the relatively small project.

Discussions with Bachy Soletanche lead to the investigation and subsequent adoption of its patented Pluss (Pipe Laying Under Slurry Soletanche) method; the project was awarded to Brian Perry Civil on this basis after a competitive tender process. While the method had been used on a number of projects in other parts of the world, it had not been used either in New Zealand or using conventional precast concrete pipes.

As with diaphragm walling, the Pluss method uses bentonite support fl uid during trench excavation.

The bentonite minimises fl uid loss into the ground by forming a thin 'fi lter cake'; this enables the application of a positive fl uid pressure to be maintained.

Figure 2 shows the system's methodology. Guidewalls, typically 1-1.5m deep, are built to support the upper soils and guide the excavation. Pre-mixed/hydrated bentonite support fluid is added to the trench as the excavation proceeds to maintain the fl uid at the required level, typically within 0.5m of ground level.

A support and guide system is fi xed to the pipes along with a cap.

The pipes are then lowered and suspended in the bentonite support fluid from the guidewalls. Pipe joints are made with the help of the guide, which makes the initial connection, and then by the application of a vacuum pressure inside which pulls the pipes together in the slurry.

The creation and maintenance of the vacuum also tests the integrity of the joint.

The bentonite support fl uid can then either be replaced or, as in this case, hardened with the addition of cement. This can be achieved by adding neat cement to the trench and mixing insitu or adding the cement at the mixer and circulating the material from the trench.

The hardened cement bentonite slurry properties depend on the locally available materials and mix design; typical unconfined compressive strengths in the range 150700KPa are achievable at 28 days with the addition of 180-300kg/m 3 of cement.

Once the bentonite cement slurry has cured suffi ciently it can be trimmed back and the road reinstated. Because the bentonite cement is of low permeability, the formation of permeable zones through the hardened slurry may be required to avoid 'damming' ground water flows.

The use of a bentonite support fluid to facilitate the construction of diaphragm walls and piles in unstable soils is common around the world although rare in New Zealand.

The nature and behaviour of bentonite support fl uid is fairly complicated and sensitive to site conditions. This beyond the scope of this paper; Hutchinson 1, Xanthakos 2 and Bariod 3 provide good guidance and experiences.

There are numerous project references over the past 30 years and, as testament to this and by way of example, the ICE Specifi cation for piling and embedded retaining walls 4 provides guidance on use and compliance testing of the material. Sitespecifi c issues such as contamination and use of additives require a greater level of understanding and experience.

Generally a trench will remain stable where the soils are normally consolidated and the bentonite fl uid level is maintained at 1-1.5m above groundwater level.

On the Wellington site, between manholes 1 and 3 (Figure 1), the excavation is adjacent to the controlled single-way traffic on The Esplanade.

Analysis of the trench, considering vertical and horizontal soil arching, was performed based upon the Huder approach 5 with an applied highway surcharge. The results of this analysis are presented in Figure 3.

The graph plots the 'active' soil and 'bentonite' fl uid pressures against depth; the latter is the 'net' fl id pressure after allowance for the groundwater pressure. From the graph it can be see that in the upper 1.2m, the 'active' is greater than the 'bentonite' pressure which highlights potential instability. As the guidewalls extend to 1.2m this is acceptable, provided the bentonite level is maintained within 0.5m of ground level.

32 Use of bentonite in New Zealand for ground support is extremely rare, so the high shear mixing unit and ancillary equipment for the project had to be put together in the workshop.

Cement silos and 90m 3 bentonite hydration/holding tanks were available. The small site area resulted in the mixer, cement silo and bentonite storage tanks being located up the Derwent Street hill; these were bunded to contain potential spillage (Figure 4). Site offi ces and the materials storage and preparation area were further up the hill.

The small, narrow site area, coupled with the overhead services, meant a 20t excavator was the biggest that could be accommodated; a small mobile crane was used to lift the 2.1t precast pipes.

This section only highlights aspects specifi c to the Pluss technique used on this project and associated issues addressed (Figure 5).

Services As with many urban trenching operations there were many services (stormwater, water, power, gas and telecom) crossing the line of the link sewer. The excavation to form the guidewalls (Figure 5a) provided an opportunity to locate and divert the shallow services.

The presence of severed services (pipes, conduits, etc) provided a flow path for the fluid bentonite with the potential for material loss, contamination and ultimately trench instability. All services, live or otherwise, needed to be relocated and sealed at both sides of the trench; this had to be completed in dry conditions to ensure a watertight seal.

Trench excavation Trench excavation was carried out under the bentonite support fluid and included the excavation of fill, dune sands and beach gravels (Figure 5b).

In the trench next to manhole 2, up to 3m of greywacke was removed. Excavated material was placed directly into sealed trucks and taken to a nearby landfill site where it was stockpiled for drying before final placement. While special measures could be required to dispose of fl uid bentonite slurry, incorporating the hardened material in the trench overcomes this issue.

The stability of the trench was good even though it remained open for several days. Regular testing of the bentonite support fluid was carried out. However, on the second section excavated there was an overnight loss of bentonite, resulting in a 1.5m drop in level and resultant instability of the trench below the base of the guidewalls.

The loss was not attributable to services but to a layer of course gravels and cobbles with no fines and a change in the bentonite properties.

Surface water on the bentonite support fl id, its general appearance and increased viscosity, suggested some fl cculation had occurred, resulting in higher filter loss.

The most probable cause in this case was cement contamination from the first hardened section.

Salt/sea water contamination was considered less likely. The addition of fi nes to the bentonite and implementation of procedures to maintain the bentonite support fluid level in the trench prevented the situation recurring.

Manholes and connections Manholes were made above ground and then lowered into a pre-dug, bentonite support fluid-filled excavation. After positioning, the hardening process was carried out as for the sewer pipe.

Connections to the manholes were readily made from inside, by breaking out a hole in the manhole riser and tunnelling through to the pipeline. The surrounding hardened cement bentonite slurry retained the unstable ground and prevented groundwater inflows. The pipe was positioned and the annulus between the riser and pipe section grouted to provide watertightness.

Hardened cement bentonite slurry Cement addition, as noted above, increases viscosity which increases the pumping effort required. This was particularly significant when pumping to the furthest point from the mixer (about 100m) as the addition of 220kg/m 3 of cement made the slurry barely pumpable and hence limited the hardened strength attainable.

The sampling of the slurry to enable UCS testing and thus verify the strength requires care to ensure it is representative of the material in the trench. Slurry from the mixer contains none of the excavated sand in suspension that serves to increase the strength.

Due to the flocculation that occurs in the upper 1m material in the trench, the slurry contains a lower solids ratio and thus will have a lower strength; this material should be removed before final reinstatement. Cement bentonite samples should be recovered at about mid-depth in the trench. Slurry samples taken from depths of 3-4m tested at 28 days achieved strengths of 220KPa.

The first use of the Pluss methodology in New Zealand incorporated its successful modifi ation to accommodate standard precast concrete pipes. After an initial learning curve the works progressed to meet the contract programme.

Successful completion of the link sewer, towards the end of 2004, in an area that has been historically problematic, is testament to the efforts of all involved.

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