Here one of the authors, James Thomson, provides a section reviewing the installation of marine wastewater outfalls using trenchless techniques including traditional tunnelling, microtunnelling and horizontal directional drilling (HDD).

The following is part one of an edited extract covering the use of microtunnelling; part two will cover HDD and will feature in the April edition of Trenchless International magazine.

Introduction

Historically, outfalls using tunnelling methods have been used where traditional installation methods were not viable because of the depth or ground conditions. The technical advances in the last 20 years, in both microtunnelling and HDD equipment and techniques, have now made their use viable for outfalls.

These methods have some inherent advantages:

The environmental impact is greatly reduced; the installation takes place underground so there is no disturbance to the beach, the approaches or the seawater. A number of outfalls located in environmentally sensitive areas have successfully used trenchless methods where installation from the surface would not be acceptable.

The impact on the community and the infrastructure for the most part is eliminated. Roads, beaches and shipping can continue in normal use without being aware of the work going on underground. The impact to the seabed is confined to a localised area where the tunnel or bore emerges and where diffusers have to be installed.

The location of the launch site for the outfall is much more flexible. For tunnelling and microtunnelling, the work is undertaken from a shaft that can be sited to minimise impact. The site can be back behind the foreshore and often on top of a cliff. Although HDD requires a greater amount of space for the equipment than the other two methods, the equipment is set up on the surface with small shallow excavations and the time of occupation is relatively short. The launch site is often located back from the shoreline and frequently on top of the cliffs.

The design issues for trenchless installations are reduced. No longer is it necessary to design for wave forces and currents and seismic concerns are reduced as the pipeline is an integral part of the soil structure.

Concerns about erosion or accretion of seabed material around the pipeline are eliminated. Scour is commonly found not only on outfalls laid directly on the seabed, but also to pipes laid on supports where, over time, support is lost and settlement occurs. Settlement can lead to excessive bending and failure of the pipe.

Pipe is installed well below the seabed which presents fewer risks. There is no concern about the risk of losing surface protection to the outfall pipes or ensuing damage by natural forces or from dragging anchors.

Trenchless methods are less affected by bad weather conditions. The only weather concerns are for the short period during the final breakthrough and when installing the diffuser section. The ability to continue working during gales and high seas greatly reduces the cost and the risks. For traditional installation, particularly in exposed locations, weather conditions can delay the program and accumulate significant cost when plant is standing idle.

Surf zone construction. The most difficult, disruptive and expensive part of an outfall installation can be the surf zone where major temporary and permanent piling is needed. Trenchless methods can carry the line out to deeper water where offshore craft can operate and traditional installation can be used. This can be an economic and less disruptive solution.

It is in installing the sections from the land to the deep water where microtunnelling and HDD have found a key role.

Microtunnelling for installing outfalls

In the last 20 years, microtunnelling methods for installing pipe diameters have become accepted practice for pipes ranging from 300–3,000 mm and sometimes greater. Microtunnelling has displaced traditional tunnelling in diameters up to 2.5 m as it is safer, quicker, cheaper and provides a better finished product.

The use of intermediate jacking stations and efficient lubrication methods now enable drive lengths of 1 km and greater to be achieved for larger diameters. The other key development that has made microtunnelling a viable method for outfalls and intakes is the ability to install both vertical and horizontal curved drives.

For outfalls, microtunnelling is normally employed in diameters between 1.2 and 2.5 m, although smaller diameters the length of drive is limited. The upper limit on the drive length is more a question of economics than technical limitation, although the nature of the soil conditions play a part. A practical maximum drive length would be around 1.5 km. One major advantage is that depth of installation and working under a body of water is not a limitation. Standard pressure balance microtunnelling machines are designed to work to depths of up to 100 m of head. Machines can be modified to provide even greater capability.

The first intakes and outfalls were installed by microtunnelling in the early 1990s and subsequently there has been an increasing use because of the advantages it offers over alternative methods.

Some specific issues can arise when designing an outfall. Figure 1 illustrates some possible scenarios for installation of an outfall from a shaft located on a cliff top. It is assumed that the outfall will be installed with a vertical curve and terminate in a reception area where the diffusers are to be installed.

To recover the TBM, it is necessary to install a bulkhead equipped with a valve in the end of the line. A reception pit is created by a dredged excavation in the bed of the ocean into which the TBM is driven. Closing the bulkhead allows the TBM to be jacked off the pipe and independent of the pipeline. Divers can connect the machine so it can be recovered onto the barge. The remainder of the work is completed using traditional ocean-going craft and equipment. For deep installations, however, a reception shaft may be created in the ocean to allow the TBM to be recovered; it also acts as a riser shaft for the outfall.

Figure 2 illustrates recovery from a seabed excavation. This ability to recover the TBM from a dredged excavation is a major advantage over traditional tunnelling methods.

Case studies

The following case histories illustrate a range of outfall constructions by microtunnelling.

The Europipe

The German Europipe was completed in 1995 and was the longest single length of pipeline ever driven by pipe jacking. It was not an outfall but a landfall for gas pipelines coming from Norway’s North Sea oilfields; however, the nature of the tunnelling was virtually the same as for an outfall. An underground solution was required as this was an environmentally sensitive shore area and a national park. Soil conditions along the alignment consisted of saturated sands, clays and peat. Figure 3 illustrates the variable soft ground marine deposits. There is a large tidal range, at high tide the hydrostatic pressure is a potential 1.5 bar.

The Europipe was driven from the shore out under the seabed to a reception shaft some 2,600 m away. The pipes that were jacked to form the line were of 3 m internal diameter. The tunnel was located at some 6–8 m below the surface and was driven on a vertical curve. Working 24 hours a day and six days a week, it took just over 100 working days to drive the 2.6 km, an average of 25 m per day.

Horden Outfall UK

The Horden Outfall is an interesting example where the advantages of trenchless and traditional construction methods were combined to provide a cost-effective solution.

Horden is located on the northeast coast of England. To meet the strict European community regulations on the discharge of wastewater to the sea, the local water authority, Northumbrian Water, designed a scheme involving a new treatment plant and a 1.8 m diameter outfall into the North Sea. As the site for the new treatment plant was located 42 m above the beach on a cliff top, and the adjacent land was a protected environmental area, trenchless installation was required.

Horden was one outfall out of four in the package. The design and contractual arrangements for the whole works were based on a three-way partnering arrangement of client, consultant and contractor. A core team, with members from all three parties, was responsible for developing the overall design and determining the cost targets. The solution accepted was based on microtunnelling a 1.8 m diameter concrete pipe. This involved a curved drive 550 m long. The first section out of the shaft was about 180 m on a downward gradient of 1:7. This was followed by a vertical curve of 1.2 km radius to a point 300 m from the drive shaft. The final 250 m was on a 1:286 downward gradient terminating in a pre-dredged channel cut into the seabed.

The TBM was driven into a dredged channel that had been excavated by the jack up dredger. All the flow and service lines to the TBM were removed and the machine was sealed using the in-line air locks. The use of two temporary pressure-equalising pipes allowed the TBM to be pressurised to the high tide pressure calculated to be 1 bar. The rear of the machine and the end of the tunnel were sealed using an inflatable air bag. The tunnel was then flooded with a head of 10 m equaling the high tide level. The TBM was then jacked off the pipes. Divers attached straps to allow the crane on the lifting barge to raise it and suspend it below the craft. It was then taken to a quay where a heavy crane was able to lift the 50 tonne machine on to a low loader.

The remaining section of the outfall was a 711 mm steel pipe extending 1.3 km further out to sea, terminating with four risers each containing four ports. These were constructed in the traditional manner using offshore craft for dredging a trench and laying the pipe.

Marbella Outfall, Biarritz, France

There were a number of constraints on the design and installation, not least being the potential negative impact of the work on tourism and residents. A second constraint was to maintain the existing outfalls during the construction. The third consideration was the difficult weather conditions that prevail at certain times of the year.

The solution developed was based on microtunnelling a new 1,600 mm outfall from a shaft set well back from the beach and the frontage property extending 780 m into Marbella Bay.

It was possible to locate this shaft so that the two existing pipelines carrying the effluent from the treatment plant, together with line carrying storm water, could be intercepted and diverted into the shaft and the new outfall.

The longitudinal profile has a 12 m diameter shaft 20 m in depth. Provision is made within the shaft for de-sanding and deodorising as well as energy dissipation for the high-level incoming lines.

The geotechnical conditions encountered were variable with sand, limestone and clay marl.

A pressure balance slurry shield was adapted for this project. The head combined both roller cutters and picks to deal with the heterogeneous conditions and with relatively large openings to allow the excavated material to pass into the shield. This shield is equipped with an air pressurised access chamber, which allows man access for changing worn face tools. A unique feature was the provision of a rear section to the shield, which was designed to seal off the tunnel and allow the front sections of the machine to be decoupled by divers and removed from the sea bed exit point.

The shield and the installation of the diffuser were recovered using a jack-up platform equipped with a drilling rig and a crane with a grab to excavate down into the marl layers below the sea bed.

Divers recovered the microtuneller and placed the diffuser within the dredged channel excavated in the sea bed.

Conclusion

Every installation and site will have its own characteristics and limitations and these will determine the most effective and economic method.

It is becoming apparent that trenchless methods can, in a number of situations, provide the best solution often in combination with traditional outfall installation methods.

Stay tuned for Part two of this extract, to be featured in the April edition of Trenchless International magazine.

James Thomson C.Emg.Ing

Email: .(JavaScript must be enabled to view this email address)

He is the author of several books and nearly 100 technical papers and has acted as Expert Witness in a number of disputes.