Temperate rainforests are found near coastlines. The largest temperate rainforest stretches over 1,609 km along the coast of the Pacific Northwest from Oregon to Alaska in North America, while smaller temperate rainforests can be found in Chile, Australia, New Zealand, Norway, Japan and Great Britain. Because temperate rainforests are near oceans, they are wet and cool, and can receive up to 100 inches of rain a year.

Huckbody Environmental Director Andrew Huckbody has 24 years experience in environmental impact assessment management of large scale infrastructure projects across 26 countries. He was Environmental Manager on the 220 km Milford Haven to Brecon Pipelines Project in South Wales, which involved pipeline construction through temperate rainforest terrain of the Brecon Beacons National Park.

Getting it right before you dig

Temperate rainforests are often given high nature conservation and landscape value, which means that pipeline developers must adhere to the local legislation and regulations designed to protect the environment during pipeline construction.

For Mr Huckbody good project design processes during the pre-construction phase can limit costly complications that may arise during construction.

“When selecting the pipeline route, the topography of the region must be considered as many temperate rainforests have low spots, swampy ground or streams, which often require trenchless technology as a crossing technique,” says Mr Huckbody.

Mr Huckbody also says that it is important to consider ecology data before commencing construction work. “In general temperate trees have deep roots compared to tropical species that are shallow rooted. Tree roots, as a rule of thumb, extend to the edge of the canopy; however different temperate species have different rooting requirements which will impact trench alignment needs.”

The protection of animal pathways are also listed by Mr Huckbody as an important element of pre-construction design as the routeing and stringing of pipe and welding of lengths can block the passage of mammals.

Drainage is another important consideration for pipeline design teams. “The high rainfall in temperate climates can mean that even dry-looking sites can turn into swamps after two days of rain. Down slope drainage is key to mitigating the impact of this, with angled channels, or water diverters, cut across the right-of-way (RoW) to divert water off the spread.

“Reduced construction widths give rise to problems of soil storage and the phased construction of the pipeline, from stringing to ditching. In difficult ground conditions, constructing a running track is a balance between importing material to make a good running surface or using in situ materials. The balance is between time and resources versus shorter construction period, while there is always a risk of wet weather stopping work,” says Mr Huckbody.

Be water wise

In Mr Huckbody’s experience, pre-construction drainage is essential to successfully completing pipeline construction in temperate rainforest terrain.

“There are a number of issues which need to be considered when designing pre-construction drainage systems. For example, in the United Kingdom there is an extensive network of agricultural drainage, some originating from over 100 years ago.

“Project managers prefer to use pre-construction drainage in temperate climates to keep the spread as dry as possible, which often results in pre-construction drains being connected to existing field drains. However water flows downhill in undulating terrain and, if permeable drainage pipe is used, dirty water can get into the drainage system, and eventually into rivers via the field drains. This is extremely difficult to stop.

“The best solution is to make sure good pre-construction drainage is installed on the up side of the RoW – and generally never on the down side where dirty water enters gravel trench and slotted pipe, and makes its way into rivers,” Mr Huckbody says.

Down slope drainage of the RoW is an issue for pipeline construction teams as Mr Huckbody notes, “Even dry slopes can turn into torrents of mud after soil stripping and grading. Therefore it is necessary to put in and maintain grips at right angles across the spread, including the running track – even if you have to use pipes or rocks – as this slows the water down. Reduced velocity means less material is transported and results in cleaner water.”

Mr Huckbody says that when designing a pre-construction drainage system, it is important to consider the location of trenches, tie-ins, and bell holes, and if any tunnelling techniques will be used during the construction phase. It is also necessary to consider where water will be pumped from excavation sites. If excavation sites are situated in pre-construction drain areas, the dirty water will enter the rivers as the field drains discharge there.

“The rule of thumb when connecting drains is only to use non-perforated pipe within the RoW, as when the spread gets wet and muddy it will enter the drains and river,” he says.

He went on to say that it is also necessary to plan dewatering of all excavations during pre-construction design, which includes considering filters, pumping distances and soil permeability. “Poor planning can result in additional cost and time trying to pump water back up hill into filtration areas to keep it out of rivers at the bottom of slopes.”

To trench or not to trench

The decision to dig trenches or use trenchless technology can have a dramatic effect on the environmental impact of the pipeline construction, as Mr Huckbody reflects, “Any trenchless technique such as horizontal direction drilling (HDD), microtunnelling, thrust boring or pipe jacking is usually good for woodlands as it is generally designed to avoid tree roots. Trenchless technology allows pipes to be laid at such a depth that tree roots cannot interfere with pipeline integrity.

“As there is no need for padding or backfill material, roots and trees are unaffected by the construction process and the landscape and ecology remain intact,” Mr Huckbody says.

However, he is also quick to point out that trenchless practices such as HDD and microtunnelling are expensive and not without environmental risk from material spillage or breakouts of mud and drilling fluids such as bentonite and polymers.

“One example I have seen is where the design team had adopted a microtunnelling solution through an incised wooded valley of conservation and landscape value – to ‘keep the water clean’ – yet failed to realise that construction of the tunnelling necessitated a running track for side booms and other equipment – which required trees to be cut and the river disturbed to put in a flume pipe crossing bridge.

“What the design team failed to realise was that dirty water will always run down to the bridge crossing, resulting in dirty water entering the river. The net result in that project was the substantial cost of trenchless technique, plus a dirty river. In this particular case it would have been better to have open cut the river in two to three days and then reinstated it, allowing the river to be kept clean,” he says.

Gaining access

For Mr Huckbody, access is key in temperate rainforest areas. “Many large forests have limited heavy road access. Heavy plant access is a balance between constructing heavy stone roads throughout the length of the RoW or installing a range of temporary access roads, which can be reinstated after construction.

“The down side of building heavy roads along sections of poor ground is time and cost. It could take weeks to import hundreds of tonnes of stone and then take it out again. In contrast, constructing some temporary accesses that can be reinstated later is cheaper and easier, but requires negotiation with land owners and potentially has a higher environmental impact.”

He also notes that existing road infrastructure is important as weight limits on bridges and narrow roads through villages can create challenges in transporting plant to inaccessible places along the project RoW. “Careful planning for soft or low-lying areas is also important, as sidebooms weighing up to 85 tonnes require flume pipes and importation of geotextile material to ensure load bearing,” he says.

Finding room to move

Pipeline projects constructed through temperate rainforests often require a narrower RoW, which makes construction work difficult as grading, stringing, trenching, lowering-in, and pipelaying operations are all affected when space is limited.

“A narrow RoW can require inventive solutions, such as fabricating sections of pipe outside the RoW, which are then carried onto the RoW using booms to drop the pipe into place. Topsoil stripping is also problematic in narrower RoWs, as there may not be enough space to store topsoil and subsoil.

“In some instances topsoil has to be trucked out and stored in boxed-out sections along the pipeline spread, then brought back during reinstatement,” says Mr Huckbody.

He says that constructing in narrow RoWs requires careful phasing of the pipeline train, as simultaneous operations often prove impossible. “For example, stringing is sometimes locked out and restricted to boxed-out wider RoWs on either side of a narrow RoW.”

Good soil management ensures good reinstatement

“Topsoil is the key to successful reinstatement, as it contains the physical and chemical material, as well as the seeds and propagules to allow regeneration of the site,” says Mr Huckbody.

He says that good stripping and storage is necessary to ensure successful reinstatement. “Research into height limits of topsoil to prevent anoxic conditions and preserve seeds is extensive; however woodland soils are more robust and can be stacked 2 m or higher for shorter periods, avoiding water logging.”

When asked why topsoil was so important, Mr Huckbody says “Good topsoil prevents erosion, which enables greenery to be established as soon as possible after the reinstatement of the soil profile. To protect the quality of topsoil it should not be mixed with underlying poorer structure and low fertility subsoils.”

Choosing the right machinery for the job is also critical. “Backhoes instead of dozers may have to be used for careful stripping in undulating wooded sites with tree roots, or in areas characterised by patchy soils such as beech forests in Europe, which develop over thin rock soils.

“Backhoes are a better tool to use in this instance as dozers tend to dig deeper, picking up more infertile subsoil. In reduced-width sites, sheets may be needed to separate stored piles of topsoil and subsoil to maintain their integrity for reinstatement.

“If the potential for soil erosion is high, all stored soils can be sheeted, or grass can be sewn to stabilise the material so it is not lost during construction period.

“Sometimes double topsoil stripping is undertaken, where fragile habitats might suffer from a mixing of seeds through the whole 30 cm topsoil profile. This requires careful consideration, especially in exposed sites, where the thin seed layer can be eroded or blow away.

“I have worked with specialist machines that use modified guillotines on the backhoe to chop a large sod of intact vegetation and lay it back like paving. This is done to preserve valuable habitats and is an expensive method, but is sometimes justified,” says Mr Huckbody.