Inspection of Figure 33 in this Materials & Construction (M&C) Defects Chapter illustrates a situation where a pipeline needs to cross below an existing interstate highway, as well as an adjacent secondary road. Comparable situations to this could involve crossing below an often travelled rail-line or navigable river, or other water bodies like wetlands (bogs and swamps) and lakes or rivers, or in the northern latitudes tundra. Hills and mountains also can be barriers that must be traversed. This Primer presents images of such barriers to illustrate some usual approaches to manage these obstacles in cross-country as well as urban settings. In closure, links to videos posted on external websites are provided for those that seek additional images or details concerning the construction of transmission pipelines.
40” (1016mm) diameter crude line in a shot-rock trench
40” (1016mm) diameter 230’ (70.1m) span arch bridge transporting crude oil
Horizontal barriers to the usual cross-country construction process range in scale depending on the nature of the barrier. Some are linear and run the width of the related easement or corridor. Such barriers include interstate highways as well as secondary roads, rail-lines, and navigable rivers and canals. Other horizontal barriers lack well defined boundaries, the scale of which might depend on the climatic conditions. Such barriers include water bodies such as lakes and wetlands, whereas in the Arctic latitudes tundra can be encountered, and the water ranges from its liquid state, to solid ice, to flowing ice, depending on the season. The challenges faced when dealing with Arctic conditions transcend all aspects of pipelines from permitting and routing through abandonment and are well documented elsewhere in the context of projects built in the Arctic [e.g., 1] and so not addressed herein.
Horizontal barriers that were overcome before the advent of horizontal bores and directional drills typically adapted the open-cut practices used elsewhere along the spreads, with bridges and early boring concepts used only where the other practices were inadequate [e.g., 2]. The next several images illustrate a typical water crossing, a pipe bridge, a concrete-coated pipe-string that will traverse a bottomland floodplain, and a view of such concrete-coated pipe that remains uncovered in a river crossing.
Traversing wetlands such as bogs and swamps using usual open cut construction requires unique practices. One approach involves the use of large timber mats that are laid out ahead of the construction equipment to create a stable working platform. The mats act to spread the weight of the construction equipment over a broad area, acting in analogy to a snowshoe. This and other adaptive practices facilitate the use of heavy equipment under otherwise unstable circumstances.
Such adapted practices have where feasible largely been supplanted by the evolving schemes used today in cross-country as well as urban construction, which include horizontal bores or ramming methods, was well as horizontal directional drills. Key in such applications is understanding and as necessary managing the subsurface geology – and as well in the case of urban construction the presence and location of the utilities. Three-dimensional (3D) maps can be developed as necessary by vertical core sampling, and/or other more sophisticated methods for the cross-country construction whereas the analog to sewer-pipe construction coupled with sophisticated tools like ground-penetrating radar and others are deployed in urban applications. Reference 2 traces the evolution of such practices. Reference 3 presents an updated view, with the images that follow being provided to supplement that mid-1980s document and illustrate some of the circumstances that can be problematic.
Urban construction is tedious as compared to cross-country practices. Once all existing buried facilities and utilities are located, the upper layers such as pavement must be cut and removed, followed by exposing all existing facilities and utilities, after which the trench is machine-cut, which then must be shored with trench boxes as the trenching advances incrementally forward. Sewer-pipe type construction then follows, joint by joint, using usual practices. Thereafter, the urban process is much like the cross-country process, except that the trench boxes must be removed, and other aspects unique to the urban process must be addressed.
Bores and directional drills are similar in some ways, but differ significantly in the nature of the crossing primarily in terms of its length, the equipment used, and in the process used for the crossing. Both bores and drills involve preparatory steps, with the setup for the drills being more involved than for the bores.
Shorter crossings are accomplished often by boring a hole through which the pipe passes. Pits are prepared either side of the obstacle after which the boring machine drills a hole under the road or other barrier to allow insertion of the pipe-string needed to bridge the gap. In some instances a casing is first installed in the hole, with the pipe-string needed to bridge the gap then inserted inside the casing. An alternative to a bore relies on a similar setup, but in lieu of the bore a prepared pipe segment is jacked or propelled by impact through the soil along the path of the pipe-string needed to bridge the gap. Impact loading while practical for some crossings can be problematic depending on the local circumstances.
Longer crossings and/or those with other than a straight route are made with directional drills. This method is feasible for crossings where the subsurface geology facilitates drilling and will sustain an otherwise unsupported tunnel, which means that formations involving rock and sand need to be avoided. If the subsurface conditions are viable, entry and exit points will be located that are consistent with the pipeline’s routing with sufficient access and space to setup for the drill. Thereafter, the crossing path and profile are established to avoid existing subsurface infrastructure while completing the crossing. After setup a drill is created in three steps. First a pilot hole is drilled, with the lead end controlled to track the desired path from the entry pit to the exit pit. Second, on pull-back of the pilot string the size of the hole is enlarged to sufficiently permit the insertion of a pipe-string sized to run end to end along the subsurface path. Finally, preceded by a bullnose the pipe-string is pulled back through the hole. While the drill is in progress the line pipe sections needed to complete the crossing are strung-out on the side of the crossing opposite of the drill rig. These are welded, inspected, coated using usual practices after which the string is hydrotested. Thereafter, it goes onto rollers (as opposed to skids) in preparation for being pulled back through the drilled out hole. Once pulled back, the cutting head is removed and the drill string tied-in to the pipeline on both ends.
Below is a video that animates this process. Similar videos can be found by a webcrawl using keywords such as ‘directional drilling video animation’.
Vertical barriers like hills and mountains differ in vertical scale and gradient, and also in regard to the geological circumstances that underlie the barrier. Illustrated are two facets of such construction – steep hills and hills with significant side-slopes both of which must be managed. The methods used in usual open cut construction depend on how steep the gradients are and the nature of the geology. Shallow gradients do not pose issues, whereas position stability when the gradients get steeper can be managed in part by capitalizing on the over-bend at the top of the hill and by the use of a yo-yo practice wherein down-slope loadings on the active side of the hill are countered by a cabled interconnection to a counterweight (e.g., bulldozers) setting on the opposing downslope. Side-slopes get managed using terracing and cuts, with shoring used as needed to stabilize the terracing (e.g., ). Where rock is encountered favorable routing is adopted with the trench either machine cut or shot using explosives, with bedding and padding used to avoid damage to the pipeline, or Rock Shield or similar products deployed to protect the pipeline. Machine bends are made in the right-of-way (RoW) as needed to accommodate directional changes.
Geographic barriers have become common place as reservoirs and supply points have become increasingly distant, forcing pipelines into RoW that traverse roads, bodies of water, and hills and valleys. As noted, Reference 1 provides insight into the many related aspects for Arctic construction. Over the years methods have been developed and refined to better negotiate such barriers, with the bores and drills emerging as best practices to avoid upsetting the environmental status quo.
- Anon., Arctic Gas Construction: Impacts on Northern Transportation Infrastructure, ProLog Canada, January, 2003.
- Hosmanek, M., “Pipeline Construction,” U of Texas Petroleum Extension Service, 1984.
- Anon., “Building Interstate Natural Gas Transmission Pipelines: A Primer,” INGAA Foundation Report 2013.01, January, 2013.
- Anon., “Mitigation of Land Movement in Steep and Rugged Terrain for Pipeline Projects,” INGAA Foundation Report 2015-03, April 2016\