Page images
PDF
EPUB
[graphic]

GEOTECHNICAL CONSIDERATIONS

Oil at a temperature of 160° F was circulated

The Interior Mountain Route from Prudhoe Bay through it for 22 months. Instruments were set up to

to Edmonton extends for 1,738 miles, while the Arctic Coastal Route is 1,706 miles. Both routes traverse approximately 500 miles of continuous permafrost and a further 900 miles of discontinuous permafrost, as shown on Exhibit 8.

The main problem in dealing with permafrost is that where the finer-grained soils occur, they often contain large amounts of ice that, on thawing, can cause settlement, or make the soil unstable on slopes and promote erosion. Therefore, by means of photo-interpretation, soil sampling and laboratory tests, the study-route terrain has been carefully classified on the basis of landforms and their subsoil and ground-ice characteristics, and the probable behaviour of each type has been predicted. Muskeg areas have been classified in detail.

In normal climates, burial is the preferred method of installing pipe. Here, it was decided that wherever burial might threaten the safety of the pipeline on the environment, it should be avoided. To arrive at clear-cut decisions on above or below-ground installation, various theoretical studies, field work and laboratory tests were conducted, notably at a test site near Inuvik, N.W.T.

Inuvik Test Site

The Inuvik Test Site lies in the permafrost zone near the Mackenzie River delta, in finegrained, ice-rich, post-glacial sediments. It was established initially to demonstrate and test two preferred designs for above-ground construction; it also provided valuable data on the behaviour of thawed permafrost soil, on pipe behaviour and on heat flow in the ground.

A 2000-foot loop of 48-inch diameter pipe, half on timber piles and half on a gravel pad covered with gravel (berm construction) was constructed to simulate the design and conditions envisioned for above-ground pipelines (Exhibits 9 and 10).

measure temperature in and around the pipe, movements of pipe and supports, strain, vibrations caused by winds and seismic effects, various soil characteristics as outlined below, and meteorological data. It was concluded that pile-supported construction would perform satisfactorily in any areas of high ice content peramfrost.

Similar observations of the berm section indicated that continuous berm construction would have useful application provided that sufficient insulation was used under the pipe. Further study might show the continuous-berm method to be advantageous in permafrost areas of low ice content soils and with ample supplies of cheap gravel, and also in areas where surface crossings of the pipeline are of major importance. A system using intermittent gravel-pad supports combined with insulation might also have attractive possibilities. (To be conservative, design and cost estimates were based on the use of pile support.)

A separate below-ground test using 90 feet of 24-inch diameter pipe indicated that about 85 percent of settlement occurred almost immediately, further long-term subsidence being negligible. Of particular importance was the observation that despite high ice content, the permafrost in thawing did not become a slurry: the melt water escaped rapidly enough for the soil to retain some strength. Therefore, within limits that can be specified, slopes would be tolerated safely.

Loading tests on steel and timber piles provided preliminary design capability for vertical and lateral loads. Timber piles were considered suitable for most applications in the colder permafrost areas.

Thermal studies were conducted with a mathematical model programmed for computer calculation. Data from the terrain type studies described above were used to assign thermal and physical characteristics representative of the types of soil

along the route. Climatic data were considered, as affecting the temperature of air and soil surrounding the pipe. Predictions were then made of the size, shape, and behaviour of the thawed area in each soil type. From all this information, the sections where pipe could be safely buried were determined.

Final Choice of Routes

The two routes described in the study were chosen on the basis of the following desirable objectives:

a) shortest length,

b) greatest accessibility,

c) most favourable topography compatible with terrain stability and avoidance of existing communities and sensitive wildlife areas, d) minimum ecological and sociological disturbance.

A comparison of the alternative routes would indicate that although the amount of above-ground construction is about the same - 360 miles in each case - the Arctic Coastal Route is shorter, more accessible and more nearly level; the Interior Mountain Route avoids more ecologically sensitive areas and has fewer river crossings but presents more difficult construction problems.

The final choice of route is still pending.

Laboratory studies included verification and expansion of field studies. Soils were tested for permeability, shear strength, thaw settlement and thermal conductivity

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed]
« PreviousContinue »