Elevated and Buried Sections of the Alaska Pipeline
Soil samples were taken from locations along the proposed route of the pipeline. Following are examples of the pipeline design to cope with these soil conditions.
1. Thaw-Stable Soil
This soil condition allows the pipeline to buried in the conventional manner, consisting of digging an 8' wide trench to a depth of 8-16’, and infilling with layers of non-sharp fines, gravel and fine loam, buffer board packers being used as required.
The pipes are laid on top of this with sacrificial zinc anode ribbons laid alongside them. As well as being an anticorrosive measure, these zinc ribbons also act as telluric current conductors, returning any electric current picked up back to earth.
2. Thaw- Unstable Soil
If this permafrost is disturbed by heat subsidence will occur, so the insulated pipeline is installed well above the soil, on specially developed supports.
In hot permafrost areas where the heat may also cause subsidence and affect the pipe supports an additional method is employed to keep the permafrost from melting.
This consists of inserting two pipes into the permafrost containing a solution of anhydrous ammonia that vaporizes in the permafrost. The vapor rises up the pipes before condensing in a heat exchanger integral with the vertical support leg, maintaining the ground temperature below the air temperature.
3. Special Soil Conditions
i. In permafrost soils where for various reasons the pipeline was required to be buried, the pipes were insulated to a thickness of 4” and buried in the normal manner.
ii. Some locations required the pipeline to be buried in a refrigerated trench; here the pipes are insulated and buried as above. Brine refrigeration is circulated around the trench through 6” pipes to maintain the condition.