Embankment side-slopes over permafrost

One of the most perplexing problems, and one for which a suitable, economically viable solution is yet to be found, concerns the side-slope of an embankment over permafrost. When an embankment for a road or airfield is placed over permafrost, the thermal balance is always changed. If the

embankment is properly designed and constructed the permafrost table beneath the central portion of the embankment will remain at or above its original level. Often it is desirable to cause the top of the permafrost to rise into the non-frost-susceptible material of the embankment. Under the central portion of the road this can usually be done with insulation or adequate embankment depth. However, as the side-slope approaches the natural terrain, it does not have enough thermal resistance to protect the permafrost from the increased heat input caused by its bare surface. To compound the problem, during the winter, snow is plowed off of the main surface of the road on to the side-slope. This produces a much deeper insulating snow cover over this area and prevents the cold winter air from cooling and reinforcing the permafrost under this portion of the embankment. The result is increased summer heating and decreased winter cooling, and as expected, melting of the permafrost under the side-slope.

If the permafrost is ice rich, the side-slope slowly subsides into the thaw channel that develops under the toe of the slope. This causes a rotation of the side-slope away from the main body of the embankment and a loss of side stability. Longitudinal road cracks develop, and any structures on the side-slope or shoulder of the road will deflect outward.

Insulation has been used in an attempt to reduce the heat flow into this area, but without complete success. Smaller side beams have been constructed at the toe so that the insulation could be extended into this area. However, this is more expensive and also suffers from the same problem at the toe of the smaller beam. The problem has simply been moved to a new location and a wider influence zone has been created. Success with this technique has also been limited.

Experimental cooling ducts have been installed in beams along the toe of the embankment. This approach consisted of a lateral pipe (duct) buried in a beam at the toe of the embankment with a vertical riser to produce a chimney effect. As the soil warms the air inside the duct, the air rises up the chimney.

This in turn draws cold air from above the snow cover through the pipe inlet and discharges it at the riser. Performance of duct cooling systems has been satisfactory unless the airflow in the ducts becomes blocked in winter.

The soil temperatures in the vicinity of the air duct are constantly colder than in either insulated beams without duct cooling. Although the warming effect of the side-slope was not eliminated, it was slowed and therefore reduced the magnitude of the subsequent problems and maintenance, at least until the ducts became blocked. Ideally the ducts should be placed below the water level for maximum cooling of the permafrost, but practically, this is not feasible as the ducts fill with water and mud.

Words and expressions:

Side-slope Боковой склон

Beneath Под, ниже

Bare surface Неизолированная поверхность

To plow off (amer.)-plough off Зд. очищать

Shoulder Обочина

Duct Трубовород, труба

Vicinity Вблизи, поблизости

Magnitude Важность

Mud Ил