Calculation methods for the construction of embankments and berths on permafrost

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Calculation methods for the construction of embankments and berths on permafrost

Construction of embankments in permafrost climates entails the need to solve a number of issues caused by thawing and freezing of soils during and after construction. Permafrost soils often have high ice content, including visible inclusions and ice lenses. When the phase state of the water contained in the soil changes, the strength characteristics of soils change, and they deform. In some cases, cohesive soils (clays, loams, etc.) completely lose their bearing capacity after thawing. Freezing of such soils leads to frost heave and affects the stress state of the structure.

Therefore, one of the issues to be solved when designing in permafrost areas is to determine the thermal state of soils in the backfill and in the embankment base.

Solving time-dependent issues on the change in temperature patterns under variable boundary conditions presents a certain challenge. This issue is further complicated by the presence of wet soil in the massif, and the presence within the massif of a moving boundary separating the melt zone from the frozen zone, where the phase change condition is set (Stephan problem).

Thermal calculations of embankments are very time-consuming and are carried out by trained specialists.

This is somewhat of a predicament for designers. They need to choose the location, the optimal structure and the construction method at the initial stage of design. To do this, it is enough to know two extreme states of the temperature pattern in the ground. The first state, for the period of construction of the structure and the first year of its operation. The second state, for the moment of time calculated by the service life of the structure. While the first state is known and specified by initial design data, the second state must be predicted, i.e. determined by calculation. Since the service life of the structure measures by decades, the quasi-steady ground temperature state in a large time range can be considered as the sum of the steady state (calculated for the average long-term temperatures at the surface of the considered area) and the non-steady state (determined by the periodic annual seasonal change of temperatures relative to the average long-term temperature).

The analysis of thermal engineering calculations of embankments shows that seasonal temperature changes at the outer boundaries of the studied area practically do not have any effect on the position of the boundary of frozen ground at the base of the structure and, therefore, such changes can be disregarded.

When setting the boundary conditions, one should proceed from the assumption that the ground temperature in flat terrain is conditioned by the average annual air temperature in this area, heat exchange conditions at the surface, and the geothermal flow from the Earth’s interior (i.e. the genesis of permafrost is neglected here, assuming that permafrost was formed under the impact of the above factors).

The conditions of heat exchange at the surface are considered taking into account the anthropogenic disturbance of the cover (caused by human activity), since the natural conditions of heat exchange at the surface may change after the construction of a structure. Quasi-steady ground temperature conditions observed in reality, with a degree of accuracy sufficient for practical purposes, can be characterized by the average annual ground temperature, equal to the temperature of the same ground at a depth of 10 m. When calculating the boundary of frozen ground at the base of a structure, this temperature (measured in wells or preliminary determined by reference data) should be taken as the boundary conditions applied to the boundaries of the studied area in contact with air.

A mathematical model for determining the boundary of frozen ground at the base of a structure is constructed by imposing additional boundary conditions (determined by the creation of the water area in front of the embankment) on this steady temperature background.

To solve this problem, a relatively simple method for calculating the thermal state of the ground was developed.

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