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Thermal resistivity soil

The thermal resistivity of the soil has a significant impact on cable ampacity of buried cables, especially when directly buried without backfill. The lower the thermal resistivity, the higher the cable ampacity for the same conductor temperature $\theta_c$.

If the thermal resistivity of soil is unknown, the values from the reference operating conditions in IEC 60287-3-1 are recommended. Or else, the IEEE Standard 442 provides values for some materials and a paper by O.E. Gouda from 2010 lists values for some tested type of soils.

The table lists the thermal resistivities calculated from the thermal conductivities of typical soils surrounding pipelines as given in the Subsea Engineering Handbook by Yong Bai and Qiang Bai, 2012.

The following table is taken from IEC 60287-3-1 ed. 2.0

Soil conditionsWeather conditions$\rho_4$
Very moistContinuously moist0.7
MoistRegular rainfall1.0
DrySeldom rains2.0
Very dryLittle or no rain3.0

Symbol
$\rho_4$
Unit
K.m/W
Used in
$d_{im}$
Non-isothermal earth surface imaginary layer of soil [m]
$D_x$
Characteristic diameter daily load [mm]
$\delta_{soil}$
Thermal diffusivity soil [m$^2$/s]
$\delta \theta_{e,t}$
Transient temperature rise outer surface [K]
$\Delta \theta_{kp}$
Temperature rise by object k [K]
$\Delta \theta_{kp,t}$
Transient temperature rise outer surface by object k [K]
$g_{dry}$
Geometric constant of circle drying zone [p.u.]
$H_{tc}$
Parameter Hc trough cover
$k_4$
Thermal conductivity soil [W/(m.K)]
$R_{q22}$
Thermal resistance 22 multi-layer backfill [K.m/W]
$R_{q32}$
Thermal resistance 32 multi-layer backfill [K.m/W]
$T_{4c}$
Thermal resistance backfill correction Neher [K.m/W]
$T_{4d}$
Thermal resistance daily load cycle [K.m/W]
$T_{4db}$
Thermal resistance backfill correction [K.m/W]
$T_{4iii}$
Thermal resistance ambient [K.m/W]
$T_{4piii}$
Thermal resistance pipe—ambient [K.m/W]
$T_{4ss}$
Thermal resistance steady-state [K.m/W]
$T_{4w}$
Thermal resistance weekly load cycle [K.m/W]
$T_{4y}$
Thermal resistance yearly load cycle [K.m/W]
$T_e$
External thermal resistance of tunnel [K.m/W]
$T_{tr}$
Thermal resistance trough [K.m/W]
$v_4$
Ratio thermal resistivity dry/moist soil
Choices
Materialmaxmeanmin
Peat (dry)5.882
Peat (wet)1.852
Peat (icy)0.529
Sand soil (dry)2.3261.7861.449
Sand soil (moist)1.1491.0470.962
Sand soil (soaked)0.5260.4630.413
Clay soil (dry)2.8572.2991.923
Clay soil (moist)1.4491.2821.149
Clay soil (wet)0.9620.7690.641
Clay soil (frozen)0.938
Gravel1.1110.930.8
Gravel (sandy)0.398
Limestone0.769
Sandstone0.6130.5310.481