Finite Element Aided Design Method for Frozen Earth Shaft Support-Paper

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The paper presents a finite-element-based method for designing frozen-earth shaft support, offering an alternative to traditional thick-walled-cylinder calculations that can be overly conservative and poorly represent actual ground conditions.<br /><br />The proposed process combines field investigation, laboratory testing, structural and thermal modeling, and construction monitoring. Fieldwork should establish soil layering, groundwater levels and flow, and the presence of a suitable impermeable base. Soil samples are tested to determine unfrozen strength and stiffness, frozen-soil strength, and long-term creep behavior. Because frozen-soil properties depend on temperature and time, creep tests are used to estimate long-term strength for design.<br /><br />For structural design, an axisymmetric finite-element model represents the shaft and its in-situ stresses. Designers iteratively assess different frozen-wall thicknesses and compare the maximum hoop stress with the frozen soil’s long-term compressive strength. The resulting factor of safety is calculated as long-term strength divided by maximum hoop stress. Staged construction can also be modeled when the shaft is excavated and supported progressively.<br /><br />Thermal finite-element analysis determines refrigeration-pipe spacing, freezing duration, and required cooling capacity. Models can use surveyed pipe locations and account for groundwater flow, which brings heat into the system and may delay or prevent wall formation. The paper notes that groundwater velocities above approximately 1 m/day can prevent a frozen wall from forming with a single row of pipes.<br /><br />Instrumentation should track ground and coolant temperatures, groundwater levels, pipe flow, refrigeration-plant performance, and ground heave or settlement. Comparing these measurements with model predictions supports quality assurance and timely adjustments.<br /><br />The author concludes that finite-element methods, supported by reliable site data and laboratory testing, better represent actual frozen structures and provide a more realistic basis for evaluating safety than older simplified methods.

Keywords

finite element method

frozen-earth shaft support

axisymmetric modeling

frozen soil strength

thermal analysis

groundwater flow

refrigeration pipes

long-term creep

factor of safety

construction monitoring