Permafrost

with frozen soil and concrete increases nonlinearly with decreasing of temperature under the
different water content condition. After the freeze-thaw the data, acquired from refrozen shear
test, shows the adfreezing force of the second loading distinctly reduces compared with the one
of first, moreover to the sample of less water content, it reduces more distinctly.
Key words: permafrost; freezing strength; bearing capacity; neutrality point
Mechanical properties of Qinghai-Tibet clay subjected to closed-system
freezing and thawing
18
Da-yan Wang, Wei Ma, Xiao-xiao Chang, Wen-jie Feng
(State Key Laboratory of Frozen Soil Engineering, Cold and Arid Regions Environmental and Engineering
Research Institute, CAS, Lanzhou 730000, China)
Abstract: A prerequisite for the pavement design, stability analysis and calculation is the
availability of mechanical characteristic of subgrade soils. It is widely proved, however, that
such factors as soil structure, water content and bulk density, grain size distribution, shape and
mineralogy, the degrees of soil-grain interlocking, grain cementation and chemical weathering,
the particle-bonding mechanisms in clay soils, and the presence of vegetation determine soil
mechanical properties. In cold regions, a further dimension is added to soil mechanical
properties through the introduction of freezing and thawing for the process of freeze–thaw
changing the microcosmic structure of soil. In this case, the soil mechanics properties are also
severely affected by ice bonding between particles during freezing and excess moisture during
thawing. These effects may in turn substantially reduce the foundation capacity of structure.
A review of the published literature reveals that, although considerable research has been
carried out on freezing and thawing of fine-grained soils and its effect on engineering properties,
most research take it as hydraulic barriers in liners and covers for landfills or caps for
remediation of contaminated sites, and emphasize freeze-thaw effects on the void ratio,
permeability and hydraulic conductivity, only limited information is available regarding the
effects of freeze-thaw on soils mechanical characteristic. However, fine-grained soils are often
used as foundation soils. If soil undergoes internal fabric changes after freeze-thaw cycles, most
likely the bearing capacity of foundation will be affected. An increased knowledge of the
mechanical behavior of soils experienced freeze-thaw cycle is therefore needed.
This paper presents an experimental study performed on compacted Qinghai-Tibet clay at
different initial confining pressure to investigate the effect of freeze-thaw on the mechanical
properties such as stress-strain behavior, failure strength, resilient modulus, cohesive force and
friction angle. The following conclusions are drawn based on this study:
(1) The height of specimen examined in this test program increased and the water content
decreased with increasing numbers of freeze-thaw cycles during the first seven cycles.
However, after seven cycles, they will keep constant with additional freeze-thaw cycles.
(2) Even though the magnitude of resilient modulus and failure strength were influenced by
increasing the number of freeze-thaw cycles, the type of the stress-strain curve was only
response to the variation of confining pressure and without regard to the increasing the