It has been recognized that freeze–thaw cycling is a weathering process which considerably changes the structure of soils and thus has great influence on the engineering properties of soils. In construction work in cold regions involving newly exposed soils, care must be taken regarding freeze–thaw cycling (Sheng, 2004). For instance, in the permafrost regions in Quebec, Canada, it has been found that the embankments constructed on soil which has never experienced freeze–thaw cycling are damaged in just one year due to the loss of bearing capacity (Leroueil et al., 1991). Therefore, newly constructed highway embankments that are left unpaved for a few years is, to some extent, a consideration of possible damage by freeze–thaw cycling (Eigenbrod, 1996).

When considering the change in engineering properties of soils subjected to freeze–thaw, changes in physical and mechanical properties are usually examined separately. In a review work by Qi et al. (2006), it was summarized that there are general agreements on the influence of freeze–thaw on physical properties, especially on density and hydraulic permeability. For instance, previous studies show that freeze–thaw has a dual influence on soil density, i.e. loose soils tend to be densified and dense soils become looser after freeze–thaw cycles (Konrad, 1989). According to this phenomenon, Viklander (1998) proposed a residual void ratio, eres, in terms of freeze–thaw, which means that both loose and dense soils would reach the same void ratio after a number of freeze–thaw cycles. However, the hydraulic permeability increases regardless of the change in density ([Chamberlain and Gow, 1979], [Othman et al., 1993] and Viklander, 1998 P. Viklander, Permeability and volume changes in till due to cyclic freeze–thaw, Canadian Geotechnical Journal. 35 (1998), pp. 471–477. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (26)[Viklander, 1998]).

With regard to the change in mechanical properties after freeze–thaw cycling, different conclusions can be found, sometimes even contradictory. This may be due to the fact that different researchers studied different soils and applied different conditions during freeze–thaw. It is recognized that there is a lack in study on the difference of the natural and reconstituted soils, on the difference of the same soil at different densities, on the difference of soil types, as well as on the difference of the freezing conditions. At the same time, the deforming process during freeze–thaw and the stress state have not been well considered. This paper takes a silty soil, the Lanzhou loess, as the study object. The soil experiences freeze–thaw under different freezing conditions and different original dry unit weights. The changes in its engineering properties are investigated. The deforming process during freeze–thaw and the stress state will also be examined. With regard to the engineering properties, focus will be firstly put on the change in density after freeze–thaw. Then changes in the strength parameters and modulus will be checked, because almost all constitutive models for soils need these three parameters. As preconsolidation pressure differentiates the elastic and elastic-plastic range for a soil in the critical state-based constitutive models, it also will be discussed. This study will essentially facilitate relatively accurate constitutive modeling of soils subjected to freeze–thaw.

Freeze–thaw cycling is a weathering process which considerably changes the engineering properties of soils. Therefore, the influence of freeze–thaw must be taken into account for modeling of stress–strain behaviors in stability and deformation analyses for slopes, embankments and cuts in cold regions with soil layers experiencing freeze–thaw cycling. In this paper, freeze–thaw induced changes in the engineering properties of a silty soil, Lanzhou loess, were studied under different freezing conditions and with the dry unit weight from 15.3 to 17.3 kN/m3. The soil samples were subjected to one freeze–thaw cycle. Then changes in dry unit weight, strength parameters, preconsolidation pressure as well as modulus were examined. The changes in mechanical properties along with both the freezing condition and the original dry unit weight, were discussed. It is found that under the same freezing condition, there is a critical dry unit weight, γdcr, in terms of freeze–thaw for some of the engineering properties. When the original dry unit weight was at γdcr, the soil density, the cohesion and preconsolidaiton pressure remained unchanged after freeze–thaw. When the original dry unit weight was larger than γdcr, these three parameters decreased; with the original dry unit weight less than γdcr, these parameters increased after freeze–thaw. However, the modulus always decreased after freeze–thaw.