Geotechnical Properties of Alpine Permafrost

Introduction

Arenson et al. (2007) present an overview of geotechnical properties of frozen soils and the many factors that have an influence on the mechanical response of permafrost soils. As high ice contents with excess ice and high air contents are often found in soils in mountainous environments, this section focuses on soils with these properties. Andersland and Ladanyi (2004) or Esch (2004) present thorough overviews for the remainder of the spectrum of frozen soils. An additional overview on most recent progress in permafrost geotechnics is presented in Springman and Arenson (2008).

Unfrozen Water Content

While water under atmospheric pressure freezes at 0°C, water in soil pores freezes at slightly colder temperatures as it is influenced by the soil skeleton. Mineralogy, particle size and pore water chemistry may influence the freezing point and the unfrozen water content so that at temperatures below zero degrees, liquid, unfrozen water and ice coexist (Anderson and Tice, 1972; Fish, 1985; Williams, 1967a; Williams, 1967b). In a mountainous environment with coarse- grained soils prevailing and generally no pore water salinity, the amount of unfrozen water below -2°C is negligible. To estimate the unfrozen water content, Smith and Tice (1988) and Tice et al. (1976) propose the following relationship.

wu = a-9p, where wu is defined as weight of water divided by dry weight of soil expressed in percentage, 9 is the temperature expressed as a positive number in degrees Celsius below freezing, and a and P are soil parameters. Typical values for gravelly materials found in mountainous environments are a = 2.1 and P = -0.408 (e.g. Smith and Tice, 1988).

General Soil Strength Considerations

The strength of frozen soil is controlled by the interaction of soil particles similar to unfrozen soils and the cementation effect of the ice matrix. However, the strength properties of the matrix are strongly non-linear and temperature- as well as loading- and deformation-

rate dependent. Generally, the strength of ice increases as the temperature decreases. At temperature close to the melting conditions, the specific surface of the soil particles affects the phase change. Because the unfrozen water affects the activation energy (e.g. Barnes et al., 1971; Fish, 1985), adopting the Arrhenius approach to account for variations in temperature will be invalid at temperatures close to the melting point and different approaches (e.g. Hivon and Sego, 1995) should be used.

When the ice content varies, a minor increase in strength is noted up to a volumetric ice content of approximately 40% (Arenson and Springman, 2005b; Goughnour and Andersland, 1968). However, research suggests that dispersed soil particles within dirty ice alters the failure mechanism, so that strength is slightly lower than in pure ice (Arenson and Springman, 2005b; Hooke et al., 1972; Yasufuku et al., 2003). Structural hindrance between the solid particles and dilation occurs as the volumetric ice content decreases, increasing the strength and reducing creep deformation significantly. At high relative soil densities, the resulting increase in strength over the same soil in its unfrozen state may be quantified as cohesion at zero stress (e.g. Arenson et al., 2004; Nater et al., 2008). However, at very large strains, icebonding fails, destroying the cohesive effect and the strength of the frozen material will be similar to the strength of the equivalent unfrozen soil.

Air voids also have a significant influence on the volumetric strain behaviour. Volumetric strains of more than 10% were recorded in triaxial compression tests at an axial creep strain of 20% for a sample with an initial air content of 25%. Experimental work on glacier ice containing air bubbles showed that the mean number of air bubbles per unit volume correlated inversely with the mean uniaxial compressive strengths (Gagnon and Gammon, 1995).

The loading regime further changes the failure mechanism and consequently the strength of the frozen soil. Fast loading of ice rich material or ice results in brittle failure, whereas low strain rates provoke a ductile response dominated by creep deformations. Arenson and Springman (2005a) present a schematic representation of this behaviour (Fig. 3).

Loading velocity (strain rate)

Figure 3. Sample response as a function of the loading rate and the ice content.

Strength parameters for frozen soils vary significantly and specific laboratory investigations are recommended for particular problems. The unfrozen strength of the frozen soil can be used as a lower boundary assuming that the cementing effect of the ice is no longer present and allowing for the effect of (unfrozen) groundwater.

Ground Characteristics

Rock and Soil Foundations

Generally, rock foundations in permafrost are no different to any other rock foundation. Therefore permafrost bedrock and rock walls only present an additional hazard when they are jointed and fissured and these discontinuities are filled with ice. Direct shear tests and centrifuge modelling on ice-filled joints imply that where the direction of dip of the joint planes is appropriate, the stability of a steep, jointed rock slope is maintained by the ice (Davies et al., 2001; Davies et al., 2000). The factor of safety decrease as the temperature increases and reached the melting point of the ice. This hypothesis implies that a jointed rock slope that is stable when there is no ice in the joints, and is also stable when ice in the joints is at low temperatures, may become unstable as the ice warms. The process can be attributed to the reduction in strength and stiffness of the cementing ice as it warms and the existence of water that may reduce the effective stress between the blocks (Gunzel, 2008). Gruber and Haeberli (2007) suggest that the enhanced creep susceptibility of warm ice may have caused the increased rockfall activity at high altitude in Europe in the summer of 2003.

Because the existence of ice in rock joints controls the strength of the rock, it is crucial that it is reported in addition to common rock mass classification (e.g. Bieniawski, 1989).

Foundations on frozen soils are challenging because the mechanical properties of the ground varies as temperatures, ice content and loading regimes changes as indicated above. Creep deformations pose particular challenges and need special attention during the design because they can influence the serviceability of an engineered structure.

It is therefore important to properly report the ground ice conditions during soil and rock classification. The heterogeneity of various permafrost soils necessitate an appropriate sample size for the determination of the volumetric ice content that depends on the maximum grain diameter for soils or the non-fissured length in rocks (Fig. 4).

Creep of Frozen Soils and Rock Joints

Creep occurs as soon as excess ice is present in the soil. In a dense soil when only pores are frozen, structural hindrance inhibits any creep deformation. However, for low particle contents the matrix can deform and creep occurs. Even thin ice layers, e.g. from segregated ice or in rock joints, may be enough to trigger creep deformations. It is important that ice lenses are identified to judge the ground's susceptibility for creep movements even in soils with low ice contents and in rocks.

Creep laws and properties of for various frozen soils are presented in Andersland and Ladanyi (2004). Creep parameters for ice-rich soils can also be found in Arenson and Springman (2005 a).

Material

Rock

Soil

Frost susceptibility:

Rock

Fissured

Intact

Non fissured length (NFL)

Frost susceptible

Soil

Frost susceptible

Non frost

susceptible

Max. grain size (Dmax)

Deciding dimension:

Non fissured length (NFL)

Max. grain size (Dmax)

Sample size:

Vol. ice content:

0%

dry

0 -10%

few*

10 - 30%

some*

30 - 60%

intermediate*

>60%

many*

* ice filled fissures

0%

unfrozen

0 - 20%

ice poor

20 - 55%

intermediate

55 -85%

ice rich

85 - 100%

dirty ice

100%

ice

Figure 4. Determination of sample size and volumetric ice contents as a function of grain size diameter and non-fissured length for soils and rocks.

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