Ring-shear studies of till deformation: Coulomb-plastic behavior and distributed strain in glacier beds

Abstract

ABSTRACT. A ring-shea r device was used to study the factors that co ntrol the ultimate (steady) streng th of till at high shea r strains. Tests a t a steady strain ra te and at different stresses norm al to th e shearing direction yielded ultim ate friction angles of26.3° and 18.6° for tills conta ining 4% and 30% clay-sized particles, respectivcly. Other tests at steady normal stresses a nd va ri abl e shear-strain rates indi cated a tendency for both tills lO weaken slightly w ith increasing stra in rate. This weakening may be due to sma ll increases in till porosity.Th ese res ults provide no evidence o f v iscous behavior and suggest that a Coulombplas ti c idealization is reasonable [or till deformation. Howeve r, vi scous behavior has often been sugges ted on th e basis of di stributed shear stra in observed in subglaci a l till. We hyp ot hes ize that deform ation may becom e distributed in till th at is deformed cyc licall y in response to flu ctu ations in basal wa ter pressure. During a deform ation event, transient dil ation of discre te shea r zones should cause a reduction in intern al pore-water pressu re that should streng then th ese zo nes rela tive to the surro unding till, a process call ed dilarant harde ning. Consequent changes in hea r-zo ne positi on, when integrated over time, may yield the obse rved di tributed stra in.

The experimental basis for interpreting particle and magnetic fabrics of sheared till

“…Evans et al (2006) have cited the variable fabric strengths of debris flows, which move under low effective stresses, as evidence of this phenomenon. However, triaxial and ring-shear testing on basal tills in their dilatant state indicate no significant deviation from the behavior of a Coulomb material (Iverson et al, 1998;Tulaczyk et al, 2000), even under effective normal stresses as small as those measured in the dilatant upper till horizon beneath Breidamerkurjökull (Boulton and Hindmarsh, 1987). Moreover, fabrics of debris flows cannot be directly compared with those of subglacial till because debris flows move many orders of magnitude faster, causing momentum exchange among particles (Iverson, 1997).…”

“…The design and function of our ring-shear device have been discussed elsewhere (Iverson et al, , 1998Hooyer and Iverson, 2000;Thomason and Iverson, 2006), so only essential details are reiterated herein. A till specimen, 0·115 m wide and up to 0·085 m thick, occupies an annular chamber with an outside diameter of 0·6 m (Figure 1).…”

“…As noted by Clarke (2005), our most secure knowledge of this subject comes from laboratory experiments. All experiments carried out to steady deformation rates and stresses indicate that till is best treated as a Coulomb material, such that its steady-state strength is essentially independent of the rate at which it shears (Iverson et al, 1998;Tulaczyk et al, 2000). The mildly nonlinear viscous rheology advocated by Boulton and Hindmarsh (1987) has lost favor due to methodological concerns associated with that field study (Clarke, 2005) and satisfactory explanations of its results using experimentally tenable Coulomb models (Tulaczyk et al, 2000;Iverson and Iverson, 2001).…”

Mechanisms leading to the 2016 giant twin glacier collapses, Aru range, Tibet

“…The glacier shape did not adjust fast 30 enough to reduce the driving stress due to strain rate limitation in the cold-based areas producing large stresses in the remaining sticky spots (Figure 8 saturated temperate surrounding and increasing deformational heat. High clay and silt content measured in the till suggests low friction-angle properties (Iverson et al, 1998) and higher sensitivity of the shear strength to water pressure.…”

“…The sustained very low basal drag under Aru glaciers (<20kPa) may be similar to ice stream mechanisms whereby water-saturated till enables fast flow at low driving stresses (≈20kPa) (Cuffey and Paterson, 2010). It has been shown that 15 glacier till behaves with a plastic rheology with a shear strength strongly dependent on the effective normal stress (Clarke, 2005;Iverson et al, 1998;Iverson, 2010;MacAyeal, 1992). Such behaviour was found to be well described by a Coulombtype friction law (Boulton and Jones, 1979;Clarke, 2005;Tsai et al, 2015) as follows:…”