Direct Measurement of Stress at the Base of a Glacier

Abstract

Contact stress transducers w er e p.1 aced in subglac ial b edrock a nd used to monitor continuously shear stress and normal pressur e changes at the contact with the overriding glacier sole 100 m beneath the surface of the Glacier d'Argentiere during periods in summer 1973 and spri ng 1975. The measured flu ctuations in normal press ure and shear st ress do n ot appear to be r elated to changes in sliding velocity. Analysis of the data reveals short-term flu ctuations in normal pressure a nd sh ea r stress which appear to be related to the passage of individual la rge debris pa rticl es or groups of p a rticl es over the transducer. The shea r stress appears to be a fun ction of the volume conce ntration of debris in the ice. The volume concentration at any point appears to be partially d ependent o n a "streami ng" process b y which basal debris-rich ice tends to flow a round the lateral flanks of hummocks on the glacier bed. Where su b-glacial cav iti es occur, this streaming effect appears to be d ependent on the extent of cavitation and thus on ice overburden pressure a nd velocity. It is suggested that this process can account for an apparent lag between changes in normal press ure and sh ear stress.The maximum ra tio b etween shear and normal stress averaged over a period of 10 min was 0.44. This is equivalent to a spatial average over 0.3 cm. D ebris concen trat ions in basal ice of up to 43% by volume occurred. It is suggested that concentrations of this order are commo n a t the base of temperate glaciers and thus that a signifi cant part of the drag a t the base of a glacier may be con tributed by frictional interactions between the basal-d ebris load and the bed. REsUME. M esures directes du cisaillement a la base d'un glacier. D es j a uges de contrainte pa r contact ont ete placees sur le lit sous-glaciaire et utilisees pour relever en continu les va ria tions de la pression normale et de la contrainte d e cisailleme nt au contact avec la fa ce inferieure du glacier a 100 m sous la surface du glacier d' Argenti ere pendant des p eriodes de I' ete 1973 et du printemps 1975. Les flu ctuations mesurees d ans la pression norma le et la contrainte de cisaillem ent ne semblent pas e tre correlees a ux variations de la vitesse d e glissement. L'analyse d es donnees revele d es flu ctuations de courte duree dans la pression normal e e t la contrainte de cisailleme nt qui sem blent liees au passage de gros blocs morainiques isoles ou de groupes de blocs au-dessus du capteur. Le cisaill ement parait etre fonct ion de la concentration en volume des materiaux m orainiques dans la glace. La concentra tion e n volume en un point sembl e dependre e n panie d'un processus d e "courant" par lequel la glace basale rich e en moraine tend a s'ecouler autour des fl a ncs la teraux d es protube rances du lit glaciaire. Lorsqu' il se produit des cavites sous-glaciaires, cet effet d e courant parait lie a I'impo rtan ce de la cavite et done a la vitesse et a la pression de la glace d e couverture. On sugbere que ce process...

Debris‐bed friction of hard‐bedded glaciers

“…In his treatment, bed-normal forces were too small to slow particles significantly. Lee-side cavities observed behind clasts [e.g., Boulton et al, 1979] are clear evidence that particle movement is retarded significantly relative to ice movement.…”

Subglacial observations from øksfjordjøkelen, North Norway

“…Subglacial observations are necessary to test these theories. Such observations have been made by Kamb and La Chapelle (1964), Theakstone (1966Theakstone ( , 1979, Vivian and Bocquet (1973), Vivian (1980), Boulton (1974Boulton ( , 1979, Boulton et al (1979), Wold and Ostrem (1979), Hagen et al (1983) and Anderson et al (1982). The nature of the subglacial environment makes access difficult and it is only rarely that the bed can be reached via natural tunnels, crevasses or marginal cavities.…”

Role of transient water pressure in quarrying: A subglacial experiment using acoustic emissions

“…There is also strong justification for assuming abrasion rate to increase with sliding velocity, which influences both the flux of abrasive particles across the bed [ Boulton , 1974; Hallet , 1979] and the force with which particles are pressed against stoss surfaces by sliding ice [ Hallet , 1979; Iverson , 1990]. Most authors, however, consider quarrying to be more important than abrasion [ Jahns , 1943; Boulton et al , 1979; Drewry , 1986; Iverson , 2002], and this claim is supported by measurements [e.g., Loso et al , 2004; Riihimaki et al , 2005] and modeling [ Hildes et al , 2004].…”