In-situ estimates of the tensile strength of snow utilizing large sample sizes

TLDR: In this paper, Zugbruehc et al. measured the tensile strength of snow by using large sample sizes of naturally deposited snow and showed that the results show less scatter in the data than previous in-Jitu estimates and lower mean strength values as a function of density.

Abstract: Experimental procedure and measured estimates of the tensile strength of snow are given by a new method utilizing large sample sizes of naturally deposited snow. Data are presented as a function of average sample density, temperature, loading rate, and snow type. The results show less scatter in the data than previous in-Jitu estimates and lower mean strength values as a function of density. The relevance of the data to tensile fracture as observed in slab avalanche release is discussed. RESUME. Estimation in situ des e{forts de traction dons la neige sur des 'ehan/ilions de .grande dimension. On donne un procede experimental et des estimations des efforts de traction dans la neige par une nouvelle methode utilisant des echantillons de grandc dimension de neige deposee naturellement. Les resultats sont presentes en fonction de la densite moyenne de l'echantillon, de la temperature, de la surcharge et du type de neige. Les resultats montrent une dispersion moindre que les estimations anciennes et des moindres valeurs moyennes d es efforts en fonction de la densitc. On discute le rapport entre les resultats et les ruptures a la traction observees dans les declenchements d'avalanches de plaques. ZUSAMMENFASSUNG. In-,itu-AbJChiit z ullgen der Zugfestigkeit von Sehnee an groJfformatigell Proben. Das experimcntdle Verfahrcn und die Abschatzungsergebnisse .. iner neuen Methode zur Bestimmung der Zugfestigkeit von Schnee, bei der grossformatige Proben natlirlich abgelagerten Schnees benutzt werden, werden mitgeteilt . Die \\OVert .. stellen sich in Abhangigkeit von der mittlcren Probendichte, der Temperatur, der Bclastungsrate und der Schnecart dar. Die Ergebnisse zeigen gcringere Streuung als fruhere Abschatzungcn und kleinere mittlere Festigkeit in Abhangigkeit von der Dichte. Die Bedeutung der Daten fur Zugbruehc, \",ie sic in Brcttlawincn zu beobachten sind, wird diskuti,·rl. INTRODUCTION The tensile strength of snow is an important quantity in snow mechanics. Destructive tensile fractures are habitually observed on the line where snow slabs break away from their anchoring zones (Haefeli, 1967; Peria, [1975]). The tensile strength of snow is a function of many variables including density, temperature, and snow type. In addition , any test introduces additional variables which must be considered including loading rate, sample size, boundary conditions at the places where the sample and testing equipment are in contact, and sample disturbance in mounting the specimens. Experiments with small sample sizes using centrifugal tensile testers consistently show large scatter in the strength values and high mean strength values (Sommerfeld, 1974). In addition, centrifugal testers cannot be used in situ and therefore may subject the sample to damage upon insertion of the sample tubes. Jarring of the samples against the walls of the tester at fast acceleration rates is also possible in centrifugal tests. The procedure used in the present paper to estimate the tensile strength of snow is intended to provide estimates for larger sample sizes closer to the expected sample size in natural slab avalanches. In addition, the tests were made in situ and the loads were applied slowly to prevent effects of jarring and sample damage. The boundary conditions at the places where the snow specimen and the testing equipment were in contact were such that the conditions expected prior to destructive tensile fracture observed in snow-slab release were approximately simulated. EXPERIMENTAL PROCEDURE The experiments were done during the winters of 1971 72 and 197273 in the Cascade Mountains, U .S.A. The apparatus consisted of a series of large tables set out on a horizontal snow surface to collect samples of snow. The tables had removable, wooden sides such that the • Present address: Environment Canada, Canmorc, Alberta ToL 01\\10, Canada. 321 322 ./O ll R:'>IAL OF GLACIOLOGY samples .were ·collec ted in a box on top of th e ta bl cs. Following collect ion of a sample, t hc top was carcfull y trimmed o ff a nd the woodcn sidcs were removed to provide uniforml y shaped and sized samples . The surface of th e tables was partly roug h and partly smooth . It was hoped that d estruc tive tensil e fracture would occur on the line between the rough and smooth sections as th e table was slowly tilt ed by lifting onc cnd resulting in slow loading by body forces. This proved to be th e casc for each test. The fra c ture surfaces \\WIT, in gcneral , smooth and perpendic ular tn the ta hle sudacc. The stress field in t he sa mples in th e present tes ts is not hornogencous. The tests welT d esigned to simulate conditions of tensil e fa ilure in slabs whose resistan ce to sustaining basal shear stress is diminished . Thcre will , therefore, be small differences whcn the present dat a arc compared to uniaxial test results . However, a small er cffec t is to be expected when comparing the results 10 fi eld es timates.