Back analysis of the Nerlerk berm liquefaction slides

Abstract

Five liquefaction slides occurred in 1983 during the construction of a hydraulically placed subsea sand berm designed to form part of a bottom-founded, offshore, hydrocarbon exploration platform at Nerlerk in the Canadian Beaufort Sea. These slides were triggered by simple static loading arising from the sand placement itself. Failures started at locally oversteepened side slopes and retrogressed to form bowl-shaped crests. The slide material came to rest beyond the berm at very flat slopes. Stability back analyses reveal that effective stress strength parameters consistent with limiting equilibrium are within the range of the collapse surface parameters determined from triaxial laboratory tests. This finding lends credibility to the collapse surface concepts introduced in another study. Back calculations show that the berm state prior to failure was much looser than the steady state, with the potential for a large strength loss. According to back calculations, the berm density was lower than that inferred from cone penetration tests.Cinq glissements resultant de la liqukfaction du sable se sont produits en 1983 durant la construction d'une berme de sable placC hydrauliquement sous la surface de la mer. Cette berme devait faire partie de la fondation d'une plate-forme d'exploration pCtrolikre reposant sur les fonds marins au large du littoral h Nerlerk dans la mer canadienne de Beaufort. Ces glissements ont Ct C dCclenchCs par le simple chargement statique rksultant de la mise en place du sable lui-m&me. Les ruptures ont debut6 localement a des points oh les pentes des c6tCs de la berme Ctaient sur-inclinCes, et ils ont rCtrogressC jusqu'h ce que les cr8tes aient la forme d'une assiette. Le matCriel du glissement s'est arr&tC au delh de la berme h un angle trks faible. Des analyses de stabilitC h rebours ont rCvClC que les paramktres de risistance en terme de contrainte effective correspondant h l'ttat d'Cquilibre limite sont situCs 1 I'intCrieur du domaine de variation des paramktres dCterminCs sur la "surface d'effondrement" au moyen des essais triaxiaux. Cette constatation ajoute du cridit aux concepts de "surface d'effondrement" introduits dans une autre Ctude. Les calculs h rebours ont dCmontrC que 1'Ctat de compacitt de la berme avant rupture Ctait beaucoup plus I2che que dans 1'Ctat stable et comportait potentiellement une perte importante de rbsistance. D'aprks les calculs h rebours, la compacitk de la berme Ctait plus faible que celle donnCe par les essais de p6nCtration au cbne.Mots clks: IiquCfaction, sable, remblai hydraulique, stabilitC de pente, essai de pCnCtration au cbne.[Traduit par la revue]Can. Geotech. J. 22,579-588 (1985)

Elastoplastic modelling of diffuse instability response of geomaterials

“…Instability and loss of controllability took place much sooner for the looser sand specimen than for the denser one. Similar findings have been experimentally produced by Sladen et al [8], Sasitharan et al [45], di Prisco and Imposimato [49] and Gajo et al [50], and numerically verified by Wan et al [51] using Gudehus' probe tests at several stress states along a conventional triaxial loading path under drained conditions.…”

“…Notable instances of this are natural landslides in very shallow angled soil slopes, [7][8][9] and the collapse of a loose sand when sheared under undrained conditions or mixed stress-strain control [10,11]. In the latter, after reaching a peak-stress ratio far away from the plastic limit surface, the loose sand succumbs to a spontaneous loss in strength in the absence of any localized deformations.…”

Undrained torsional shear tests on gravelly soils

“…The effects of fines contents on the liquefaction resistance of sand-fines composite material previously reported are summarized as: -fines have little or no effect on liquefaction resistance (Ishihara 1993), -fines increase liquefaction (Chang et al 1982;Kuerbis et al 1988;Yasuda et al 1994), (Troncoso and Verdugo 1985;Sladen et al 1985;Vaid 1994), and -fines reduce liquefaction resistance until a certain threshold in fines content, and then increase liquefaction resistance (Koester 1994;Lade and Yamamuro 1997).…”

Factors affecting pre-failure instability of sand under plane-strain conditions

“…Several researchers have related instability conditions to the state parameter (Sladen et al, 1985;Anderson & Riemer, 1995;Yang, 2002). However, as observed by Chu et al (2003) and Wanatowski et al (2010), the state parameter concept may not be used directly for prediction of the instability behaviour of sand.…”

“…The possibility of static liquefaction or instability occurring for dilating sand has also been observed in several case studies. In a discussion to identify the mechanisms that caused the failure of the Nerlerk berm in the Canadian Beaufort Sea (Sladen et al, 1985), Been et al (1987) concluded that the failure could have occurred for dilatant sand. Several other cases of flow slide failure in dilatant sand have also been presented by Been et al (1988).…”