1Y82 1201: A depositional model for stratigraphic complexes and facies superimposition in lodgement tills. Boreas. VOI. 11,. OSIO. ISSN 0300-9483.A typical stratigraphy below a streamlined till plain in Northumberland, England, consists of cross-cutting lodgemcnt till units, within and between which occur repeated shoestring interbeds of 'cut and fill' channels. I i I I units have erosional lower contacts; in certain cases marked changes in erratic content and local ice flow direction arc evident from one till unit to another. These lodgement till complexes have hitherto been described by 'tripartite' schemes of lower grey till(s) and upper reddened till(s) identified with respect to 'middle' fluvial horizons; regional correlation proceeding on the basis of matching 'middle' horizons, with the whole sequence commonly interpreted as evidence for multiplc glaciation. Data indicates, by way of contrast, that thLse lodgement till complexes were deposited during a single phase of subglacial deposition. T i 1 1 deposition was not continuous but was interrupted by erosional episodes.Changes in the mix of bedrock lithologies transported by the glacier down a single flow line or by lateral displacement of basal ice flow units within the glacier result in till units of different facies to be emplaced when deposition recommences. a process referred to as 'unconformable facics superimposition'. Subgla-cia1 meltwater flow was also a characteristic of the glacier bed; channeled glaciofluvial sediment bodies are found as ribbon-like inclusions in the till and appear to have been deposited rapidly. These so-called 'middle' fluvial horizons occur repeatedly in section, their lateral extent at any given exposure being dependent upon the orientation of the exposure with respect to former ice flow direction. These lenses act as internal drainage hlankcts and have accelerated postglacial soil formation in the drier climatc of eastern Britain accounting for the reddened colour of upper till(s). It is suggested that this model of subglacial deposition can be employed in other areas of northern England characterized by subglacial (lodgement till plain) terrains.

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)

The range of densities achievable by hydraulic placement of sand straddles the boundary between values giving acceptable potential performance and those giving unacceptable potential performance. This has led to concerns over the safety of structures using hydraulic fills, such as the artificial drilling islands in the Canadian Beaufort Sea. Liquefaction failures of hydraulically placed sand have occurred at four or more of these islands. Until recently, the factors affecting in situ density were little understood. Data obtained from several artificial islands are presented and these are used to demonstrate the overwhelming influence of method of placement on in situ density. The possible reasons for this influence and the implications for design are discussed. Recommendations are made for research that, together with conclusions drawn in the paper, should allow hydraulic fills to be used with more confidence in the future.Le domaine de variation des densites qui peuvent &tre atteintes par mise en place hydraulique du sable chevauche la limite entre une performance potentielle acceptable et une inacceptable. Ceci a cause certaines inquiktudes quant a la securite des structures utilisant des remblais hydrauliques, telles que les iles artificielles dans la mer canadienne de Beaufort. Des ruptures par liquefaction du sable mis en place par la methode hydraulique se sont produites dans au moins quatre de ces iles. Jusqu'a recemment, les facteurs affectant la densite in situ Ctaient peu compris. Les donnies obtenues sur plusieurs de ces iles artificielles sont presentees et elles sont utiliskes pour dimontrer l'influence prkpondkrante de la mithode de mise en place sur la densite en place. Les causes possibles de cette influence et les implications pour la conception sont discutees. L'on y fait des recommandations de recherches qui, combinkes avec les conclusions tirees de cet article, devraient permettre dans le futur l'utilisation des remblais hydrauliques avec plus de confiance.Mots clis : sable, remblai hydraulique, liqukfaction, essai de penktration au c8ne.[Traduit par la revue]Can. Geotech. J. 26, 453-466 (1989)

Considerable attention has been given to the inference of sand density from indirect tests, particularly the cone penetration test (CPT). Data from tests performed in large-scale chambers have demonstrated that for a given sand there is an approximately unique relationship between CPT tip resistance, density (or void ratio) and effective stress level. Data have been presented to support the supposition that CPT tip resistance, if normalized by division by the mean effective stress, is uniquely related to the difference between the current void ratio and the void ratio at the steady state at the same mean stress level. The Paper presents a study of chamber test data for the Ticino sand and demonstrates that there is no such unique relationship for this sand. Use of published correlations that fail to recognize this and other potentially important factors could result in interpretations of sand state that, if carried through to design, could in some circumstances be catastrophic. A series of flow slides that occurred during the construction of an hydraulically placed subsea sand berm at Nerlerk, in the Canadian Beaufort Sea, highlights these difficulties and provides an opportunity to compare field performance with laboratory data. On a beaucoup étudié la détermination de la densité du sable à l'aide d'essais indirects, particulièrement a partir de l'essai de pénétration au cône. Des données obtenues au cours d'essais effectés dans des chambres de grande capacité ont démontré qu'il existe une relation à peu près unique entre la résistance à la pointe du cône, la densité (ou l'indice des vides) et le niveau effectif des contraintes. On a présenté des données d l'appui de l'hypothèse que si la résistance à la pointe est normalisée en la divisant par la contrainte effective moyenne elle est reliée de façon unique à la différence entre l'indice instantané des vides et cet indice pour l'état stationnaire et pour le même niveau moyen des contraintes. Cet article étudie les résultats d'essais effectués dans de telles chambres sur du sable du Tessin et démontre qu'il n'existe aucune telle relation dans le cas de ce sable. L'emploi de corrélations publiées qui ignorent ce facteur entre d'autres aspects d'importance potentielle pourrait conduire à des déterminations de l'état du sable qui seraient catastrophiques pour la construction dans de certaines circonstances. Ces difficultés sont soulignées par une série de glissements par liquéfaction qui eurent lieu lors de la construction d'une risberme mise en place sous la mer par moyen hydraulique dans la Mer de Beaufort (Canada) et qui donnent l'occasion de comparer les performances sur place avec les données de laboratoire.

Summary Glacial tills in coastal Northumberland deposited by last glaciation (Devensian) ice ( c . 25,000 years B.P.) exhibit a reddened post-depositional weathering profile up to 8 m thick, which has previously been misinterpreted as a separate stratigraphic unit, or ‘Upper Boulder Clay’. Weathering profiles are developed in grey lodgement tills that form an extensive coastal lodgement till plain deposited beneath a wet-based ice sheet. The tills contain intraformational sands, gravels and laminated clays deposited in subglacial meltwater channels and lakes. These sediments have often been partially eroded and incorporated within overlying till resulting in lateral variations in the depth of post-depositional weathering. Sand and gravel lenses, for example, act as drainage layers within the till thereby increasing the depth and degree of weathering and often forming a sharply defined lower boundary to the weathering profile. These features are seen as an apparent tripartite stratigraphy consisting of an upper red till, middle sands and gravels and a lower grey till. Where the depth of the till section is less than between 2 and 3 m only a single reddened weathered till is found. Weathering takes the form of oxidation followed by leaching of carbonates and is characterized by colour change, increasing content of rotten boulders, and prismatic gleyed jointing. Within the weathering profile four distinct weathering zones can be recognized. Clay and moisture contents increase with degree of weathering giving rise to systematic zonal changes in Atterberg indices and shear strength parameters. Deep weathering (≏ 8 m) of Devensian till is limited in its geographical distribution to eastern Britain and can be related to the glacial erosion of sulphide-bearing lithologies. The oxidation of disseminated and comminuted sulphides in these Devensian tills is accelerated by the soil cracking that accompanies the development of summer soil moisture deficits under the lower rainfall conditions of eastern Britain.