On the potential of gassy marine soils triggering submarine slope instabilities

The development of debris flows and turbidity currents in the course of a submarine slope failure event can cause major damage in offshore infrastructure. Additionally, the tsunamogenic potential of large slope failures at continental margins poses a direct threat to coastal communities. Therefore, the trigger mechanisms of submarine slope failures have been thoroughly investigated in the past. However, the influence of free gas in the sediment, which has been observed close to several slide events, remains unexplained. In order to evaluate the potential of gassy marine soils to precondition or trigger slope failure the mechanical behaviour of gassy soils is assessed based on an extensive literature review. It is found that gas-induced excess pore pressures can lead to liquefaction failure in sands, while cohesive, gassy soils show a less conclusive response. Hence, fine-grained soils and approaches to implement the gas impact into relevant existing constitutive soil models are assessed in greater detail. Concludingly, based on the predominant boundary conditions in failure prone regions at the continental margins, free gas occurrence can be defined as a preconditioning factor rather than as a definite trigger mechanism.

Morphological signature of gully development by rapid slide retrogression in a layered coarse-grained delta foreslope

Coarse-grained deltas are often characterized by steep foreslopes (often more than 10°) that are traversed by delta-front channels. The channels thus erode into relatively steeply inclined bedding. In this context, the slopes flanking the channels can be steeper than the friction angle since they include a component of dip related to the delta-front slope as well as the channel-related erosion slope. In this study, part of the Busu River delta (Papua New Guinea) was imaged using a high-resolution multibeam bathymetry survey over an area where the angle of the slopes flanking the channels locally reaches 50°. A detailed analysis of the delta slope morphology has revealed an additional source of instability due to erosion within the main channels. In some places, erosion cuts into the channel flank forming a local knickpoint inclined in a direction approaching that of the bed dip. The cut can then initiate breaching or static liquefaction failure from that point up to the crest of the interfluve resulting in a V-shaped gully.

Studies on Liquefaction Failure Forms and its Damage to Engineering

The liquefaction failure forms are reviewed, including foundation strength failure, sand boil, seismic settlement, large ground displacement and flow slide. Taking pile foundations and buried pipeline as examples, there suggested some measures to reduce hazards of liquefaction.

Influence of soil nail orientations on stabilizing mechanisms of loose fill slopes

Soil nailing has been used to upgrade substandard loose fill slopes in Hong Kong. Due to the possibility of static liquefaction failure, a typical design arrangement comprises a structural slope facing anchored by a grid of soil nails bonded into the in situ ground. Numerical analyses have been conducted to examine the influence of soil nail orientations on the behaviour of the ground nail–facing system. The results suggest that the use of steeply inclined nails throughout the entire slope could avoid global instability, but could lead to significant slope movement especially when sliding failure prevails, for instance, due to interface liquefaction. The numerical analyses also demonstrate that if only subhorizontal nails are used, the earth pressure exerted on the slope facing may cause uplift failure of the slope cover. To overcome the shortcomings of using soil nails at a single orientation, a hybrid nail arrangement comprising nails at two different orientations is proposed. The numerical analyses illustrate that the hybrid nail arrangement would limit slope movement and enhance the robustness of the system.

SEABED INSTABILITY AROUND CAISSON BREAKWATERS

A sensibility numerical study, considering the soil conditions found at Frontera Port in Tabasco, Mexico was conducted to identify the variables that govern the response of a seabed-foundation-structure system subject to wave loading. Among all the possible causes of instability, this study deals only with those associated with liquefaction failure of silty-sands due to cyclic shear stresses generated by regular waves. This research was prompted by the accidents that have occurred near Frontera Port, the most serious of which took place in October 2007 when the Usumacinta oil platform settled, causing 21 fatalities. Previous analysis of this accident (Leis, et al, 2007) suggested that the platform did not present any structural failure and that the accident was a result of an unexpected behavior of the seabed; probably liquefaction. In order to offer results regarding coastal protection activities in the study area, the analysis presented here was developed simulating a vertical breakwater similar to that constructed in 2001 at Barcelona instead of the oil platform. Puzrin, et al, 2009 report that in November 2001 four caissons of this breakwater failed due to seabed liquefaction. The adaptation of the design of the vertical breakwater to the study area conditions was estimated by means of the Goda, 1985 formula.