Design guidance for protection of geomembrane liners in landfill applications

A new method of evaluating strains and predicting required protection layers that are placed over geomembranes is developed based on the combined effects of the clay strength and stiffness and the cushioning effect of a nonwoven protection layer. Plots giving the required geotextile protection for a different maximum strains are presented for expected landfill pressures under 300 kPa for angular, 38 mm gravel placed above a geomembrane liner for both drained and undrained loading conditions of the clay. A similar plot is also given for tire derived aggregate placed above the liner for pressures under 500 kPa for undrained loading conditions. All tests were conducted at room temperature. For all cases, nonwoven geosynthetic protection layers with mass per unit areas (MUA) exceeding at least 1200 g/m2 were required to limit long term strains below current threshold levels, such as a 4% strain target as given by Rowe and Yu (2019). The MUA of the protection layer, for the particular aggregates and geomembranes tested, is dependent on the loading rate, water content relative to optimum, the silt content, and the activity of the clay below.

Why is the Tuaheni Landslide Complex a spreading failure?

<p>Numerous subaqueous landslides exhibit spreading failure morphologies which are typically characterized by repetitive patterns of parallel ridges and troughs oriented perpendicular to the direction of movement. Whilst these spreading failures are commonly attributed to (i) downslope removal of material causing unloading of the temporary stable slope or (ii) significant loss of shear strength of the substratum allowing blocks of overlying sediment to detach and slide downslope, their movement rates and potential triggers remain poorly constrained. <!-- Suggest wording this to set up motivation of research -->Spreading appears to be a dominant failure mechanism within the Tuaheni Landslide Complex (TLC)<!-- I’m not wedded to this name but makes some sense to stick with the same --> on the Hikurangi Subduction Margin off the coast of Gisborne, New Zealand. A combination of swath bathymetric, 2D and 3D seismic data, drilling investigations and laboratory experiments on sediments recovered from the TLC indicate that this geomorphology has been generated by translational failure. Failure could occur through episodic cycles of movement-arrest in response to either elevated pore fluid pressures or undrained loading during earthquakes<!-- We now have some data from dynamic experiments which would be good to include in the model -->. <!-- You will know better than me Morelia but it might pay to be a little more circumspect at this stage? -->We developed numerical models that integrate this unique data set to explore the processes that lead to spreading failure and determine how large shear strains can be accommodated without accelerating to catastrophic failure. The results provide a novel approach that demonstrates how seafloor morphology can, in part, be controlled by the underlying failure processes</p>

Enhanced landslide mobility by basal liquefaction: The 2014 State Route 530 (Oso), Washington, landslide

Landslide mobility can vastly amplify the consequences of slope failure. As a compelling example, the 22 March 2014 landslide near Oso, Washington (USA), was particularly devastating, traveling across a 1-km+-wide river valley, killing 43 people, destroying dozens of homes, and temporarily closing a well-traveled highway. To resolve causes for the landslide’s behavior and mobility, we conducted detailed postevent field investigations and material testing. Geologic and structure mapping revealed a progression of geomorphological structures ranging from debris-flow lobes at the distal end through hummock fields, laterally continuous landslide blocks, back-rotated blocks, and finally colluvial slides and falls at the landslide headscarp. Primary structures, as well as stratigraphic and vegetation patterns, in the landslide deposit indicated rapid extensional motion of the approximately 9 × 106 m3 source volume in a closely timed sequence of events. We identified hundreds of transient sand boils in the landslide runout zone, representing evidence of widespread elevated pore-water pressures with consequent shear-strength reduction at the base of the slide. During the event, underlying wet alluvium liquefied and allowed quasi-intact slide hummocks to extend and translate long distances across the flat valley. Most of the slide material itself did not liquefy. Using geotechnical testing and numerical modeling, we examined rapid undrained loading, shear and collapse of loose saturated alluvium, and strong ground shaking as potential liquefaction mechanisms. Our analyses show that some layers in the alluvium can liquefy when sheared, as could occur with rapid undrained loading. Simultaneous ground shaking could have contributed to pore-pressure generation as well. Two key elements, a large and rapid failure overriding wet liquefiable sediments, enabled the landslide’s high mobility. Basal liquefaction may enhance mobility of other landslides in similar settings.

Modelling of the Response of Partially Saturated Non-cohesive Soil Subjected to Undrained Loading

The paper deals with the modelling of the undrained response of non-cohesive partially saturated soils subjected to triaxial compression. The model proposed is based on an incremental equation describing the pre-failure response of non-cohesive soils during shearing. The original model, developed by Sawicki, was modified by taking into account pore fluid compressibility. The governing equation makes it possible to simulate effective stress paths under undrained conditions. Numerical results are compared with experimental data.