Thin Weak Layer Research Articles

Snow Profile Observations for Avalanche Forecasts using the New Generation Rammsonde.

A new generation rammsonde has been developed recently for detection of snowpack property. It measures the reflectivity of red light, the penetration force on a cone 'and the electrical conductivity around the cone simultaneously in a few minutes. Field studies have proved that the new instrument enables detection of weak snow layers, tracing snow layer structures and classification of snowpacks for avalanche forecasts. However, the instrument is incapable of detecting very thin weak layers which were detected by the shovel shear test but cannot be seen by the naked eye.

The Impact of surface perturbations on snow-slope stability

Measurements and observations by others indicate that a potential slab avalanche consists of a relatively cohesive slab of snow overlying a thin weak layer that coniains flaws where locally the shear stress from the overburden is not fully supported. Under favorable conditions, snow will shear strain-soften, which provides the basis for applying a slip-weakening model to examine the size of flaw needed to initiate sub-critical crack propagation along the weak layer. Using typical values for snow properties, the model predicts sub-critical crack growth can initiate from a relatively small flaw well before the shear stress from the overburden approaches the peak shear strength at tin-bed. The occurrence of small flaws or imperfections in the basal layer would explain field measurements which usually indicate that avalanching occurs before the applied shear stress exceeds the shear strength at the basal layer.Widespread slab-avalanche activity often increases significantly soon after the onset of rain on new snow. Measurements of temperature and mechanical properties show that only the upper 0.15 m or less of the slab has been altered at the time of avalanching; alterations at the sliding layer have not yet been detected. Results from the slip-weakening model indicate that the rain-induced alterations would reduce the size of flaw needed to initiate sub-critical crack growth by 10–20%. The observations and model results show clearly the importance of the slab properties; it is evident that both the slab and the weak layer act together to control slope stability. A further implication is that the stability of freshly deposited snow is often close to critical, because a relatively small surface perturbation is often sufficient to cause avalanching. This is not surprising, because it is well known from field observations that new snow on slopes should be treated with caution.

The Impact of surface perturbations on snow-slope stability

Measurements and observations by others indicate that a potential slab avalanche consists of a relatively cohesive slab of snow overlying a thin weak layer that coniains flaws where locally the shear stress from the overburden is not fully supported. Under favorable conditions, snow will shear strain-soften, which provides the basis for applying a slip-weakening model to examine the size of flaw needed to initiate sub-critical crack propagation along the weak layer. Using typical values for snow properties, the model predicts sub-critical crack growth can initiate from a relatively small flaw well before the shear stress from the overburden approaches the peak shear strength at tin-bed. The occurrence of small flaws or imperfections in the basal layer would explain field measurements which usually indicate that avalanching occurs before the applied shear stress exceeds the shear strength at the basal layer.Widespread slab-avalanche activity often increases significantly soon after the onset of rain on new snow. Measurements of temperature and mechanical properties show that only the upper 0.15 m or less of the slab has been altered at the time of avalanching; alterations at the sliding layer have not yet been detected. Results from the slip-weakening model indicate that the rain-induced alterations would reduce the size of flaw needed to initiate sub-critical crack growth by 10–20%. The observations and model results show clearly the importance of the slab properties; it is evident that both the slab and the weak layer act together to control slope stability. A further implication is that the stability of freshly deposited snow is often close to critical, because a relatively small surface perturbation is often sufficient to cause avalanching. This is not surprising, because it is well known from field observations that new snow on slopes should be treated with caution.

Failure Mechanism of Landslide with “squeezing-out” deformation

The Nebukawa-station Landslide was triggered by the Kanto earthquake in 1923. Both geological and geotechnical studies point out that the landslide failed rapidly by shearing (compression failure) in the weak pumice layer which was accompanied with the unique deformation named “squeezing-out” that is, the upper layer, mainly massive lava flow, subsided and squeezed out the underlying pumice layer to the front of slide, and the slope collapsed successively.The elastic-plastic FEM analyses both on the landslide and the simplified three-layers model indicate that the “squeezing-out” is one of the common deformation type on slope which consists of thin weak intertrappean layer (e. g. pumice) and gentle dipping (less than 10 degree) thick rigid layers (e. g. lava).Several geologic evidences support the idea that the landslide with the “squeezing-out” deformation occurs in dissected slope of stratovolcano, and sometimes leads to the run-off of debris, high speed and long distance; these will then provide basic information useful for disaster prevention planning.

Experience with Shear Frames

AbstractThe shear frame is a simplein situdevice for indexing the shear strength of thin weak layers. The index is sensitive to shear-frame geometry, rate-of-pull, and shear-frame mass. It is time-consuming to carefully align the device on theGleitschicht(shear failure plane) in a slab avalanche zone. The ratioshear frame index/shear stressof theGleitschichthas a high variance, and may not be a fundamental measure of slab avalanche stability. Corrections for the normal stress on theGleitschichtreduce the variance only slightly. Despite these limitations, the shear frame is a useful tool for gathering statistical data on strength distributions and anisotropies of theGleitschichtuntil a more fundamental technique is developed.

Experience with Shear Frames

AbstractThe shear frame is a simple in situ device for indexing the shear strength of thin weak layers. The index is sensitive to shear-frame geometry, rate-of-pull, and shear-frame mass. It is time-consuming to carefully align the device on the Gleitschicht (shear failure plane) in a slab avalanche zone. The ratio shear frame index/shear stress of the Gleitschicht has a high variance, and may not be a fundamental measure of slab avalanche stability. Corrections for the normal stress on the Gleitschicht reduce the variance only slightly. Despite these limitations, the shear frame is a useful tool for gathering statistical data on strength distributions and anisotropies of the Gleitschicht until a more fundamental technique is developed.

Shear fracture precipitated by strain softening as a mechanism of dry slab avalanche release

Snow stratigraphy studies at the fracture lines of dry slab avalanches often indicate a situation that consists of a harder thick layer overlying a thin weak layer. Prefracture stress conditions and progressive fracture of snow slabs are discussed here from the point of view that slow shear failure in the weak layer results in Griffith‐like critical lengths there, resulting in rapid shear fracture and subsequent avalanche release. The important aspects of the timing of dry slab avalanche release and the relationship between the applied shear stress and the shear strength at the weak layer are discussed in this context. The results show that the main experimental facts on snow and field observations of snow slab avalanches are explainable from this approach, although direct, concrete proof is not presently available.