Method: Detecting subsurface slipface shear
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Fig 1. Contrasting sand emplaced

Fig 2. Displacement by avalanche

When an avalanche tongue flows down a slipface, does it flow over the previous surface or does it entrain underlying sand, creating sub-surface shear? The following technique was developed to answer that question, and if there is a shear zone, locate it.

The technique is to insert a pencil-thin column of sand of contrasting colour into the slipface, and then see how the column is affected by an avalanche.

The insertion is performed by loading an ordinary drinking straw (about the diameter of a pen or pencil) with sand of a contrasting colour, and then inserting the straw perpendicularly into the slipface and pulling out the straw. As the straw is withdrawn, the sand will leave the straw and form a column.

It's good to insert and withdraw the straw with as little lateral movement as possible so that the column is cylindrical.

Subsequent shearing of the sand containing the column will displace the column, as shown in figure 2.

Example: Finding the shear surface beneath an avalanche

Fig 3. Red sand column in sand over which an avalanche flowed. Displacement of the red sand indicates subsurface entrainment and shearing occurred. A region of unwetted sand made this excavation imperfect.

A column of red sand (from south Utah) was emplaced in the lower slipface of a transverse dune at Great Sand Dunes National Monument, in Colorado, USA (figure 3, to the right). A thin bamboo stake was inserted at a known distance (2 cm) laterally on the slope from the red sand column, with marks at measured intervals, aligned with the surface. An avalanche was induced (by throwing a stone at a cornice inflection point).

After the avalanche flow, water is used to stabilize the sand for excavation, with the goal of wetting the sand around the marker stake and the red sand column. The marker stake will also help stabilize the moistened sand. A barrier is inserted upslope, and dry sand is carefully removed from around the moistened sand. Then the moist column is carefully sliced away, millimeter at a time from the top or side, until red sand is encountered. That point can be compared with the mark on the bamboo stake which was aligned at the former surface of the slipface. If they differ in depth, then there was subsurface entrainment and shearing (as was the case illustrated in figure 3).