Rock, debris and earth spreads (including landscape creep)

Additional scientific description

Spreads are tensional features that are susceptible to changes in their geotechnical properties at the stratal boundary, commonly at low angles of dip. They manifest as an extension of cohesive soil or rock mass, commonly combined with subsidence. Typically, subsidence may occur because of liquefaction, suffosion, tectonic spreading, uplift, or hydrological change leading to creep or deep erosion, which may be naturally occurring or due to anthropogenic activity. Related phenomena can include the formation of fractures, gulls and lineations.

In rock spreads, solid ground extends and fractures, pulling away slowly from stable ground and moving over the weakened layer without necessarily forming a recognizable surface of rupture. The softer, weaker unit may, under certain conditions, squeeze upward into fractures that divide the extending layer into blocks. In earth spreads, the upper stable layer extends along a weaker underlying unit that has flowed following liquefaction or plastic deformation. If the weaker unit is relatively thick, the overriding fractured blocks may subside into it, translate, rotate, disintegrate, liquefy, or even flow.

Spreads can also be triggered in sensitive clays (also called quick clays or Leda clays) and involve the rapid translation and dislocation of a coherent soil mass in blocks in the form of horst and graben found above a weak horizon of remoulded, i.e., liquefied, sensitive clay (Cruden and Varnes, 1996; Hungr et al., 2014). When the soil mass completely liquefies, it will flow, which is then termed a flowslide (Hungr et al., 2014). In either case, the failure is retrogressive, meaning that the mass movement mainly progresses upslope (Richer et al., 2020). Such slope failures have been observed in the northern hemisphere in Scandinavian countries, USA (e.g., Alaska, Maine).

Spreads in sensitive clays usually subside in the form of a crater downslope leaving intact blocks of material within the crater. If the sediment completely liquefies, the crater is emptied as the sediment flows outside (Richer et al., 2020).

Landscape creep is the slow downward movement of slope-forming soil or rock, typically less than 1 m deep. Movement is caused by shear stresses that are sufficient to produce permanent deformation, but too small to produce shear failure (adapted from Hutchinson, 1968; Varnes, 1978). Three types of creep are recognised: within the depth affected by seasonal change in soil moisture and temperature; continuous where shear stress continuously exceeds material shear strength; and progressive where slopes are reaching the point of failure for other mass movement failure processes.

Solifluction in permafrost zones represents the slow mass-wasting associated with freeze-thaw action whilst the saturated soil movement associated with ground thawing is gelifluction (Washburn, 1979 and French, 1996).

In seismic zones, liquefaction may temporarily weaken the lower layer. Other triggering factors include ground loading, riverbank and coastal erosion and plastic deformation of unstable material at depth, e.g. salt.

Metrics and numeric limits

Landscape creep can be regional (tens of square kilometres) or confined to small areas, e.g. due to aspect. Creep rates are slow (0.5 to 15 mm/ year according to Saunders and Young, 1983). In contrast liquefaction-related movement is rapid, subsiding as post-seismic drainage occurs, but it can also be regional in extent.

An example of a spread in sensitive clays occurred in Sant-Jude, Québec, Canada on May 10th, 2010 where a family of four perished in their house during the rapid mass movement. Its volume was estimated at 520 000 m3, with a length of 80 m and width of 275 m. The slope gradient varied between 12° and 20° (Locat et al., 2017). The trigger was thought to be high porewater pressure during spring thaw.

Moreover, 14 historical cases were documented in detail in Québec where the width varied between 145 to 955 m and distance of retrogression ranged from 75 to 675 m. In sensitive clay spreads, the width is generally larger than the length (Locat et al., 2018).