Essay About Position Of Thrust Sheets And Margins Of Landslide Toe Blocks

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The position of thrust sheets around
the margins of landslide toe blocks, and their
morphology and direction of thrusting, suggests that
they were formed as a result of toe block pressing and
movement in the surrounding sand. Toe-thrust sheets
therefore can be considered as the morphological
expression of ongoing instability at the landslide
toe. The upthrust nature of these sheets at West
Runton suggests that rotation of toe blocks,
generating forward movement of the surrounding loose
beach sand, is the principal process of toe-thrust
sheet formation (fig. 6). Passive pressing of toe
blocks into the surrounding sand under gravity is
unlikely to result in either brittle failure of the
sand or differential movement of the sand away from
the toe blocks (i.e., different thrust sheet widths).
The presence of thrust sheets therefore suggests that
landslide blocks are actively excavating into the
softer and unconsolidated beach sand that is displaced
outward as a result of this process.
The size and extent of the thrust sheets can be used
as a proxy for the scale, rate, and timing of block
movement. For example, the bigger landslide blocks are
associated with more extensive thrust sheets, and
sheet width is likely associated with excavation
depth. The presence of multiple and overlapping thrust
sheets that vary in extent along the front of
landslide blocks (figs. 4, 5) also suggests that
different parts of the toe are active at different
times and therefore that sliding rates and volumes
averaged across the entire landslide (Waltham and
Dixon 2000) likely conceal wide spatial and temporal
variations.
[Graphic omitted] The presence of delicate toe-thrust
sheets within the intertidal zone at West Runton is of
interest because these features are easily destroyed
by waves and will be wiped out by every high tide.
Figure 6 is a schematic cartoon illustrating a
possible formation mechanism for these features.
During high tides, the elevated position of the
external water plane (mean high water level) against
the landslide toe means that there is a small
difference in head, and low hydraulic potential
gradient, between the landslide toe and its external
environment (fig. 6a). The depth of marine water also
likely increases interstitial pore water pressure both
within the submerged beach sand and within the
fine-grained landslide sediments and influences
effective pressure (cf. Dixon and Bromhead 2002).
Elevated external water pressure at high tide helps to
hold back toe advance (Hutchinson 1988).
At low tide, when the ground water table is located
within the beach sand and is under lower (atmospheric)
interstitial pore water pressure, a large difference
in head and therefore steepened hydraulic potential
gradient exists between the landslide toe and the
external environment (sea level) (fig. 6b). Under
these conditions, sliding and the formation of
toe-thrust sheets can occur. The depth of thrusting
may be limited