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 WestRunton 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.
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[Graphic omitted] The presence of delicate toe-thrust sheets within the intertidal zone at West Runt on 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. 6 a).
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. 6 b).
Under these conditions, sliding and the formation of toe-thrust sheets can occur. The depth of thrusting may be limited to the position of the ground water table within the upper beach. This model suggests that landslide instability and toe sheet thrusting may be rhythmically driven by tidal unloading and loading of beach and slide sediments and by the position of the external water plane (mean high / low water positions and position of water table).
The elevation of the external water plane also has implications for effective pressure within the beach sand. Pore water pressure is unlikely to vary significantly within the fine-grained glacial till, which has low and lows hear strength (Bell 2002), so water level variation in the surrounding environment (i. e. , by the rise and fall of tides) is the likely driver of changes ineffective pressure and susceptibility of beach sand to failure.
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Formation of toe-thrust sheets likely occurs at low tide when there is the steepest hydraulic gradient between toe sediments and sea level and where interstitial water can drain from beach sand (fig. 6).
Conversely, formation of toe-thrust sheets has not been observed during high tide, which is known to be associated with a slowdown in flow velocity of coastal landslides (Hutchinson 1988).
[Graphic omitted] This tidally dependent model can also help explain the intermittent and cyclic nature of landsliding in coastal settings (Grainger andKalaugher 1987; Hutchinson 1988).
Support for the model comes from previous studies that consider the role of pore water pressure variations in controlling landslide movement. For example, many studies show a clear relationship between precipitation (in increasing head and reducing effective pressure and friction) and sliding rate (Julian and Anthony 1996; Ka laugher et al.
2000; Waltham and Dixon 2000; Guzzetti et al. 2004).
Dixon and Bromhead (2002), using data, described the sensitivity of coastal landslide toes to changes in the external environment including coastal erosion, slide morphology, and tidal loading.