Fig 1.
Ghost crabs and their burrows.
Panels: (a) Burrow entrances of ghost crabs on a sandy beach on Ishigaki Island, Okinawa Prefecture, Japan (arrows point to the burrow entrances), (b) the horn-eyed ghost crab Ocypode ceratophthalma, and (c) a plaster mold (cast) of a ghost crab burrow obtained from in Ishigaki Island, where Ocypode ceratophthalma dominants the ghost crab fauna.
Fig 2.
(a) Map of Japan with locations of Tsu, Mie Prefecture, Japan and Ishigaki, Okinawa Prefecture, Japan. Inset maps (b) of Tsu (Kawage Beach) and (c) Ishigaki (Sakieda Beach). Survey area dimensions at (d) Tsu and (e) Ishigaki. For Panel (f), we used the measured quantities and X-Z coordinate system (inset), where X denotes the slope distance from shoreline (along the surface of the beach). Z denotes the vertical distance (depth) from the surface to a given point at depth.
Fig 3.
Schematic of an experimental container used to determine stability of a horizontal tunnel through wet granular substrate.
This horizontal tunnel was 20 mm in diameter with a 20 mm of overlaying substrate. This container had the following dimensions: x = 100 mm, y = 50 mm, and z = 120 mm, but container sizes and tunnel diameters varied slightly to test different effects of sands on various tunnel diameters.
Table 1.
Size of experimental containers (microcosms).
Table 2.
Experimental parameters.
Fig 4.
Grain size distribution of beach sand collected at the Tsu site.
Panels: (a) The horizontal axis is distance from shoreline (X), and (b) is substrate depth (Z). Beach sand samples were sifted into four diameter particle sizes: d < 0.3 mm (turquoise bars), 0.3 < d < 0.71 mm (green bars), 0.71 < d < 1.4 mm (yellow bars), and 1.4 mm <d (pink bars). Wt = percent weight.
Fig 5.
Water content and packing fraction of sand with distance from shoreline and at depth for beach sand sampled at the Tsu site.
Panels (a) and (c) depict sample results of water content (W) and packing fraction (ϕ) at depths (Z) 10 cm for each distance from shoreline (X). Panels (b) and (d) depict water content (W) and packing fraction (ϕ) measured at shoreline distances (X) of 15, 20, and 25 m.
Fig 6.
(a) Histogram of burrow diameters (Db) at the Tsu site. The maximum and minimum of tunnel diameter are 38 mm and 10 mm, respectively. (b) Histogram of the vertical depth (H) of burrows at the Tsu site. The maximum and minimum of depth values are 43 cm and 10 cm, respectively.
Fig 7.
Burrow characteristics at the beach sites.
Panels: (a) Cumulative number of burrows whose entrance positions are closer than the depicted distance from the shoreline (X) and the water content of sand at distance X measured from the shorelines (and the depth Z = 10 cm) at the Tsu site (solid curves) and Ishigaki site (dashed curves). (b) Cumulative number of burrows shallower than depth H at 17.5–22.5 m distance from shoreline (green squares) and water content of sand versus tunnel depth at 20 m from the shoreline (blue triangles), measured at Tsu site.
Fig 8.
Depth of burrows relative to groundwater elevation and distance from shoreline (Tsu site).
Burrows tend to become deeper as distance from shoreline increases. About 15 m landward from shore, the depth of burrows coincides with the groundwater elevation. However, burrows are shallower than the groundwater level at location more than 15 m from the shoreline. Semi-buried burrows (depicted as exes) indicate relatively old (seemingly inactive) burrows that are almost buried.
Fig 9.
Experimental results on the initial stability of a tunnel in wet granular layer.
Panles: (a) Static stability diagram for glass beads and sand. The ranges in water content where a stable tunnel structure can be maintained are indicated by circles and blue color. Unstable regions are indicated by exes and red color. Although a certain amount of liquid is necessary to keep a tunnel stable, too much liquid prevents the formation of a stable tunnel. (b) Three characteristic outcomes are depicted schematically (immediate collapse, stability, and collapse due to exuding water).
Fig 10.
Phase diagram of tunnel deformation/collapse scenarios obtained from experiments with sifted sand collected from the Tsu site (d < 0.3 mm).
Symbols indicate the three modes of tunnel deformation [shrink (circles), shrink with collapse (triangles), and collapse-by-subsidence (ex)]. The mode of tunnel deformation clearly depends on tunnel diameter (D0) but is almost independent of water content (W0) as was the case for glass beads [8]. The collapse-by-subsidence scenario is only observed at a water content (W0) of 0.02 and tunnel diameter (D0) of about 80 mm. The range of the actual burrow diameters and corresponding water contents is depicted by the yellow rectangle.
Fig 11.
Temporal variations of τ obtained by compression experiments with sand collected at the Tsu site.
Sand grain diameter (d) was < 0.3 mm. The mean (solid curves) and standard deviation (thin bands of matching color) are depicted for three experimental conditions with four tunnel diameters (D0). Stress τ(t) curves are depicted for sands with five different water contents (W0). Panels: (a) D0 = 10.1 ± 0.2 mm, (b) D0 = 20.6 ± 0.2 mm, (c) D0 = 39.3 ± 0.1 mm, and (d) D0 = 77.1 ± 0.1 mm.
Fig 12.
Effective yield stress (τyield) and maximum stress (τmax) in wet sand of two diameters (from the Tsu and Ishigaki sites) relative to various tunnel diameters subjected to compression of overlying sand substrate.
Panels: (a) Water content (W0) relationship between effective yield stress (τyield) using sand collected at the Tsu site (d < 0.3 mm) relative to various tunnel diameters D0, (b) water content (W0) relationship between maximum stress (τmax) using sand collected at the Tsu site relative to various tunnel diameters (D0), (c) water content (W0) relationship between effective yield stress (τyield) using sand collected at the Ishigaki site (sand diameter 0.3 < d < 0.7 mm) relative to tunnel diameter (D0) of 20.5±0.3 mm, and (d) water content (W0) relationship between maximum stress (τmax) using sand collected at the Ishigaki site.
Fig 13.
Relationship between tunnel diameter and depth of actual burrows (circles) and an experimentally obtained pressure yielding depth Hyield (plus marks).
Tunnel diameter (D0) relationship with Hyield was computed from experiments (plus marks) with sand collected at the Tsu site of grain diameter d < 0.3 mm. Colors differentiate various water contents in sand (W0). Circular symbols indicate the relationship between the thickness of sand overburden (H) and tunnel diameter (Db) of actual crab burrows observed in the field.