Coupled thermo-hydro-mechanical modelling of crack development along fossil dinosaur’s footprints in soft cohesive sediments

The fossil footprints have been used to back calculate the properties of the soil in the Age of Dinosaurs. The interpretation of fossil footprints requires the simulation of the processes during as well as after the footprint was generated. Some radial and circumferential cracks were observed occurring on track walls of the footprints. It is supposed that the origin of these cracks can be elucidated by means of footprint’s drying after the footprint is generated. In order to verify this hypothesis and to allow for a precise interpretation of the dinosaur tracks, a series of laboratory and numerical simulation tests was carried out. The tests were designed to mimic the shape evolution of the footprint of the dinosaur during drying. Within the experiments the change of environmental humidity and temperature was monitored and recorded. The laboratory experiment showed that both the radial and circumferential cracks appear during the drying process. The numerical simulation has been performed to better understand and to account for the cracking mechanism in dinosaur’s tracks. In this study the behaviour of a silty soil during drying is numerically simulated by means of a 3D model and performing coupled thermo-hydro-mechanical analysis utilizing the finite element program CODE BRIGHT. Based on the analysis of the tensile stresses along the sample, it was found that the highest tensile stress is on the track wall and it is due to soil shrinkage. It can be concluded that the high tensile stress induced during drying is the most possible reason for cracks to appear in radial and circumferential direction along the foot’s imprint

Coupled thermo-hydro-mechanical modelling of crack development along fossil dinosaur’s footprints in soft cohesive sediments

The fossil footprints have been used to back calculate the properties of the soil in the Age of Dinosaurs. The interpretation of fossil footprints requires the simulation of the processes during as well as after the footprint was generated. Some radial and circumferential cracks were observed occurring on track walls of the footprints. It is supposed that the origin of these cracks can be elucidated by means of footprint’s drying after the footprint is generated. In order to verify this hypothesis and to allow for a precise interpretation of the dinosaur tracks, a series of laboratory and numerical simulation tests was carried out. The tests were designed to mimic the shape evolution of the footprint of the dinosaur during drying. Within the experiments the change of environmental humidity and temperature was monitored and recorded. The laboratory experiment showed that both the radial and circumferential cracks appear during the drying process. The numerical simulation has been performed to better understand and to account for the cracking mechanism in dinosaur’s tracks. In this study the behaviour of a silty soil during drying is numerically simulated by means of a 3D model and performing coupled thermo-hydro-mechanical analysis utilizing the finite element program CODE BRIGHT. Based on the analysis of the tensile stresses along the sample, it was found that the highest tensile stress is on the track wall and it is due to soil shrinkage. It can be concluded that the high tensile stress induced during drying is the most possible reason for cracks to appear in radial and circumferential direction along the foot’s imprint

Triaxial test results for the determination of stiffness E₅₀ [kN/m²] of Rhine sand with an initial density of e = 0.6.

<p>Blue, green and grey line: Deviatoric stress is plotted against axial strain for experiments conducted at 50 kN/m², 100 kN/m², and 150 kN/m² confining pressure, respectively. The stiffness E₅₀ is the secant stiffness over the first 50% of the deviatoric stress.</p

Sequence of footfalls in elephant walk after [5].

<p>The static loading conditions (loading steps 1 to 4) simulated by FEA are marked and quantified within the sequence. The leftmost loading step is loading step 1, with the elephant at a standstill. Black bars indicate ground contact of the respective foot. fl  =  left forefoot, fr =  right forefoot, hl  =  left hindfoot, hr  =  right hindfoot. See text for a detailed description of the loading steps.</p

Results of dry density and water content profile measurements.

<p>Soil samples were obtained from the prepared test field by manual sampling with a metal tube. Samples were taken inside and outside several footprints, indicated by differing sampling depths, i.e., differing starting points of the top of the tube. Footprints are displayed schematically, for detailed information see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077606#pone-0077606-g011" target="_blank">Figure 11</a>.</p

Geometry and generated mesh of the FEA model and interfaces.

<p>See text for a detailed description of the model.</p

Capture of elephant footprints geometry using 3D laser scanner.

<p>A total of six footprints were scanned, i.e., three pairs, each of them consisting of one forefoot imprint (right) and one hindfoot imprint (left). Each pair is pictured by a photograph (top), 3D surface plot (center), and a 2D longitudinal section plot (bottom).</p

Grain-size distribution of Rhine sand.

<p>Grain sizes are given for characteristic values, i.e., for 10% (), 30% (), and 60% () of the sand passing the corresponding mesh size by weight.</p

Vertical sections of FEA model at loading steps 2 to 5.

<p>Colors indicate amount of deformation.</p