Clay smear in Sandbox experiments
by J. Schmatz & J.L. Urai
The aim of this study is to improve the knowledge of the mechanical and
kinematic evolution of clay smear in order to enhance the potential of
quantitative fault seal predictions. The experimental models consist of layered
sand-clay sequences with carefully characterized material properties, which
are water- saturated and deformed in a novel underwater sandbox. During the
experiments sand is drained (pore pressure is everywhere hydrostatic) and
clay is undrained with development of overpressure in some experiments.
Results show a strong dependence on the geometry (number of layers and
layer thickness) of the layers and on mechanical properties of the clay.
Movies:
6.2- Open Fracs in Sand – soft clay:
Description:
Multilayers of soft clay develop relays between basement fault-related
deformation bands which initially propagate in directions related to the elastic
stress field and later evolve into kinematically more favourable positions.The
sheared sand and clay layers become stabilized and thicken by grain-scale
mixing.
6.2- Clay smear experiment – firm clay:
Description:
Models in which the clay is strong enough to develop tensile fractures, the
clay layers fracture and rotate in the fault zone as angular boudins. In models
with intermediate clay strength the boudins are progressively eroded and
softened to become a soft clay gouge.
Open fractures in Sandbox experiments
by Heijn van Gent, Marc Holland & J.L. Urai
The aim of this study is to explore of the mechanical and kinematic evolution
of normal faults in cohesive rocks, such as carbonates near the surface, but
also in basalts. The experimental models consist of hemihydrate with carefully
characterized material properties, with different geometries of intercalated
sand or hemihydrate/graphite-mixtures. While pure Hemihydrate is relatively
firm, with a true cohesion and tensile strength, the sand layers are weaker
and the mixture layers are a little stronger. The effect of this mechanical
stratigraphy was studied in an experimental series. Results show open
fractures in the upper 2/3 of the box with a strong dependence on the
geometry (number of layers, layer material and layer thickness) of the layers.
The compaction of the hemihydrate and the related increase of strength in the
deformation box results in changes in failure mode with depth. The inclusion
of sand layers leads to the formation of “clay smear like” structures in the
deformation zone.
Movies:
6.2-Open Fracs In Sandbox-Pure:
Description:
Hemihydrate (CaSO4 • ½ H2O) is a powder with a true, measurable cohesion
and tensile strength. It is therefore capable to sustain open fractures along the
fault plane. Similar cavities in nature have a huge impact on fluid flow.
Model: 20cm pure Hemihydrate
6.2-Open Fracs In Sandbox-Sand:
Description:
The inclusion of sand layers in cohesive Hemihydrate (CaSO4 • ½ H2O)
powder leads to much wider and complex deformation zones. The sand layers
smear into structures similar to clay smears in these zones.
Model: 20cm pure Hemihydrate with four sand layers of 0.5cm thickness
6.2-Open Fracs In Sandbox-Mixture:
Description:
The inclusion of layers with hemihydrate/ graphite mixtures in cohesive
Hemihydrate (CaSO4 • ½ H2O) powder leads to much wider and complex
deformation zones. Cavities form along the main faults in the model.
Model: 20cm pure Hemihydrate with four hemihydrate/ graphite mixture
layers of 1cm thickness