Characterization of Fillite as a potential Martian regolith simulant

This thesis presents the results of a research program characterizing a soil
simulant called Fillite, which is composed of alumino-silicate hollow microspheres
harvested from the pulverized fuel ash of coal-fired power plants. Fillite is available in
large quantities at a reasonable cost and it is chemically inert. Fillite has been selected by
the National Aeronautics and Space Administration (NASA) Glenn Research Center to
simulate high-sinkage/high-slip environment in a large test bed such as the ones
encountered by the Spirit rover on Mars in 2009 when it became entrapped in a pocket of
soft, loose regolith on Mars. The terms high-sinkage and high-slip used here describe the
interaction of soils with typical rover wheels. High-sinkage refers to a wheel sinking with
little to no applied force while high-slip refers to a spinning wheel with minimal traction.
Standard material properties (density, specific gravity, compression index,
Young’s modulus, and Poisson’s ratio) of Fillite were determined from a series of
laboratory tests conducted in general accordance with ASTM standards. Tests were also
performed to determine some less standard material properties of Fillite such as the small
strain shear wave velocity, maximum shear modulus, and several pressure-sinkage
parameters for use in pressure-sinkage models. The experiments include an extensive
series of triaxial compression tests, bender element tests, and normal and shear bevameter
tests.
The unit weight of Fillite on Earth ranges between 3.9 and 4.8 kN/m
3
, which is
similar to that of Martian regolith (about 3.7 – 5.6 kN/m3
) on Mars and close to the range
of the unit weight of lunar regolith (about 1.4 – 2.9 kN/m3
) on the Moon. The data
presented here support that Fillite has many physical and mechanical properties that are
similar to what is known about Martian regolith. These properties are also comparable to
lunar regolith. Fillite is quite dilatant; its peak and critical angles of internal friction are
smaller than those of most other simulants. Smaller shear strength, coupled with much
smaller bulk unit weight as compared to other simulants, results in smaller bearing and
shearing resistances allowing for better simulation of the intended high-sinkage, high-slip
behavior for rover mobility studies.
The results of the normal bevameter tests were used to determine parameters for
two models available in the literature - the Bekker model and the New Model of Mobility
(N2M) model. These parameters were then used to predict the sinkage of a Spirit rover
wheel if the rover were to be used on Fillite. The predicted sinkage of a Spirit rover
wheel in Fillite was 84% of the wheel diameter, which was within the observed sinkage
of 50 to 90% of the wheel diameter of the Spirit rover on Mars. Shear bevameter tests
were also performed on Fillite to assess the shear stresses and shear deformations
imparted by wheels under torsional loads. The results compared well to the estimated
shear stresses and deformations of Martian soil caused by the wheels of the Spirit rover.
When compared to other simulants (e.g. GRC-1), the pressure-sinkage and shear stressshear
deformation behaviors of Fillite confirm that Fillite is more suitable for highsinkage
and high-slip rover studies than other typical simulants derived from natural
terrestrial soils and rocks.