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Physical and mechanical properties of lunar soil


|V.Gromov |
|VNIITRANSMASH, |
|2, Zarechnaia street, St. Petersburg, 198323, Russia|
| |
|Tel. +7 (812) 1359837 Fax +7 (812) |
|1461618 |
|E-mail: rclvvg@mail.ru |


Resume

The purpose of this paper is to systematise and review the series of
investigation concerning the physical and mechanical properties of the soil
on the Moon. The results of these investigations permit a deeper
understanding of the soil-forming processes of the uppermost layers on the
Moon and on the other planets. They are also needed to clarify general
trends and to provide basic data and engineering models in order to develop
new techniques for planetary exploration. This seems to be of vital
importance nowadays, because we are on the eve of a new stage in the
development of missions to the Moon and the investigation of other planets.



Introduction


The knowledge of the physical and mechanical properties of the soil is
of fundamental importance because it is the basic for engineering activity
aimed at construction of lunar bases and for mineral resource exploration.
During manned flights the astronauts are operating and walking along the
surface and knowledge about surface properties determines considerably the
safety of the entire mission. Therefore, for further investigation and
exploration of the Moon and on the other planets it is vital to know the
physical and mechanical properties of the soil and it is evident that such
studies should proceed in advance to avoid unnecessary risk and expenditure
of certain missions. The first task is to study thoroughly the data
accumulated and to outline the methods for their solution (Gromov, 1999).




1. Basic studies of the physical and mechanical properties of lunar soil

The study of the physical and mechanical properties of lunar soil had
been started even before the first flights to the Moon were realised. The
surveys were mainly based on the results of radio-telescopic investigation
of the lunar surface as well as on studies of terrestrial soils and
artificial materials with the same optical, thermal, and electrical
characteristics as the lunar soil. Those data served as the basis for
designing unmanned lunar spacecraft (Krotikov and Troitsky, 1963).
A new step in the study of lunar soil occurred with the landing of
unmanned spacecrafts on the lunar surface. The information acquired during
these missions provided us with a true picture of the properties of lunar
soil. Based on those experiments, new models of lunar soil were developed,
and also closest analogues of terrestrial soil were selected. Fresh igneous
deposits in the vicinity of intensive volcanic activity proved to be quite
good terrestrial simulates of lunar soil (Cherkasov et al., 1967).
No less important step in the study of lunar soil was the delivery of
soil samples to Earth, followed by their detailed and comprehensive
investigation in terrestrial laboratories.
A large number of studies on physical and mechanical properties of
lunar soil were also performed by means of unmanned vehicles Lunokhod-1 and
Lunokhod-2 and by "Apollo" astronauts. The investigation of lunar soil was
carried out at landing sites which appear to be geomorphologically typical
for the lunar surface.
Studies of physical and mechanical properties of lunar soil were
grouped as follows:
1. Testing of returned lunar soil samples in order to reveal the main
trends and relation of the physical and mechanical properties with respect
to the bulk density (Gromov et al., 1971, Carrier et al., 1973, Leonovich
et al., 1975, Kemurdjian et al., 1976, Gromov et al., 1979);
2. Studying the physical and mechanical properties of lunar soil under in
situ conditions and correlating with the geomorphological environments
conditions (Bazilevsky et al., 1984, Carrier et al., 1991, Kemurdjian et
al., 1976, Gromov et al., 1986, Kemurdjian et al., 1993);
3 Selecting and studying terrestrial analogues of lunar soil for conducting
tests on the Earth (Krotikov et al., 1963, Cherkasov et al., 1975, Gromov
et al., 1992).

2. Lunar soil granulometric composition

One of the main soil characteristics that governs its physical and
mechanical properties is the granulometric composition (i.e., size and
shape of the particles). Particle-size distribution of the soil samples
delivered to the Earth was defined by dispersion, by conductometric method
and by microphotography (Stacheev, 1979). The general conclusion is that
the particle-size distributions are relatively uniform, even though the
samples were taken from different regions of the lunar surface. The
samples, for the most part, consist of small mineral particles that differ
in shape. The particles easily stick to each other to form separate clods
and aggregates. In its granulometric composition, lunar soil resembles
dusty sand.
In the papers of Carrier (1973) and Stacheev (1979), data concerning
lunar soil samples granulometric composition are cited. These data allow to
come to the following general conclusions:
a) the granulometric composition of the lunar soil samples taken from the
different regions of the Moon is sufficiently well submitted to the
logarithmic - normal law of particle size distribution depending on
particle relative contents by weight in soil;
b) the average particle size and particle size standard deviation values
expressed in logarithmic units are well correlated and standard deviation
increases with the average particle size;
c) the particle size standard deviation (in logarithmic units) at the site
of sampling on the Moon depends on a regolith layer overall thickness
(Stacheev, 1979).

Summarized data of granulometric composition parameters of the lunar soil
are listed in Table.1. The soil granulometric composition degree of
inhomogeneity considerably influences the soil physical and mechanical
properties. The degree of inhomogeneity can be represented as an average
particle size to effective particle size ratio. An effective particle size
is the size of such particles the total area of which in the soil mass unit
is equal to the total area of all the soil particles.



Table 1 - Parameters of the lunar soil granulometric composition

|Soil |Average |Particle |Effect|Degree of |
|sample|particle |size |ive |inhomogene|
| |size |standard |partic|ity |
| | |deviation |le | |
| | | |size, | |
| |da, |lg da|(in |de, mm|K=da/de |
| |mm | |logarithmi| | |
| | | |c units) | | |
|Luna-1|0,085|-1,07|0,623 |0,0303|2,81 |
|6 | |1 |0,816 | |5,83 |
|Luna-2|0,077|-1,11|0,620 |0,0132|2,77 |
|0 | |3 |0,586 | |2,49 |
|Apollo|0,098|-1,00|0,677 |0,0354|3,38 |
|-11 | |8 |0,536 | |2,15 |
|Apollo|0,118|-0,92|0,885 |0,0474|7,97 |
|-12 | |8 |0,747 | |4,41 |
|Apollo|0,138|-0,86| |0,0409| |
|-14 | |0 | | | |
|Apollo|0,061|-1,21| |0,0284| |
|-15 | |5 | | | |
|Apollo|0,153|-0,81| |0,0192| |
|-16 | |5 | | | |
|Apollo|0,079|-1,10| |0,0179| |
|-17 | |2 | | | |


More detailed investigation of the particles distribution by their size
for the lunar soil at the sites of the stations "Luna-16" and "Luna-20"
landing revealed increasing average particle size with the depth of soil
sampling. But the effective particle size changes a little, with a.
variation of 0,06- 0,09, which is 3-6 times smaller than the variation of
the average particle size. That is why the effective particle size can
serve as one of the main factors defining soil granulometric composition.
Granulometric composition inhomogeneity factor values depend on regolith
layer thickness at the regions of soil sampling. The same correlations are
revealed at the sites of the soil sampling within the "Apollo" program.
Thus despite a wide range of granulometric composition of soil samples from
different lunar regions, there are common regularities; the most important
for the evaluation of physical and mechanical properties are the relatively
constant particle size at the sampling site and granulometric composition
inhomogeneity factor dependence on the regolith layer thickness.

3. Bulk density and void ratio

The main factor that determines the physical characteristics of a lunar
soil sample is the degree of packing, as estimated by the void ratio (i.e.,
ratio of void volume to solid volume). In Table 2, the bulk density, void
ratio and relative deformation for soil samples delivered by Apollo 11, 12,
14, 15 and Luna 16,20 spacecrafts are listed.


Table 2 - Bulk density and void ratio

|Lunar soil |Bulk |Void ratio |Density of |
|sample |density, | |grains for |
| |g/cm3 | |void ratio |
|Condition |Loos|Compac|Loos|Compac|g/cm3 |
|of soil |e |t |e |t | |
|Apollo 11 |1.36|1.8 |1.21|0.67 |3.01 |
|Apollo 12 |1.15|1.93 | | | |
|Apollo 14 |0.89|1.55 |2.26|0.87 |2.9 |
| | |1.51 | |0.94 |2.93 |
| |0.87| |2.37| | |
|Apollo 15 |1.1 |1.89 |1.94|0.71 |3.24 |
|Luna 16 |1.11|1.793 |1.69|0.67 |3 |
| |5 | | | | |
|Luna 20 |1.04|1.798 |1.88|0.67 |3 |
| |0 | | | | |


In the papers (Carrier, 1991) the following best estimates for the
average bulk density of the lunar soil in the intercrater areas of the
lunar surface have been recommended (Table 3).



Table 3 - Average bulk density of the lunar soil in the intercrater areas

|Average bulk density, |Depth range, cm |
|g/cm3 | |
|1.5 |0 - 15 |
|1.58 |0 - 30 |
|1.66 |0 - 60 |
|1.74 |30 - 60 |
|1.9 |300 |


4. Compressibility and shear strength

Compressibility and shear strength parameters of the lunar soil samples
were measured under different packing conditions, thus permitting the
determination of general trends and relations of the physical and
mechanical properties. Average values of the physical and mechanical
properties of the lunar soil samples when compressed under static pressure
are given in Table 4.

Table 4 - Average compressibility and shear strength parameters of
delivered lunar soil samples

|Soil Parameters |Void Ratio |
| |>1.3 |1.3 -|1.0- |<0.9 |
| | |1.0 |0.9 | |
|Coefficient of | | | | |
|compressibility, |>40 |20 |8 |<3 |
|(1/MPa) |<1 |1-1.5|1.5-2.|>2.5 |
|Cohesion, (kPa) |<10 | |5 |>20 |
|Angle of internal | |10-15|15-20 | |
|friction, (deg) | | | | |


The basic characteristics of the compressibility and shear strength
parameters of the lunar soil samples are the following:
1. The physical and mechanical properties of lunar soil samples returned
from different regions of the Moon are rather similar;
2. The maximum compression of the soil occurs during the initial loading.
Elastic rebound of the soil proved not be high, being on the average lower
than one percent of the initial strain value. Under repeated loading, the
additional compression is of no consequence either;
3. The main factors that control the lunar soil packing process is particle
sliding and tighter compression of soil particles and aggregates. When
compressed by a pressure ranging from 50 to 100 kPa, the process of
bringing together the particles as well as increasing the number of their
contacts is nearly complete, and further soil packing proceeds due to
distortion of the particles at the points of contact;
4. The shear strength of the soil is described clearly enough by the Mohr-
Coulomb formula. However, the parameters of shear strength depend
considerably upon the degree of soil packing. In a loose state the soil
has a small cohesion and angle of internal friction. As it is packed more
tightly, the angle of internal friction and cohesion increase, and non-
linearity is apparent in the shear strength envelope;
5. Soil deformation due to a localized load, applied to loose soil,
corresponds most fully to local shear failure or punching failure. For
dense soil, the deformation to a large extent corresponds to general shear
failure;
6. The study of physical and mechanical properties of lunar soil makes it
possible to determine the range of objective regularities that are,
combining granulometric composition parameters and physical and mechanical
factors. Relative setting, angle of internal friction, are in good
agreement with the logarithm of effective particle size. Cohesion of soil
models is in good agreement with the degree of inhomogeneity.

The in situ properties of lunar soil were obtained, using spacecraft
(Luna-9,13, Surveyor, Lunokhod-1,2 rovers) and by "Apollo" astronauts. The
Lunokhod operations resulted in making 1000 measurements of physical and
mechanical properties of the soil surface. The measurements were made in
different locations on the lunar surface, including craters, stone
scatterings, isolated stones laying on the horizontal parts of the surface,
as well as on the slopes.
The in situ lunar soil structure was estimated by visual evaluation of
the nature of its distortion caused by the Lunokhod's chassis.The
approximate evaluation of granulometric composition of the soil was done
according to the wheel tracks in the surface. Noticeable imprints in the
soil can only occur if the average particle size is significantly smaller
then the parts of the wheel that are is contact with the soil.
When all of those factors were considered, it was concluded that the in
situ soil is similar to a category of dusty sands and is subject to
considerable packing under the impact of natural factors of the near-Moon
space and the processes of the lunar surface formation. In addition, the
variations in the global geomorphological settings between the mare and
highland regions on the Moon have little influence, if any, on the
processes of deposition and formation of the uppermost layer of soil.
Therefore, typical soil properties can be defined.
The in situ physical and mechanical properties of lunar soil are
summarized in Table 5.


Table 5 - The in situ physical and mechanical properties of lunar soil

| |Void ratio |
|Soil |>1.3 |1.3-1.0 |1.0-0.|0.9-0|<0.8 |
|paramet| | |9 |.8 | |
|ers | | | | | |
|Bearing|<7 |7-25 |25-36 |36-55|>55 |
|capacit| | | | | |
|y, kPa | | | | | |
|Cohesio|<1.3 |1.3-2.2 |2.2-2.|2.7-3|>3.4 |
|n, kPa | | |7 |.4 | |
|Angle |<10 |10-18 |18-22 |22-27|>27 |
|of | | | | | |
|interna| | | | | |
|l | | | | | |
|frictio| | | | | |
|n, deg.| | | | | |
|Relativ|0.005 |0.25 |0.3 |0.3 |0.15 |
|e | | | | | |
|frequen| | | | | |
|cy of | | | | | |
|occurre| | | | | |
|nce (%)| | | | | |
|Typical|Isolate|On edge |On |Inter|In areas|
|locatio|d bumps|of fresh|elemen|- |of |
|ns on |and |craters |ts of |crate|shallow |
|the |small |with |very |r |depth of|
|Lunar |beds of|small |eroded|areas|re-worke|
|surface|fine-gr|dimensio|crater| |d soil; |
| |ained |ns; on |s | |stone-li|
| |materia|steep | | |ke |
| |l |slopes | | |formatio|
| | | | | |ns, |
| | | | | |isolated|
| | | | | |stones |

The main factors that controls the mechanical properties of soil in
situ is its degree of packing, characterized by its void ratio. The void
ratio for soil in situ was determined on the basis of experimental
measurements of bearing capacity versus. void ratio made on terrestrial
simulants chosen according to the results of studying the physical and
mechanical properties of lunar soil samples delivered to Earth.
On the lunar surface a void ratio of 0.8-1.0 is most frequently
encountered. This in situ conditions are characteristics of location with a
relatively even surface and uniform relief. Looser soil may be observed in
locations that exhibit crater-forming processes and other forms of relief,
which produce considerable slopes. Extremely loose lunar soil is hardly
found in situ. The location of very dense soil with void ratios less then
0.8 are distributed over the lunar surface, which is evidence of diverse
processes which took place in the formation and the packing of the
uppermost lunar layer.
In most cases (80%), the lunar soil is homogeneous to depth of 10 cm.
Inhomogeneity of the soil structure is accounted for by hard outcrops,
conglomerations of stones on the surface, particularly near certain craters
and probably by marked stratification as well.
Lunar soil with a bearing capacity of 25-55 kPa is the most widespread
(60%). These locations are characteristics of a relatively even surface and
intercrater areas. Bearing capacity of less then 25 kPa can be observed on
the rims of crater formations and on slopes steeper than 10 degrees
(Bazilevsky et al., 1984).
Recommended typical values of lunar soil cohesion and friction angle
are given in Table 6 (Carrier et al., 1991).

Table 6 - Typical values of lunar soil cohesion and friction angle

|Depth range, |Cohesion, kPa|Friction |Void |
|cm | |Angle |Ratio |
| |Avera|Range |Avera|Rang| |
| |ge | |ge |e | |
|0-15 |0,52 |0,44-0|42 |41-4|1,07+0,07|
|0-30 |0,90 |,62 |46 |3 | |
|30-60 |3,0 |0,74-1|54 |44-4|0,96+0,07|
| | |,1 | |7 | |
| | |2,4-3,| |52-5|0,78+0,07|
| | |8 | |5 | |


The physical and mechanical properties of a soil on depth are defined by
peculiarities of soil layers. The information received from seismic
experiments shows presence of several typical layers. The top layer has
thickness of 2-12 m. With a speed of distribution of longitudinal waves of
100 m/s. The last layer has a thickness of 18-38 m and the speed of
longitudinal waves in this layer is 300 m/s.
On the whole, the results of the lunar study of soil physical and
mechanical properties based on the samples delivered to the Earth and
measurements made in situ are in very good agreement and demonstrate that
the processes of lunar soil formation have very much in common over vast
areas. The data acquired may serve as a basis for developing soil simulants
intended for setting up modern space technology for further investigation
and exploration of the Moon.
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