INTRODUCTION
Some
paleontological excavations require the removal of massive overburden composed
of hard rock bodies. Collecting in such conditions often requires the use of
explosives (that could damage the fossils) or heavy machinery. Moreover,
explosives and heavy machinery (e.g. bulldozers, crane trucks) may not be a
solution due to security factors, permission issues, terrain conditions, and
rock properties. On the one hand, crane trucks are not always able to access the
fossil sites and explosives are also avoided due to high accident risk, the need
of specific training, requirement of formal permissions, and the danger of
damaging the fossils. Due to several constraints, some rocks and layers have to
be removed by hand, using a pneumatic or electric jackhammer, which is
time-consuming, causes rapid equipment deterioration, and is physically
exhausting.
This paper
introduces two techniques that may be a rapid and useful alternative for
breaking big rock blocks and overburden. Combining both demolition agents and
stone-splitters allows removal of considerable large-dimension blocks (fig. 1)
without the drawbacks of traditional methods.
Figure 1.
Procedure to use expansive cement: A- drill the holes using a hammer drill; B-
transfer the expansive demolition agent to a clean bucket; C- mix homogeneously
with the same proportion of water; D- pour the expansive cement in the drill
holes previously opened.
These
techniques were tested in Portugal, where most of the dinosaur finds are made on
the seashore cliffs in Late Jurassic terrestrial sediments, which comprise a
sequence of intercalated hard sandstone and mudstone. We have successfully
applied both expansive agents and stone-splitters during the paleontological
excavations of 2004 and 2006 (fig. 2).
Figure 2.
(a)
Aspect of the sandstone layer A, above the bone-bearing layer B. Main block
removed with combined technique stone-splitters and expansive cement, viewed
from (b) top before removal, and (c) side after removal (drill holes still
visible).
EXPANSIVE DEMOLITION AGENT
When mixed
with water, expansive cement forms an aqueous slurry that can reach
expansion stresses up to 11MPa – unit of pressure force by unit of area –
forcing the rock to break (information from Crackamite®). When properly
used, it can create a clear splitting line. Much information about the
usage, application and composition of expansive demolition can be extracted
from the patent record (see references below). Most of the patents describe
expansive demolition agents made of quicklime crystals sizing 10 to 100μm
(Kawano et al. 1982, Sato et al. 1987, Makino et al. 1988) reacting by
hydration with water. However, this reaction can be extremely quick and
release a lot of heat up to a point where the slurry reaches its ebullition
point, producing gas and increasing the risk of explosion (Kawano et al.
1982, Sato et al. 1987, Makino et al. 1988). The reaction can hydrate
rapidly and therefore some formulae add a fluidization agent and
hydroxycarboxylic agent, in order to reduce the reaction speed. A careful
reading and comprehension of the expansive agents instructions should be
accomplished before any procedure. Nowadays some formulae do not present
these risks (e.g. Sato et al. 1987), being constituted with
calcium-alumino-ferrite, free lime and magnesia and reaction moderator
components.
Procedures: A successful detachment of the block from the main rock body
is a function of the following variables (fig. 3, table 1): (i) the physical
position of the splitting line (i.e. how many tons of rock are going to be
detached); (ii) the number of drill holes (i.e. distance between holes);
(iii) depth of the drill holes (i.e. amount of cement used and the diameter
of the drill); (iv) the hardness and thickness of the layer to be demolished
(or in technical terms the tensile and compressive stress intrinsic to the
rock); (v) the angle of the drill hole in relation to the rock body (80 to
90º); (vi) ratio between water and expansive demolition agent (normally the
expansion capabilities decrease with a larger amount of water. This depends
on the technical specifications of each product but, in general, 0.3
water/demolition agent); (vii) the diameter of the drill (the larger the
drill, the smaller the expansion power of the demolition agent, or in other
words the smaller the hole, the more expansive power potentially is
available); (viii) the amount of time given for the demolition agent to
react (optimum is 24h); (ix) rock and water temperature (the cooler the
temperature, the smaller the expansive stress). Crackamite® and other
companies provide technical graphics demonstrating the relation between
these variables. As an empirical rule, around midday may be the best time of
the day to use expansive demolition agents since it is expectable to be the
hottest day temperature, and will coincide with the maximum expansion rate
of the expansive agent.
After
considering the factors explained above the procedure is simple. Drill the
holes using a hammer drill. Each hole takes about thirty minutes to one hour
in hard sandstone to make with a hammer drill. While drilling, pour water in
the holes and switch systematically from one hole to the other. Once the
mixture has the right viscosity, every drill hole can be filled almost to
the top of the opening. Mix homogeneously the demolition agent and water
using an appropriate mixer (although hands can also be used). Pour the
slurry in the holes up to the top and wait 24 to 48h, according to the
recommendations of the product. Sometimes the effect of the demolition agent
might not be enough, and stone-splitters or iron bars working as levers may
be need to be used to remove the block (fig. 3; table 1). During expansion,
the expansive agent does not extrude through the top of the drill hole.
After expansion and breaking, the expansive agent transforms into a
non-cemented powdered secondary product, easy to remove and dispose of.
There are
no apparent environmental problems associated with usage of expansive
demolition agents. At the end of the applications a small amount of
apparently innocuous powder remains. In comparison with explosives, using
expansive demolition agents is environmentally friendly (in terms of
vibration, residues, and sound disturbance) and fossils and surrounding
formations are less likely to be damaged.
Table 1
– Type of rock versus distance between drill holes using different diameter
drills (adapted information from CBA® EXPANSIVOS).
Type of rock |
Distance between
drill holes, using drill diameter of 4cm, in cm |
Distance
between drill holes, using drill diameter of 4.5 cm, in cm |
Less hard
rock (like mudstones) |
45 |
60 |
Hard stone
(like sandstones) |
30 |
40 |
Very hard
rock (fresh granite) |
20 |
30 |
STONE SPLITTERS
Stone-splitters (also known as feathers and wedges) are a simple and old
device used since the Roman Empire. They consist of three metal pieces:
one wedge and two escorts or guides (fig. 3). The wedge is rectangular
in cross-section, with a straight shaft that narrows downwards. The
escorts are semicircular in cross section, with the rounded side for
contact with the rock and the flat opposite face in contact with the
wedge. The shaft is straight, except at the top, where it bends to the
sides. Since such metal pieces are simple in their architecture they can
be produced by any blacksmith, being adapted to the diameter of the
drill holes. Besides being shock-resistant, the metal should be smooth
in order to avoid friction with the rock and between metal pieces
(fig.3).
Figure 3.
Stone-splitters: A- general aspect of the three parts of the stone-splitter; B-
cross-section; C- inserting stone-splitters in the rock; C- putting oil between
the wedge and the feathers facilitates the sliding.
Procedures:
Existing rock cracks and weak areas are preferable when deciding the position of
the splitting line. Several drill holes have to be made along the splitting line
using a hammer drill, where the distance between the drills holes depends on the
layer thickness and hardness of the rock. In general, drill holes may be about
20 cm apart, but harder or thicker stones require smaller distances between
drill holes. The depth of the drill holes should be at least one third of the
thickness of the layer of stone to be broken. Harder stone requires deeper drill
holes. When drilling, some water should be poured in the hole in order to cool
the drill.
The diameter of
the hole should be similar to the stone-splitter diameter, and wide enough to
adjust the two escorts of the stone-splitter. Harder rocks may require larger
stone-splitters (up to 3 cm thick).
Once the two
escorts are inserted in the hole, the thinner end of wedge is adjusted in
between. Oil may be applied to facilitate the sliding of the wedge between the
escorts when the wedge has already firmly penetrated in the drill hole. Oil
should be used with caution in order to avoid unwanted jigs of the
stone-splitters. Since each drill hole has a stone-splitter, one should hammer
down the wedge, making it penetrate between the escort. A few hammer hits on
each wedge should be enough, avoiding differential penetration (meaning
stone-splitters penetrating more than others) along the splitting line. If
everything works normally, the stone splits a few minutes to an hour later,
depending on the size, thickness and
hardness
of the rock. Soaking the splitting line with water helps to weaken the crack.
The stones splitters can be re-used.
Cliff slopes
can be unstable and potentially harmful for workers. Nevertheless, double
caution when working nearby cliff slopes face when the stone splitters or
expansive agents are being used since they would get even more unstable.
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