Building with earth - Gernot MINKE (1)

a 10 = 100 √
d
+ 10
D
The curve derived from this modified formula
for a maximum grain size of 4 mm is
shown in 4.8.
Preparation
The compressive strength of a mix is affected
by the type and amount of preparation,
as well as by the proportion of water used
in the preparation, a fact that is neither wellknown
nor well-researched.
At the Institute for Building Technology of
the Swiss Federal Institute of Technology in
Zurich and at the BRL, it was proven that a
slightly moist loam, when free from lumps
and compacted in a soil block press, usually
has a smaller compressive strength than the
same loam combined with sufficient water,
mixed by hand, and then simply thrown into
a mould (as is done when making adobes).
In one experiment at the BRL, handmade
adobes had, on an average, a compressive
strength 19% higher than if produced in a
soil block press which imparted a pressure
of 20 kg/cm 2 to the material. The belief of
many researchers and practitioners that
pressing in a soil block press leads to an
increase of compressive strength may only
be true for limited cases. As a rule, it is not.
The “secret” of loam lies in the lamellar
structure of the various clay minerals and
their internal electrical attraction, which is
activated only by water and movement. This
means that by kneading loam in a plastic
state, the clay minerals are able to come
together in a denser, parallel layered packing,
achieving greater binding force, and
when dry, higher tensile and compressive
strength.
Using the compacting apparatus shown in
4.9, developed at the BRL to test samples
of equal defined density, cylindrical samples
were produced that were 76 mm in diameter
and 100 mm in height. The samples
were then compacted by ten strokes of a
4.5 kg weight falling onto them from a
height of 0.45 m. The volume of a freshly
dug earth sample was thus compacted by
about 30% to 40%. The same silty soil was
mixed with some water in a mechanical
force mixer for two minutes and 15 minutes
respectively, and then filled in a cylindrical
form of the same size in a pasty state. After
drying, the sample that was not compacted
had an average compressive strength of
28% and 38% respectively, higher than
those that were rammed. This test demonstrates
that preparation can be much more
relevant to the strength than the compaction.
However, it should be noted that
the sample mentioned above was silty,
whereas this difference is not as large with
loams of high clay or sand content.
Compaction
Compacting loam under static force in order
to increase its compressive strength is generally
less effective than beating or ramming
while vibrating (by dynamically applied
forces). When a heavy object falls onto it,
waves are generated, causing soil particles
to vibrate.
This in turn creates movements that allow
the particles to settle into a denser pattern.
Furthermore, if there is sufficient water, clay
minerals have the ability to form parallel,
denser, and more ordered structures due to
electrical forces, resulting in higher binding
and compressive strength.
Loam Specific Vibration Compressive
weight
strenght
[kg/m 3 ] [rpm] [N/mm 2 ]
silty
sandy
2003
1977
2005
2003
2009
2024
0
1500
3000
0
1500
3000
3.77
4.11
4.17
2.63
2.91
3.00
4.10
Table 4.10, based on the various tests done
by the BRL, shows the comparative effectiveness
of dynamic versus static compaction.
Here it can be seen that the compressive
strength of a sandy loam under constant
pressure for ten seconds and vibrating at
3,000 cycles per minute is enhanced by
14%. For each technique of preparation,
there is an optimum water content that can
be determined only by testing. According to
4.9 Compaction apparatus
for soil samples
developed at the BRL
4.10 Compressive
strengths after static and
dynamic compaction of
sandy loam (clay 15%, silt
29%, sand 56%) and silty
loam (clay 12%, silt 74%,
sand 14%)
4.11 Deriving the Proctor
Curve with a multi-point
method (Voth, 1978)
4.12 Proctor Curves of a
silty loam with and without
the addition of lime
(Voth, 1978)
4.9
44
Improving the earth