Loose materials - The Ecology of Building Materials - page 125

Table 8.4: Estimating the clay content of earth

Thickness when rolled Percentage weight of clay

Cannot be rolled out Less than 2

3-6 mm-thick rolls 2-5

Approx. 3 mm rolls 5-15

Approx. 2 mm rolls 15-25

Approx. 1-1.5 mm rolls 25-40

Approx. 1 mm rolls 40-60

Rolls thinner than 1 mm More than 60

(Source: Låg, 1979)

Correct perception has become a tradition. The approximate clay content can be
estimated through rolling out clay samples and judging their thickness, as shown
in Table 8.4.

Deciding technical properties
Many methods have been developed to test the properties of earth. The following is based
on a method recommended by the German industrial standard (DIN 18952). There are
quicker and simpler methods, but their results are not always reliable. There are also more
chemically based methods.

Assessing the binding tensile strength
As with concrete, it is an advantage to have an even proportion of different-sized particles
within the earth, no larger than the small stones in shingle. A well-graded clay will bind
better as smaller particles fill the gaps left between the larger particles.
There are usually two tests to assess the binding tensile strength - in both tests the
moist earth samples are kept under a damp cloth for 6-12 hours before testing:

• The ball test tests stiffness. A sample of 200 gm of earth is rolled into a ball, which is

then dropped from a height of 2 m over a glass surface. If the diameter of the flattened
ball is less than 50 mm after impact, then the earth is good enough.

• The figure-of-eight tests the cohesion between the particles. A fracture test is car-

ried out on a piece of earth formed into the shape of a figure eight. This method
was once used for testing concrete. The earth is knocked into the figure eight form
with a wooden hammer (see Figure 8.5). The mould has specific proportions and
can be made of either hardwood or steel. At the narrowest point it has an area of
5cm 2 . The thickness of the mould is 2.23 cm. The hammered piece of earth is
taken out and hung in a circular steel ring. It is then loaded with weight in the form
of water in a small vessel. An earth with a binding strength of less than
0.050 kp/cm 2 is unusable.