Building with Earth: Design and Technology of a Sustainable Architecture

40 Improving the earth

Addition of fibres
The shrinkage ratio of loam can be reduced
by the addition of fibres such as animal or
human hair, fibres from coconuts, sisal,
agave or bamboo, needles from needle
trees and cut straw. This is attributable to
the fact that relative clay content is reduced
and a certain amount of water is absorbed
into the pores of the fibres. Because the
fibre increases the binding force of the mix-
ture, moreover, the appearance of cracks is
reduced. Some results of tests conducted at
the BRL are shown in 4.5.

Structural measures
The simplest method for reducing shrink-
age cracks in earth building elements is to
reduce their length and enhance drying
time. While producing mud bricks, for
instance, it is important to turn them upright
and to shelter them from direct sunlight
and wind to guarantee a slow, even drying
process.
Another sensible method is to design
shrinkage joints that can be closed sepa-
rately, and which avoid uncontrolled shrink-
age cracks (see chapters 5, p. 56; 8, p. 76;
and 14, p. 113).

Stabilisation against water erosion

In general, it is unnecessary to raise the
water resistance of building elements made
from earth. If, for instance, an earth wall is
sheltered against rain by overhangs or shin-
gles, and against rising humidity from the
soil through the foundation by a horizontal
damp-proof course (which is necessary
even for brick walls), it is unnecessary to add
stabilisers. But for mud plaster that is
exposed to rain, and for building elements
left unsheltered during construction, the
addition of stabilisers may be necessary.
Theoretically, a weather-resistant coat of
paint is sufficient as protection, but in prac-
tice, cracks often appear on the surface or
are created by mechanical action. Further,
there is the danger of rainwater penetrating
the loam, causing swelling and erosion.

The rule of thumb says that cement and

bitumen as stabilisers are good for loam

with less clay, and lime for clayey loams. This

rule, however, does not take into considera-

tion the type of clay. For instance, Montmo-

rillonite and Kaolinite clay react quite differ-

ently, as described in chapter 4, p. 45. The

stabilisers cover the clay minerals and pre-

vent water from reaching them and causing

swelling. In this chapter, common stabilisers,

used traditionally and up to the present,

are described. Other stabilisers that mainly

increase the compressive strength are men-

tioned in this chapter, p. 45 and 47.

Water resistance can also be raised by

changing the grain distribution of silt and

sand, as this author has demonstrated using

three mud bricks (shown in 4.6) onto which

ten litres of water were poured for a period

of two minutes. The brick in the middle,

with high silt content, showed extreme ero-

sion up to 5 mm depth. The brick on the

right, with a higher clay content (ca. 30%)

showed erosion up to 3 mm depth; the

brick on the left, with the same clay content,

but less fine and more coarse sand, exhibit-

ed very little erosion.

Mineral stabilisers (binders)

Cement

Cement acts as a stabiliser against water,

especially in soils with low clay content. The

higher the clay content, the more cement is

needed to produce the same stabilising

effect.

Cement interferes with the binding force of

the clay and therefore it is possible that the

Increasing compressive strength

soil is less than that of the same soil without

cement, as shown in this chapter, p. 45.

Linear shrinkage (%)

Bentonite

Kaolinite

Silty loam mortar

Sandy loam mortar

Fibre added (%)

Coir Flax straw Rye straw

4.4

4.5

4.6

4.4Reduction of shrink-

age by adding sand to

Kaolinite and Bentonite

4.5Shrinkage ratio of

loam mortars with additi-

on of fibres

4.6Erosion test on green

bricks