Building with Earth: Design and Technology of a Sustainable Architecture

This is easiest if the clay is available in pow-

der form and just mixed into the wet loam.

In some countries, Bentonite is available in

bags like cement. This consists of 80% to

90% pure clay and contains about 70%

Montmorillonite. The dry density of the

powder is about 800 kg/m^3. It should be

kept in mind that while Montmorillonite has

a very high bending strength, it also has a

characteristically high swelling and shrinking

behaviour. It is often easier to get clay

powder from ceramic industry suppliers or

extremely clayey soils from brick-making

plants. Rich clods of clay need to be kept in

water to form slurry, and then mixed into

the loam with a mixer (see chapter 3).

Additives

The binding force of lean loams can be

increased by whey, fat-free white cheese,

fresh cheese, urine, manure, double-boiled

linseed oil, or lime-casein glue. The results

have to be tested in each case before using

these additives in a building element. Some

of the data compiled by the BRL may be

seen in 4.1.

Increasing compressive strength

Loam for building normally has a compres-

sive strength of 20 to 50 kg/cm^2. The per-

missible compressive stress for walls accord-

ing to the German standard DIN 18954 is

3 to 5 kg/cm^2. In practice, it is very seldom

required to enhance compressive strength,

this being necessary only in highly stressed

elements used in structures taller than two

storeys (which are not permissible by most

standards anyway). With earth components,

the edge strength against impact is very

important and often needs to be increased.

Rigidity of corners against breakage

depends upon compressive as well as

bending tensile strength. This “edge impact

strength” is very important during construc-

tion, when bricks or blocks are being trans-

ported, moved or stacked.

The compressive strength of a loam type

depends mainly upon its soil grain size

distribution, water content, the static or

dynamic compaction imparted to it, and the

type of clay mineral present. If the sand and

gravel particles are distributed so as to give

a minimum packing volume, and the silt and

clays are such that the inter-granular spaces

of the sand and gravel are fully filled by

them, then maximum density (and hence,

compressive strength) has been achieved.

Optimum grain size distribution

It is not commonly known that the com-

pressive strength of a mix can be enhanced

by merely optimising and varying the pro-

portion of silt, sand and gravel particles, but

without increasing the clay content.

In concrete technology, we speak of an

ideal grain size distribution, “Fuller parabola,”

or well-graded mix, given by the expression:

a = 100 √

where a is the weight of all grains with

diameters less than d, expressed as a pro-

portion of the total mass which has the

largest grain of diameter D.

Boemans points out that this grading for-

mula is not directly useable for earth con-

struction, since according to it, the clay con-

tent given is only 2% to 3%, which is obvi-

ously low for earth construction (Boemans,

1989). He claims this formula to be valid

only for particles larger than 0.002 mm,

while also suggesting a base minimum clay

content of 10%. This modification leads to

the expression:

43 Improving the earth

100
90
80
70
60
50
40
30
20
10
0
0.001 0.002 0.06 0.01 0.02 0.06 0.1 0.2 0.6 1 2 6 10 20 60 1 00

Clay Silt Sand Gravel

Sedimentation Sieving

Fine Medium Coarse Fine Medium Coarse Fine Medium Coarse

d

D

4.8