Building with Earth: Design and Technology of a Sustainable Architecture

the German standard DIN 18127, t h e opti-

mum water content is said to be the one at

which a maximum dry density is achieved.

The compaction is to be done with a Proc-

tor hammer. In order to obtain this optimum

water content, samples with varying water

contents are compacted in this way and

their densities determined. The water con-

tent which gives the highest density is called

the optimum water content. The curve

obtained by connecting these points is

called the “Proctor Curve” (4 .11).

In earth construction, however, the maxi-

mum density or compaction, and therefore,

the so-called optimum water content, do

not necessarily lead to maximum density or

compaction. Therefore the so-called opti-

mum water content does not necessarily

lead to the maximum compressive strength,

nor is it the most decisive parameter. On the

contrary, the decisive parameters are worka-

bility and binding force; hence it is recom-

mended that loam should not be used with

optimum water content as per DIN 18127,

but instead with a water content somewhat

higher than the optimum so derived. In fact,

this so-called optimum water content may

be treated, in practice, as a minimum water

content. With compressed soil blocks, it has

been shown that a water content 10%

higher than the optimum gives better results

than the so-called optimum. Boemans also

stated that the optimum water content

does not usually result in maximum com-

pressive strength. He also discovered that if

there is lesser compaction and higher water,

then the same compressive strength may

be achieved by using higher compaction

and less water (Boemans, 1989, p. 60 ff.).

At the Labor Géomatériaux of the Ecole

Nationale des Travaux Publics de l’Etat

(ENTPE) in Vaulx-en-Velin, France, it was

found that the type of clay minerals involved

also influence the compressive strength

after compaction. For instance, by raising the

static pressure from 2 to 8 MPa when pro-

ducing soil blocks using a press, the com-

pressive strength rose by about 50% with

Kaolinite, and by about 100% with Mont-

morillonite (Oliver, Mesbah, 1985).

Mineral additives

Lean clayey loam can reach a higher com-

pressive strength with the addition of Mont-

morillonite clay. At the BRL, tests were con-

ducted with sand enriched with 17% by

weight of Kaolinite and Bentonite respec-

tively. (Bentonite contains about 70% Mont-

morillonite). With Kaolinite, the compressive

strength reached was 5 kg/cm^2 , and with

Bentonite, 12 kg/cm^2.

The addition of lime and cement, usually

intended to increase the weather resistance

of loam, also generally increases compres-

sive strength. As described here, however,

compressive strength may also be

decreased by these additives, especially in

amounts lower than 5%. This is because

lime and cement interfere with the binding

force of clay minerals. The greater the clay

content, the higher must be the amount of

lime or cement added.

Tests have shown that as a rule, lime offers

better stabilisation with rich clayey loams,

while cement gives better results with

leaner loams. Furthermore, cement is more

effective with Kaolinite and lime with Mont-

morillonite. In practice, it is always recom-

mended that relevant tests be conducted.

When doing so, the following points are to

be kept in mind:

1. When loam is stabilised with cement or
   lime, some pores should remain. Only the
   points of contact of the larger particles
   should be cemented together, but fewer
   pores should be filled than with concrete.


2. When the cement hydrates, free lime is
   formed. This reacts with the silicate acids of
   the clay minerals so that in addition to the
   early stabilisation caused by cement, a
   longer lasting hardening also occurs. Unlike
   cement concrete, therefore, the strength of
   cement-stabilised loam increases a little
   even after 28 days.


3. When adding hydraulic lime, an ion
   exchange between the clay minerals and
   the added calcium ions takes place, lasting
   between four and eight hours. The addi-
   tional hardening process caused by the
   reaction of the hydrated lime with the car-
   bon dioxide from the air occurs very slowly.

45 Improving the earth

4 .11

4 .12

Pd t/m^3

1.83

1.70

+ 6%Kalk

+ 3.5%

13.0 16.5

t’Su 0.13 t /m 3

7 10 15 20 25

1.80

1.70

1.60

1.50