1. STRENGTHS OF NATURAL
SOIL BRICKS
By Dr-Ing. John K. Makunza
University of Dar es Salaam
Department of Structural Engineering
12th December 2006
1

2. 1.0 Introduction
An adequate shelter is a basic human need, yet about 80%
of the rural population in developing countries still live in
spontaneous low quality settlements, as they cannot afford
the high cost of building materials which could produce
better shelters. One alternative for the expensive materials
is to use natural soil stabilized bricks because they have
been identified as low cost material with the potential of
reducing the problem of living in poor or sub-standard
houses. The technology uses the available soil on site, which
is stabilized with a small amount of cement or/and lime
depending on the characteristics of the soil so as to improve
the engineering properties of the produced bricks.
2

3. Problem Statement
Residents of most areas of Southern Highlands in Tanzania
have been using mud bricks and interwoven timber,
saplings or bamboo daubed with mud to build their houses
and sometimes they apply cement plaster on the walls.
Problems of the houses:
- vulnerable to weather especially during rain season
- soil material can expand and loose cohesiveness,
particularly with cement plaster
- they are not sufficiently strong, although they somehow
last long even for more than ten years.
- it has been learnt that thieves can easily chop-out some
bricks or part of the mud wall and break-in then steal
properties.
Therefore the houses are not safe and are less reliable.
Examples of the houses are shown in Figures 1, 2 and 3.
3

4. Figure 1: Traditional house constructed of mud-walls and
roofed with grass
4

5. Figure 2:House built of pure soil bricks (from non-stabilized soil)
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6. Figure 3: Pure soil brick-wall in which part of the cement
plaster has fallen away 6

7. 2.0 Objectives
The main objective of this study was to assess the
suitability of bricks made of stabilized soil, as to
whether they can be used for the construction of
strong, safe and reliable low cost houses, or otherwise.
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8. 3.0 Adopted Methodology
The methodology adopted in carrying out the study
included the following:
• literature review: Through reading different
books, journals, some publications, and
information obtained about properties of soils
and soil stabilization,
• site visits and sample collection in which
samples S1 and S2 were collected
• laboratory tests and data analysis in which
the samples were tested and the data
obtained analyzed to check the characteristics
and adequacy of the soils as well as the bricks.
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9. 4.0 Soil Stabilization
In many developing countries especially in rural areas, soil is used
for the construction of houses. Natural compacted soil has good
insulation and fire resistant properties[1]. It is, however,
vulnerable to moisture and the erosive effects of weather. Walls
constructed out of well compacted soil, have adequate
compressive strength under dry conditions; however they will
loose their strength under adverse moisture content. Soil durability
and strength can also be improved by:
(i) Changing the distribution of grain size; gradation control
(ii) Compacting the soil;
(iii) Adding stabilizers or chemicals;
(iv) Mixing all of the above.
Soil can be improved and used as a building material for various
types of structures by adding substances known as stabilizers,
and the product is called stabilized soil. A properly stabilized,
consolidated, well-graded soil that is adequately moisturized,
mixed, and cured will provide a strong, stable, waterproof and
long-lasting building bricks. 9

10. Functions Stabilizers in Soils
Stabilizer material in the soil will do the following;
(i) cementing/binding the soil particles together making
the product stronger
(ii) water proofing - reducing the amount of voids and
water which can be absorbed by the soil
(iii) reducing the shrinkage and swelling properties of soil
(iv) increase the tensile strength of soil.
Some Commonly Used Stabilizers are;
(i) cement
(ii) lime
(iii) combination of lime and cement
(iv) combination of lime and Pozzolana
(v) asphalt
(vi) burning clay bricks*** 10

11. 5.0 Soil Tests and Results
5.1 Bottle test
The aim of this test was to determine the approximate amount of
clay, silt, sand and gravel presence in the soils.
Table 5.1 Summary of bottle test results
Soil description S1 S2
Height of clay/silt content (mm) 250 220
Total height of the content (mm) 480 480
Percentage of clay/silt (%) 52.1 48.5
5.2 Atterberg limits
Soils can have various states of consistency, liquid, plastic or solid.
The boundaries separating the limits and indices, are expressed as
percentages by weight of the moisture content.
Table 5.2: Summary of Atterberg limit test
Soil sample S1 S2
Liquid limit (wL) % 43 37.6
Plastic limit (wP)% 21 20
Plasticity index (IP)% 22 17.6 11

12. 5.3 Box (linear shrinkage) test
This test is used to determine the amount of cement or lime to
be used for stabilizing a particular soil. The test results for soil
samples S1 and S2 were as follows;
Table 5.3: Summary of linear shrinkage test results
Sample soil S1 S1
Shrinkage (mm) 12 18
5.4 Grain size distribution
Table 5.4: Summary of particle size distribution
Soil description S1 S2
Clay (%) 34 25
Silt (%) 23 23
Sand (%) 40 51.3
Gravel (%) 3 0.7
Solid density (kg/dm³) 2.65 2.65
Soil description Gravelly silty Gravelly silty
clayey SAND clayey Sand
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13. 6.0 Production of Bricks
(a) (b) brick size: 300 x 114 x 145 mm
Figure 4: (a) Hand steel mould, (b) Stabilized soil bricks
Mix Ratio: Cement: lime: soil 1:1:15 batch by volume
cement: water 1:4 (water cement ratio) 13

14. 7.0 Testing of Bricks
Compressive Strength Test
This test was done in order to determine the
compressive strength of the bricks against
vertical loading. The procedure for testing
each brick was done as follows:à External
dimensions of each sampled brick were
taken and the brick was marked for
identification
• The brick was weighed and recorded in
grams
• The brick was placed into the machine
and all set-up procedures were properly
Fig. 5: A Brick under test
done
• The brick was gradually loaded until it
failed
• The ultimate load was recorded.
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15. Results – Sample S1
Densities of soil Bricks - S1
1.8
1.7
Desnity [g/cm³]
1.6
1.5
1.4
1.3
1.2
1.1
1
1 2 3 4 5 6 7
Sample No []
Figure 6: Densities of bricks – sample S1
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16. Co m pre ssive stre n gth o f S oil Brick s - S 1
T e st Re sults BS 562 8 Va lue
6
5
Stress [N/m m ²]
4
3
2
1
1 2 3 4 5 6
Sa m ple No
Figure 7: Compressive strength for bricks from sample S1
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17. Results - Sample S2
Densities of Soil Bricks - S2
2
1.8
Density [g/cm³]
1.6
1.4
1.2
1
1 2 3 4 5 6
Sample No []
Figure 8: Densities of soil bricks from soil sample S1
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18. Compressive strength of Soil Bricks - S2
Test results BS 5628 Value
6.00
5.00
Stress [N/mm²]
4.00
3.00
2.00
1.00
0.00
1 2 3 4 5 6
Sample No []
Figure 9: Compressive strength for bricks from sample S2
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19. Water absorption test results
The test was carried out in order to assess the water
absorption of the bricks.
Results:
Average water absorption for bricks from sample S1 = 12.3%
Average water absorption for bricks from sample S2 = 9.86%
Specification control values : not greater than 20%
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20. 8.0 Compressive Strength Test
of Sample Walls
Load bearing walls
may be designed to
carry in plane
horizontal loads
induced by wind,
bracing effects or
earthquake; the
loads are transferred
to the walls
primarily via
diaphragms such as
floors or roofs.
Figure 10: Wall specimen under test
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21. Results:
Average maximum load: 137.00 kN
Average compressive strength: 1.22 N/mm2
From practical point of view, the strength of
walls for normal residential buildings is less than
0.60 N/mm2
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22. Application of stabilized soil bricks
Figure 11: Guard’s hut built from stabilized soil bricks 22

23. 9.0 Concluding Remarks
• Basing on the results obtained in this study, it has been
found that bricks made from stabilized soil with a mix
ratio of 1:1:15 for cement: lime: soil have
compressive strengths greater than that specified in the
British Standard (BS 5628 Part 1) of 2.80 N/mm2.
• It has also been learnt that this technology uses less
amount of cement in the production of bricks when
compared with cement sand blocks
• Building with stabilized soil bricks is a technology which
offers a good possibility for enabling low income groups
to build their own houses at low cost
• stabilized soil bricks have more advantage when
compared to burnt bricks because they are
environmentally friendly since there is no felling of trees
for burning the bricks.
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24. Burnt Bricks
Figure 12: Burnt brick site near Ifunda – Iringa region
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25. Recommendations
To achieve a successful dissemination of this technology,
the following recommendations should be implemented.
• Promoting stabilized soil bricks through advertising and
pilot housing, so that many people could have much
knowledge about this technology.
• To prepare operation manual for the soil preparation,
use of the bricks press and building process.
• Training people about the production of these bricks.
• Further research on use of stabilized soil bricks for low-
rise buildings is required. In this case a study on the
strength of walls against lateral loading and earthquake
effects is deemed necessary.
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26. Steel Mould for Bricks Production
Figure 13: Steel mould for bricks production 26

27. Fig. 14:
Map of
Tanzania
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