Tutorial: What is Interlocking Stabilised Soil Blocks ISSB?

Interlocking Stabilised Soil Blocks (ISSB) are made of a compacted mixture of soil and a stabilising agent such as lime or cement. They are formed in moulds that form grooves within the blocks such that they interlock horizontally and or vertically. The Eco-friendly Building Material.
This technical tutorial focuses on the use of Interlocking Stabilized Soil Blocks as an alternative building material.

Environmental
ISSBs are cured in the sun hence there is no need for fuel such as wood thus saving the environment from degradation.
Financial and Economic
1. Due to the interlocking nature of ISSBs, far less mortar is required thus saving on construction costs.
2. Since the blocks can be made on site, costs associated with their transportation are eliminated.
3. Due to their appearance, plastering of the walls can be avoided further reducing construction costs.
4. Using the blocks results to fast construction since they are largely stacked and have no curing time.
5. ISSB blocks are easy to manufacture and can be done by a small group of people.
6. Youth and women who provide both skilled and unskilled labour in the technology are able to
earn a living and be economically empowered.
Aesthetics
1. The blocks have an appealing appearance with an elegant profile, uniform size and face-brick look that takes the natural colours of the soils used that does not require plastering.
Challenges of ISSBs
1. Availability of suitable soil for block making is such challenge.
2. The quality of the blocks is determined by the quality of the raw materials introduced into the mould, the method used for mixing and the moisture content of the mix.
Types of ISSB machines
There are two main types of machines namely:
1. the manual / hand pressed and
2. hydraulic block making machine
When choosing the most appropriate machine, the following factors need to be considered:
1. The type and scale of the building /structure to be constructed;
2. Ease of maintenance of the machine;
3. Availability, reliability and cost of electricity;
4. The cost of the end product.
Of the two options, the manual / hand pressed machine is the most preferred option especially in a rural setting since it is manually operated and easy to use.
ISSB production process
In order to achieve quality blocks, proper selection of the raw materials must be made. These materials that include soils, sand, water and stabilisers (cement or lime) must be carefully selected.
1. soil selection
i. Carrying out a site evaluation ensures that suitable soil is available for the preparation of the blocks.
ii. It is recommended to use sub-soil of a fine quality.
iii. The soil must be free of organic material and should not contain harmful quantities of salts.
iv. Soils with a high clay content result to cracked blocks and should be not used. However, if they must be used, then they must be mixed with a blending agent (such as sand or quarry dust) and a higher cement content to prevent the blocks from cracking.
2. stabiliser
i. The most commonly used stabilisers are cement and lime. Cement is best used with soils that have low clay content to achieve greater strength quickly. Lime is recommended for high-clay content soils but takes longer to harden and to produce strong blocks.
ii. Sand and gravel may be added into high clay content soil to increase its density.
iii. Fibrous material such as dried grass, or synthetic fibers can be introduced into the soil mixture as reinforcers.
3. mix preparation
i. The soil must be sieved to remove foreign elements and organic matter.
ii. Mix the sieved soil with stabilisers at predetermined ratios until the mixture has a uniform colour. Mixing can be done either by hand or by using a mixer.

4. compaction
i. The soil mixture is then loaded into the machine’s mould and compacted to ensure strength and quality.
ii. The resulting block should be removed carefully from the mould and checked for texture and quality.
5. curing
i. Curing is the process of hardening the soil blocks so that they can gain maximum strength.
ii. The blocks should be placed on a flat surface with adequate spacing between the stacked rows.
iii. The blocks can be stacked in layers of five and covered either with grass or polythene paper to protect from direct sunlight and to reduce the rate of evaporation in order to maintain proper humidity.


Its constituents and the water curing process undertaken after production determine the strength of an ISSB. The composition of stabilized soil block consists of 60 - 70% soil, 20 - 30% coarse sand and 8 - 10% cement.

Types of ISSBs
Depending on the machine, different types of ISSBs can be produced:
1. Straight Double Interlocking Block
2. Curved Double Interlocking Block
3. Wide Format Interlocking Block
4. Straight Single Interlocking Block