Interlocking Stabilized Soil Blocks (ISSB)

Arup produced a report for compressed earth blocks in general that includes information for soil preparation, stabilizers, block dimensions, compression or extrusion, curing, block property testing, construction, finishing, and design parameters such as wall thickness, percentage of windows in a wall, and considerations when building in non-seismic and seismic areas.

UN Habitat in collaboration with Good Earth Trust (GET) has a report specifically for ISSBs and presents similar information in a series of simple diagrams.

Most ISSBs in East Africa are made with an ISSB machine that compresses the soil into the interlocking form. Depending on the machine, the following blocks can be produced: Straight Double Interlocking Block, Curved Double Interlocking Block, Wide Format Interlocking Block, Straight Single Interlocking Block, or Grooved Double Interlocking Block.

Planning the production of ISSBs starts with the site and properties of the soil there. A sedimentation test can be performed to determine the percentages of clay, silt, sand, and gravel in the soil. Other tests include shrinkage tests, sending the soil to the laboratory, or using a portable soil-testing kit.

Stabilizers include cement lime or bitumen, fibrous natural materials, chemicals and resins, or sands and gravels. It is important to refer to more detailed material to determine what stabilizer is most appropriate for the type of earth mix that is being used.

The process of producing the blocks includes steps of training the labour force (quality assurance is extremely important for ISSB production), excavation, sieving, mix preparation, mixing, measuring the mix, compressing the mix, removing the block, dry and stack, and quality check.

Technical support can be found from the manufacturer or distributer, or from the reports listed in the Design specifications section. Another resource is the Auroville Earth Institute, and on their links page they list their full network of earth-building specialists including CRAterre, Cartoterra, Dachverband Lehmbau (the German association for building with earth), Earth Block International, and Rural Housing Knowledge Network, among others.

Though the full lifespan is unknown, historical precedent and testing signify that if built to a high level of quality and with the most appropriate soil mix and detailing, ISSBs and ICEBs can last decades similar to the lifespan of fired earth bricks.

Academic institutions have performed numerous calculations and tests to analyse strength properties, thermal conductivity, and introduction of bacteria or alternative stabilizers among many more. It is not necessarily the case that these tests inform construction.

Workers are subject to the general dangers of a construction site including working from heights and with sharp tools. ISSBs do not require operation of any large machinery.