The city of Tehran is founded on Quaternary alluvium, which has been geologically classified by Rieben. The city is located at the foot of the Alborz Mountain Range, which is basically composed of Eocene pyroclastic deposits (green tuff) and other volcanic rocks. The geology and the morphology of the Tehran region is similar to that for other cities located at the foot of mountains. Rieben divided the Tehran alluvia into four categories, identified as A, B, C and D (from oldest to youngest). In the Rieben geological classification system, which is widely used in Iran, the age and general geological characteristics of alluvia are considered, rather than engineering properties. The Rieben and other geological classification systems are described in this paper and geological factors that affect the geotechnical characterization of the Tehran alluvium are discussed. Because of the nature of the Tehran soils, undisturbed samples for laboratory testing are difficult to obtain and the execution of large-scale in situ tests is difficult, expensive and not practical for the majority of construction sites. Accordingly, a geological–geotechnical classification system is required to assess the engineering properties of coarse-grained soils for use in small to medium-sized construction projects. To determine the geotechnical properties of the Tehran alluvia, a number of in situ tests have been undertaken. The test results have been compared with published research results and the Rieben classification system has been extended to cover geotechnical properties. A similar framework could be used to create local geotechnical–geological classification systems of other coarse alluvia in other locations.

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