Testing and Classification
Test before you trust
Testing turns a borrow pit into a specified material. Field tests - jar sedimentation, ribbon, roll, ball drop, and dry strength - are quick screens, not final proof. Laboratory tests normally include particle-size distribution, Atterberg limits, linear shrinkage, compaction by Proctor to find maximum dry density and optimum moisture content, and compressive strength on cured specimens. For stabilised mixes, also test durability against wetting and drying.
Classification for earth building is not identical to a geotechnical soil class. What matters is grading, fines content, plasticity, shrinkage, and compacted density. A soil labelled CL may still work if blended, while an SM may fail because it lacks binder.
Traps: relying on one field test; testing a disturbed or unrepresentative sample; comparing strengths at different moisture contents; and confusing optimum moisture content with the water needed for workability. Another common error is assuming that a higher dry density always means a stronger wall regardless of clay content.
Worked example: a Proctor test returns a maximum dry density of 1.90 Mg/m³ at 10% moisture. A field sample weighs 1.78 Mg/m³ at 13% moisture. The field soil is wet of optimum and under-compacted; either dry it or add dry mix, then re-compact in trial panels. Do not simply add cement to fix wet soil.