Same clay.
Different structure.
Triaxial Testing of Mixed and Layered Speswhite and Polwhite Kaolin
City, University of London
Supervised by Professor Sarah Stallebrass
Does arrangement
change behaviour?
Natural soils are rarely uniform. My final-year project investigated how the same clay blend responds to loading when it is mixed evenly or arranged in horizontal layers.
I tested one mixed and two layered specimens using consolidated undrained triaxial tests: first allowing consolidation, then applying axial loading with drainage closed. Stress, strain and pore pressure measurements revealed how the internal structure influenced the response.
Structure leaves
a measurable trace.
In the selected comparison, layered specimens retained a higher specific volume — a less dense arrangement — than mixed specimens. Their fitted compression and critical-state lines were approximately parallel.
Normal compression
Mixed: v = 3.15 − 0.182 ln(p′)
Critical state
Mixed: v = 3.07 − 0.188 ln(p′)
Reconstructed from the fitted equations on p. 25 of the dissertation; these are fitted lines, not raw test traces. The volume comparison uses selected layered tests from 2025 and mixed tests from 2024 (pp. 23–25).
Look beyond
composition.
The study shows why internal layering matters when interpreting clay behaviour. Composition alone does not describe the response of a structured soil.
This was a small experimental study. Layer proportions after trimming, water-content measurements and instrument uncertainty limit how widely the results can be generalised. Further tests across different ratios and layer orientations were recommended.