This study investigates the effects of organic matter on lime and cement stabilized Ariake clays with emphasis on their mechanical properties and microstructure. The study also focuses on the effect of humic acid, which is a part of organic matter, on the strength development of stabilized clays. The results show that the strength and the yield stress of lime stabilized clay with high humic acid are low due to the obstruction of the pozzolanic reaction. Consequently, the cementing products are not visible in the micrographs.The humic acid has a greater effect on the strength reduction of lime stabilized clay than that of cement stabilized clay. The effect of humic acid on strength of stabilized clays decreased with increasing salt concentration because the humic acid becomes inactive at higher salt concentration.

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A number of oedometer and triaxial consolidated undrained triaxial shear tests were performed on both undisturbed and reconstituted specimens of natural soft clays. Based on a comparison of strength behaviour between undisturbed and reconstituted specimens of soft clays, a new interpretation of the loss of soil structure is proposed. This new definition indicates that the resistance of soil structure disappears completely when the consolidation stress is larger than the yield stress in oedometer compression (i.e. the post-yield stress state). The difference in behaviour of compression and strength between reconstituted and undisturbed specimens in the post-yield stress state is explained as being caused by the difference in water content, whereas the mechanical behaviour in the pre-yield stress state (i.e. consolidation stress smaller than yield stress in oedometer compression) is affected by both the water content and the resistance of the soil structure. Experimental data from mercury intrusion porosimetry for a strongly structured diatomite are also used to verify the dramatic change of microstructure in the vicinity of the consolidation yield stress.

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It has been well documented that natural marine Ariake clays are sensitive clays. In this study, extensive data of marine Ariake clays are obtained to investigate the gravitational compression behavior for sensitive clays. Analysis results indicate that the compression behavior of remolded Ariake clays is not different from that of other remolded=reconstituted soils. But natural Ariake clays do not follow the gravitational compression pattern reported by Skempton (1970) for natural sedimentary soils. At a given value of effective overburden pressure, the void ratios of natural Ariake clays are almost independent of liquid limits. Most natural Ariake clays lie above the sedimentation compression line proposed by Burland (1990). When the liquid limit is larger than 90% and the ratio of natural water content over liquid limit ranges 0.8-1.1, the natural Ariake clays lie around the sedimentation compression line. In addition, the natural Ariake clay with higher value of the ratio of natural water content over liquid limit lies above the natural Ariake clay with lower value of the ratio of natural water content over liquid limit. Salt removal is the most probable cause for such a phenomenon.