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Flocculation by chemicals can adequately reduce the clogging effect, and the bending effect of prefabricated vertical drains can be eliminated by laying a prefabricated drain in the horizontal direction. Various settling column tests with different contents of chemicals, polyacrylamide (PAM) and hydrated lime (Ca(OH)2), were initially conducted to find the optimum dose for the fastest settling. Then, the combined effect of using both PAM and lime on the dewatering behavior of dredged clay slurry is studied using a prefabricated horizontal drain (PHD) and vacuum pressure of 80 kPa. Initially, sedimentation and self-weight consolidation with the optimum dose of PAM and lime are allowed to occur, and then further vacuum preloading is carried out. The combined effect of lime and PAM is very effective in increasing the dewatering rate, taking only 207 hours in comparison to 247 hours for the soil slurry treated with only PAM and 411 hours for the soil slurry without any chemicals. After the treatment, the shear strength substantially increases to 58.5 kPa for the soil with lime and PAM in comparison to 42.8 kPa for the soil with PAM only and 16 kPa for the soil without any chemicals. [ABSTRACT FROM AUTHOR]

An experimental investigation was undertaken in order to assess the effect of sodium (Na2SO4) and calcium (CaSO4·2H2O) sulphates and curing period on stress–strain curves and failure modes of grey (GS) and red (RS) clayey soils stabilised by lime (L), natural pozzolana (NP) and their combinations (L–NP). Several soil–L–NP mixtures were studied to be used as subgrade soils for road pavements. Stress–strain curves were obtained from unconfined compressive strength (UCS) test made on several soil–L–NP specimens after curing for 7 and 120 days. Tests results showed that the use of L or L–NP without sulphates produced a significant increase in peaks stress of both clayey soils and then modified their stress–strain curves from nonlinear to linear behaviour almost up to 70% of peak stress after a longer curing period. However, the presence of 2% Na2SO4 or any CaSO4·2H2O content provided beneficial effects on peaks stress and stress–strain curves of both stabilised clayey soils and then improved their linearity almost up to 95% of peak stress after curing for 120 days. In contrast, the presence of 6% Na2SO4 caused undesirable effects. In addition, both sulphates greatly affected the failure modes of soil–L–NP specimens, particularly at a later stage. [ABSTRACT FROM AUTHOR]