Plastic limit and compaction characteristics of fine- grained soils A. SRIDHARAN* and H. B. NAGARAJ † *Department of Civil Engineering, Indian Institute of Science, Bangalore, India; † BMS College of Engineering, Bangalore, India Laboratory determination of the compaction characteristics of soils is important for use in earthwork constructions. The purpose of compacting earth fills such as earth dams and embankments (highway, railway and canal) is to produce a soil mass that will satisfy the two basic criteria: reduction in settlement and increase in shear strength. For preliminary design and assessment, correlations with the liquid limit have been attempted by various investigators. However, from the present study it is seen that the correlations are not up to a satisfactory level. In this paper the results of detailed investigations carried out are examined to find which of the index properties correlate well with the compaction characteristics. In the analysis, apart from the authors’ experimental results, results from the available literature have also been considered. It is found that the plastic limit bears a good correlation with the compaction characteristics, namely optimum moisture content and maximum dry unit weight, much better than liquid limit or plasticity index. Keywords: clays; compaction; earth fills; embank- ments; ground improvement; laboratory tests La de ´termination en laboratoire des caracte ´ristiques de compaction des sols est importante pour les constructions a ` terrassement. Les remblais de terre compacte ´e comme les leve ´es de terre et les talus (autoroutes, voies ferre ´es et canaux) ont pour but de produire une masse de sol qui satisfera deux crite `res de base : la re ´duction du tassement et l’ame ´lioration de la re ´sistance au cisaillement. Pour la conception pre ´liminaire et l’e ´valuation, divers chercheurs ont tente ´ d’e ´tablir des corre ´lations avec la limite liquide. Cependant, cette e ´tude montre que les corre ´lations ne sont pas d’un niveau satisfaisant. Dans cet expose ´, nous examinons les re ´sultats de ces recherches de ´taille ´es afin de trouver celles des proprie ´te ´s indice qui correspondent le mieux aux caracte ´ristiques de compaction. Dans cette analyse, outre les re ´sultats expe ´rimentaux obtenus par les auteurs, les re ´sultats disponibles publie ´s ont e ´galement e ´te ´ pris en compte. Nous avons trouve ´ que la limite plastique montre une bonne corre ´lation avec les caracte ´ris- tiques de compaction, nomme ´ment la teneur en eau optimum et le poids unitaire sec maximum, bien mieux qu’avec l’indice de limite liquide ou l’indice de plasticite ´. Notation e void ratio G S specific gravity of soils I P plasticity index I S shrinkage index ¼ w L  w S OMC optimum moisture content S r degree of saturation w water content w L liquid limit w P plastic limit w S shrinkage limit ª d dry unit weight of soils ª d max maximum dry unit weight of soils ª w unit weight of water (ª d ) P dry unit weight at plastic limit water content Introduction Soil compaction is the process whereby soil particles are constrained to pack more closely together through a reduction in the air voids, and generally through mechanical means. The purpose of compacting earth fills such as earth dams and embankments (highway, railway and canal) is to produce a soil mass that will satisfy the two basic criteria: reduction in settlement, and increase in shear strength. Many other en- gineering structures constructed on soils, such as highways, railway subgrade and airfield pavements, also require com- paction. Compaction increases the strength characteristics of soils, which in turn increases the bearing capacity of founda- tions constructed over them. It also decreases the amount of undesirable settlement of structures and increases the stability of slopes of embankments. Compaction plays a vital role in the preparation of a good compacted soil liner in waste impound- ment sites to make them relatively impervious to leachates and thereby reduce the threat of groundwater pollution. Thus compaction is used as a practical means of achieving the desired strength and compressibility and also hydraulic conductivity characteristics of the soils used. The compaction characteristics of a soil as obtained from a laboratory compaction test are maximum dry unit weight Ground Improvement (2005) 9, No. 1, 17–22 17 1365-781X # 2005 Thomas Telford Ltd (GI 3173) Paper received 17 February 2003; last revised 22 March 2004; accepted 21 April 2004