Comprehensive design of columnar reinforced foundations M. Bouassida* This paper presents the results obtained after investigations conducted on reinforced soils by columns through funded collaborative Tunisian French research program. The predictions of bearing capacity, based on limit analysis approaches, and settlement estimation, based on linear elastic behavior, by using the group of columns model, constitute essentially the first requirement to undertake and optimized design of columnar reinforced foundation. Further, the prediction of settlement evolution based on a poroelastic model and improved soft soil characteristics due to stone column installation appeared as interesting components to complete the aimed design in more realistic manner. The illustration of results is discussed after recorded data from experimental studies and case histories. Finally, new trends for the design of columnar reinforced foundations are presented and some perspectives of the research program to be pursued on this subject are announced. Keywords: bearing capacity, desing, linear elasticity, settlement reinforcement by columns Introduction The reinforcement by columnar inclusions of weak and very compressible soils became, since the seventies, well prac- ticed by using different techniques of installation like sand compaction piles, stone columns, and soil cement columns. Previous contributions had focused usually on a specified reinforcement technique and a given verification either the bearing capacity or settlement. The design can also involve consideration of the acceleration of consolidation settle- ment of the host soil when the columns behave like vertical drains, as well as the liquefaction potential of the host soil, which is relevant mostly for loose saturated sands. The design of columnar reinforced foundations either with emphasis on oriented to bearing capacity or on settlement verification was undertaken in different ways by using various mechanical models. These models are: the isolated column (trench), the unit (or composite) cell, and the group of columns. The same models also served for investigating new aspects of reinforced soils by columns, as the improvement of soft soil characteristics due to stone column installation, the evolution of consolidation settle- ment as a function of a loading history. For example, Priebe’s method (1995) was dedicated to settlement prediction of soft soils reinforced by stone columns, and Broms’ method (1982) for bearing capacity and settlement verifications was dedicated to deep mixing technique. The isolated column or trench model is composed of a single inclusion surrounded by unlimited initial soil volume. The unit cell model is conceived from the distribution of a group of columns installed in regular pattern. Geometrically, it is a reproducible volume of reinforced soil which includes one column. As examples, for columns installed in square and triangular grid patterns correspond respectively to parallelepiped and hexagonal cylinders as periodic volumes of unit (or composite) cell models. Then, in order to carry out the well known axisymmetric model for bearing capacity and settlement calculation, an equivalent cylindrical unit cell having circular cross-section is adopted (Balaam and Booker, 1981). This paper chronologically retraces the stages of a long research program along which several results were obtained by the use of the three mechanical models and by performing different approaches and methods of calculation aimed at the design of foundations on columnar reinforced soils. The bearing capacity of soils reinforced by columns is first addressed. Results obtained by performing limit analysis direct approaches and homogenization method are recalled and interpreted. Comparison and assessment with existing results are discussed. Further, the settlement estimation follows by focusing on the use of variational methods in linear elasticity in order to introduce the apparent Young modulus of reinforced soil by a group of columns. The advantages of such a modeling, with respect to prior contributions which had mostly considered the unit cell model, are highlighted. Then, new trends for analyzing the design of columnar reinforced soils are Universite ´ de Tunis El Manar, Ecole Nationale d9Inge ´ nieurs de Tunis. 05/UR/ 11–06. BP 37 Le Belve ´de ` re, 1002 Tunis, Tunisia *Corresponding author, email Mounir.bouassida@enit.rnu.tn ß 2013 W. S. Maney & Son Ltd Received 12 February 2012; accepted 26 April 2012 156 DOI 10.1179/1938636213Z.00000000014 International Journal of Geotechnical Engineering 2013 VOL 7 NO 2