In recent years, reinforcing of soils by group or network of micropiles have found widespread applications. This type of reinforcement is used in overcoming difficulties associated with differential settlement, unstable slopes, etc. In this thesis, within the frame of the National Project FOREVER (Reinforcing of Soils by Micropiles), different numerical methods for analysis of axially and laterally loaded isolated micropiles, micropile groups and networks are presented. For analysis of axially loaded micropiles, we have selected two methods for the investigation of the soil-micropile interface. The first method correlates Ménard pressuremeter results with interface parameters. The second one based on the continuum media concept with a new definition of magical radius in non-homogenous soils. Three full scale experimental micropiles, realised for a bridge of TGV Paris-Brussels (1994), were successfully modelized using these two methods. A new approach for modelization of non-linear behaviour of micropiles submitted to lateral loads is suggested. This is achieved by using of non-dimensional moment-strain curve for outer fibre of reinforcement. This model called "Multizons" allows to vary progressively the rigidity modulus of micropile around the position of maximal moment until plastic hinge. This new method is implanted in existing numerical code PILATE-LCPC which uses a 'p-y' method. Calculation results of this new program called PILATEP are compared successfully with finite elements (code CESAR-LCPC) as well as with experimental data from the tests on small diameter piles by MAHMOUD and BURLY (1994). In order to analyse the micropile groups and networks, a new hybrid method type "z factor" and "y factor", based on application of transfer matrix, is presented. This model uses, on the one hand, load transfer approach ('t-z' & 'p-y' models) for analyzing individual micropiles in group and, on the other hand, continuum concept for evaluation of micropile-soil-micropile interaction. This method is programmed in numerical code GOUPIL-LCPC which analyses micropile groups by load transfer method. A comparison is made of results obtained from this new code called GOUPEG by experimental data from a full scale test of a group contains three micropiles (in Rueil-Malmaison, 1994) and effectiveness of this program is proved. In a network of micropiles, they are simultaneously loaded axially and laterally. A coupling between axial and lateral group effects is established. Finally, the results obtained are confirmed with a good agreement, by different methods based on the continuum concept and finite elements method (code CESAR-LCPC).