An approximate value of the horizontal effective stress in an overconsolidated clay deposit is often determined from an estimate of the effective stress ratio, K0, using the equation [Formula: see text]. It is also generally considered that the α parameter is approximately equal to sin [Formula: see text]. For evaluating K0 in sensitive clays of eastern Canada, a testing program has been initiated on three different sites representing a large range of overconsolidation ratios. Three different in situ devices were used: the Cambridge self-boring pressuremeter, the flat dilatometer, and hydraulic fracturing. The results show that the different techniques give essentially the same K0 values. They also give K0 values larger than those expected, with an α parameter typically between 0.75 and 1.15. Key words : clay, dilatometer, horizontal stress, hydraulic fracturing, in situ tests, self-boring pressuremeter.