The present invention relates to immprovements to the methods and apparatuses for in situ measurements of the swelling characteristics of soil (14) by using equipment comprising a probe (11) that includes at least two superposed portions (111,112)that are expandable independently from each other, means (8) for inserting said probe into the soil (14), and maans (13) for expanding said portions and for controlling the radial pressure exerted on the soil (14). According to the present invention said probe (11) includes a one-piece central body (18) for its two independintly inflatable portions (111,112) each of which comprises a measurement membrane (19) fixed on said body (18), the probe also includes a middle ring (16) secured to said central body and forming a peripheral groove (20) that is upwardly directed and into which a soaking fluid (15) is delivered, which fluid flows uniformly into the ground (14).