For partly saturated soils, the Mohr failure envelope is curved at low confining pressures.

The curved part is due to presence of undissolved air voids, which is compressible.

As the air voids compress with increasing confinement, the envelope continues to become flatter.

When all air is dissolved in the pore water, the specimen is completely saturated, and the envelope becomes horizontal.

The undrained shear strength obtained at full saturation depends on the initial degree of saturation.

References:

P.V.Lade (2016). Triaxial Testing of Soils: John Wiley & Sons.