A longitudinally yieldable support for underground roof support includes a first support section with a first elongate outer shell partially filled with a solid compressible filler material and a second support section at least partially filled with a second solid compressible filler material. The second support section is at least partially disposed within the first elongate outer shell and is telescopically extendable relative thereto. A bladder is interposed between the first and second support sections that is fillable in situ to cause extension of the second support section relative to the first support section when filled under pressure with a filler material.