The type-I clathrate Rb6.15Si46 with partly empty cage sites has been studied up to 36 GPa using Raman spectroscopy, synchrotron x-ray diffraction in diamond-anvil cells, and ab initio total-energy and lattice-dynamics calculations. A first phase transition is observed at 13±1GPa and a ``volume collapse'' transition within the clathrate structure is then observed at 24±1GPa . Pressure-induced amorphization into a high-density amorphous (HDA) state occurs above P=33±1GPa . The HDA form transforms into a low-density amorphous polymorph during decompression. During the compression study using angle dispersive synchrotron x-ray diffraction techniques we measured bulk modulus parameters for rocksalt-structured TaO, included adventitiously in the clathrate sample [ K0=293(3)GPa and K0'=5.4(3) ].