One solution using mat2cell, splitting every 10 rows. Probably easier to understand, because no 3d-matrices are used:
cell2mat(mat2cell(x,repmat(10,size(x,1)/10,1),size(x,2))')
Second solution using reshape and permute, should be faster but I did not try it.:
reshape(permute(reshape(x,10,[],size(x,2)),[1,3,2]),10,[])