though opening 20 at the opposite end of panel 10 to an ac circuit of an adjacent wall panel 10 this demonstrates one advantage of both transverse and longitudinal conduit passages fig1 also shows plumbing pipes 70 and 72 running through the wall panel 10 through openings 18 and 19 respectively fig1 illustrates a preferred method of making a panel through the use of an expander unit 80 and liner 81 as shown in fig1 expander unit 80 is preferably comprised of four members 80 a d linked together as shown in fig1 14 members 80 a d can be spread apart or brought closer together manually or by pneumatically operated or electrically operated devices known to skilled persons as shown in fig1 liner 81 is placed on the collapsible structure 80 to form the cavities once the concrete or similar material has been poured and adequately set or cured the expander unit 80 is collapsed and removed the liner 81 is then preferably removed a few hours later another preferred method is shown in fig1 where cross sectional bar 83 is inserted in each location where openings are required this cross sectional bars 83 passes through holes 81 h in liner 81 shown in fig1 and through passages in expander unit 80 once the concrete or similar material has adequately set or cured bars 83 are removed prior to collapsing the expander unit 80 alternatively liner 81 is made without any holes and a transverse interconnection opening can be made by suspending and compressing a polystyrene or similar compressible material shown as.