One can also drive equations from first principles but they have to be validated experimentally. The problem you posed is somewhat complicated because not only there is a mixture of two materials but also there is the packing factor which is dependent on the size distribution of sand granular in the mixture. If you cannot find references, you can set up and conduct a simple experiment. Take a plastic tube, about 20 mm in ID and about 200 mm in length. Steadily heat one of the copper disks at one end and cool the other copper piece at the other end.
Moderate heating is sufficient.
Plot the measured temperature as a function of distance along the tube length. The other end of the tube can be immersed in cold or ice water. Consult an introductory text book on Heat Transfer for more information. Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.