[U-Boot] Load a kernel image

Hi list, I'm working with an embeded system. I'm sure you know about the microprocessor NIOS2. I have chose u-boot and it runs perfectly in my platform. But it can't read my memory SD card how i have it configured. I would like to know how to load the kernel image (uclinux) in memory correctly by myself, i mean without u-boot, or where i can find some information about it. I got the image of kernel uclinux in elf format and i get errors with gzip magic numbers when i try to load my image at his linked address. I would like also to develop a SPI driver for u-boot in order to read SD cards memory, i don't know if it's already developed. Regards, Iván.