The US geological survey sp-en glossary gives "piled up, heaped" as the translation for "regazado".

Together with the short description of "regazado" in this wiki article on mining, I'd say that in this context it simply means loading material for disposal. In other words, that your initial idea is correct.

http://es.wikipedia.org/wiki/Mina_(minería)#Carga_.28Rezagad...

I think you're right about what it means, and that the equivalent mining term in English is "mucking":

"Rezagado" is logically the process of "rezagar", and "rezagar" means "to muck". "Rezaga" is the muck itself. These terms seem to be Mexican. "Muck" is the rock and earth debris left after blasting, and "to muck" is to remove it from the face. It's done either by hand, with shovels, or with a mucking machine.

"muck (min), marina, escombros, descombro, [Pe] desmontes, [Mx] rezaga, roca o mineral fracturado por los explosivos en un disparo"
http://www.infomine.com/dictionary/hardrockminers/spanish/we...
"rezagar (min), mucking machine"
http://www.infomine.com/dictionary/hardrockminers/spanish/we...
The latter definition seems odd, since a mucking machine should logically be a rezagadora, but it places us in the right area.

"Rezagar el material pétreo de extracción de minas de cantera al transporte para su procesamiento. [...]
Apoyar en el retiro y carga de rezaga de mineral de forma manual a los equipos de extracción."
http://www.conocer.gob.mx/perfiles_ocupacionales/PDFs/MONOGR...

"muck
n
4. (Mining & Quarrying) earth, rock material, etc., removed during mining excavations
v
remove muck, clear away muck, as in a mine"
http://www.thefreedictionary.com/mucking

"The photograph depicts a key activity in the operation of a hard rock gold mine. Mucking in mining terms refers to the removal of the rock and ore after a dynamite shot has been fired. The ore is then crushed and further processed."
http://consortiumlibrary.org/archives/Photographs/FavoritePh...

Mucking includes both clearing and loading the muck:
"Hard rock mining requires three steps: drilling and blasting the rock so that it can be physically removed, mucking (loading) the rock in some form of container, and hauling and hoisting the rock to the smelter."
http://www.workinglives.ca/hard-rock-mining/index.html

"A team of women from Missouri University of Science and Technology has won another world championship in mucking. [...] The debris left over from a mining blast is known as a muck pile, and that's where the term mucking comes from."
http://news.mst.edu/2011/03/st_women_finish_first_in_minin.h...

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Note added at 8 hrs (2012-10-20 12:23:09 GMT)
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A couple of examples to illustrate the sequence:

"Report for drilling, blasting and mucking. This work was performed to hold claims for 1996 and 1997."
http://www.geologyontario.mndmf.gov.on.ca/mndmfiles/afri/dat...

"A sinking cycle includes the following operations: (R. Tatia 2005)
Drilling
Blasting
Mucking and hoisting
Support or shaft lining"
http://en.wikipedia.org/wiki/Shaft_construction#Activities_r...

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Note added at 7 days (2012-10-28 01:03:13 GMT) Post-grading
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Further confirmation of the meaning of "rezagado" for the benefit of future users:

"Carga (Rezagado)
Por carga se entiende la recogida de la roca arrancada del suelo, y su traslado hasta un medio de transporte. En el arranque mediante maquinaria esta operación se realiza a la vez que el arranque. Así, por ejemplo, una pala excavadora utiliza su cazo para arrancar y cargar.
En las primeras minas la carga se realizaba a mano, con la ayuda de palas."
http://es.wikipedia.org/wiki/Mina_(minería)

"Video for Mucking
Monitor para Rezagar"
http://gts-mx.com/gtsint/index.php/es/productos/mineria/reza...

"Cargue (rezagado): Es una operación que se realiza después del arranque y que consiste en colocar el material en un medio de transporte, ya sea manual o mecánico."
http://www.ternium.com.mx/saladeprensa/glosario

"Dentro de las operaciones básicas en cualquier tipo de mina son tres: arranque (tumbe), carga (rezagado) y transporte (acarreo)."
http://www.maquinariaparatodos.com/content.php/36-Uso-de-maq...

There is really no doubt about it.