Various methods of mining and reclaimation may have been used. Typically the depths of disturbance and recovery measures would not be the same. It takes about 100 years to develop an inch of topsoil, so it is unlikely what you are sampling is apt to be mining spoils that may have been reconditioned, neutralized, and perhaps fertilized. If there are old mining operation and mitigation plans to review, material or water tests, that may help decide how best to sample. You might find some professional soil science advice from NRCS, USFS or the state mining or geology agency. If you are setting up research, you probably would want to compare results with an unmined control site. A statistician is often helpful in setting up a statistical study if you are interested in publishing, etc.

One of the many methods or protocols you could follow is the "Soil sampling for habitat recreation and restoration" by Natural England. See link: http://publications.naturalengland.org.uk/publication/31015

Right update by the previous researchers

I would recommend the ISO 18400 series, particularly the following protocols:

ISO 18400-100 Soil quality – Sampling – Guidance on the selection of sampling standards

ISO 18400-101 Soil quality – Sampling – Framework for the preparation and application of a sampling plan

ISO 18400-102 Soil quality – Sampling – Selection and application of sampling techniques

ISO DIS 18400-104 Soil quality – Sampling - Sampling strategies

ISO 18400-105 Soil quality – Sampling - Packing, transport, storage and preservation of samples

ISO 18400-201 Soil quality – Sampling – Physical pretreatment in the field

ISO DIS 18400-203 Soil quality – Sampling – Investigation of potentially contaminated sites

Thank you all for your answer. I will look into everything mentioned here