Dear colleague,

As you may appreciate from my profile, I am an (experienced)  EIA/SEA practitioner and reviewer; I have dealt with proposed hydro-power dams, for example in Pakistan and in my current country of residence; from that experience, I guess you may need the physical soil parameters as required by an erosion model (e.g. USLE) in case the reservoir is situated in a relatively dry climate.

Are you reporting on the scoping phase? Have the potential environmental impacts already been identified?

For soil analysis I recommend to look in the USDA and EPA web pages, 

http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1244466.pdf

This book could be very useful for you, too

http://www.planta.cn/forum/files_planta/methods_of_analysis_212.pdf

For EIA, the offcial DoE web page has recommendations about the analysis (properties and parameters needed) for EIA

If your work is focused on a Hydro power dam construction, geological, hydromechanical, and other related studies are needed. Regarding soil analysis: lithology, mineralogy, texture, granulometry, salinity, CE, CEC, pH, OM, agregates, water holding capacity, clay content and clay type, etc... are some of the parameters to be performed

Hello, you also find various information about guidelines and analysis standards at the FAO, the Food and Agriculture Organization of the United Nations: www.fao.org

Here, you can get thresholds for parameters regarding soil quality etc.

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/297676/scho0309bpqm-e-e.pdf

http://www.forestry.gov.uk/pdf/LRU_BPG02.pdf/$FILE/LRU_BPG02.pdf