USER DEFINED FORMULA TRENCH DEPTH

Question

Hello guys, 
 I am quite stuck and as far as I know, most of you have experience in the water sector. My question is more on the science behind trench depth calculation for rural water pipelines. Is there any formula that guides the designer to decide on the depth of excavation while laying water pipelines in the ground. I came across one of the designs 
 PDF 
 Check the attached pdf, Does anyone has a formula in mind that can help me to calculate the trench depth and input it as a USER DEFINED FORMULA in Watergems so that I can achieve results as shown above and finally be able to draw the graph as an offset measured from the normal ground elevation curve that Watergems produces? 
 ANALYSIS.rar 
 Or, Can anyone help me to interpret the above results on the trench depth figures and be able to come up with a formula that will be applicable to other pipes of different diametres. The profile was produced in the Civil 3d enevironment but I believe its easier to do all that in Watergems than civil 3D.Anyone farmiliar with trench depth calculations and sewer systems your input is welcome.The results seem to be connected to the slope gradient/O.D times a certain factor.

Yashodhan Joshi · Answer

Hello, 
 The trench depth or depth of excavation depends on the standards followed for your particular country. These standards vary from area to area and are based on different parameters, such as soil characteristics, depth of ground water table, topography etc. 
 While laying a sewer pipeline there are other components involved too such as bedding, offsetting, casing etc. which involves use of varied materials such as concrete, rubble, stone etc. The bedding or casing depends on the site topography and soil & ground water parameters. 
 This again varies based on the sewer pipe used. Hence different conditions exist for different pipe types (HPDE, Concrete, PVC etc.) 
 As far as actual trenching depth is considered, it would be an offset from the invert of the pipes. Again this would depend on the bedding, slope stabilization etc. at the actual site. 
 The best way to move forward here would be to refer your local standards and look for the construction practices followed. Depending on those you might be able to get a relation based on invert depth of the pipes. 
 Hope this helps.