Dinga. From your profile & your email address I assume that you emanate from Australia. I also assume that you are talking about Septic trenches.
With the differing requirements of Land application we would need to know from wot State & Shire you live.
Normally in Qld it is a requirement to engage a Soil Engineer to do a site analysis & then carry the work out to strict guidelines, remembering that in Qld the Trenches are classed as Sanitary Drg & can only be carried out by Licensed Persons.