Australian Standards 1546:2000 & 20012 provide a table for the calculation of the basal area (where the treated water contacts the natural soil) using the daily loading rate and the capacity of the soil to absorb water.
In order for the soil to receive the water its structure must be open. This applies to all soils. When the excavation is carried out a tooth bucket must be used so as to leave the area scarified with an open structure. The importance of this increases as the structure of the soil gets heavier such as Cat 5 and 6 soils.
The photo to the left shows a correctly prepared basil area ready for placement of a layer of system sand before driving excavation equipment on it. This allows the basal area to receive the water easily. Again in heavy cat 5 & 6 soils the standard may recommend application of gypsum to improve the soil structure.
This photo to the right shows the worst possible basal area preparation. The excavator operator has done a good job of the excavation, top soil striped and placed up slope ready for cover on the back filled system and then to make the excavation look even better has tracked rolled to entire basal area. The practice of rolling excavated soil is often used on road construction to prevent water soaking into the soil. (Sealing it off) or to build dam walls, not installing a land absorption system. As per AS1547: 2000& 2012 the receiving soil must be left scarified by digging with a tooth bucket or ripping with single tine.
In Cat 5 & 6 soils adsorption area preparation is as per the notes in table 4.2 A1 on page 116 of AS/NZS 1547:2000 or table L1. on page 145 of AS/NZS 1547:2012 which may both include application of gypsum.