Clients and non geotechnical engineers frequently want to build pavements over organic contaminated cohesive soils. Organic contaminated soils have two outstanding characteristics; they loose considerable strength when they become saturated, 1 or 2 orders of magnitude, often essentially flowing, and they are not volume stable. Much of their dry strength is due to negative pour pressure developed in the organic material when dry, aka, desiccation cementing.
Build over unknown amounts of peat organic topsoil, A horizon material, or even B horizon organic contaminated clays or silts, is possible as long as the client will accept unknown amounts of settlement. Peat will commonly settle to 25 percent of it's original volume. CNR estimates the amount of settlement over time equal to the depth of fill placed on it. Building over peat is a subject unto it'self.
Road pavements and parking lot pavements are dependent on good drainage for a good life. 2 percent grade in 2 directions is functionally the minimum grade that one can expect will maintain drainage with the typical amounts of settlement that we see in parking areas. Even this will not compensate for buried topsoil, poorly compacted trench backfill and local overloads, rutting, and the like. Any thing less than 2 percent guarantees birdbaths over time.
Pavements at 1 % is less than the paver accuracy, so birdbaths are guaranteed upon completion of paving.
To compensate for buried organic fill, at least in one direction, the grade should be increased. I would suggest 4%, however, this may be reduced and reduction will result in shorter pavement life. Anywhere water will puddle will shorten pavement life as asphalt is permeable to water, especially if the AC has low density or is cracked as a result of rutting or settlement.
For pavements to function fairly well, it is recommended that over peat or burred topsoil, a minimum of 0.9 metres of clean clay fill be placed. For heavier traffic, trucks, 1.2 is recommended. ATU requires 1.5 metres for primary highways. This assumes that 0.3 metre lift will support compacters, to achieve 98% cSPD.
It is the opinion of the author that these may be reduce 0.3 or 0.45 metres where organic contaminated clay is the offending organic. All these depths are below subgrade elevation. Standard 300 mm subgrade preparation is assumed, or cement stabilized. If the client wants to use 150 mm subgrade, then only reduce 0.3 metres, so the minimum is 0.6 metres.
These are typical rules of thumb. Specific information and detailed analysis may alter these suggestions.