On-Site Wastewater Treatment and Disposal Systems - Forced ...

If a perched water table condition occurs in the surface soils that are
to be moved, provisions should be made to prevent this water from flow-
ing into the filled area. Curtain drains, perimeter drains or barrier
trenches may be necessary upslope or around the filled area to remove
this water (see Section 7.2.6).
b. Fill Selection
The fill material should be similar in texture to the underlying sand or
loamy sand. The backfill material used to cover the system should be
finer textured to shed surface runoff. It may be the original soil that
was removed.
7.2.5.4 Construction
Care should be exercised in removing the unsuitable soil prior to fill-
ing to prevent excessive disturbance of the sandy soil below. The
machinery should always operate from unexcavated areas. The top few
inches of the sand or sandy loam soil should be removed to ensure that
all the unsuitable soil is stripped.
The exposed surface should be harrowed or otherwise broken up to a depth
of 6 in. (15 cm) prior to filling. This eliminates a distinct interface
forming between the fill and the natural soil that would disrupt liquid
movement.
Once the fill has been placed, construction of the absorption system can
proceed as for trenches or beds in sands. However, if the fill depth is
greater than 4 ft (1.2 m), the fill should be allowed to settle before
construction begins. This may require a year to settle naturally. To
avoid this delay, the fill can be spread in shallow lifts and each me-
chanically compacted. This must be done carefully, however, so that
layers of differing density are not created. The fill should be com-
pacted to a density similar to the underlying natural soil.
7.2.5.5 Operation and Maintenance
The operation and maintenance of fill systems are identical to trenches
and beds constructed in sands. The fill system lends itself very well
to treatment with chemical oxidants or reconstruction in the same area.
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