Depth of Exploration. Extend all explorations 5 ft (1.53 m) below
the expected elevation of the concrete trench invert or the depth of the pre-
engineered system to determine ground water conditions.
Special Ground Considerations.
Give particular attention to the
a) The possibility that the ground below a backfilled piping
system may not be able to absorb runoff that has seeped into it.
b) Areas where ponding may occur, either along a sloping surface
or in low flat areas.
The permeability of the ground below the system (see below).
Perform field permeability tests as follows:
a) Space field permeability tests (percolation) along the line of
a trench at intervals of approximately 100 feet (30.5 m). When available
information indicates uniform subsurface conditions, longer intervals may be
allowed for larger projects.
b) Dig holes approximately 1 foot2 (0.093 m2) to a depth of
2 feet (620 mm) below the approximate bottom of a trench.
c) Fill each hole with water to the bottom elevation of the
d) After the water has completely seeped away, immediately refill
each hole with water to the same depth.
e) If it requires 20 minutes or less for the water to drop 2
inches (51 mm), consider the soil dry; otherwise, consider it as saturated at
Use test results as follows:
a) If the soil is saturated, no further tests are required.
A underground conduit systems for wet soils must be used.
b) If the soil is dry, as defined above, deepen permeability test
holes an additional 3 feet (920 mm) to determine if the water table is within
5 feet (1.53 m) of the trench bottom.