(1) Water table located 1 to 2 feet (305 to 610 mm) below
lowest point of water entry. The criteria of para. 188.8.131.52 a) applies.
(2) Water table located 2 or more feet below lowest point of
water entry: Concrete shallow trench systems with noncontinuous bottom
(tunnel constructed of noncontinuous concrete bottom with openings provided in
bottom at intervals of 4 feet (1220 mm) or more to permit drainage into the
semipervious/pervious soils) may be used. Special considerations are required
when the concrete shallow trench would traverse open drainage ways or swales
where the water table would be less than 2 feet (610 mm) below the concrete
trench bottom. The designer may elect to reroute the system, place fill to
bring the system out of the unsuitable conditions, or provide a continuous
bottom trench floor for this area of the site.
c) Swelling Soils (material with high swell potential): If the
specific site conditions are such that these alternatives are not viable,
consider aboveground low profile or a direct buried system of the
prefabricated or preengineered type in accordance with NFGS-02694 for those
areas. Design the concrete shallow trench system in materials having high
swell potential in accordance with para. 184.108.40.206 a). Soils having a liquid
limit (LL) greater than 50 and a plasticity index (PI) greater than 25 require
testing (consolidation swell) to determine the swell characteristics. When
the results of the swell test indicate high swell potential, special
considerations such as over excavation (width and depth) and replacement with
nonexpansive fill, under-trench drainage system or other methods of minimizing
differential heave will be provided. The design of special features such as
described above will be in accordance with instructions provided by the
geotechnical engineer who performed the detailed site classification survey.
Design of joint spacing and joint details to accommodate movements will also
be provided when required.
Settlement of Trenches. Generally, settlement of concrete trenches
will not be a problem since the unit load of the shallow trench system will be
similar to the existing unit overburden load. Backfill adjacent to the
concrete trench must be thoroughly compacted to prevent settlement which would
create ponding. Positive slopes away from the concrete trench are desirable.
Special care of backfill and compaction is required where the system crosses
existing streets to preclude settlement and cracking of the roadway adjacent
to the trench from repeated traffic loads.
Under-Trench Drainage Systems. Use concrete trench subdrain
systems as required. When subsurface conditions are of differing soil types,
(fine grained and coarse grained) and those differing soil conditions will
cause blocked drainage either horizontally or vertically adjacent to the
concrete trench, provide subdrains to ensure drainage to prevent ponding or
entrapment of water adjacent to the shallow trench system. Base the design of
the subdrain system on the instructions provided by the geotechnical engineer
who performed the detailed site classification survey and classified each