Environmental Engineering Reference
In-Depth Information
Frost
penetration
Impervious
layer
Gutter
Common
backfill
12 in.
minimum
filter if
required
Drainage blanket
Weep holes 5 ft
maximum vertical
spacing between
rows, gravel
pocket at intake
Clean
GW,GP,
SW,SP.
Longitudinal
drains
Impervious
fill
Longitudinal
drain
(a)
(b)
(c)
FIGURE 8.51
Examples of retaining wall drainage: (a) complete drainage; (b) prevention of frost thrust; (c) minimum
drainage. (Geosynthetics are used as blankets and filters.) (From NAVFAC, 1982.)
Open Excavations
Support by walls
is normally not required if relatively steep slopes remain stable and if
there is adequate space within the property for the increased excavation dimension. In any
event, groundwater control is usually required to:
Provide for a dry excavation
●
Relieve pressures along the slope face, or against walls
●
Relieve bottom pressures to control piping and heave
●
Sloped, unsupported excavation dewatering systems
may include:
Sump pumping in relatively shallow excavations in soils with moderate flows
●
Single-stage wellpoints to depths of 15 ft in sandy soils
●
Multistage wellpoints at 15-ft-depth intervals in sandy soils
●
Deep wells for free-draining soils at substantial depths, used in conjunction with
wellpoints placed at shallower depths as illustrated in
Figure 8.46.
●
Supported Excavations
Sheet piling
(Figure 8.52)
or soldier piles and lagging are sometimes used in conjunction with
wellpoints. Driving the sheet piling to penetrate impervious soils provides a partial seepage
cutoff, as does driving the sheeting into sands below the excavation bottom to a depth equal
to at least half the head difference between the water table and the excavation bottom.
Slurry walls backfilled with concrete (diaphragm walls) provide an impervious barrier
and may become an integral part of the final structure, as illustrated in
Figure 8.53.
Care
is required to ensure that no openings occur in the wall through which groundwater can
penetrate. If they occur, they can usually be corrected with injection grouting.
Building Basements
Uplift protection
for building basements may be provided by
Adequate basement slab thickness
●
Slab tied down with anchors
●
Gravity wells to relieve a perched water condition
●
Underdrain system
●
Underdrain systems
must be designed for the life of the structure, removing the maxi-
mum flow of water without significant soil loss or clogging. The system consists of several
