Page 39 - Construction Waterproofing Handbook

BELOW-GRADE WATERPROOFING 2.3
accumulations and natural capillary action of soils. Waterproof materials must be applied
in heights sufficient to prevent infiltration during temporarily raised groundwater levels.
With every below-grade installation, a system for collecting, draining, and discharging
water away from envelopes is recommended. Foundation drains are effective means for
proper collection and discharge. They consist of a perforated pipe installed with perfora-
tions set downward in a bed of gravel that allows water drainage. Perforated drain piping
is usually polyvinyl chloride (PVC). Vitreous clay piping is sometimes used, but it is more
susceptible to breakage. Drain piping is installed next to and slightly above the foundation
bottom, not below foundation level to prevent the washing away of soil under the founda-
tion that can cause structural settlement.
Coarse gravel is installed around and over drainage piping for percolation and collection
of water. Additionally, mesh or mats are installed over the gravel to prevent soil buildup,
which can seal drainage piping perforations and prevent water drainage. Collected water
must be drained by natural sloping of pipe to drain fields or pumped into sump pits.

PREFABRICATED FOUNDATION AND SOIL DRAINAGE SYSTEMS

These field-constructed foundation drainage systems are obviously very difficult to build
properly and often perform poorly over time due to infiltration into the drainage piping by
silt, sand, and soil that will eventually clog the entire system. Manufacturers have responded
by developing “idiot-proof” systems to replace these now-antiquated field constructed
systems. These prefabricated systems are relatively inexpensive and make them completely
reasonable for use as additional water control for practically any construction project
including residential, multifamily, commercial, and civil structures. These systems add
superior protection for minor costs to any project. For example, concrete slabs without
reinforcing can withstand hydrostatic pressure equal to approximately 2.5 times the slab’s
thickness. In practically every structural design, it becomes much more economical to add
under-slab drainage than to increase the thickness of the slab.
Prefabricated plastic soil drainage systems are available from a number of manufactur-
ers (refer to Chap. 16). These products are manufactured in a variety of plastic composite
formulations including polypropylene, polystyrene, and polyethylene. Figure 2.2 pictures
a typical manufactured drainage product. The systems combine specially designed
drainage cores covered with geotextile fabric in prepackaged form that eliminates all field
construction activities except trenching and backfilling operations.
The systems are idiot-proof in that the product is merely laid into the area designated for a
drainage field. Only appropriate sloping of the trench to collection points is required. Figure 2.3
presents a simplified isometric detail of a drainage system installation. The product is puncture-
resistive to protect its performance during backfill. Manufacturers also provide ample accessories
(including termination and transition detailing) to complete the installation. Figures 2.4 and 2.5
show available accessories including a tee connection to join one branch of a drainage to
another, and an outlet connection for collection of water that terminates at a drain box or culvert.
Materials are available in a variety of widths (up to 36 in) and lengths provided in rolls
of up to 500 ft long. The product should be puncture-resistant with some elongation capa-
bility for movement after installation, and be resistant to the natural or human-made elements
to be found within the intended service area.