Page 282 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SITE UTILITY SYSTEMS

6.4 CHAPTER SIX

layer is at atmospheric conditions. The top of the layer is the water table, which fluctuates
with the amount of water present.
When an aquifer is found between impermeable layers, the water is confined in the same
way as it would be inside a pipe. Because of the confining strata, the aquifer is under pres-
sure greater than atmospheric. This aquifer is called an aquiclude, but is better known by
the more popular terms confined aquifer or artesian aquifer. Hydrostatic pressure within an
artesian aquifer is sometimes high enough to cause water in a well to rise above the surface
level of the ground above it.
When a confining bed is located beneath an aquifer, it prevents water from continuing
its downward movement. The limited amount of water that accumulates above this confin-
ing bed is known as a perched aquifer.

SUBSURFACE DRAINAGE

This section discusses the drainage aspects of subsurface water entering excavations and the
drainage requirements resulting from intrusion of subsurface water into footing drains.
When water in any form interferes with the construction of a project or could potentially
cause damage to any structure or installation placed in the ground, it becomes a problem rather
than a resource and must be removed. Problems occur when the presence of water could cause
a structure to float or when groundwater could cause soil-bearing resistance to be lost.
Subsurface water is removed by placing a drainage system below the level required to
be kept dry. In many cases, this drainage system will remove excess water by gravity. Two
of the most common systems make use of trenches filled with pervious backfill and drain
pipes. The drain pipes may have holes in them or may be installed with open joints to allow
water to enter, and are pitched to provide a flow path. A special layer of pervious backfill
is placed above the trench bottom or drain pipes to allow subsurface water to flow easily
through them. This pervious backfill is often called filter material.
There are three general methods of draining subsurface water, categorized according to
their purpose. The first is subbase drainage, which is generally used for draining road sub-
bases. This system consists of shallow drain pipes laid near and parallel to the edge or end
of the pavement to be drained. The second is called subgrade drainage, which is generally
used when it is necessary to lower a high water table from around and inside the project
area. It consists of either trenches filled with filter material or pipes placed to collect and
route the water away. The third is called an interceptor drainage system, which consists
of pipes or trenches placed in a manner that will intercept the groundwater before it gets
to the project site.
When none of the gravity drainage methods are possible, a pumped system is required.
The most common method for draining excavations of a project under construction uses
well points, which serve the same purpose as intakes for a well. A well point is a reinforced,
pointed metal cylinder with holes in it for water to enter. The points are driven into the
ground below the level of the excavation and connected to the surface with a discharge
pipe. The top of the pipes are connected by a header to the suction side of a pump, which
collects the groundwater and discharges it to an approved location. The groundwater level
is brought below the bottom of the excavation, keeping it dry. A typical well-point system
is illustrated in Fig. 6.3.

INFLOW INTO EXCAVATIONS

The development of predictive methods for the inflow of water into excavations or from
foundation drains has not kept up with the solutions to other problems in groundwater

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