Page 206 - Urban Construction Project Management

Logistics 161
DEWATERING

Dewatering is the process of removing water from the construction site, utilizing berms
to keep surface water away, and pumps to drain the subsurface of the site and lower the
water table so that the work can be performed in a dry site. When building in an urban
environment, the CM/GC often encounters subsurface water conditions given that the
major cities around the world were originally founded near bodies of water to facilitate
access and transportation. The specifications, local building codes, and project safety
usually require that excavation and foundation work be performed in a dry site. In addi-
tion, given the limited amount of space to build on in an urban environment, buildings
tend to be taller and have several sub grade levels, thus having a greater tendency to
encounter subsurface water.

Water tables are usually found below grade level. Dewatering is usually accomplished
by pumping the water from the site, from sump pits at the bottom elevation of the site.
In order to ascertain the level of the water table and the type of sub-grade soil and rock
conditions that exist at the site, borings need to be done at the beginning of the project.
This will indicate the existing water table elevation. The PM needs to be aware that if
the construction site is near a body of water that is affected by the tides, then the water
table may vary depending on the tidal conditions and whether the site is in the zone of
influence of the tidal body of water. Exhibit 6-40 is a diagram of a typical well point
installation.

Methods for Dewatering
Depending on the type of soil encountered, two different types of approaches are gen-
erally utilized. The first approach is utilized when sand and silt are being excavated.
Sand and silt have a tendency to seep into the sump where the water is being pumped,

Monitoring Monitoring Exhibit 6-40
well Discharge to Pump Pump Discharge to well
storage pond storage pond Well point
dewatering.

Water-tight
sheathing Original water
table

Pipe

Water table during
well point dewatering
Screen