Environmental Engineering Reference
In-Depth Information
The purpose is to identify potentially hazardous or unfavorable conditions such as active
faults, unstable slopes, sinkholes, and deep deposits of weak soils that would cause con-
struction to be either risky from the aspect of safety, or extraordinarily costly. Investigation
is made with stage 1 and often with parts of stages 2 and 3.
The data obtained have a number of useful applications. At a high-hazard site, the deci-
sion may be made to abandon it and search for one with more favorable conditions. If the
site is marginal, preliminary planning can consider optimum utilization of favorable areas
and the avoidance of unfavorable areas to minimize construction costs. Depending upon
the nature of the project, unfavorable areas may be characterized by shallow rocks, a shal-
low water table, deposits of weak soils, or expansive or collapsible soils. On sites with no
obviously severe constraints, preliminary designs and cost estimates may be prepared.
The data gathered provide the basis for intelligent planning of more detailed investiga-
tions, because the selection of proper methods and tools is highly dependent on geologic
conditions.
Preliminary Design Phase
The preliminary design phase usually proceeds when the locations, dimensions, and load-
ings of the proposed construction have been defined. Its purpose is the comprehensive
determination of the distribution of the geologic materials and their engineering charac-
teristics, and the evaluation of hazardous or constraining conditions.
Investigation includes stages 2, 3, and 4, and on many projects may be combined with
the feasibility phase. In routine, straightforward projects, investigation may only involve
a preliminary phase.
The data obtained are used for the selection and dimensioning of foundations, retain-
ing structures, groundwater control, slope stabilization, and tunnel and pavement sup-
port systems. If the project involves mineral extraction, the effects on the surface are
evaluated and methods formulated to avoid detrimental effects such as subsidence and
collapse.
Final Design Phase
Sites with difficult conditions are subject to a final design phase. The designer may be
required to change building locations or dimensions or to confirm or modify designs, pri-
marily to realize economies. The purpose of this phase is to procure supplementary data,
usually from additional undisturbed samples for laboratory tests or full-scale load tests, to
confirm or improve design criteria. Investigation requires detailed information and uses
stages 2, 3, and 4.
Construction Phase
Unforeseen geologic conditions may be encountered, common on projects with deep exca-
vations or deep foundations, or unconventional designs may be used with contingency
plans. Either case requires additional investigation during the construction phase.
Investigation stages 2, 3, 4, and frequently 5 are performed.
Instrumentation is installed to monitor changing groundwater levels; stability and
movements of slopes and retaining structures; settlements of fills, embankments, and
foundations; ground subsidence and its effect on adjacent structures; and deformations
and stresses in underground support systems. Instrumentation provides an early warning
system and the data base for decisions for invoking contingency plans when deformations
and stresses exceed those anticipated during design.
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