Two papers presenting the details of a relatively simple analytical model
for estimating the magnitude and rate of dissipation of construction
generated excess pore pressures are available over the internet, free of
charge. These are application oriented papers containing a level of detail
that normally does not survive the editorial process for journal articles
and conference proceedings. The papers, which have been peer reviewed and
copyrighted, are intended specifically for practicing engineers looking for
practical, cost effective methods to analyze the problem. The method was
specifically developed to permit the analyses to be performed using
commercially available spreadsheet software, thereby eliminating the need
for special application software. As a companion to each paper a
spreadsheet has been prepared by the author, in Microsoft Excel (97-2000 &
5.0/95 compatible formats), illustrating the solution to an example problem,
and both spreadsheets are also available free of charge.
The problem of estimating construction generated excess pore pressures in
soil can be thought of in terms of three interdependent components. The
first involves establishing the stresses applied to the soil. The second
involves defining the pore pressure response of the soil to the stress
changes that occur as the load is applied. The third is the consolidation
process, which addresses the dissipation of excess pore pressures during
construction. The primary components of the analytical model consist of:
(1) an approximate method for estimating the stresses in the foundation or
embankment resulting from the applied loads, (2) a method for computing the
pore pressures generated in the foundation or embankment, and (3) a
two-dimensional finite difference consolidation algorithm to account for
pore pressure dissipation during construction.
For the purpose of illustrating the basic method the first paper is limited
to the case of construction generated excess pore pressures in a fully
saturated soil foundation upon which an earth structure is constructed.
Since the method is capable of modeling the actual embankment construction,
the first paper includes a simple procedure for estimating the rate of load
application, which typically parallels the project construction schedule.
The second paper discusses how the model can be extended to account for an
unsaturated soil, as well as the case of a moving drainage boundary, and
illustrates how to estimate construction generated pore pressures within the
core of a zoned earth dam.
It is the author's hope that engineers will attempt to apply the procedures
described in these papers, with care and good judgment, to the design of
projects that are sufficiently instrumented to ensure that pore pressures
generated during construction do not exceed the values predicted by the
model. While the method shows promise as a design tool, additional
comparisons should naturally be made between the proposed model and actual
field data collected from other projects, thereby either further validating
the model or providing a mechanism through which the method can be modified
and improved.
The papers are available from the author's website:
www.geotechconsultant.com
David J. Kerkes, Ph.D., P.E.
Consulting Geotechnical Engineer
Houston, Texas (USA)
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