Project Completion Report OWRT Project A- 076 -ARIZ
The work upon which this publication is based was
supported in part by the United States Department
of the Interior as authorized under the Water
Research Act of 1964, as amended. / Constant head borehole infiltration tests are widely used for the in
situ evaluation of saturated hydraulic conductivities of unsaturated soils
above the water table. The formulae employed in analyzing the results of
such tests disregard the fact that some of the infiltrating water may flow
under unsaturated conditions. Instead, these formulae are based on various
approximations of the classical free surface theory which treats the flow
region as if it were fully saturated and enclosed within a distinct envelope,
the so- called "free surface." A finite element model capable of solving
free surface problems is used to examine the mathematical accuracy of the
borehole infiltration formulae. The results show that in the hypothetical
case where unsaturated flow does not exist, the approximate formulae are
reasonably accurate within a practical range of borehole conditions. To
see what happens under conditions closer to those actually encountered in
the field, the effect of unsaturated flow on borehole infiltration is investigated
by means of two different numerical models: A mixed explicit -
implicit finite element model, and a mixed explicit -implicit integrated
finite difference model. Both of these models give nearly identical results;
however, the integrated finite difference model is considerably faster than
the finite element model. The relatively low computational efficiency of
the finite element scheme is attributed to the large humber of operations
required in order to reevaluate the conductivity (stiffness) matrix at each
iteration in this highly nonlinear saturated -unsaturated flow problem. The
saturated -unsaturated analysis demonstrates that the classical free surface
approach provides a distorted picture of the flow pattern in the soil. Contrary to what one would expect on the basis of this theory, only a
finite region of the soil in the immediate vicinity of the borehole is
saturated, whereas a significant percentage of the flow takes place under
unsaturated conditions. As a consequence of disregarding unsaturated flow,
the available formulae may underestimate the saturated hydraulic conductivity
of fine grained soils by a factor of two, three, or more. Our
saturated -unsaturated analysis leads to an improved design of borehole infiltration
tests and a more accurate method for interpreting the results of
such tests. The analysis also shows how one can predict the steady state
rate of infiltration as well as the saturated hydraulic conductivity from
data collected during the early transient period of the test.
| Identifer | oai:union.ndltd.org:arizona.edu/oai:arizona.openrepository.com:10150/610826 |
| Date | 03 1900 |
| Creators | Stephens, Daniel Bruce, Neuman, Shlomo P. |
| Publisher | Department of Hydrology and Water Resources, University of Arizona (Tucson, AZ) |
| Source Sets | University of Arizona |
| Language | en_US |
| Detected Language | English |
| Type | text, Technical Report |
| Source | Provided by the Department of Hydrology and Water Resources. |
| Rights | Copyright © Arizona Board of Regents |
| Relation | Technical Reports on Natural Resource Systems, No. 35, http://hdl.handle.net/10150/191055 |
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