settlement_of_shallow_foundations_on_granular_soils (Lutenegger ang DeGroot)

9.0 RECOMMENDATIONS
Based on the results of the extensive and exhaustive review of previous work performed in
the area of estimating settlement of shallow foundations on granular soils deposits it is obvious that
a number of significant improvements have been made in the last ten years on the understanding of
the interaction between foundations and granular soil deposits. It appears that there are two primary
sources of error associated with the ability to accurately predict the settlement performance of
foundations resting on granular soil deposits. These are: (1) lack of a complete site investigation to
provide sufficient test results on the nature and variability of granular deposits at a particular site;
and (2) the use of simplistic, empirical and generally outdated methods of analysis which tend to
give erratic results which are not of a sufficiently general nature and generally do not recognize the
important factors contributing to the deformation characteristics of granular soils under foundation
stresses imposed by shallow foundations.
As a result of this work, a number of recommendations are suggested to improve the
approach ofMHD to predicting the settlement of transportation related structures, (e.g., bridge piers
and abutments) supported by shallow foundations on granular deposits.
9.1 Improvements in Site Characterization
A major attempt must be made to improve the scope of site investigations for proposed
bridge structures in which shallow foundations may be a viable option to significantly more
expensive deep foundations. The potential cost savings associated with using a shallow foundation
system will in most cases significantly offset the additional expense associated with an increase in
effort during the site investigation. The number of test boring or test locations and the frequency of
field tests performed within the zone of most significant influence of the foundation needs to be
increased substantially (on the order of double the current amount). Additionally, the type of field
tests performed needs to be modified. In light of recent advances in the availability of various in situ
tests that may be used to improve the quality of settlement predictions, the use of tests such as the
pressuremeter and plate load test should be implemented as routine tools to supplement the more
conventional approach using test boring with Standard Penetration Tests. Detailed recommendations
regarding the various in situ tests used in practice are given in the Appendices of this report.
The use of the CPT is not considered a particularly significant advantage in the approach to
a site investigation, provided that the SPT practice used by all contractors for MHD can be
standardized and rigidly enforced. The only advantage to using the CPT is to provide a more
continuous sounding, however it is felt that in this case this would only be achieved at considerable
expense of equipment and manpower to the state and could actually create a slowdown in data
reduction and interpretation for typical projects. Additionally, there may be significant problems
associated with deploying the CPT in typical granular deposits located throughout the state.
The use of the Drive Cone Test to supplement the SPT is however, considered an appropriate
test to use to help rapidly and economically identifY site variability. As with the SPT, the DCT test
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