An evaluation of the utility of four in-situ test methods for transmission line foundation design

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Virginia Tech

A major powerline is typically supported by many widely spaced structures. Each structure is, in turn, supported by a foundation or foundations. The prevailing philosophy behind transmission structure design to date has been based on the notion that information for geotechnical conditions is sparse and relatively simple in nature. Within this context, it is useful to note that one mile of construction for a routine lattice tower line, can involve twenty five to thirty separate foundations. More accurate soils data can allow for more efficient (smaller) foundation designs with consequent reductions in construction and material costs for the construction.

This research examines four existing in-situ soil strength testing methods; standard penetration test (SPT), the cone penetrometer (CPT), the flat plate dilatometer (DMT), and the pressuremeter (PMT). Soils data were collected at eight separate sites using each of the devices. The test sites were chosen to mirror soil conditions encountered within the service territory of Virginia Power, the project sponsor. A total of 19 standard soil borings, 30 cone penetrometer soundings, 26 dilatometer soundings, and 33 pressuremeter tests were undertaken in residual, alluvial and marine clay soil conditions.

The testing program was conducted with five areas of concern: (1) comparison of the penetration/ stiffness data from the four tests, (2) comparison of values of undrained shear strength and angle of internal friction developed from each of the test methods, (3) determination if pressuremeter data can be correlated to and thereby developed from one of the more rapid tests, (4) comparison of indirect soil type identifications from the cone and dilatometer with laboratory identifications from the standard borings, (5) development of information on the relative effort required for each test.

Comparison of the penetration resistance stiffness data produced useful correlations among the CPT and DMT, with the SPT data yielding more erratic results. Shear strength data was most consistent for the marine clay sites, while the CPT and DMT returned useful friction angle data in the alluvial sands. PMT data correlated well to both the CPT and DMT test results. Correlation of PMT results to the SPT was more erratic. Indirect soil identification from the CPT and DMT was fully adequate for transmission line foundation design purposes, and finally, useful comparative data on the relative testing time required for the four insitu tests was developed.