It is not unusual for transmission line structure foundations to be designed without soil borings at each structure location. Project pressures such as a tight schedule or budget constraints are usually the reasoning behind such decisions, but this thinking can cost the utility money in the long run.
A statistical analysis reveals that substantial time and cost savings happen when borings are obtained at each location, due to foundations that would have been over- or under-designed with fewer borings. In some cases, foundations could have been designed without accounting for adverse subsurface conditions, such as shallow rock, which would have resulted in construction delays if encountered without structure-specific boring information.
Soil boring data was obtained at more than 2000 structure locations for Ameren’s Illinois Rivers 345 kV transmission line project, encountering variable subsurface conditions over its 375-mile (603-km) length, crossing central Illinois from Missouri to Indiana. Foundation designs for tangent and light-angle pole types based on structure-specific boring information were compared to theoretical designs resulting from presumptive design parameters or parameters from borings obtained at various intervals. Soil and rock parameters for hundreds of structure locations were regenerated, applying assumptions to reflect those that would be made if boring data was not available at every structure.
Presumptive design parameters used for preliminary design and construction cost estimates were estimated using the geotechnical characterization mapping prepared for the project, based on publicly available geospatial information. An analysis of a simulated situation in which borings were completed on a portion of the structures, instead of each structure along one segment of the alignment, was also completed. For this simulation, specific structure locations were chosen to represent where borings would have been performed. The design parameters based on the boring data at structure locations where borings would have been performed were used for presumptive foundation designs for several structures on either side of these locations.
Geotechnical Data and Costs
For the actual project, geotechnical parameters used for the foundation designs were based on borings that were drilled at every structure location. (Drill rig access was generally straightforward.) Final designs were completed based on the soil-structure interaction, with respect to axial and lateral loads and deflection criteria.
The transmission line analyzed for the study traverses glacial till plains that are dissected by streams and rivers where alluvium is present. The variations in soil strength that resulted in the differences in final designs for the study are found within geographic regions with similar geologic origin. In other regions where the geologic origins of the soil are different, variations in the size of deep foundations with similar loading conditions could be larger than those found in the study.
For the purposes of the study, generalized values of $600 per cubic yard of reinforced concrete (installed in soil), and $2,000 per soil test boring, with associated laboratory testing and design parameter reporting, were used for the cost comparisons indicated. Although not included in the cost estimates completed for the study, substantial additional costs would be applicable to cases when an inadequate design is identified, which, if recognized prior to construction of the foundation, would result in redesign and project delays. Additional potential costs associated with a foundation failure resulting from an inadequate design (that is not identified prior to construction) were not addressed in the study.
Foundation and Design Data
The data analyzed and summarized in this article has been separated into two groups for clarity. Group 1 represents 816 structure locations along the entire alignment and consists of foundations with equal diameter and similar structure type and height. In addition, they have similar wind span and the foundations are entirely in soil, with the depth to bedrock greater than the foundation length. Also, foundation lengths were used to determine cost savings, because the foundation diameters are equal for this data group. The second group contained foundations with variable bedrock depths within one segment of the alignment.
For the purpose of discussion, the results were segregated into three cases to consider for the comparison of costs. Case 1 includes conservative design based on estimated parameters. The result was considered conservative if the presumptive design based on estimated or presumptive soil parameters at a specific location resulted in a larger (volume of concrete) foundation than the design based on the actual boring data at that same location.
Instances of inadequate design, where the design based on estimated soil parameters resulted in a smaller (volume of concrete) foundation than the design based on the actual boring data for a specific location, represents Case 2. Equivalent design is Case 3. The result was considered equivalent if the design based on estimated soil parameters results in a foundation of approximately the same size as the design based on the actual boring data for a specific location. A more complete discussion of the information presented in this article can be supplied by contacting the authors.
Group 1 Results – Tangent and Light Angle Structure Foundations in Soil
The lengths of drilled pier foundations designed based on soil borings at each structure location for commonly used tangent and light-angle poles are presented in Figures 2a and 2b, respectively. The figures show the maximum and minimum drilled pier lengths grouped by poles with similar design loadings. Because the foundation diameter and loading are the same for each group, the variability in the foundation lengths represents the variability of the soil encountered, specific to each location. Data scatter was represented by plotting the second standard deviation of each data subset to represent the margin of error in the variability of drilled pier lengths with respect to the average, maximum, and minimum lengths for each group.
For projects where borings are planned to be completed at each structure location, budgetary cost estimates can be reasonably based on the average length of each foundation type and loading condition with respect to the expected soil design parameters, based on averages obtained from available data prior to performing borings.
Positive and negative variations in the estimated construction costs are expected but would result in fairly accurate construction cost estimates if the average values used are representative of the average soil conditions encountered for final design. Figures 2a and 2b show that the variations are generally evenly distributed on either side of the average, as expected for foundations with similar size and loading designed for construction in naturally occurring soils.
However, for projects where soil borings are not planned to be completed at each structure location, the soil parameters used for design or budgeting should not simply be based on expected average soil parameters. Designs based on expected average parameters are anticipated to be inadequate or conservative for a large percentage of the foundations because of the scatter in soil design parameters for foundations along an alignment several miles in length. Without boring data, the use of conservative lower bound estimates for soil parameters is necessary to minimize the risk of producing inadequate designs.
The result of using lower bound soil parameters to minimize inadequate designs is to substantially increase the number of conservative designs for all foundation designs. For the purposes of the study, it was assumed that the use of predicted lower bound estimated soil design parameters would result in foundations represented by the upper-bound pier lengths indicated in Figures 2a and 2b. The added construction cost due to the resulting conservative designs can be represented by the difference between the upper bound pier lengths and the average pier lengths plotted on these figures.
Based on this analysis, the net foundation construction cost savings represented by the actual foundation lengths, based on borings at each structure location, compared to the upper- bound lengths to represent the use of conservative, lower bound soil parameters, are 19.8% for the tangent poles and 17.7% for the light-angle poles. The costs of performing soil borings at each structure are 6.1% and 3.3% of the construction costs of the foundation, respectively for tangent and light-angle poles, with the reduced percentage cost for borings for the light-angle pole locations indicative of the larger foundations and resulting higher construction costs at these locations as compared to the tangent pole locations.
The approximate net cost savings (construction cost savings less soil boring costs) of 13.7% and 14.4% would result in total dollar savings of approximately US$2,996,500 for the 735 tangent poles and US$689,200 for the 81 light-angle poles considered for this study, or an average of about US$4,077 and US$8,508 per foundation, respectively, based on the stipulated unit costs.
Group 2 Results – Structure Foundations In Areas of Shallow Bedrock
This data group was from a segment of the project alignment where available subsurface information indicated that bedrock could be encountered within the expected foundation depths. The cost of excavation for foundations that penetrate bedrock is typically much greater than the cost of foundations where the excavation only penetrates soil.
For the project segment analyzed as Group 2 of the study, the available geospatial data indicated that bedrock would be encountered such that foundations with rock sockets would be required at an estimated 52 structure locations. Bedrock was actually encountered at 60 of the borings completed within this segment, although the final designs at these locations were evaluated to minimize costly rock excavation, and rock sockets were completed at only 19 locations.
There were 49 locations in this segment where bedrock was indicated at depths that would require rock sockets for preliminary design, based on estimated subsurface conditions, but not encountered in the actual borings — resulting in only three locations where rock sockets were presumed and actually constructed. However, there were 16 locations where the depth of bedrock was estimated to be greater than the necessary foundation length but was encountered in the borings at depths that necessitated rock sockets.
In locations where bedrock is not expected but is encountered during a foundation excavation, there is an increased cost to excavate unexpected bedrock, a cost for redesign of the foundation to incorporate the higher strength parameters and minimize rock excavation, and the possibility of significantly increased cost due to delaying the project during the foundation redesign and contractor retooling activities.
In areas where bedrock is expected but not encountered, there are possible cost savings because the budgeted cost of rock excavation is typically greater than additional costs for redesign plus the soil excavation and reinforced concrete materials for a longer foundation in soil. However, there are also costs associated with construction delays that would likely exceed the savings.
Data Saves Time and Money
Soil borings and the analyses of the data can save time and money. Ameren has found that performing geotechnical borings at all locations that are accessible to drilling equipment is beneficial. There is a high probability for substantial cost savings related to the construction of deep foundations due to the natural scatter in soil strength along transmission line alignments. Additional analysis will better quantify the value of structure-specific borings as compared to variously spaced borings; however, the risk of “inadequate” foundation designs will still exist. For alignments where excavations for deep foundations may penetrate hard, sound bedrock, the benefit of performing borings at each structure location is intuitively obvious but difficult to quantify. Additional analyses that include the costs of excavation, materials, delays, and redesigns are necessary to quantify cost savings related to structure-specific borings for foundations constructed in bedrock, but the potential cost and schedule implications to a project indicate that borings should be completed at each structure location in areas where bedrock is expected within the range of potential foundation depths.