Use of Engineering Geophysics in the Design of Highway Passing Lanes

The addition of passing lanes has been a long standing need on Idaho Route 55. This narrow, winding road is one of the few routes that links Boise, Idaho, with destinations to the north. The roadway is bounded by a steep rising slope on the east, and a precipitous drop to the Payette River on the west. Additionally, the area is known for landslide hazards. Given these hazards and uncertainties in the subsurface geology, Idaho Transportation Department (ITD) commissioned an engineering geophysical survey to aid in the design of the passing lanes just south of Banks, Idaho. The engineering design required that several issues be addressed. These included the following: 1. Slope Stability 2. Amount of blasting required. 3. Need for retaining walls. 4. Volumes of soil and rock to be excavated. To help design engineers address these questions, P-wave refraction mapping was used to produce cross-sections perpendicular to the roadway. A novel approach was needed, since traditional in-line shooting would have required offsets greater than were available. The solution was to perform a 3-D, ground consistent, delay-time inversion. Soil and rock properties, and variations in the thickness of overburden were determined. These results were found to be in agreement with track-hoe trenching done at selected locations along the roadway. The geophysical work revealed that the site differed significantly from assumptions used in the initial design stages of the project. The potential for landslides was found to be significant. Given the updated geologic picture and projected costs, ITD decided to abandon this site for the passing lanes. The project was terminated, thus avoiding excessive costs that would have been experienced due to the differing site conditions. INTRODUCTION Differing Site Conditions and Risk Knowledge of the soil profile is essential to the engineering design of projects like roadways, tunnels, and foundations. A major risk for such projects is that the subsurface geology will differ from the assumptions made in the initial design and cost estimation stages of the project. Such miscalculations due to Differing Site Conditions (DSC) can be costly. For example, in the case of an earthen dam project, Green Construction Co. v. Kansas Power and Light Co., unexpected soil moisture content cost Green $420,000 (Steigler, 1994a). In the case of a 3.3 mile road project, Brown Bros. v. Metropolitan Government, "unclassified excavation" involved excavation of rock instead of soil. Brown sought $281,541 as a cost overrun, but lost in court (Steigler, 1994b). The cost of a 3 mile geophysical survey might easily be viewed as a bargain in hindsight. The burden of these additional costs can fall on several parties, including owners, engineering design firms, and construction contractors. For that reason, the inclusion of a DSC clause in contracts is becoming common. To further reduce risk, subsurface data are also appearing in contracts. In most cases, one should go beyond presenting raw data. A growing practice is to include a Geotechnical Baseline Report (GBR) as an addition to contract documents (Austin, 1994). Engineering geophysics should be considered as a cost effective means to reduce the exposure of responsible parties to financial risk. When used in conjunction with test borings, trenching and soil testing, the combined knowledge and resulting geologic picture can be used to avoid costly mistakes, and provide the information needed for better designs. In the case of the Banks passing lane proposal, significant uncertainty existed about the soil profile. Initial field observations of surface geology revealed a soil profile consisting of a poorly compacted, silty sand overburden with shallow granitic bedrock below. Adding passing lanes to the existing two lane roadway would require either cutting into the steep up dip slope, or adding fill and expanding down slope towards the Payette river. Components of Site Investigation The site investigation consisted of three parts. These were as follows: 1. Field Mapping of outcrop fractures and historic landslides. 2. Soils testing. 3. Geophysical mapping of overburden and bedrock. The geologic mapping documented the past landslides and addressed hazards related to potential rock falls. In general, hazards are greater when the fracture pattern parallels the roadway. In most cases, the joint planes mapped were suborthogonal to the roadway. Samples for soil testing were collected by Idaho Transportation Department (ITD) during test borings and trenching activities. The borings and trenchings also provided a check on the geophysical depth determinations to bedrock. Soil testing included the following: 1. Mechanical Analysis (Grain Size) 2. Compaction (AASHTO T-99) 3. Water Content 4. Direct Shear 5. Atterberg Limits (when enough clay was present) 6. In situ compaction and unit weight (Nuclear Density tool) The above tests, when used in conjunction with the geophysical mapping of overburden thickness, help in the evaluation of landslide hazards. Further, both geophysical mapping of soil boundaries and soils testing aid in the design of engineered structures. The geophysical work was conducted to map the thickness of the silty sand overburden, and the subsurface elevation of the bedrock. ITD provided CAD files with survey data. The geophysical results were presented in a report with geologic cross sections, contour maps, and CAD files with contours added to the basic ITD survey. In projects like this, the geophysical results are most helpful if integrated into the CAD environment of the end user. In particular, we have found that significant effort may be required to convert coordinate systems and relate geophysical results to the roadway. That effort is well spent in terms of end user satisfaction. Figure 1 is an index map of the project generated from the CAD files.