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“Geotechnical Investigations for Above-Ground Electric Transmission Structures in New England”

Gary R. McAllister, P.E., Terese M. Kwiatkowski, P.E. GZA GeoEnvironmental, Inc.

Abstract This paper discusses goals and challenges to be considered in planning a geotechnical investigation program for the design of high voltage and extra high voltage overhead electrical transmission line structure foundations in New England, based on practical lessons learned on recent projects. The topics presented herein are not necessarily unique to overhead transmission projects or to New England; however, in combination, the topics help illustrate the scope and key objectives of a successful exploration program, which may be distinct from general building, transportation, and other infrastructure projects in other locales. Keywords: Geotechnical, Transmission, Foundations, Subsurface, Investigation, New England. Introduction Since the publication of Karl Terzaghi’s Erdbaumechanik (Soil Mechanics) in 1925, which was the first text to elaborate on the comprehensive mechanics of soils and discuss methods of investigations, numerous texts, papers, guidance documents, and technical references have been prepared on the topic of geotechnical investigations. Geotechnical investigations for overhead transmission projects face several unique challenges. Many of these challenges revolve around the physical conditions and other constraints imposed on the subsurface exploration team tasked with completing test borings and field testing along a given alignment. Although the present paper is targeted toward a specific project type and locale, and identifies several key issues within that context, it is insufficient to address all of the aspects of geotechnical exploration that should be considered for a New England overhead transmission project. Moreover, this paper does not discuss geophysical methodologies, the geotechnical report, foundation design, or construction observation. In short, this paper is intended as a general “roadmap” for owners and managers preparing for such a geotechnical investigation. Geotechnical Investigation Objective “Of all building materials, soil is the least uniform and most unpredictable. Design and analysis must be based upon sound engineering principles and relevant experience.” (IEEE, 2001) “The performance, economy and safety of any civil engineering structure ultimately is affected or may even be controlled by its foundation.” (Holtz, 1981) The above quotes highlight the importance of having a clear and well defined understanding of the types, depths, and degree of variability in soil and bedrock, and in groundwater conditions, in order to derive

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engineering properties for the design of a structure foundation. Since a complete characterization of subsurface conditions along the entire alignment is neither practical nor economically feasible, the geotechnical study should be targeted toward the locations and types of the most critical and representative proposed structures, as well as the anticipated methods of construction. Just as importantly, a geotechnical investigation should add economic value to the project. The project will benefit from adequately characterized and clearly communicated subsurface conditions, by facilitating more efficient foundation designs and effective construction approaches, and reducing risks associated with “unforeseen” subsurface conditions. The Geotechnical Engineer’s Role The geotechnical engineer’s role can vary from project to project based on the Owner’s preferences and on the contractual structure of the team. Nonetheless, it is to the design team’s advantage to integrate the geotechnical engineer into the project at an early stage, sharing routine communications with the owner, designer, contractor, and public. The disciplined geotechnical engineer will strive to understand and to follow the preferences of both the Client and the Owner regarding the proper flow of communications. The perceptive and proactive geotechnical engineer will use the information gained through this early participation on the team to help prepare and execute a thorough and efficient geotechnical investigation. Understanding the geology and geography of the project alignment as discussed later will allow the geotechnical engineer to advise the project team early in the planning and preliminary design process on feasible foundation types and methods of construction, including potential risks. New England Geology Bedrock geology in New England is comprised of multiple geologic provinces, generally oriented northeast-southwest, parallel to the coastline and the Appalachian Mountain chain. Bedrock ages range from Precambrian (older than 600 million years) to Tertiary (less than 2 million years). In general, most New England bedrock is crystalline in nature, being either igneous rock (e.g., granite or basalt) or recrystallized metamorphic rock (e.g., marble, gneiss, or schist). The major exceptions to this are the sedimentary rocks (sandstones, siltstones, conglomerates, etc.) of the Connecticut Valley and the Boston and Narragansett basins.  

The visible landscape, and to a large extent the surficial geology, of New England primarily reflect the multiple glaciations of the area over the past million years or so, culminating with the retreat of the last (Wisconsin) ice sheet between roughly 30,000 and 10,000 years ago. The substantial erosive power of glacial ice not only removed most pre-glacial soils, but also deepened and widened pre-glacial bedrock valleys and left New England a legacy of alpine landforms (e.g., the “Knife Edge” arête on Mt. Katahdin in Maine, and the New Hampshire cirque known as Tuckerman Ravine). Glacial till, an unsorted mixture of grain sizes ranging from silt and clay to large boulders, was deposited directly by the ice sheet, and overlies bedrock in virtually all areas where bedrock is not actually exposed at the surface. Subsequent deposits of silt, sand, gravel, and cobbles by glacial

Figure 1-Knife Edge (LaFreniere)

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meltwater filled in the low areas in the post-glacial landscape; these glacial deposits constitute the majority of the large aquifers tapped by municipalities for drinking water. Finally, post-glacial deposits, including aeolian, coastal dune, alluvium, and organic soils, as well as anthro-transported materials (i.e., fill) make up the remainder of New England’s soils.

Fortunately, New England’s geologic setting is not conducive to certain problematic conditions found elsewhere in the United States such as expansive clays, collapsible soils, and karst terrain. However, the variability over short distances and the wide range of strength and deformation characteristics of the region’s soil and rock can prove challenging. The lower-strength water-borne deposits often pose design challenges in deriving sufficient geotechnical foundation resistance; whereas the clayey to boulder tills and the crystalline bedrock can pose significant excavation and construction challenges. In addition, New England, like much of the east coast, is a generally humid environment with ample precipitation resulting in plentiful water bodies and wetlands,

and groundwater that is commonly encountered within the depths of foundation excavations. Transmission Structures This paper is focused primarily on explorations for overhead 115 kV and higher voltage, angle and dead-end transmission structure foundations. These structures are subject to significant dead and live loads, warranting comprehensive geotechnical information. Thermal resistance and constructability concerns associated with underground electrical transmission, are worthy of discussion in a separate paper. Geotechnical Investigation Scope Preparation “The methods of soil exploration must be chosen in accordance with the type of soil profile at the site of the construction operations.” (Terzaghi, 1967). Site Information

Developing an understanding of the history and geologic depositional environment of the project alignment area during the early stages of a project offers insight into the soil types likely to be encountered and their respective engineering characteristics. The well-planned geotechnical investigation will be targeted toward the anticipated project-specific geologic conditions and construction means and methods. The investigation scope should be further based upon an understanding of physical constraints such as the Right-of-Way (ROW) topography, access restrictions and sensitive abutters and public groups. A desktop study of published geologic data can be a relatively simple process that assists the geotechnical engineer in anticipating subsurface conditions along the ROW. In GIS form, the geologic data can be geo-referenced to proposed structure locations, current and historical aerial photography, and topography.

Figure 3-Mapped Glacial Till, Bedrock Outcrops, and Swamp

Deposits along a ROW

Figure 2-Groundwater in Caisson Excavation

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The geotechnical firm’s library might also offer subsurface data of other nearby projects. If possible, this data review should be coupled with a site walk to better visualize anticipated subsurface conditions, topography, and access restrictions. When properly interpreted and effectively communicated, this information is also useful to the owner’s cost estimating and sensitivity analyses during the project’s early conceptual and preliminary design phases. Conversely, assumptions with significant cost and schedule implications (i.e., minimizing planned outages, winter work, conflicts with other construction projects, etc.) identified during cost estimating should also be considered when preparing the investigation scope. Project Information The geotechnical investigation should obtain the minimum technical information required by the design engineer. The investigation can bring economic value to the project, both in terms of supporting an efficient foundation design as well as reducing the project risks to be undertaken by the owner and the contractor. Examples include identifying locations where conventional shallow footings can be used in lieu of drilled piers, identifying excavation-related challenges such as boulder obstructions or unstable sands. The scale and relative cost of the geotechnical investigation should be commensurate with the estimated foundation- and civil-related construction costs. A modest contingency in the program’s cost and schedule will also allow the geotechnical engineer to modify the exploration and testing program in response to the actual conditions encountered. For example, deeper exploration and more frequent sampling will help in evaluating liquefaction susceptibility in areas of loose saturated alluvial sand deposits. Also key to the successful scoping of the exploration program is a sound understanding of the transmission line design procedures. These include anticipated structure loads, proposed structure types and configurations, input parameters specific to the preferred design software, and the project specific design criteria (e.g. controlling load combinations, and allowable structure deflections). Variations in structure selection and framing standards may result in a wide variety of foundation design reactions. For example, large self supporting four-legged steel lattice towers and cross braced H-Frame type structures impart large uplift and compression loads on foundations, as compared to self-supporting single steel monopole type structures, which load their respective foundations with large shears and overturning moments. Knowing the applicable foundation design considerations and anticipated loads allows the geotechnical engineer to strategize the preferred sampling, testing, and reporting plan for the project, which will best equip the foundation design engineer with the key soil data required.

Explorations The number, frequency, and depth of explorations for overhead transmission structures are often a project-specific determination made by the owner and/or structural engineer. Experience has shown that the basic investigation program will consider at least one subsurface exploration at each proposed individual or group of dead end angle structures. Greater exploration spacing may be appropriate at in-line tangents and lighter running angle structures located within similar geologic settings.

Figure 4-Boulders in Caisson Excavation

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The geotechnical engineer should consider anticipated subsurface conditions and access restrictions, such as existing energized overhead lines, when selecting exploration methods. For example, air-rotary probes and test pits can offer several advantages over conventional test borings in bouldery unsaturated soils underlain by shallow bedrock.

Field Testing

Standard Penetration Testing (SPT) is a common sampling technique that approximately measures of in-situ relative density or consistency, while concurrently retrieving samples for classification and testing. The field SPT values indicated on the test boring logs can be misleading at face value and should be properly corrected and interpreted by the Geotechnical Engineer. Thin-walled sampling is necessary to properly sample soft to medium stiff clays (i.e., cohesive alluvium, glacio-marine, and glacio-lacustrine soils) in preparation for strength and compressibility lab testing. N-size rock cores advanced an adequate depth into the bedrock are necessary to distinguish between dense tills, boulders, and bedrock, and to obtain samples for classification, Rock Quality Designation, and laboratory testing. Other methods of exploration and testing, such as vane shear, Pressuremeter and dilatometer testing, cone penetration testing (CPT), and geophysical methods, offer some specific benefits, and although less commonly employed, should still be considered.

Soil and Rock Classification Simple visual-manual classification of soil and bedrock, when performed by a qualified and experienced engineer or geologist in an accepted and reproducible manner, such as the Unified Soil Classification System (USCS) and the International Society for Rock Mechanics (ISRM) systems, can communicate useful and reproducible information. The Modified Burmister classification system, in conjunction with the USCS, is generally accepted in New England due to the familiarity of the local civil engineering and construction industry with this classification system.

Laboratory Testing Index testing on representative samples of discrete strata, combined with empirical correlations and local experience, can be sufficient to derive many soil engineering properties with reasonable precision. In many cases, however, additional laboratory testing, combined with a site-specific analysis based on an understanding of soil mechanics principles, may be necessary to confirm or adjust the empirical correlations, especially where heavier reliance on the strength and deformation properties of the soil is needed. For example, consolidation testing of “undisturbed” glacio-marine samples may be warranted to evaluate the feasibility of supporting drilled shaft type caissons within an overconsolidated glacio-marine crust. Similarly, data from unconfined compression strength testing of rock cores is useful information for bidding contractors to select appropriate tooling, evaluate alternate methods for foundation construction, and to estimate productivity.

Figure 5-Drilling in Boulder-Laden Soils

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Field Exploration Considerations With the rare exception of cases where a new ROW is being established, explorations will be performed within or adjacent to existing transmission ROW properties, occupied by energized overhead / underground transmission lines, active substations and switchyards, and potentially energized grounding grids and cables. ROW‘s are also commonly shared by other underground utilities. The respective ROW may be

owned in Fee by the electrical utility or held as an easement across the property of an adjacent land owner. Within rural or urban locations owned or leased by the utility, the ROW land use can take on almost limitless form, varying from congested development with traffic and pedestrian concerns to quarries, golf courses, farmland, or otherwise undeveloped land. “New England … is a region of high land values and considerable population density. Due to the proximity of high population areas, even rural communities have experienced a proliferation of environmental groups with concomitant enthusiasm for regional planning and critical watch over land use.” (Barton, 1982) In most cases, ROW owners and neighbors have a vested interest in the ROW land, and rarely is equipment access to the proposed structure locations unobstructed. Further, ROWs traverse multiple jurisdictions, and cross roads and water bodies. Establishing and following approved protocols for safety, environmental protection, and public relations is essential to a successful geotechnical field investigation. A successful project will involve effective planning and communication with all stakeholders, including other members of the project team (owner, civil and structural engineers, local, state and federal permitting agencies, and natural resources firms at a minimum), other utility and property owners, interest groups, and the general public. The information provided in preliminary construction access plans, including updated topography and natural resource inventories, in conjunction with site visits, offers a valuable resource to the geotechnical investigation field team.

This due diligence will inevitably identify added requirements to be incorporated into the exploration program. For example, some or all exploration locations may require the use of low-impact rubber-tracked equipment and matting, erosion and sediment controls, emergency spill kits, biodegradable equipment fluids, and special handling and disposal of drill spoils. Supplying sufficient drill water, properly grounding equipment, maintaining unobstructed access for emergency

maintenance crews, and securing against vandalism are among several logistical concerns for the exploration equipment

contractor. A rigorous health and safety plan with daily tailboard meetings, suitable personal protection

Figure 6-Gas Pipeline Marker within 345 kV ROW

Figure 7-Sediment & Erosion Controls

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equipment, and an electrically trained safety observer are fundamental safety elements to the program. In the event of undesired results due to either a breakdown of field protocol or to unplanned circumstances, a timely root cause analysis and updated field protocol shared with all involved team members is warranted to avoid recurrences. A continual flow of field data into the office allows the geotechnical engineer to review assumptions made in the exploration program, share details with the project team, and make desired modifications to the scope while the equipment is still on site. Conclusion A properly planned and executed geotechnical investigation exploration program will benefit the project with adequately characterized and clearly communicated subsurface conditions, by facilitating more efficient foundation designs and effective construction approaches, and reducing risks associated with “unforeseen” subsurface conditions. Careful planning, execution, and revision of the exploration program will improve the overall likelihood of the program’s success. Acknowledgment The paper was prepared with contribution from Mr. Joe Drouin, P.E., Power Engineers, Inc. and Mr. Lawrence Feldman, P.G., GZA GeoEnvironmental, Inc. Explorations shown in photos were performed by New Hampshire Boring, Inc., and Drilex Environmental, Inc. References AASHTO. (1988). Manual on Subsurface Investigations. Washington, D.C.: AASHTO. Barton, W. (1982). Siting for Aggregate Production in New England. Geotechnology in Massachusetts (pp. 375-380). Amherst: University of Massachusetts. Cornell University. (1995). Reliability-Based Design of Foundations for Transmission Structures. Palo Alto: EPRI. FHWA. (1996). Geotechnical Guideline No. 15 - Differing Site Conditions. Washington, D.C.: FHWA. Glossary of Geology. (1972). Falls Church: American Geological Institute. Goodman, R. (2002, October). Karl Terzaghi's Legacy in Geotechnical Engineering. Geo-Strata . Holtz, R. K. (1981). An Introduction to Geotechnical Engineering. Englewood Cliffs: Prentice-Hall, Inc. IEEE, A. /. (2001). IEEE Guide for Transmission Structure Foundation Design and Testing. New York: IEEE. Kulhawy, M. (1990). Manual on Estimating Soil Properties for Foundation Design. Palo Alto: EPRI. LaFreniere, P. (n.d.). www.mountwashington.org. Retrieved 12 18, 2010, from Mount Washington Observatory. Poulos, H. (2000). Foundation Settlement Anlaysis - Practice Versus Research. The Eighth Spencer J Buchanan Lecture, (p. 34). College Station. Samtani, N. (2006). Soils and Foundations Reference Manual. Washinton, D.C.: FHWA. Terzaghi, K. P. (1967). Soil Mechanics in Engineering Practice. New York: John Wiley & Sons.