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and economic gain for the company. Additionally, geothermal pile systems reduces the emission ofgreenhouse gasses, it can account for the entire energy use of a typical office building for heating andcooling, and it makes better use of a necessary structural component therefore reducing the life cycle costof the system.The purpose of this Major Qualifying Project is to design a geothermal system to support a series ofoffice buildings in Boston, Massachusetts and evaluate the system’s pay back return period. Thebackground includes the preliminary research that was completed to ensure a thorough understanding ofeach component involved in the design framework. After this research was completed, an outline of thedesign process was prepared. This enabled a smooth design flow without leaving out any key designparameters. The methodology guides the reader through the design process. The use of AASHTO andFHWA codes were used to complete structural analysis. Additionally, the use of specialized computersoftware was used to aid with the geothermal design. To understand how economical this system is, a costbenefit analysis was performed to calculate how long in energy savings it would take to repay theadditional installation costs of the system. Ultimately, it should become clear to the reader that everyoneinvolved in the construction process can easily achieve success with this technology. Systems of this kindare beneficial to both the construction industry as well as the environment and should be considered forall new commercial buildings.12

2. Background2.1 IntroductionTo begin this project, an immense amount of literature research was necessary. It was important toevaluate and consider every aspect of the combined structural and heat exchange system. Everycomponent picked could change the outcome. There were a lot of components to consider. On thestructural side of the project there was the site exploration program, producing a soil profile, and sizing anadequate micropile system to support anticipated structural loading. As far as the geothermal heat transfersection a big part was the selection of the type of geothermal system itself. Following this different typesof transfer medium, thermally enhanced grout, heat pumps, and the total length of geothermal pipes hadto be investigated.2.2 MicropilesThe micropile considered in this project is a type of drilled shaft. Drilled shafts are used to transfer load tosubsurface soils and rock. They are typically large diameter cylindrical shafts which support column loadsfrom the above structure. Micropiles are a type of drilled shaft that are much smaller in diameter, usuallyless than 12 inches. From a construction standpoint, the same goal, to support the structure, can beaccomplished in a much smaller area. Also, since it is easier to drill a smaller diameter, the micropile canbe extended to greater depths, resting on stronger soil. Micropiles are often the preferred technologywhere traditional drilled shafts would be too large, such as in the case of underpinning buildings, earthexcavation support and slope stabilization.Micropiles are a high capacity, small diameter, drilled-in-place, steel reinforced foundation support. Thequality (i.e., strength and stiffness) of soil usually improves with depth, therefore excavating throughweak surface soils can provide greater bearing capacity from deeper soils or rock. A drill rig is used toexcavate a cylindrical hole to a calculated required depth. This depth is calculated based on the subsurfaceconditions, materials used, supported load and settlement considerations. To support higher loads andavoid settlement a steel cage or center reinforcing steel bar can be implemented. The drilled shaft is thenfilled with concrete or grout. The grout bonds the soil to the pile as well as to the center reinforcing steel.13

2.3 Structural ComponentsThe term foundation can have many meanings in different fields. We will take the term foundation tomean the formation that connects a structure to the ground (Coduto, 1999). All civil engineered structuresrequire foundations. I am considering the foundation of a mid-sized office building. Buildings impart theirweight and other loads (wind, snow, seismic) onto their foundations. The loads must be safely andeconomically dispersed to the ground below the structure. To begin the design of a foundation, theunderlying soil conditions must be investigated. This is done by the means of in-situ testing.The structural evaluation of the micropile system was deemed the most vital design component. Thesystem resulted in a structural failure is the worst case scenario and must be avoided at all costs.2.3.1 Site Exploratory and In-Situ TestingIn-situ means, “in-place” soil testing as to not disturb the sample being analyzed. An important aspect forthe design of heat extraction and rejection energy systems is to physically analyze the exact ground thatthe system will be installed in. Taking measurements from literature and charts online is not enough. Toobtain particular ground conditions, the testing equipment is brought to the field and the soil is tested inplace. This eliminates disturbance and contamination that could occur from bringing the sample to alaboratory to test. Testing the soil in its natural condition maintains characteristics, temperature leading tooverall more accurate results. Additionally, subsurface conditions change over time. Relying solely ondata can turn up to be inaccurate, resulting in failure.The main in-situ test used for this project is the Standard Penetration Test (SPT). This is one of the mostcommon in-situ testing procedures, developed in the 1920s. The procedure was standardized in 1958 inthe ASTM standard D1586 (Coduto, 1999).Due to the nature of this testing procedure there are many variables that can contribute to variations. Theprimary variations are the method of drilling, how well the bottom of the hole is cleaned before the test,presence or lack of drilling mud, diameter of the drill hole, location of the hammer, type of hammer,number of turns of the rope around the cathead, actual hammer drop height, mass of the anvil that thehammer strikes, friction in rope guides and pulleys, wear in the sampler drive shoe, straightness of the14

drill rods, presence or absence of liners inside the drill sampler, and lastly the rate at which the blows areapplied.Corrections to the hammer are made to account for the variations. This corrected value is known as theN60 number. The charts and formulas to account for these variations can be seen in Appendix D.For this project, a boring report by GEI consultants was used to estimate the soil conditions of theproposed office building in Boston, MA. GEI followed this ASTM standard to obtain true subsurfaceconditions. The exploration was completed in January 2007, at the (proposed) Moakley building at theBoston Medical Center, Albany Street, Boston MA. The boring logs completed at this site are included inAppendix C.2.3.2 Soil ProfileAfter analyzing the in-situ test results a soil profile can be created to summarize underlying soilcharacteristics. This simplifies the design process by analyzing according to one summarized soil boringinstead of an array of different logs.2.4 Geothermal Design ParametersA Ground Source Heat Pump (GSHP) is a technology used to heat and cool buildings by extraction of thesubsurface thermal energy as well as rejection of heat to a cooler subsurface. This technology makes useof geothermal energy, heat stored below the earth’s surface. Ground Source Heat Pumps first started tosteadily emerge in Sweden and Switzerland in the 1980’s. A GSHP system needs to address three maincomponents: how to transfer heat to and out of the ground, a heat pump to convert the heat to anappropriate temperature, and the building to transfer the heat or cold into the rooms (EGEC, 2008).The geothermal design parameters are important to determine how to most effectively transfer heatenergy from the ground to the heat pump and into the building as well as reject heat to the ground insteadof using traditional air conditioning. Each individual parameter must be considered on the basis ofperformance, availability and cost. First, the different types of ground source heat pump system have to15

be considered. After this, the components within the system were evaluated; the transfer medium solution,thermally enhanced grout, pile spacing, and the heat pump itself.2.4.1 GSHP SystemsThe earth provides a steady as well as immense heat source and heat medium for thermal transfer use.Deep vaults containing a mercury thermometer were installed beneath the Observatoire in Paris in the late17th century by the French chemist and physicist Lavoisier. These vaults were 27 m deep. In 1778, it wasdetermined through observation and data collection over time that the readings in the earth were constantthroughout the year (EGEC, 2008).Figure 1, summarizing a result of a study completed by Virginia Tech, shows the mean annual earthtemperature throughout the United States. The mean annual earth temperature is found at about 30 feetbelow the surface, and where the subsurface temperature remains relatively constant.Figure 1: Annual Mean Earth Temperature, United State (Virginia Tech).16

Over time, different methods of GSHPs were created. Figure 2 shows the different GSHP systems. Thesemethods include ground water wells, horizontal coils with direct expansion, horizontal coils with brinecircuit, vertical borehole heat exchangers in co axial form, borehole heat exchangers in U-pipe form, andborehole heat exchangers in spiral form.Figure 2: GSHP – Schematics (EGEC, 2008).Ground Source Heat Pumps are also classified in another way: open and closed loop systems. Open loopsystems require a groundwater or surface water supply to be present. This is a direct one way heat transferwhere the water source will flow through the heat pump system and be discharged. Closed loop systemswork as an underground network. This is a system of ground loop piping, commonly made of a highdensity plastic (HDPE) filled with a working fluid. The fluid flows to deeper points in the earth, nevercoming in direct contact with the soil, acting as the earth-coupled heat exchanger. As the working fluidflows back to the heat pump the actual fluid never leaves the pipes and is re-circulated.17

Different restrictions could prohibit different types of systems. Without a water source, open loop systemscannot be attained. Depending on land availability, certain types of horizontal systems may take up toomuch land. An effective system can be chosen once the parameters of the site are determined.2.4.2 The Heat Transfer MediumThe heat transfer medium is the flowing fluid in a GSHP system. If freezing could present a problem, anantifreeze mixture is used.Due to the extreme low temperatures in Boston during the winter months, antifreeze will be added to thewater medium to ensure that it will never freeze towards the surface. It is suggested that an antifreezesolution be used in any instances where the temperature will be below 40 degrees F, which will withoutdoubt be the case in Boston. There are many different types of antifreeze additives to consider. All ofthem fall into three different categories: alcohols (methanol, ethanol), glycols (ethylene glycol, propyleneglycol), and salts (sodium chloride, calcium chloride, and potassium acetate) (Virginia Tech).The alcohol water solutions will have a low viscosity and higher heat transfer capabilities. In terms of thesystem this translates to mean lower pumping power and a shorter ground loop. Due to the toxicity ofalcohol, this type of solution presents a high level of danger. Methanol is highly volatile and flammablein pure form. Despite the toxicity, methanol has successfully been used over a decade in the heat pumpindustry. Some local regulations prohibit the use of methane, and this must be investigated before use.Ethanol cannot be purchased in pure form, only in denatured form and denaturants have the possibility ofchemically attacking the HDPE piping.Ethylene glycol mixtures are typically used to cool automobiles. One danger in using this substance isthat its sweet taste can attract animals and children and cause harmful ingestion. Due to its toxicity it isgenerally not used for ground heat pump systems. Propylene glycol is a less toxic but far more expensivemixture

Additionally, due to the subsurface characteristics of Boston, MA, deep foundations such as micropiles . 5 are often used to support higher loads for building foundation support. . Engineering design goes beyond following existing standards and specifications. It is the concept of using what you know and more importantly adapting to learn .