Studying the Geology of a Building Site - Case Study Example

and Number of the Teacher’s ROLE OF GEOLOGY IN THE CONSTRUCTION OF HOOVER DAM Introduction The Hoover Dam, earlier dedicated as Boulder Dam by President Roosevelt in 1935, is a concrete arch-gravity dam in the Black Canyon of the Colorado River in the south-west of the United States of America. It was constructed in the early 1930s during the Great Depression. At that time, the dam was the country’s highest statement of modern progressive construction. As the first and largest of the country’s dams in the 1930s, the structure rose 727 feet above bedrock, “with a base thickness of 600 feet of concrete and a width at its gently curved crest, of nearly a quarter of a mile” (McGovern 2000: 143). Studying the geology of a building site is a vital first step before designing and constructing any structure, particularly formidably large and heavy works like concrete dams. For both civil engineers and environmental scientists, a study of geology is vital, for understanding the soil mechanics and its functions as a building site, and for understanding the environmental impacts of a construction project. Thesis Statement: The purpose of this paper is to investigate why geology is important for a civil engineer or an environmental scientist, using a case study of the Hoover Dam. The Hoover Dam: A Construction Miracle of the Twentieth Century The Colorado River was chosen for constructing the Hoover Dam because of its steep gradient of 11,000 feet in 1,450 miles, more than any of the larger rivers. It was expected that the sharp fall of the Colorado river would facilitate the spinning of turbines at the highest speeds. Black Canyon was selected as the site for the dam, and not the earlier choice of Boulder Canyon. Both sites were on the lower Colorado River beyond the mouth of the Grand Canyon; however the advantages with Black Canyon lay in “a more solid and therefore safer bedrock foundation” (Powell 2008: 72), which is reiterated by Stevens (1990) who states that the bedrock at Black Canyon had less jointing and faulting than at Boulder Canyon, after diverting the river there would be less silt and debris to clear, tunneling through the canyon walls would be much easier, and the dam would require less concrete for building because the gorge was narrower. Further, the advantages of constructing the dam at Black Canyon included its easier access, and facilitation for sediment accumulation at the deepest part of the reservoir. The Hoover Dam was unprecedented in the number of hazards and problems the construction project presented. The pressure caused by the reservoir of over nine trillion gallons of water was countered by using the principle of the arch shape to the dam. To anchor the dam firmly, its base was designed to be several times thicker than its crest. Additionally, to keep the dam site dry during construction, the river had to blocked with a coffer dam and the water diverted through bypass tunnels bored through the bedrock (Powell 2008: 74, 58). Further, tremendous measures had to be undertaken for overcoming natural obstacles such as jagged mountains of volcanic rock, the rapid flow of the Colorado River rushing through a narrow gorge, and the rugged and inaccessible Black Rock Canyon between Nevada and Arizona situated against a desert background infested with wildlife including rattlesnakes. The dam site was also in an isolated area, away from human habitation, with the model town for workers being about eight miles from the dam site. There were sharp differences in temperature, from 140 degrees on the canyon floor in summer to below 20 degrees in the winter. Other daunting construction problems included the necessity to “divert the Colorado river through freshly made tunnels in nearby rocks and a temporary coffer dam while the site was cleared and the dam built” (McGovern 2000: 144). Additionally, the possibility of stress imposed by temperature changes and the use of large volumes of concrete in relatively small areas, resulting in cracks in the dam had to be countered. This was solved by using refrigerated water through tubes inserted into the concrete thereby cooling the concrete and sealing it effectively only seventy-two hours after it was poured (Wilson 1985: 482). The complexity and ostentatious daring of the project invoked wonder at this marvel of engineering, which appeared to be heralding a new world by “Taming the Untamable at Boulder Dam” (Adams 1935: 5). Engineers and geologists work together when planning and designing a project. Site investigations and studies of the soil composition, soil mechanics, examination of the bedrock and its constituents, and the water table are basic requirements before a site can be selected for construction, particularly for major projects. The Site: Geology and Water Mechanics of the Soil and Bedrock The Black Canyon and Colorado regions have had a long and complex history of volcanism. Lake Mead is the reservoir formed behind the Hoover Dam. The lake extends over southern Nevada and north-western Arizona (Geological Survey 1960: 3). A minimum of four generations of volcanic rocks have been identified in the area south of the dam, with thick formations tilted, eroded and buried by later accumulations. The volcanic section is composed of lavas, breccias, tuffs and glasses. The lavas range from dark basalt, brownish andesite, other intermediary types, or light coloured rhyolites. “Fragmental materials from coarse breccia to fine-grained tuff, record explosive activity” (Geological Survey 1960: 18). The layers of volcanic rocks are thousands of feet deep. The walls of Black Canyon near Hoover Dam reveal great thickness of lavas and related volcanic rocks; besides igneous rocks of many kinds and sedimentary formations rich in marine limestones. The Hoover Dam area “contains exposures of Precambrian metamorphic rock, Tertiary volcanic and plutonic rock, and Quaternary gravels” (Mills 1989: 1). Paleozoic rocks are restricted to roof pendants in Tertiary plutons and xenolithic blocks in mafic lava flows. With few exceptions, rocks of the Hoover Dam area have been broken along several late Miocene faults with complex slip components and the Mead Slope left-lateral strike-slip fault. Well exposed, high faulted Miocene rocks at Hoover Dam provide an opportunity to study the paleostress history of a small area in a region where earlier geologic studies indicate “clockwise rotation of paleostress and a nearness to a major strike-slip fault zone boundary. Studies were conducted at the Hoover Dam site by Angelier, Colletta, and Anderson (1985). The sense of slip was detected on nearly 500 separate faults. The fault slip data show internal consistency in relation to lithology, size of faults, and location within a small area. However, with respect to fault-slip, the data reveals an inhomogenous mixture of primarily strike-slip and dip-slip motions. From this mixture, it is possible to resolve two distinct stress fields with a difference of 60 degrees in their directions of extension. From the relevant calculations, it is evident that these relationships suggest that strike-slip and dip-slip faulting belong to the same tectonic regime. Similar studies of other alternatives resulted in the conclusion that within the small area studied, a choice between alternatives to determine which is correct, cannot be made. Sites overlying gypsum, limestone deposits, coarse-grained sandstone or other coarse materials such as gravel have to be avoided, to prevent the escape of confined fluids. Further, rock or clay foundations helped to reduce seepage losses from reservoirs, whereas loam or volcanic soil as the underlying material cause seepage losses to increase. Subsurface materials being inconsistent, if any areas of the banks or bottom of the reservoir, or a channel leading to some other drainage basin is formed of sand or gravel, even if it is covered with a blanket of fine soil, “there is grave danger of excessive seepage loss” (Colton 1998: 217). When planning the construction of the Hoover Dam, the engineers and geologists had to take into consideration the pressures of water against retention structures, and prevent seepage that would cause deterioration to the structure. The designers of dams and restraining structures such as levees and dykes employed specific engineering formulas to identify the path of potential seepage and accordingly prepared their designs to prevent it. The Darcy formula denoted the association between pressure or hydraulic gradient and soil conditions or permeability, and discharge velocity, which equals the constant of permeability times the difference of head or hydraulic pressure divided by the length of column. This formula was used for forecasting the yield of a fluid under a known pressure, and along with other calculations the basic physics of groundwater flow indicated that moisture would flow underneath an earthen dam or embankment following an elliptical direction. (Colton 1998). “Beyond subterranean flow, moisture would saturate a dam in a line descending from the point of contact downward toward the opposite side of the dam” (Colton 1998: 199). To reduce the speed of water percolation through an embankment, engineers used the relationship of the Darcy formula, by which velocity could be decreased by lengthening the distance. For this purpose, an impervious blanket could be placed at the inner toe of the dam, or sheet piling could be inserted extending below the base of the dam. When implementing construction work below the surface water level or the water table, engineers had to drain the construction site. For the Hoover Dam as for other similar projects, a coffer dam was built so that work could proceed at an underwater level. In still water, workers filled earthen material in the form of a dyke around the site, and pumped out the water. Other situations where currents were involved, required more complex arrangements. The options included driving sets of parallel wooden piles into bed of the river or flowing water around the site, and then placing sheet piling walls between the timber anchors, frequently filled with a mud puddle for added strength. Another method generally used on bedrock featured a sheet-metal coffer wall reinforced by a timber and rock-filled crib for stability. All these methods took the base material into consideration, and erected a barricade between the work site and the standing water (Colton 1998: 206). Some leakage through and beneath the coffer dams were expected outcomes of the temporary barriers. The main objective of a coffer dam was to reduce infiltration to the extent that a moderate amount of pumping with sump pumps would keep the construction area free from the water. When the construction of foundations encountered the water table, ditches were prepared to drain the area and lower the upper level of groundwater. If the foundation extended deep into the subsoil, and surface drainage would not be able to eliminate adequate quantities of water, well points would be used to facilitate drainage. Well points are wells bored around a construction site. They use the principle of the cone of depression; that is, by removing groundwater faster than recharging takes place well points lower the level of groundwater in a cone-shaped area around the well. By creating coalescing cones of depression through several such wells, engineers lowered the water table under the construction site. This technical procedure required knowledge regarding “the depth of the aquifer to be drained, its recharge rate, and the permeability of the earth material” (Colton 1998: 207). In the process of preparing for waterproofing, engineers routinely assessed seasonal water-table levels. Further, various concrete mixtures could slow leakage by reducing the porosity of the concrete. However, “no concrete mixture could stop all movement of water through joints or cracks” (Colton 1998: 207). Although impermeable coatings may reduce leaks, coatings would be used as a last resort because their doubtful effectiveness. Engineers and geologists prefer to prevent leakage by proper planning in the original design. A well-planned foundation would have drainage channels similar to those used in agricultural fields, for the water to flow away. Multiple layers of water-impermeable membranes along with a drainage system, and water-resistant jointing improves the waterproofing efficiency at the foundation. Conclusion This paper has highlighted the Hoover Dam, and investigated the role of geology in its construction. In designing the dam’s reservoir, the geology of the reservoir site, the availability of construction materials, and the relative cost and value of water storage were taken into consideration. Upon selection of the site, a detailed geological investigation to prevent seepage loss was conducted before the designing stage. Thus, reservoir sites which were overlain with gypsum, limestone or coarse materials were not preferred. Rock and clay foundations help to minimise loss of reservoir water due to seepage. Because of the Black canyon area having a long history of volcanic activity, the subsoil of the Hoover Dam was found to have a great thickness of lavas in the form of volcanic rocks, besides numerous kinds of other igneous rocks. Thus, the narrow width of the gorge, the velocity of the Colorado River rushing down its greatly inclined path, the ideal location away from townships and human habitats, the ideal soil and bedrock compositions, and the lower presence of jointing and faulting in the bedrock of the site, facilitated the construction of the Hoover Dam. This impressive structure was the first of its kind, and still continues to be regarded as a distinctive marvel of engineering. Works Cited Adams, Mildred. Taming the untamable at Boulder Dam. The New York Times Magazine, February 24th, 1935. Angelier, Jacques, Colletta, Bernard & Anderson, Ernest. Neogene paleostress changes in the Basin and Range: A case study at Hoover Dam, Nevada-Arizona. Geological Society of America Bulletin, 96.3 (March 1985): pp.347-361. Colton, Craig E. Industrial topography, groundwater and the contours of environmental knowledge. The Geographical Review, 88.2 (1998): pp.199-218. Geological Survey. Geological survey professional paper, Issue 295. Geological Survey (U.S.). Washington: The United States Government Printing Office. (1960). McGovern, James R. And a time for hope: Americans in the Great Depression. Connecticut: Praeger Publishers. (2000). Mills, James G. Geologic history of the Hoover Dam 7.5’ quadrangle. Text and references accompanying NBMG Map 102. (1989). Retrieved on 30th April, 2011 from: http://www.nbmg.unr.edu/dox/m102text.pdf Powell, James L. Dead pool: Lake Powell, global warming, and the future of water in the west. California: The University of California Press. (2008). Stevens, Joseph E. Hoover Dam: An American adventure. The United States of America: The University of Oklahoma Press. (1990). Wilson, Richard G. Machine-age iconography in the American West: The design of Hoover Dam. Pacific Historical Review, 54.4 (November 1985): pp.463-493. Read More