The emperor's new clothes - opportunities and limitations applying AEM to geotechnical design work
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In summer 2015, we acquired close to 6.000 km of Helicopter, time-domain airborne electromagnetic (AEM) data for regional geotechnical mapping for the Norwegian National Rail Administration. This survey and further experience from related Norwegian road planning projects demonstrated the unprecedented accuracy of modern AEM data. The extent of geotechnical site investigations can be drastically reduced, both in terms of time schedule, and costs if AEM derived bedrock models are included when soil investigations are planned. Geotechnical projects demand high resolution (meter scale) and AEM data is to some extent capable of delivering that. Some of our data matched the resolution of corresponding ground geophysics data. Here we present the way in which AEM can be used as bedrock models, sensitive clay delineation and to determine bedrock types. Our discussion leads us to the missing link between high vertical resolution in the first tens of meters for geotechnical work and the focus on simple, sub-vertical structures in exploration AEM. Ultimately, we should strive for the best of both worlds, shouldn't we?Keywords:
Bedrock
Geotechnical investigation
the bedrock geologic map portrays the current interpretation of the distribution of various bedrock stratigraphic units present at the bedrock surface the bedrock surface is buried by unconsolidated surficial sediments mostly quaternary over most of its extent but this surface coincides with the modern land surface in areas of bedrock exposure the map is consistent with all available data including drill records and well samples as well as surface bedrock exposures both natural and man made and shallow to bedrock soils units nrcs county soils maps mapped stratigraphic intervals are portrayed primarily at the group level i e a grouping of bedrock formations each characterized by distinctive lithologies rock types summarized in the map key and associated metadata the distribution of bedrock units was mapped to conform to the current map of bedrock topography elevation of the bedrock surface the structural configurations of relevant stratigraphic datums were intercepted with the bedrock topographic surface to produce the map contacts the line style shown on the bedrock geologic map qualitatively reflects both data density and degree of certainty of individual stratigraphic contacts downloadable
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Lithology
Geologic map
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Bedrock
Longwall mining
Surface mining
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Analysis of topographic and bedrock surface data from 41 sites 23.3 km2 (9 m2) within Midwestern areas glaciated during the Late Wisconsinan identifies an average drift thickness-maximum bedrock relief transition equation for the presence or absence of bedrock surface influences on topographic detail. For sites with average drift thicknesses between 15 and 35 m, the transition occurs when average drift thicknesses exceed 0.5 maximum bedrock relief + 10 m. This equation may provide a practical tool in searching for buried bedrock valleys.
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Skyscrapers in Manhattan need to be anchored to bedrock to prevent (possibly uneven) settling. This can potentially increase construction costs if the bedrock lies deep below the surface. The conventional wisdom holds that Manhattan developed two business centers--downtown and midtown--because the depth to the bedrock is close to the surface in these locations, with a bedrock valley in between. We measure the effects of building costs associated with bedrock depths, relative to other important economic variables in the location of early Manhattan skyscrapers (1890-1915). We find that bedrock depths had very little influence on the skyline; rather its polycentric development was due to residential and manufacturing patterns, and public transportation hubs.
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Downtown
Skyline
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Through the mine pressure measurement was studied with the method of shenhua group daliuta coal mine 22614 fully mechanized working face in thin bedrock thick coal bed mining conditions of mine pressure regularity.In the surface soil for loose sand,face the upper bedrock ups and downs change and support's stress characteristics together integrated analysis for the realization of shallow buried depth of fully mechanized working face safety and efficiency mining provides the basis.
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There is a misconception by some in the geologic and non-geologic communities of Florida that photolineaments and vertical bedrock fractures are one and the same. The main objectives of this paper are (1) to document a case study where a comprehensive geophysical and geotechnical exploration program was undertaken to verify a high-quality photolinear analysis; and, based on the case study, (2) evaluate the validity of photolinears as indicators of vertical bedrock fractures in the covered karst terrain of west-central Florida. The case study, an investigation by Upchurch et al. (1999), was an analysis of photolineaments at a 445-ha site intended for construction of an above-grade reservoir in west-central Florida. The photolineaments were ground truthed using ground penetrating radar (GPR), refraction and reflection seismic profiling, standard penetration testing (SPT), and cone penetrometer testing (CPT; Dobecki and Upchurch 2010). The post hoc review, based on the comprehensive site geophysical and geotechnical testing and resulting data, determined that fifty-eight percent of the photolinears do not correspond to vertical fractures in the limestone bedrock. This review demonstrates the fallacies of assuming all photolinears represent vertical bedrock fractures in the covered karst terrain of west-central Florida. Based on this case study and the post hoc review, it is our belief that in the covered karst terrains of Florida, all photolinear evaluations should have some form of field verification in order to equate them to vertical bedrock fractures or karst features.
Bedrock
Penetrometer
Geotechnical investigation
Ground-Penetrating Radar
Standard penetration test
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Alpine engineering presents a unique set of problems with highly variable soil and bedrock conditions. Several subsurface cavities were found beneath the site to be developed for 4 three-story condominium buildings at the Beaver Creek Resort. These conditions required a special investigation program and special foundation system. The site to be developed was underlain by Eagle Valley Evaporite bedrock, which can contain cavities due to the soluble nature of the bedrock. During the geotechnical investigation several subsurface cavities measuring up to 13 feet (4 m) in height were found at or near the surface of the bedrock. The resulting recommendation to the client was to support the proposed structures on a special caisson foundation system. The caissons would be supported by the competent bedrock beneath the cavities and the portion of the caisson through the cavity would be permanently cased. After the foundation system was designed, an additional geotechnical investigation was conducted by investigating the subsurface conditions at each of the 103 caisson locations. The location and depth of cavities for each of the caissons were then documented. Cavities varying in height from 2 feet (1 m) to 13 feet (4 m) were encountered at depths varying from 25 feet (8 m) to 85 feet (26 m) in 15 of the 103 caisson locations. As a result of the additional investigation the caisson contractor was able to provide a more accurate bid and complete the caisson construction on schedule. The drilling and installation of each caisson was observed and recorded. The four buildings have shown no movement since construction in 1987.
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Bedrock
Foundation (evidence)
Geotechnical investigation
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Significance Hilly landscapes are typically mantled with soil and underlain by a weathered bedrock zone that may extend tens of meters beneath the surface before reaching fresh bedrock. The weathered bedrock zone influences water runoff to channels, the chemistry of that water, the rates and processes of erosion, and atmospheric processes due to plant uptake of moisture and return to the atmosphere. However, the spatial pattern of the underlying fresh-bedrock surface is essentially unknown. We present a testable model that predicts hillslope form and the depth to fresh bedrock. The depth increases upslope and depends strongly on the porosity and permeability of the bedrock and the rate of channel incision at the base of the hillslope.
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It has now been well established that the depth of bedrock is a key parameter in assessing the impact of local site conditions on seismic hazard analysis. Where conventional geotechnical testing like standard penetration test (SPT) or cone penetration test (CPT) requires a far greater cost and manpower to be used for such purposes, geophysical testing like ground-penetrating radar (GPR) and multichannel analysis of surface waves (MASW) may provide the researchers with more viable options to achieve conclusive evidence on bedrock depth. Application of geophysical techniques has become more and more extensive and advanced in many geo-morphological studies since the early 2000s. Geophysical techniques require less time and effort, and the easy processing of the obtained data is the primary reason for their popularity. However, due to variability in subsoil mechanical properties, wave attenuation and dispersion and diverse geological boundary conditions, the results obtained through geophysical techniques are often ambiguous and non-unique. The interpretation of the obtained data also requires skill and experience, as the range may vary widely and more often than not consensus is difficult to achieve. In this paper, an endeavor has been made to coalesce the results of two widely used geophysical techniques, namely GPR and MASW to derive more conclusive evidence for the detection of bedrock depth in a part of Garhwal Himalayas. The study area comprises of two different cities of Uttarakhand, India. Both the sites possess different geo-morphological attributes and thus prove to be a perfect platform to conduct the experimentations. Both GPR and MASW testing have been performed and results are shown in graphical format. A comparison of the GPR survey with a conventional geotechnical testing (SPT) is also presented here. This study shows that GPR and MASW can provide complementary results in estimating bedrock depth.
Bedrock
Ground-Penetrating Radar
Geotechnical investigation
Subsoil
Underpinning
Standard penetration test
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