Academic literature on the topic 'Shallow Bedrock Sites'

This paper presents seismic site classification practices for urban centres in Australia, China, and India with special emphasis on their suitability for shallow soil sites. The geotechnical aspects of seismic site classifications play a critical role in the development of site response spectra, which is the basis for the seismic design of new structures and seismic assessment of existing structures. Seismic site classifications have used weighted average shear wave velocity of top 30 m soil layers, following the recommendations of National Earthquake Hazards Reduction Program (NEHRP) or International Building Code (IBC) site classification system. The site classification system is based on the studies carried out in the United States where soil layer may extend up to several hundred meters before reaching any distinct soil-bedrock interface. Most of the urban centers in Australia, China, and India are located on distinct bedrocks within few meter depth of soil deposits. For such shallow depth soil sites, NEHRP or IBC site classification system is not suitable. A new site classification based on average soil thickness, shear wave velocity up to engineering bedrock is proposed. The study shows that spectral value and amplification ratio estimated from site response study considering top 30 m soil layers are different from those determined considering soil thickness up to engineering bedrock.

A new simulation-based site amplification model for shallow sites with thickness less than 30 m in Korea is developed. The site amplification model consists of linear and nonlinear components that are developed from one-dimensional linear and nonlinear site response analyses. A suite of measured shear wave velocity profiles is used to develop corresponding randomized profiles. A VS30 scaled linear amplification model and a model dependent on both VS30 and site period are developed. The proposed linear models compare well with the amplification equations developed for the western United States (WUS) at short periods but show a distinct curved bump between 0.1 and 0.5 s that corresponds to the range of site natural periods of shallow sites. The response at periods longer than 0.5 s is demonstrated to be lower than those of the WUS models. The functional form widely used in both WUS and central and eastern North America (CENA), for the nonlinear component of the site amplification model, is employed in this study. The slope of the proposed nonlinear component with respect to the input motion intensity is demonstrated to be higher than those of both the WUS and CENA models, particularly for soft sites with VS30 < 300 m/s and at periods shorter than 0.2 s. The nonlinear component deviates from the models for generic sites even at low ground motion intensities. The comparisons highlight the uniqueness of the amplification characteristics of shallow sites that a generic site amplification model is unable to capture.

Pedons were described, sampled and classified at 5-m intervals along a 130-m transect in an area typical of the southern Laurentian Highlands in order to assess short-range soil variability. Orthic Ferro-Humic Podzol was the dominant subgroup (25 of 27 sites). Differences in depth to bedrock and in soil water regime resulted in four soil families; bedrock was exposed at one site. At 21 of the 27 sites, however, the pedons were classified in one family: Orthic Ferro-Humic Podzol, coarse loamy, cold, humid. Differences in thicknesses and sequences of horizons resulted in a total of at least nine soil series. At scales of 1:20 000 or somewhat smaller, the soils of map units in the area would be most appropriately indicated as slope phases (10–40% slopes) of families. The dominant family would be the one indicated above with inclusions of shallow to extremely shallow phases and bedrock outcrops. Key words: Soil variability, Podzolic soils, classification of pedons

Possible sources of SO4 in soil, drift and bedrock at two saline sites in southern Alberta were investigated using sulfur and oxygen isotope analyses. Slight differences (0.6–5.2 per mil) between δ34S values of SO4 and insoluble S in the geologic material were attributed to kinetic fractionation during oxidation of organic S. Negative δ18O values of SO4 in the shallow (<6 m) bedrock, drift and soil reflected oxidation of reduced S. In contrast, positive δ18O values in the deep (>6 m) bedrock indicated sulfate reduction. The exact source of SO4 could not be determined in this preliminary study. The isotope approach, however, shows promise and warrants more detailed study. Key words: Salinization, sulfates, sulfur isotopes, oxygen isotopes

Shallow-soil sites in the Nipigon-Beardmore area of northern Ontario are widespread and pose unique operational problems for forest managers. Several working definitions of shallow soils are used in Ontario. More accurate regional definitions are required in order that silvicultural decisions for these sometimes fragile sites may be refined or new ones developed. Harvesting and silvicultural options for shallow-soil sites are limited in the North Central Region.A program of Forest Ecosystem Classification (FEC) for the North Central Region has been designed to provide better understanding of shallow sites and a framework of standardized definitions. The FEC describes several shallow-soil types that occur over boulder pavement or bedrock, and differentiates mainly on the basis of depth-to-rock contact, surface organic material thickness and texture of the primary mineral soil particles. Operational application of this classification may require identification of complexes of shallow-soil types.

Among all-natural hazards, earthquakes are the most damaging in terms of loss of lives and damage to infrastructure. Amplification and liquefaction are the major effects of earthquake that cause massive damages to infrastructures and loss of lives. Subsurface soil layers play a very important role in ground shaking modification. These ground shaking modifications when a seismic wave passes through soil are estimated by understanding of site effects. Site effects are the combination of soil and topographical effects, which can modify (amplify and deamplify) the characteristics (amplitude, frequency content and duration) of the incoming wave field. There are two stages of site effect evaluation. First is site characterization which is done by classifying the site based on soil properties. Next step is to estimate amplification of possible motions through these site classes. This is done by amplification factors or site coefficients in various provisions. Widely it has been agreed that site effects/amplification are different for deep and shallow soil deposits. Classifying the sites based on 30 m average shear wave velocity (Vs30) is useful for zonation studies because site amplification factor was defined as a function of Vs30 such that the effect of site conditions on the ground shaking can be taken into account. However, the definitions of site classes in different codes are not consistent. Seismic site classification and amplification for seismic micro zonation are obtained on basis of Vs30 irrespective of bedrock depth in Asia. As shear stiffness and the time period of soil column affect soil response most, parameters representing them are used for classification worldwide. Various codes use shear velocity (Vs) 30 and SPT-N as defining parameter for the purpose. Despite their wide use, the seismic site classification schemes considering top 30-m soil layers are being applied to dissimilar bedrock profile and are under significant research scrutiny. In this study, an attempt has been made to estimate amplification of shallow bedrock sites in Bangalore, Chennai, Coimbatore, and Vizag city in the Intraplate region using most appropriate input parameters. Initially dynamic properties of shallow bedrock sites are estimated by carrying out experimental studies of Multichannel Analysis of Surface Wave and Standard Penetration Test (SPT). Then new shear modulus (Gmax) versus SPT N correlation has been developed to overcome limitations in the existing similar correlations. Further for response calculation purpose, input layer, dynamic model curves and suitable Gmax correlations for different soil types have been identified by parametric study. Finally, a nonlinear site response has been carried out and amplification factors for shallow bedrock sites are estimated and presented in this thesis. In the first part of this work, preliminary site response analysis of hypothetical shallow bedrock sites has been carried out and mismatching of site classification and amplification factor is highlighted. Further, limitation of routinely used shear modulus correlation for stiffness estimation, dynamic models (shear modulus and damping curves), the input level of site response has been highlighted. Characterization of the subsurface and estimation of dynamic properties requires understanding of site effects and amplification. Even though several destructive earthquakes have caused extensive damages in shallow bedrock sites in Peninsular India, which is part of intraplate region, very limited systematic attempt has been made to estimate the dynamic properties of several sites in these regions. Soil dynamic properties in the form of shear wave velocity or shear modulus are required to estimate site effects and amplification. Experimental studies are carried out in shallow bedrock regions of South Peninsular India to obtain shear velocity and SPT N profiles. For these profiles, average 30 m values as per NEHRP and average till soil column, are estimated. It is observed that these values are different than average values up to rock because of inclusion of rock shear wave velocity values of the soil average values in shallow bedrock sites. Hence, 30 m average concept results in stiffer site class than soil average values. Misinterpretation of site class and following NEHRP provisions in shallow bedrock sites can lead to incorrect site coefficients and hence incorrect design force parameters. Literature review shows that several site response studies are being carried out by estimating shear modulus of soil layers considering SPT N values. Even though SPT N data are widely used for site response and seismic micro zonation, very few studies are available for in situ correlation between shear modulus versus standard penetration test (SPT) N values using field experiments. It is found in this study that many of the currently used Gmax correlations are not suitable for a particular soil type. Hence, in this study available correlations between SPT N and shear modulus are compiled and reviewed. New correlations are proposed by combining author’s data with available old data from each researcher separately in two ways (a) using all the data and (b) eliminating assumed and extrapolated data i.e. measured data. This study shows that correlations using measured data are better than correlations using all the data (including extrapolated). Further, another set of correlations is developed by combining three and more data sets by considering all the data and measured data separately. Three and more data combinations give the best correlation when compared to the original independent correlations and two data combined correlations. A new correlation has been developed considering measured, old and new, data from Japan and India, where N values are measured with hammer energy of 78%. Modification factors for old and new correlations are suggested for the other regions, where SPT N values are measured with different hammer energy. Representative evaluation of soil response requires the input parameters to be close to the physical behavior of soil column in the site. Several site response studies are carried out in South India considering limited representative parameters such as intraplate recorded earthquake data, soil specific shear modulus correlation, best soil dynamic model and input layer. Several site response analyses are being carried out using existing few shear modulus reduction and damping curves without knowing their suitability. Similarly, input ground motion for site response is being given at the depth of rock layer or borehole termination depth or 30 m. As per our knowledge, there is no clear cut guideline regarding the use of suitable Gmax correlation for the specific soil column, best shear modulus reduction and damping curves for typical soil and appropriate input layer in the site response study of shallow bedrock sites. As part of this study, an attempt has been made to identify suitable Gmax correlation for different types of soil column such as sand, clay and gravel alone or the mixture of all (sand, clay, gravel, sandy soil). Sites with earthquake data recorded at the surface, soil profiles along with SPT N values and shear wave velocity are selected from K-NET (Japanese website) data set for this study. Collected earthquake data consist of moment magnitude (Mw) of 5.0 to 9.0, which are recorded at different epicentral distances. Site response analysis has been carried out by considering earthquake data recorded at a rock site as an input ground motion to the soil profiles published in K-NET data site. Surface ground motion and response spectrum are obtained from different Gmax correlations. The results obtained are compared with surface recorded earthquake same event. The study shows that peak ground acceleration (PGA), response spectrums (RS) and amplification factor (AF) obtained from very few Gmax correlations are comparable with the recorded PGA, response spectrum and amplification factor. Over the years, several researchers have presented a variation of shear modulus and damping ratio with shear strain for different materials. Of several modulus and damping curves available for different soil type from existing literature, set of curves are selected such that only few input parameters are required to use this curve in site response analysis. Selected curves are then used for representing corresponding soil type in the evaluation of soil response. Soil profiles of sites having a surface and bedrock motion recordings are selected from the Kiban-Kyoshin Network data (KiK-net, http://www.kyoshin.bosai.go.jp/). Site response study has been carried by giving rock recorded data as input and surface response is evaluated for each site by changing modulus and damping curves. The evaluated surface response is compared with recorded data for different soil types. Comparison concentrated to identify the best match regards the shape of spectral curve and PGA value. Based on analysis, appropriate dynamic model curves for each soil type has been identified. Further an attempt is made to identify the input layer shear wave velocity beyond which change in response is insignificant. For the purpose, soil density and modulus and damping curves were kept constant by a parametric study by giving input at recorded level. Using same Kik-net data, site response analysis is carried out by considering constant properties and changing input level. Estimated response by giving input at different depth is compared with original response, the layer in which response changes are considerable is considered as cutoff layer. Shear wave velocity of cut off layer is almost similar in most of profiles considered in the study. This study shows that the input given below the soil layer of having shear wave velocity 500 (±100) m/s and above is predicting response close to recorded data irrespective of the soil type. Parametric study results obtained in this chapter are used as an input / guideline for site response studies of shallow bedrock sites. Shear stiffness of the column above the input level has been estimated using shear wave velocity profiles discussed previously. It is found that few locations do not have Vs profiles, hence few SPT N profiles are selected and added. In total 64 shallow bedrock sites are considered for analysis. In previous studies on subject, for most of the site response analyses was carried out in the intraplate shallow bedrock sites, considering synthetic data or active region ground motion data were considered. In this study, for first time available intraplate data are compiled and acceleration time histories are selected based on regional seismicity. A total of 13 intraplate motions recorded in stable continental regions is selected and baseline corrected. These motions have PGA varying between 0.05-0.17g and is in accordance with the hazard maps suggested for these regions by various researchers. Site response calculations are done using a one-dimensional non-linear approach in DEEPSOIL v5.1 software. Water table information and Index properties in the study are obtained from soil reports for the corresponding bore logs. In total, 832 analysis has been carried out and surface spectral values are compiled. Amplification factors are calculated using Ratio of Response Spectra (RRS) method from the spectral results for different time period ranges. Initially amplification factors are evaluated considering the period ranges 0.1-0.5 and 0.4-2.0 s which are similar to the International Building Code (IBC) and compared. The study shows that IBC period range does capture variation of the spectrum of intraplate shallow bedrock sites. Hence the new period range has been derived by considering spectral signatures of input and surface response spectrum. Amplification factors are calculated for new period range 0.01-0.06 s and 0.05-1.0 s. Significant amplification is observed in 0.05-1.0 s period range and amplification factor corresponding to this range is proposed as a final result. Soil profiles used in the study are grouped as five groups based on the stiffness of the soil column above input based on similarity in spectral signatures. The five groups are G1 with shear modulus <50 MPa, G2: 50-100 MPa, G3: 100-150 MPa, G4: 150-250 MPa and G5: >250 MPa. Average amplification for each group has been estimated and compared with previous studies. This study shows that amplification of short period range is comparable with PGA ratio amplification factor estimated in the region. Spectral amplification for the period range 0.05-1.0 s is less than short period amplification and IBC values. These values are calculated for each stiffness group and are decreasing with an increase in stiffness, with 3.24 for the group with modulus less than 50 MPa to 1.84 for the group with modulus greater than 250 MPa.

Estimation of reliable site effects and amplifications are important for seismic design. There are several 1D and 2D large scale site response analyses were reported in the literature and very limited shallow bedrock small scale 2D site responses were carried out. Hence in this study, an attempt has been made to generate the small-scale subsurface profiles in 2D using an integrated geophysical approach and a site-specific 2D nonlinear site response analysis at selected shallow bedrock sites in Peninsular India. Tarapur, Kalpakkam, Bangalore, Hampi, and Vizag are the shallow bedrock sites selected for the study. The subsurface profiling of these locations was done by an integrated geophysical approach by carrying out Multichannel Analysis of Surface Waves (MASW) survey, Ground Penetrating Radar (GPR) survey, Electrical Resistivity Tomography (ERT) and conventional drilling boreholes and measurement of Standard Penetration Test (SPT) N values. The study shows none of the important sites are having a uniform and homogenous subsurface layer even though likewise is assumed in several numerical models. The subsurface profiles generated by the integrated geophysical approaches are modeled in the FLAC - 2D program and site response parameters are estimated. Thirteen representative intraplate ground motions from all around the world are selected for the site response analysis based on the seismicity of the region. Results of the site response analysis were expressed in terms of short-period amplification factor (Fc) and long period amplification factor (Fs) for the intraplate shallow bedrock region. The results show that the Fc gives higher values than Fs in the case of stiff soil and it is showing a reverse trend in the case of soft soil. The trend of Fc is becoming more complex than that of the Fs especially in the case of stiff soil sites. Also, the 2D site response results were compared with conventional 1D nonlinear site response results using the program DEEPSOIL. This study shows that the actual 2D response parameters are highly varying with spatial variability of the subsurface and cannot be captured through the conventional 1D site response analysis. Further, a parametric study has been carried out to quantitatively evaluate the effect of different parameters on 2D site response analysis. Finally, from all these studies, we can conclude that stiffness of the subsurface profile, amplitude of ground motion, predominant period of ground motion, depth of subsurface profile, shape (spatial variability) of the subsurface layers, and inclination of the subsurface layer are the major factors that highly influencing the 2D response parameters of shallow bedrock sites.

Posiva Oy is responsible for implementing the program for geological disposal of spent nuclear fuel in Finland. Olkiluoto in Eurajoki has been selected as the primary site for the repository, subject to further detailed characterization which is currently focused on the construction of an underground rock characterization and research facility (the ONKALO). An essential part of the site investigation program is analysis of groundwater flow and solute transport to depth by means of numerical flow modeling as part of the 2011 site descriptive model. Groundwater flow in the crystalline rocks at Olkiluoto takes place predominantly in the void space of the interconnected fractures in the bedrock. Many of the hydraulic characteristics of flow through such a system can be represented by hydrogeological Discrete Fracture Network (hydro-DFN) models, since they capture some of the details of fracture geometry, size, connectivity and openings. The paper describes how field data is used to derive such models, ultimately for use in safety assessment. Surface based site investigations at Olkiluoto include 53 vertical or inclined deep core drilled boreholes and 27 shallower sub-vertical boreholes. The characterization of fractures is a key objective. Fracture geometrical data (fracture positions and orientations) are determined from drill core mapping and/or borehole TV images, while hydraulic data are determined with the Posiva Flow Log (PFL). The PFL method is a geophysical logging device developed to detect continuously flowing fractures in sparsely fractured crystalline bedrock by means of difference flow logging, providing the effective transmissivity of individual fracture intercepts. A methodology developed originally in the Swedish program for constraining hydrogeological DFN models based on the PFL high resolution hydraulic test data has been extended to incorporate data from underground. The approach integrates deterministic models of the main hydro-structures with a stochastic model of the bedrock between. An exploration of conceptual uncertainties in the representation of the very scarce distribution of flow at repository depth, c. 400m, has been performed. This has been greatly informed by the analysis of 15 additional sub-horizontal boreholes drilled ahead of the ONKALO tunnel excavations. Conditions for PFL hydraulic tests performed in these pilot holes provide a lower detection limit and reveal the very low magnitude of groundwater flow that may be expected around the repository depositional volumes.

The Building Code of Japan (the Building Standard Law of Japan) has been changed from the former prescriptive into performance-based type in June, 2000. This paper presents the evaluation procedures of seismic performance of seismically isolated buildings against major earthquake motions newly introduced to the Building Code of Japan in October, 2000. The evaluation procedures apply the single-degree-of-freedom (SDOF) system, equivalent linearization and response spectrum analysis. The basic concept of seismic design spectra for major earthquake motions is: 1) basic design spectra defined at the engineering bedrock, and 2) evaluation of site response from geotechnical data of surface soil layers. The principle of evaluation procedures is that the predicted response values should not exceed the estimated limit values. The evaluation criteria of each portion of a building are: 1) the isolation system shall be designed so that the maximum response displacement does not exceed the design limit displacement, and 2) the structure above the isolation system and the foundation and structural elements below the isolation system shall be designed so that the working stress does not exceed the allowable stress. The scope of application of the evaluation procedures is: 1) buildings not exceeding 60 meters in height, and 2) base isolation buildings on the soil layers excluding very soft soils.

This Focused Condition Assessment examines the impacts of the recent 2000–2020 long-term drought on the vegetation and soils of Glen Canyon National Recreation Area (GLCA). With support from the NASA DEVELOP Program, summer (June–August) Normalized Difference Vegetation Index (NDVI) values from 1995 to 2017 (excluding 2012 which was not available), measuring greenness and phenology in the vegetation, were analyzed for two periods. The first period from 1995–1999 included the pre-drought period, when precipitation was average to above average. Most years of the second period, 2000–2020, were drier than average as part of the severe drought that began in late 1999 and has continued to present (Lukas and Payton 2020). The NDVI values included mean values and were analyzed for 42 soil units, 20 associated NRCS Ecological Sites (ecosites), and the 10 most widespread vegetation alliances derived from the GLCA vegetation classification. Unvegetated rock outcrops, other exposed bedrock areas, and cliffs, which are extensive in GLCA, were not included. With the exception of some riparian areas, mean NDVI values for all upland soils, ecosites and alliances declined from pre-drought conditions. The areas showing the largest declines were clay soils, shallow sandy loam and other shallow soils and associated ecosites and alliances. Talus vegetation and mid- to upper elevation pinyon-juniper (Pinus edulis-Juniperus osteosperma) woodlands showed the smallest declines. Deeper sandy and sandy loam sites showed intermediate declines. Particularly large declines occurred in shallow soil arid sites dominated by shadscale (Atriplex confertifolia) and other saltbush species. Blackbrush (Coleogyne ramosissima), one of the dominant species in the park, showed moderate declines, primarily on shallower soils. No evidence for widespread death in either blackbrush or pinyon-juniper woodlands were noted, although recent severe drought and a weakened Arizona Monsoon since 2018 may be causing impacts to the woodland species. Relationships with livestock grazing are also examined, based on data collected on long-term monitoring plots established between 2008 and 2020. There is evidence that areas with intensive livestock grazing have shown larger declines than ungrazed areas, but these impacts need to be explored more fully at the local allotment and pasture level, and correlated with actual grazing animal unit months (AUM)’s. Several management recommendations are made, including additional plot-based long-term monitoring, exploration of cultural resource inventories and erodible soils, how these observed changes can affect livestock grazing management decisions in the park, and further exploration using NDVI data from 2018 and forward.

Offshore wind farms typically host tens to hundreds of turbines that are individually sited on foundations or anchored if floating. These are connected by inter-farm cables which feed into one or more marine-based substations, further feeding one or more shore-connected high-voltage cables - all infrastructure that requires knowledge of water depth, metocean conditions, and seabed/subsurface geology. With this industry set to establish itself on the continental shelf of Atlantic Canada, knowledge of the geological conditions from the seabed to tens of metres below will be essential for farm layout and foundation design. Thus, geoscience questions addressing regional geomorphology, Pleistocene glacial retreat and sea-level change, the characteristics of key individual stratigraphic layers, and the magnitude and patterns of sediment mobility are important. In Atlantic Canada, ongoing efforts to address these questions are using legacy data, but new data is required to further our understanding of the shallower portions of the shelf. Examples include: what is the distribution of buried tunnel valleys under offshore banks, and might their complex facies infill affect foundation conditions? How and where would the organic sediments, left by a coastal suite of landforms drowned during transgression, affect foundation or landfalling cable stability? How active is salt diapirism, and could it be considered a geohazard? Are demonstrated sediment mass failures also a risk? What is the current understanding of sediment mobility in shallow waters, and how does that affect infrastructure armouring/depth of burial? What is the variability of the geotechnical properties of our offshore sediments? What is the foundation suitability of offshore Tertiary semi-consolidated bedrock? To conclude, the initial scope of a developing regional foundation suitability model will be presented for the Eastern Scotian Shelf.