Competent Bedrock Research Articles

Brecciation of glacially overridden palaeokarst (Lower Aare Valley, northern Switzerland): result of subglacial water‐pressure peaks?

Water pressures at the base of active glacial overdeepenings are known to fluctuate strongly on various time scales. Rapid peaks in basal water pressure can lead to fracturing of the glacier bed, a process that has been described at numerous sites around the world, mostly based on large hydrofracture systems. This article presents drill‐cores from the base of a >100‐m‐deep glacial overdeepening in the Lower Aare Valley in northern Switzerland that were investigated with high‐resolution imaging (including X‐ray computed tomography) as well as compositional and microstructural analyses. The drill‐cores recovered Jurassic limestones hosting palaeokarst voids infilled with blue clay. We identify this clay, based on its kaolinitic composition, as siderolithic Bolus Clay but in a rather atypical variety formed under reducing conditions. The surfaces of the palaeokarst walls show smoothly undulating as well as brecciated sections with form‐fit interlocking clasts, which are the result of an in situ brecciation process. We argue that these particular fractures are not related to (glacio‐)tectonics or frost action. Instead, we suggest an explanation by involving water‐pressure peaks that were transmitted to the void walls by the clayey karst filling, resulting in hydrofracturing. In addition to pervasive karstification and tectonic overprinting, this water pressure‐driven fracturing may have enhanced the deep incision of the overdeepening into the rheologically competent bedrock.

Comprehensive ambient noise analyses for seismic microzonation of sarpol-e-zahab after the Mw 7.3 2017 Iran earthquake

This study is intended to investigate ambient noise recordings in form of array and single-station measurements at Sarpol-e-zahab city after the destructive Mw = 7.3 earthquake of 12 November 2017 in Kermanshah province, Iran. The ESPAC, f-k, HVTFA, and HVSR methods are applied to analyze the ambient noise and obtain useful information on three key factors of site seismic properties, i.e. the geometry of ground layers, their mechanical characteristics and site resonance frequencies. The results of this study are presented as maps of the fundamental frequency of the ground, depth of bedrock, the thickness of top cover materials, as well as the average shear wave velocity (Vs)700 and (Vs)30 for Sarpol-e-zahab. The analyses show that the single value of (Vs)30 does not represent seismic ground response, and the depth of competent bedrock, which dominates the low-frequency seismic response amplifications is located far beyond 30 m in this city. The most effective grade II seismic microzonation of Sarpol-e-zahab is provided by combining the results of these analyses.

Performing horizontal to vertical ratio testing in stiff soils in and around Grand Teton National Park

Horizontal to vertical spectral ratio (HVSR) testing was completed at two cross sections in and around GTNP. The HVSR testing produced reliable estimates of the fundamental frequencies for many of the sites tested. The goal of the testing was to determine a depth of soil above competent bedrock. However the fundamental frequencies recorded yielded predicted depths that are much shallower than expected. Also the predicted depths did not increase at greater distance from the Teton Range, which would be expected at these sites. Based on these predictions the authors do not believe the frequencies recorded are a good indication of the depth of the soil above bedrock but instead it is believed that the depths correspond with a layer of softer topsoil/overburden above a stiffer gravel layer. Although the goal of measuring the depth of soil above bedrock was not met, HVSR produced results that may be useful to others for determination of a fundamental frequency of resonance at our testing locations.
 
 Featured photo by Anna Cressman, taken from the AMK Ranch photo collection. https://flic.kr/p/2jjWZGT

Low-temperature specific heat capacity measurements and application to Mars thermal modeling

Data returned from Martian missions have revealed a wide diversity of surface mineralogies, including in geological structures interpreted to be sedimentary or altered by liquid water. These terrains are of great interest because of their potential to document the environment at a time when life may have appeared. Intriguingly, Martian sedimentary rocks show distinctly low thermal inertia values (i.e. 300 - 700 Jm−2K−1s−1/2, indicative of a combination of low thermal conductivity, specific heat capacity, and density). These low values are difficult to reconcile with their competent bedrock morphologies, whereas hundreds of bedrock occurrences, interpreted as volcanic in origin, have been mapped globally and display thermal inertia values > 1200 Jm−2K−1s−1/2. Bedrock thermal inertia values are generally assumed to be driven by their bulk thermal conductivity, which in turn is controlled by their micro- and macro-physical properties (i.e., degree and style of cementation in the case of detritic rocks, horizontal fractures and layering, etc.), and not by their density (well-known from terrestrial analog measurements, and with modest variability) or specific heat capacity (generally uncharacterized for non-basaltic materials below room temperature). In this paper, we demonstrate that specific heat capacity cannot be a potential cause for the differential thermophysical behavior between magmatic and sedimentary rocks through a series of experimental Cp(T) measurements at 100–350 K using differential scanning calorimetry. The results on 20 Martian-relevant minerals investigated in this work indicate that these materials exhibit very similar specific heats, ranging from 0.3–0.7 Jg−1K−1 at 100 K to 0.6–1.7 Jg−1K−1 at 350 K. When used in a Martian thermal model, this range of Cp values translate to very small surface temperature differences, indicating that uncertainty in composition (and its effect on the specific heat) is not a noticeable source of thermal inertia variability for indurated units on Mars. We therefore conclude that the low thermal inertia value of sedimentary rocks compared to magmatic/volcanic rocks is likely due to their low apparent bulk conductivity, which bears information on their internal physical structure. Future work combining the analysis of thermal observations acquired at various local times and seasons will help further characterize this heterogeneity.

Investigation into the types of fracture and viable depth to substratum of a housing estate using geophysical techniques

Mapping of the configuration of bedrock is paramount in civil engineering and hydrogeological settings. In civil engineering, areas that are extensively fractured (either shallow or deep) are adjudged as weak zones whereby these provinces are regarded as copious zones for groundwater exploration. Very Low Frequency Electromagnetic (VLF-EM) and ground magnetic survey were carried out in Olupona Housing Estate along seven traverses with interstation spacing of 20 m. For the two methods, traverses 1 to 3 were taken in North-South azimuth which covered distance 300 m while traverses 4 to 7 were acquired along East-West azimuth which covered distance 400 m. VLF-EM results revealed Type-1 fracture along traverse 1, traverses 2 to 4 showed no fracture but a fairly competent bedrock. Type-2 fractures were revealed on traverses 5 to 7 while Type-3 fracture was shown on traverse 5 alone. Ground magnetic results depicted that the depths to biotite or biotite muscovite granite are relatively deep. The mineral rocks with very thin, intermediate, and very thick bodies have their depths range as 14.7 m, 11.0 m and 8.8 m respectively. On the average, the depths to these mineral rocks from the surface varied from 6.5 to 19.6 m. The trends of the identified fractured zones are NE-SW and NW-SE directions. It is concluded that in order to avoid building collapse in future, construction of high-rise buildings is unadvisable in the study area. If the identified fractured zones are properly maximized for groundwater exploration, the fractured zones can be optimized such that reservoir(s) can be constructed so as to supply water to the houses for domestic usage.

Numerical Modeling Evaluation of Drilling Mud Loss Risk for a HDD Borehole in a Low-Strength Competent Bedrock

AbstractHorizontal directional drilling (HDD) plays an important role in pipeline construction. Drilling mud loss, or the maximum allowable drilling mud pressure, in soil and sand layers during HDD...

Application of Electrical Resistivity and Ground Penetrating Radar Techniques in Subsurface Imaging around Ajibode, Ibadan, Southwestern Nigeria

Integrated geophysical investigation involving Electrical Resistivity (ER) and Ground Penetrating Radar (GPR) techniques were carried out around a site underlined by Basement Complex rocks of southwestern Nigeria. The study was aimed at imaging the subsurface lithological units and delineating shallow geologic structures for the purpose of characterizing the area for construction suitability. A total of twenty five (25) Vertical Electrical Sounding (VES) data using Schlumberger array, ten (10) traverses of Electrical Resistivity Imaging (ERI) using Wenner array and Ground Penetrating Radar (GPR) surveys were carried out along the established traverse lines within the area. The VES data were quantitatively interpreted using partial curve matching technique and subsequently improve upon by inversion software using IPI2Win, to obtain the layer geoelectric parameters. The ERI data was inverted and interpreted using Res2Dinv and Res3Dinv inversion software’s respectively to generate 2D and 3D resistivity image of the subsurface. The GPR data was processed into radar section using RadExplorer software. Vertical electrical sounding results delineates typically three to four geologic layers which are the topsoil/lateritic hardpan, weathered basement (consisting clay and sandy clay) and fractured/fresh basement with layer resistivity value ranges of 10-2684 Ωm, 12-242 Ωm and 229-3213 Ωm respectively and thickness value ranges of 0.5-2.1 m and 4.0-14.1 m respectively. 2-D inverted resistivity results also delineated three major geologic layers which are the topsoil, weathered basement and fresh basement and correlates well with the results obtained from the VES results. Layers 1 to 3 of 3D inverted resistivity slice results show high degree of variation in resistivity distribution at shallow depth, consisting of highly resistive material towards the eastern part with low resistivity material concentrating at the south-western part. Results of the GPR survey also delineated three to four geologic layers which include the topsoil/lateritic hardpan, weathered basement and fractured/ fresh basement. The study area was categorized to have semi-competent to competent basement rock based on the resistivity value of the underlying material within the area. Bedrock depression delineated at some location could pose threat of differential settlement to construction works within the study area. Thus, it should be ensured that foundation is designed to sit comfortably on the competent bedrock or by employing suitable foundation work, such as piling to ensure foundation stability and prevent structural failure. Thus, electrical resistivity and ground penetrating radar techniques are versatile tools in site characterization.

Geophysical and hydrogeological characterisation of the impacts of on-site wastewater treatment discharge to groundwater in a poorly productive bedrock aquifer

Contaminants discharging from on-site wastewater treatment systems (OSWTSs) can impact groundwater quality, threatening human health and surface water ecosystems. Risk of negative impacts becomes elevated in areas of extreme vulnerability with high water tables, where thin unsaturated intervals limit vadose zone attenuation. A combined geophysical/hydrogeological investigation into the effects of an OSWTS, located over a poorly productive aquifer (PPA) with thin subsoil cover, aimed to characterise effluent impacts on groundwater. Groundwater, sampled from piezometers down-gradient of the OSWTS percolation area displayed spatially erratic, yet temporally consistent, contaminant distributions. Electrical resistivity tomography identified an area of gross groundwater contamination close to the percolation area and, when combined with seismic refraction and water quality data, indicated that infiltrating effluent reaching the water table discharged to a deeper more permeable zone of weathered shale resting on more competent bedrock. Subsurface structure, defined by geophysics, indicated that elevated chemical and microbiological contaminant levels encountered in groundwater samples collected from piezometers, down-gradient of sampling points with lower contaminant levels, corresponded to those locations where piezometers were screened close to the weathered shale/competent rock interface; those immediately up-gradient were too shallow to intercept this interval, and thus the more impacted zone of the contaminant plume. Intermittent occurrence of faecal indicator bacteria more than 100m down gradient of the percolation area suggested relatively short travel times. Study findings highlight the utility of geophysics as part of multidisciplinary investigations for OSWTS contaminant plume characterisation, while also demonstrating the capacity of effluent discharging to PPAs to impact groundwater quality at distance. Comparable geophysical responses observed in similar settings across Ireland suggest the phenomena observed in this study are more widespread than previously suspected.

The spatial variation of weathering and soil depth across a Triassic sandstone outcrop

AbstractIn this study, we used an archive of borehole logs from the British Geological Survey to collect information on the spatial structure of weathering that extends from the surface to competent bedrock across the Triassic Sherwood Sandstone Group outcrop (750 km2), in the East Midlands, UK. The borehole logs were used to estimate the thickness of the soil (n = 280) and soil and saprolite (S&S) to competent rock (n = 500). The weathering profile of the sandstone consisted of soil (median thickness ∼ 1·5 m) overlying a transition zone of compacted and weakly cemented weathered sandstone saprolite over bedrock. Topographic analysis using a NEXTMAP 5 m × 5 m digital elevation model (DEM) revealed no significant relationships between slope properties (relief, flow length, flow accumulation or slope angle) and soil or S&S thickness. A weak, but statistically significant correlation was found between the thickness of the soil and S&S (rs = 0·25, p < 0·001, n = 192). The variation in soil thickness may be related to changes in current and historic and land‐use, variation in sandstone properties and the influence of glacial/peri‐glacial processes. The thickness of the saprolite was more variable towards the southern part of the study area, where it increased to a maximum 40 m. We hypothesize and provide evidence that the greater weathering thickness is related to the occurrence of increased faulting in this part of the study region, allowing increased access to meteoric waters. A possible source of increased water supply is meltwater from Quaternary ice sheets; the overburden of ice may have increased sub‐glacial pore water pressure, with the fractures and faults acting as a drainage system for the removal of dissolved weathering products. British Geological Survey © NERC 2010

Microfracture Surface Geochemistry and Adherent Microbial Population Metabolism in TCE-Contaminated Competent Bedrock

A TCE-contaminated competent bedrock site in Portsmouth, NH was used to determine if a relation existed between microfracture surface geochemistry and the ecology and metabolic activity of attached microbes relative to terminal electron accepting processes (TEAPs) and TCE biodegradation. The bedrock is a metasandstone and metashale of the Silurian Kittery Formation. Eleven microfractures (MF 01-11) were extracted from cores of competent rock from 2 boreholes (BBC5 and BBC6) at depths > 21.3 m below ground. The host rock had 3 nominal pore width sizes (131.1, 1.136, and 0.109 μ m), a porosity of 0.8%, and a permeability of < 1 μ d. Microfracture surface precipitates were polycrystalline with grain sizes ranging from 10 to 100 μ m. Petrography and XRD revealed that carbonates and quartz were the dominant microfracture surface precipitates. Mineral distribution was heterogeneous at the 10 μ m scale. Oxidized and reduced iron species were identified with XPS on the microfracture precipitate surfaces. Carbon functional groups characteristic of NOM were also identified. SIMS mass fragment fingerprints suggested that TCE, PCE and/or VC were possibly adsorbed to NOM on the microfracture surfaces. Packer waters were alkaline (131–190 mg/L as CaCO3, pH 8.8 to 9.6), mildly reducing (Eh of −208 to 160 mV, DO of 0.4 to 2.5 mg/L), with low NPDOC values (0.8–1.7 mg/L), and measurable Fe (II) (0.1 mg/L) and Fe (III) (0.02 to 0.3 mg/L). Sulfate was the dominant anion in the packer sample water (110–120 mg/L). No sulfide was detected. H 2 was present in a number of the BBC wells at the site (2.2–7.3 nM). Amplification with specific primer sets of seven microfractures from BBC5 showed the presence of bacteria, Archaea, anaerobic dehalorespirers (Dehalococcoides sp.), sulfate reducing bacteria, and iron reducing bacteria (Geobacteraceae). Redox zonation may exist relative to spatial distance from within the microfracture network to the open fracture system. The microfracture surface precipitates, frequently spatially complex and comprised of a variety of C-, Fe- and S-containing minerals, may be another region for redox zonation. Fe was the dominant microfracture surface element and active Fe cycling is suspected. However, the primer data suggest that the microfracture network may have been more reducing than the open fracture system. In this case, the microfracture network may constitute a zone where more reductive metabolic processes occur, making this system similar to biogeochemical redox zones found in other environments.

Detection of near-surface horizontal anisotropy in a weathered metamorphic schist at Llano Uplift (Texas) by transient electromagnetic induction

The use of transient controlled-source electromagnetic prospecting to detect buried, steeply dipping foliation in metamorphic rocks is illustrated with data acquired over the Precambrian Packsaddle schist in the Llano Uplift of central Texas. The azimuthal variation of the transient voltage at a given transmitter–receiver separation about a fixed central point is consistent with the forward model response of a homogeneous halfspace exhibiting horizontal electrical anisotropy. The loop–loop exploration configuration is ideally suited to probe horizontal anisotropy. A quantitative match of the forward response to the observed data produces reasonable electrical conductivity values and coefficient of anisotropy for resistive, crystalline geological materials. The most conductive direction consistently lies within a few degrees of the geologically mapped foliation strike direction. The electrical anisotropy is strongest below the near-surface weathered layer, within the more competent bedrock. The agent responsible for generating the anistropy cannot be definitively determined, because it is likely to be a combination of geological factors, such as weathering, compositional banding and microcracking, all of which enhance electrical conductivity parallel to the plane of foliation. The transient electromagnetics is supplemented by DC resistivity and seismic surveys. The elastic anisotropy is evident in the near-surface weathered layer, but it may not persist very deep into the underlying competent schist. The exposure of foliated schist at the surface is not sufficient to rule out a possible role for systematically aligned macrofracture sets as a secondary cause of the observed anisotropy.

Delineating a shallow fault zone and dipping bedrock strata using multichannal analysis of surface waves with a land streamer

The multichannel analysis of surface waves (MASW) seismic method was used to delineate a fault zone and gently dipping sedimentary bedrock at a site overlain by several meters of regolith. Seismic data were collected rapidly and inexpensively using a towed 30-channel land streamer and a rubberband-accelerated weight-drop seismic source. Data processed using the MASW method imaged the subsurface to a depth of about [Formula: see text] and allowed detection of the overburden, gross bedding features, and fault zone. The fault zone was characterized by a lower shear-wave velocity [Formula: see text] than the competent bedrock, consistent with a large-scale fault, secondary fractures, and in-situ weathering. The MASW 2D [Formula: see text] section was further interpreted to identify dipping beds consistent with local geologic mapping. Mapping of shallow-fault zones and dipping sedimentary rock substantially extends the applications of the MASW method.

Delineation of contamination plume around oxidation sewage-ponds in Southwestern Nigeria

Integrated surface electrical resistivity and electromagnetic (EM) surveys were conducted in a hard-rock terrain of Southwestern Nigeria in the vicinity of active oxidation sewage treatment ponds. The aim was to detect soil contamination due to the spread of sewage effluent, locate possible leachate plumes and conductive lithologic layers, and access the risk of groundwater pollution in the vicinity of the sewage-ponds. Dipole–dipole resistivity profiling and very low frequency (VLF) data were acquired at 10-m intervals along five 200-m long east-west geophysical traverses. Resistivity sections obtained revealed four subsurface geologic layers comprised of lateritic clay, clayey sand/sand, weathered/fractured bedrock, and competent bedrock. A distinct low resistivity zone corresponding to the contamination plume (labeled B) was delineated from all the resistivity sections. This low zone extends into the weathered bedrock and possibly suggests contamination of this layer. The filtered real component of the processed VLF data detected three distinct anomaly zones that are representative of fractured zones filled with conductive fluids and/or lithologic boundaries that possibly serve as conduits for the movement of contaminated effluents. The results obtained from the two methods suggest possible contamination of the subsurface soil layers and groundwater in the vicinity of the sewage-ponds. The existence of this contaminated plume poses a serious threat to the ecosystem and health of the people living in the vicinity of the sewage-ponds.

Response of the Tarzana strong motion site during the 1994 Northridge earthquake

Exceptionally high ground motions (horizontal peak ground acceleration (PGA) of 1.82 g) were recorded at the Tarzana Station during the main shock of the 1994 Northridge earthquake (moment magnitude 6.7 at an epicentral distance of 6 km). At the time of the main shock, the instrument was located near the edge of a 21 m-high ridge with side slopes ranging from 3H:1V to 15H:1V. The ridge is underlain by shallow fill and soft rocks of Medelo Formation. The objectives of this study were to (1) identify the relative contributions of various factors such as local geology, topography, source mechanism, and travel path on the large ground motions recorded at Tarzana Station and (2) develop an analytical model that could adequately predict observed ground motions at the Tarzana site during the Northridge earthquake and at similar sites during future earthquakes. This study is an integral part of a series of inter-related studies referred to as the ROSRINE research (Resolution of Site Response Issues during Northridge Earthquake) project. The PGA at the surface of competent bedrock (1 km/s shear wave velocity found about 100 m below ground surface) is estimated by Silva [ROSRINE Study (2000)] at 0.46 gravity ( g). To identify the source of ground motion amplification, one-dimensional ( shake), two-dimensional (TELDYN and SASSI), and three-dimensional (SASSI) analyses were conducted using both recorded aftershock data and an estimated ground acceleration time histories at a 100 m depth. The results of the analyses indicate that (1) local geology and topography could only partially account for the observed ground motion amplification, and (2) the PGA and response spectra at a point near the edge of the ridge (the location of the instrument at the time of the main shock) is in good agreement with recorded values when the angle of incident of shear waves (SV waves) at 100 m depth is assumed at 30° from vertical. Considering the local geology and variation of shear wave velocity with depth, the 30° incident angle at 100 m depth corresponds to an 8° incident angle of shear waves at the ground surface. This observation is, in general, consistent with the incident angles of shear waves reported from study of the recorded aftershock data.

Hydrological response of hillslope soils above a debris-slide headscarp

The Jamieson Creek debris slide initiated in thin soils over a competent bedrock surface, on a planar section of hillslope, during a heavy rainstorm in November 1990. An array of automated piezometers and tensiometers was placed along a 22 m wide section of the headscarp in 1997 to monitor the temporal variation of pore-water pressures. Interpretation of the data addresses the hydrologic response to the storms in October and November 1997. The piezometers, which were designed for installation by driving, reveal very localized responses in what otherwise appears to be a uniform soil matrix. Peak positive pressures occur at the time of maximum rainfall intensity. The tensiometers indicate the hydrological response at the ground surface appears uncoupled from that at the bedrock interface. Implications of the extreme spatial variability in pore-water pressure are evaluated for conceptual models of hillslope hydrology. The assumption of parallel seepage flow is widely adopted in translational slope stability analyses, imposing a linear distribution of pore-water pressure with depth. None of the reported field data are consistent with such a linear distribution with depth or a uniform response across the slope.

Seismic site response in the greater Vancouver, British Columbia, area: spectral ratios from moderate earthquakes

Three-component, digital recordings of two recent moderate earthquakes provide valuable new insight into the response to seismic shaking in the greater Vancouver area, particularly on the Fraser River delta. The 1996 M = 5.1 Duvall, Washington, earthquake (180 km southeast of Vancouver) triggered strong-motion seismographs at seven sites and the 1997 M = 4.3 Georgia Strait earthquake (37 km west of Vancouver) triggered instruments at 13 sites in the greater Vancouver area. The latter data set is especially important because it contains the first three-component recordings made on bedrock in greater Vancouver. Both data sets represent weak ground motion, with peak horizontal accelerations of 0.5-1.5% gravity (g) for the Duvall earthquake, and 0.2-2.4% g for the Georgia Strait earthquake. Using the method of spectral ratios, we estimate the site response for each of the strong-motion instrument soil sites. On the Fraser River delta amplification is observed over a relatively narrow frequency range of 1.5-4 Hz (0.25-0.67 s period), with peak amplification of 4-10 (relative to competent bedrock) for the thick soil delta centre sites, and about 7-11 for the delta edge sites. Relative to firm soil, the peak amplification ranges from 2 to 5 for the thick soil delta centre sites, and 2 to 6 for the delta edge sites. At higher frequencies, little or no amplification, and in many cases slight attenuation, is observed.Key words: seismic site response, Fraser delta, earthquakes.

Hydrological response of hillslope soils above a debris-slide headscarp

The Jamieson Creek debris slide initiated in thin soils over a competent bedrock surface, on a planar section of hillslope, during a heavy rainstorm in November 1990. An array of automated piezometers and tensiometers was placed along a 22 m wide section of the headscarp in 1997 to monitor the temporal variation of pore-water pressures. Interpretation of the data addresses the hydrologic response to the storms in October and November 1997. The piezometers, which were designed for installation by driving, reveal very localized responses in what otherwise appears to be a uniform soil matrix. Peak positive pressures occur at the time of maximum rainfall intensity. The tensiometers indicate the hydrological response at the ground surface appears uncoupled from that at the bedrock interface. Implications of the extreme spatial variability in pore-water pressure are evaluated for conceptual models of hillslope hydrology. The assumption of parallel seepage flow is widely adopted in translational slope stability analyses, imposing a linear distribution of pore-water pressure with depth. None of the reported field data are consistent with such a linear distribution with depth or a uniform response across the slope.

What is a reference site?

Abstract Many methods for estimating site response compare ground motions at sites of interest to a nearby rock site that is considered a “reference” motion. The critical assumption in these methods is that the surface-rock-site record (reference) is equivalent to the input motion at the base of the soil layers. Data collected in this study show that surface-rock sites can have a site response of their own, which could lead to an underestimation of the seismic hazard when these sites are used as reference sites. Data were collected from local and regional earthquakes on digital recorders, both at the surface and in boreholes, at two rock sites and one basin site in the San Jacinto mountains, southern California. The two rock sites, Keenwild and Piñon Flat, are located on granitic bedrock of the southern California peninsular ranges batholith. The basin site, Garner Valley, is an ancestral lake bed with watersaturated sediments, on top of a section of decomposed granite, which overlies the competent bedrock. Ground motion is recorded simultaneously at the surface and in the bedrock at all three sites. When the surface-rock sites are used as the reference site, i.e., the surface-rock motion is used as the input to the basin, the computed amplification underestimates the actual amplification at the basin site for frequencies above 2 to 5 Hz. This underestimation, by a factor of 2 to 4 depending on frequency and site, results from the rock sites having a site response of their own above the 2-to 5-Hz frequencies. The near-surface weathering and cracking of the bedrock affects the recorded ground motions at frequencies of engineering interest, even at sites that appear to be located on competent crystalline rock. The bedrock borehole ground motion can be used as the reference motion, but the effect of the downgoing wave field and the resulting destructive interference must be considered. This destructive interference may produce pseudo-resonances in the spectral amplification estimates. If one is careful, the bedrock borehole ground motion can be considered a good reference site for seismic hazard analysis even at distances as large as 20 km from the soil site.

Emergency drainage system at the dam 3 – Outardes 4 development

A sudden increase (outburst) in the seepage discharge was recorded at the downstream toe of dam 3 of the Outardes 4 project. The normal rate of seepage of about 22.5 L/min increased suddenly to 18 000–22 500 L/min. The flow seems to have increased to its maximum rate in a period of not more than a few hours, followed subsequently by a gradual decrease to about 45 L/min in about 20 days.The rockfill dam, with a slightly inclined till core, has a maximum height of about 30 m and rests directly on a very competent bedrock. The quality of the grouted bedrock and the properties of the dam's construction material make it extremely unlikely that seepage through the rock or piping through the core and the transition zones could have caused this event.The presence of a roadfill or "berm" on the downstream rockfill zone, used as a temporary access road during construction and composed of a variety of unselected overburden and rock materials, could have acted as a barrier at the downstream toe of the dam. Thus, the recorded water outburst most likely has been caused by storage of the normal seepage water in the voids of the downstream rock shell and the sudden opening of a drainage channel through this roadfill barrier.Considering the risk of a such ponding in the downstream shell, an emergency drainage system was installed to avoid the repetition of a similar event in the future.The present paper deals with the design and construction of the drainage system whose major component was a tunnel excavated in the bedrock under the dam and illustrates the results of the tests carried out to monitor the ponding tendency in the downstream shell. Keywords: rockfill dam, drainage, tunnel (excavation), blasting (controlled).

A comprehensive study of long-period (20 to 200 seconds) earth noise at the high-gain worldwide seismograph stations

abstract The characteristics of long-period (20 to 200 sec) earth noise have been investigated at the 11 high-gain, long-period (HGLP) seismograph stations using as many as 3 years of analog seismograms and digital data from these stations. A major contribution of this study is the significant increase in the number of long-period earth-noise spectra that are available from areas of vastly differing geological and tectonic settings outside the contiguous United States. This information is important to the study of the long-period portion of the spectra of earthquakes and to the discrimination of earthquakes and underground explosions. A significant feature of earth-noise spectra based on data from all the HGLP stations is a minimum between about 25 and 45 sec. This minimum spectra level is most pronounced at sites with more than 200 m of overburden. The level of earth noise generally recorded by vertical-component seismographs at both the shallow and deep stations is independent of local meteorological conditions if the seismographs are located on competent bedrock and are subject to rigid environmental controls. The level of earth noise recorded by horizontal component seismographs at HGLP stations, however, is dependent on local meteorological conditions. This dependency decreases with increasing depth of overburden. The decay rate with depth of this horizontal earth noise is such that approximately 150 m of competent overbuiden is usually sufficient to attenuate about 90 per cent of the earth noise caused by local atmospheric conditions.