Subsurface Liquefaction Research Articles

Seismic Design of Offshore Wind Turbines: Good, Bad and Unknowns

Large scale offshore wind farms are relatively new infrastructures and are being deployed in regions prone to earthquakes. Offshore wind farms comprise of both offshore wind turbines (OWTs) and balance of plants (BOP) facilities, such as inter-array and export cables, grid connection etc. An OWT structure can be either grounded systems (rigidly anchored to the seabed) or floating systems (with tension legs or catenary cables). OWTs are dynamically-sensitive structures made of a long slender tower with a top-heavy mass, known as Nacelle, to which a heavy rotating mass (hub and blades) is attached. These structures, apart from the variable environmental wind and wave loads, may also be subjected to earthquake related hazards in seismic zones. The earthquake hazards that can affect offshore wind farm are fault displacement, seismic shaking, subsurface liquefaction, submarine landslides, tsunami effects and a combination thereof. Procedures for seismic designing OWTs are not explicitly mentioned in current codes of practice. The aim of the paper is to discuss the seismic related challenges in the analysis and design of offshore wind farms and wind turbine structures. Different types of grounded and floating systems are considered to evaluate the seismic related effects. However, emphasis is provided on Tension Leg Platform (TLP) type floating wind turbine. Future research needs are also identified.

Investigation of coseismic liquefaction‐induced ground deformation associated with the 2019 Mw 5.8 Mirpur, Pakistan, earthquake using near‐surface electrical resistivity tomography and geological data

ABSTRACTAn electrical resistivity tomography survey was conducted to assess the subsurface conditions associated with the coseismic liquefaction phenomenon in the epicentral region following the Mw 5.8 Mirpur earthquake (Pakistan) on 24 September 2019. The Mirpur earthquake produced extensive coseismic liquefaction‐induced surface deformations, including: sand blows, ground failure and lateral spreading along the Upper Jhelum Canal and in the nearby villages. Electrical resistivity data were acquired along three profiles and calibrated with available borehole data. The inverted electrical resistivity tomography profiles reveal three regional geoelectric layers, which consist of an upper 2–‐5‐m‐thick discontinuous zones of medium resistivity values ranging from 25 Ωm to 60 Ωm, underlain by a 7–8‐m‐thick zone of low resistivity (<10 Ωm) and a basal layer of high resistivity (> 100 Ωm). Based on geological and geophysical data, we infer that. (1) disrupted geoelectric layers in the shallow subsurface and spatially extended low electrical resistivity (<8 Ωm) layers document the elevated groundwater table due to sudden increase in pore‐water pressure triggered by the Mirpur earthquake. These lenses of high conductivity may represent potential hazards in the case of future earthquakes in the study area. (2) Fracture azimuths vary between 120° ± 15° and 335°–45° (subparallel and orthogonal to the strike of the Himalayan Frontal Thrust. (3) Common coseismic deformational features (e.g., sand blow and ground fracture) are located within the zone of maximum‐recorded ground shaking (intensity of VI) and underlain by Quaternary alluvial sediments. (4) Mega fractures (1.60 m wide and up to 187 m long) oriented parallel to the canal resulted from lateral spreading. We conclude that high resistivity structures extending from depth to the shallow subsurface resulted from either intrusion of air or eruption of sands from layer three. We suggest that high‐resolution geoelectrical imaging is a valuable complementary tool for evaluating the extent of subsurface liquefaction features and in understanding coseismic deformation during earthquakes, which can help with seismic hazard analysis and mitigation in seismically active regions.

When the earth blisters: Exploring recurrent liquefaction features in the coastal system of Christchurch, New Zealand

AbstractThe 14 February, or Valentine's Day, 2016 earthquake located offshore of Christchurch produced a new generation of liquefaction in the coastal environment of Christchurch, an environment of recurrent liquefaction during the 2010–2011 Canterbury Earthquake Sequence. The Valentine's Day earthquake occurred while trenches were open at a site in the coastal environment for studying the 2010–2011 liquefaction features. Observations in the 2016 trenches provided insights into the formation of a suite of surface and subsurface liquefaction features characteristic of the dune deposits, related to the persistent shallow water table, well‐sorted sands within the liquefiable layer and the soil above. Given these conducive conditions, there is a high potential that all earthquakes capable of inducing liquefaction may have been recorded during formation of the dunes near Christchurch. However, overprinting by subsequent generations of liquefaction features may be common and present challenges in unravelling the event history.

Formation of chaotic rock units during primary accretion processes: Examples from the Miura–Boso accretionary complex, central Japan

AbstractChaotic rock units exposed in the upper part of the accretionary complex preserve detailed tectonic information related to the periods before, during, and immediately after accretion. Based on the detailed survey in the upper Miocene Miura–Boso accretionary complex, central Japan, three types of chaotic rock units were identified on the basis of the grain sizes and characteristics of blocks and surrounding matrices. The chaotic rock units composed of silt matrices and sandy to pebbly blocks (Type 3) formed by gravity‐driven slumping upon the seafloor. The slumping occurred contemporaneously with deposition of the Misaki and Nishizaki Formations within the Izu–Bonin forearc. Vertical variations in the direction of slump vergence represent successive changes from an initially flat seabed to tilting to the northwest and finally to the southeast. Slumping with a northwest vergence indicates landward tilting of the seafloor immediately prior to accretion, whereas vergence to the southeast reflects oceanward tectonic tilting that occurred once the sediments had crossed the deformation front. Other chaotic rock units that have matrices abundant in sand and pebbles (Types 1, 2) formed as a result of subsurface liquefaction and injection associated with large earthquakes that occurred during and after accretion of the sediments. These chaotic rock units are useful in examining surface/subsurface changes such as tectonic tilting of the seafloor and earthquake events during the initial accretion process.

Origin of postseismic streamflow changes inferred from baseflow recession and magnitude‐distance relations

The effects of earthquakes on baseflow, the component of streamflow provided by groundwater, are analyzed for streams that exhibit large coseismic increases in streamflow and discharge large volumes of excess water. Although discharge Q may increase rapidly, the rate of baseflow recession dlogQ/dt is unchanged suggesting that the hydraulic conductivity of groundwater system providing baseflow does not change. For (at least) the studied streams, the coseismic increase of discharge therefore requires increased hydraulic head gradients resulting from the rapid release of water from some source of storage. The relationship between the response of streams, earthquake magnitude, and the distance of the drainage basin from the epicenter, is consistent with subsurface liquefaction being the mechanism that increases head gradients.

Liquefaction at the Meloland Overcrossing During the Imperial Valley Earthquake of 1979

The Meloland Overcrossing is located 0.3 mi from the Imperial fault and was shaken by ground surface motions of up to 0.3 g during the Imperial Valley earthquake of 1979 (magnitude M = 6.4). While there was no significant damage to this instrumented highway bridge or any reported surface evidence of subsurface liquefaction, it is assessed that liquefaction occurred in up to three layers at the site at various times during the first eight seconds of shaking. A modified level ground liquefaction analysis after Seed and others (1983) was used to assess such time dependent development of liquefaction. Results from such evaluations were used to assess and explain anomalies in the lateral and rotational responses of the central pier pile foundation demonstrating that the central pier of the Meloland Overcrossing came periously close to complete collapse during this event. This is contrary to the bridge designer9s belief that this highway bridge performed quite well during the earthquake as characterized solely by the lack of observable damage to the bridge. It is further suggested that the time dependent evaluation of liquefaction demonstrated herein can be used on other projects where one likewise wishes to assess the amount of deformation (e.g., the rotationally induced settlement in the case of Meloland) that is likely to ensue.

Pore-Water Pressure Changes during Soil Liquefaction

An analytical procedure is presented for evaluating the general characteristics of pore-water pressure buildup and subsequent dissipation in sand deposits both during and following a period of earthquake shaking. It is shown that in layers of fine sand, excess hydrostatic pressures may persist for an hour or more after an earthquake. However evidence of subsurface liquefaction may not appear at the ground surface until several minutes after the shaking has stopped and the critical condition at the ground surface may not develop until 10 min-30 min after the earthquake. However, for coarse sands and gravels with an impedance of drainage due to the presence of sand seams or layers, pore pressures generated by earthquake shaking may dissipate so rapidly that no detrimental build-up of pore pressure or a condition approaching liquefaction can develop. Improving the drainage capability of a sand deposit may thus provide an effective means of stabilizing a potentially unstable deposit. Analyses of the type described also provide the means for assessing whether subsurface liquefaction will have any serious effects on structures supported near the ground surface.

Effect of Subsurface Liquefaction on the Strength of Surface Soil

After a subsurface layer of soil is liquefied completely, the excess pore water pressure therein is dissipated through the overlying layer of soil. The Terzaghi consolidation theory is applied to the problem to determine the excess pore water pressure, and numerical solutions are obtained using the finite element method for a variety of initial and boundary conditions. If the compressibility of the surface soil is one order of magnitude smaller than that of the initially liquefied soil, the maximum pore pressure in the surface soil is not significantly affected by the initial excess pore pressure in the surface soil, the relative thickness of the surface soil with respect to the initially liquefied soil, the drainage condition at the lower boundary of the initially liquefied soil, or by the depth of the water table. The smaller the permeability and compressibility of the surface soil, the smaller becomes the shear strength of the soil due to the upward seepage. The surface soil can even be liquefied if its permeability and compressibility are sufficiently low as compared to those of the initially liquefied soil.