Overburden Pressure Research Articles

Correlations between p-wave velocity and SPT-N for saturated and unsaturated alluvial clays using dimensional analysis

The aim of this research is to establish practical and robust correlations between dilatational wave velocities (p-wave velocities, Vp) and the Standard Penetration Test blow count (SPT-N). Very low levels of correlation are recorded with the conventional single variable approaches, in consequence, a multivariable study is needed for higher precision. A dimensional analysis was conducted including other soil parameters that condition this physical phenomenon, such as the void ratio (e), the degree of saturation (Sr), the effective overburden pressure (σ′v), and the particle density (γs), for greater precision with Vp and SPT-N correlations. Over 100 seismic and SPT tests were performed around the coastal city of Juchitán de Zaragoza (Oaxaca), Mexico, to gather information for this research. As a result, two dimensionless indexes -DSIu (Undrained Dilatational Stiffness Index) and MDSIu (Modified Undrained Dilatational Stiffness Index)- yielded two alternative expressions between both indexes and Vp. In this way, a strong relationship between Vp and SPT-N was obtained with an adjusted coefficient of determination R2 value of 0.93.

Dolomite dissolution and porosity enhancement simulation with acetic acid: Insights into the influence of temperature and ionic effects

Dolostone reservoirs are crucial for global oil and gas storage, holding a significant portion of hydrocarbon reserves within carbonate formations. Understanding the chemical mechanism of dolomite dissolution with organic acids and associated porosity development in these reservoirs is vital for optimizing hydrocarbon recovery. In this work, two sets of simulation experiments are designed to understand the controls of temperature and ionic effects on porosity enhancement during the dissolution of dolomite with acetic acid. In specific, we discovered a three-step reaction process between dolomite and acetic acid, involving acetic acid dissolution in water, acetic acid dissociation, and dolomite dissolution. Burial environments with higher temperatures reduce the degree of acetic acid dissociation, leading to a corresponding decrease in dolomite dissolution in acetic acid solutions. The presence of salt ions in the solution influences the reaction process, with magnesium chloride (MgCl2) having the most significant enhancing effect on dolomite dissolution, followed by sodium chloride (NaCl), while calcium chloride (CaCl2) has a limited impact. Sodium sulfate (Na2SO4) enhances dolomite dissolution at lower temperatures; however, at higher temperatures (>100 °C), it may reduce dolomite dissolution due to the precipitation of CaSO4. Despite a relatively short open-flow time, episodic burial dissolution of dolomite by acetic acid fluids under overburden pressure leads to a "net increase" in the reservoir space and enhances its connectivity attributes. During burial diagenesis, geological forces episodically drive organic acid fluids to the dolomite reservoir, predominantly dissolving dolomite along pre-existing pore development zones and geological interfaces, exhibiting inheritance, episodicity, and localization characteristics. Overall, this study provides insights into the dissolution and diagenetic processes of dolomite in the presence under acetic acids, from the point of dissolution simulation experiments. Collectively, these findings enhance our understanding of pore-system evolution and fluid-rock interactions in deep carbonate reservoirs.

ON THE BEHAVIOUR OF THE CA-SO4-H2O SYSTEM

Thermotropia, barotropia and hygrotropia of the Ca-SO4-H2O system is reviewed and discussed and a three-dimensional representation of the phase diagram for system at atmospheric pressure is derived. It is concluded that the behaviour of the system is a coupled thermo-hydro-chemical phenomenon in which water activity is the key factor for existence of high concentration of Ca2+ and SO2-4 and confinement pressure plays a secondary role.

Formation of the Siberian Yamal gas emission crater via accumulation and explosive release of gas within permafrost

AbstractThe Arctic landscape has experienced many dramatic forms of change due to anthropogenic warming. Large crater‐like forms surrounded by fragmented blocks scattered outward to great distances with precursor mounds appeared in the continuous permafrost zone of the Yamal and Gydan peninsulas of Western Siberia. The morphologies of these craters, in addition to the abundant evidence of active and intense gas emissions across the region, indicate that they formed due the subsurface accumulation, pressurization, and explosive release of gasses trapped within the permafrost. Therefore, these craters were termed “gas emission craters” (GEC). However, some have suggested that in addition to gas accumulation, these features form in a manner similar to ice‐cored pingos and require pressurization due to the freezing of subsurface ice. This is despite the fact that not all GECs form in areas conducive to pingo‐like ice accumulation. Here, we test whether the pressurization and explosive release of methane gas alone can reproduce the observed morphology of the first‐discovered and most intensely studied GEC, Yamal crater. We use the available field and satellite data to constrain the initial dimension parameters of a model of the explosion process and consider several plausible configurations for the unknown interior cavity shape, the overlying permafrost cap thickness, and gas chamber volume. The explosion process is modeled in phases: gas migrates upward and accumulates in the subsurface until the gas pressure fractures the overlying cap, the expanding gas and cap blocks are initially accelerated outward together, and then the blocks are launched as projectiles. The sizes and locations of the ejected blocks found at Yamal crater are used to determine the initial gas pressures required to launch those projectiles to the observed locations. Finally, we estimate the amount of gas released in each of the multiple model runs testing different plausible internal cavity geometries. For the range of plausible block sizes and densities, and a subset of gas chamber volumes considered, we find that the gas pressure (0.6–2.6 MPa) required to launch the Yamal crater blocks to their observed distances was within the range of the sum of the ice/permafrost tensile strength and the lithostatic pressure (0.22–2.87 MPa). Thus, the observed blocks could have been launched by the energy required to fracture and displace the overlying permafrost cap. We show that the mechanism of formation of this GEC does not require pressure from the freezing of ice in the subsurface to crack and explode the overlying permafrost—gas pressure alone can produce these GECs. We expect that as our planet warms, the Siberian permafrost will continue to warm, weaken, and release gasses such as the greenhouse gas methane, and contribute to a permafrost carbon‐positive feedback cycle that would lead to the formation of even more explosive GECs.

3D shear-wave velocity structure of the Eastern Indian Shield from ambient noise tomography: Ultramafic lower crust underneath the Singhbhum Craton

Singhbhum Craton (SC) in the Eastern Indian Shield (EIS) is known to have preserved Archean crust which might provide insights into the Precambrian tectonics. In this study, ambient noise cross-correlation of about 1 year of continuous data recorded in 15 broadband seismic stations across EIS is performed. Rayleigh wave group velocities at 4–22 s periods obtained by frequency-time analysis of the cross-correlograms were subjected to tomographic inversion to produce group velocity maps. The spatial resolution of these maps was found to be ∼100 km. Inverted 1D shear-wave velocity (VS) models at regular grid points of these tomographic maps were combined to construct the first 3D VSmodel of the SC and adjoining regions within the EIS. The 3D model highlights a high VS(3.6–3.8 km/s) structure in the 4–20 km depth range and an ultra-high VS(∼4.0–4.2 km/s) structure at depths ≥ 20 km within the SC only. The VS of the upper layer and its regionalisation match with the well-studied granite batholith within the SC. The ultra-high velocitylayer in the lower crust, spatially spanning ∼100 km × 150 km, is distinctly imaged for the first time and its VS indicates probable ultramafic composition. The existence of ultramafic composition at ∼800 MPa lithostatic pressure requires ultra-high temperature metamorphism, possible through mantle upwelling/melting. The previous studies of upwarped crust-lithosphere beneath the SC and the lower-crustal ultramafic composition inferred in this study probably confirm the delamination of the lithosphere beneath the SC at some point during the Precambrian eon.

Investigating temperature change rate and pore confinement effect on thermal properties of phase change materials for de-icing and low-temperature applications in cementitious composites

Incorporation techniques of phase change materials (PCM) in cementitious composites have a significant influence on thermal properties. This study investigated the thermal behavior of low-temperature PCM when subjected to varying temperature change rates and pore confinement inside the porous network of lightweight aggregates (LWA) and encapsulation using melamine-formaldehyde-based polymer. Three categories of thermal energy storage (TES) specimens were prepared: (i) Bulk PCM (i.e., liquid PCM), (ii) micro-encapsulated PCM (MPCM), and (iii) four different LWAs infused with PCM (PCM-LWA). The thermal properties of small-scale individual TES specimens were analyzed using a low-temperature differential scanning calorimeter (LT-DSC) to evaluate the effect of ramp rates. Dynamic vapor sorption (DVS) analysis was utilized to characterize the pore structure of LWAs. LT-DSC results show that undercooling of the PCM significantly increases with the rise in ramp rate for all the specimens; temperature change rate affects the nucleation and crystallization growth process during the phase transition. Pore structure characterization of LWAs indicates that the majority of the pores (> 92 %) were larger than 17.3 nm (i.e., macropores). Confined liquid properties are subjected to modification due to interaction with the confining surfaces, as explained by Gibbs-Thomson theory. PCM incorporated in the LWA porous network experienced variable degree of supercooling during phase transition; magnitude of confinement pressure is dependent on the pore diameter, structure, and tortuosity. Experimental evidence suggested that PCM-LWA will exhibit gradual expulsion of enthalpy of fusion over a larger temperature range (i.e., ∼−5 °C to 4.28 °C) in comparison to MPCM (i.e., ∼4.28 °C).

Deformation and structure strength anisotropy of undisturbed loess in tunnels on high-speed railway, China

During tunnel excavation in loess, the surrounding loess at vault and waist exhibits deformation and structural strength anisotropy. Structural strength plays an important role in the mechanical and deformation characteristics of loess, which are clearly discussed in this paper. Undisturbed loess samples were taken from the Wangjiagou and Gongjiawan Tunnels in northwestern China. A series of tests including triaxial shear, unconfined compression, tensile strength, loading–unloading cycles, and confined compression were conducted on undisturbed loess in the vertical and horizontal sampling directions. Considering the influence of loess structure on its anisotropy, a scanning electron microscopy (SEM) test was also performed to analyze the main reasons for the anisotropy of undisturbed loess. The results indicated that the ratio of strain in the horizontal direction to that in the vertical direction decreased with increasing confinement pressure, but first increased and then decreased with increasing normalized deviated stress. The deformation parameters of undisturbed loess were anisotropic. The ratio of vertical to horizontal elastic modulus, the lateral compression modulus, the pre-consolidation pressure, and the compression coefficient of undisturbed loess were respectively 1.20, 1.15, 1.35, and 0.88. A geometric model of the structural strength anisotropy of undisturbed loess was also constructed, and related structural strength parameters were also proposed.

Finite Element Modeling Aspects of Buried Large Diameter Steel Pipe–Butterfly Valve Interaction

Buried flexible pipes are allowed to deflect up to 2–5% of the pipe diameter, which can become problematic for the connected direct-bury, large-diameter butterfly valves. The complex behavior of the pipe–valve–soil system makes it difficult to predict the deflection of the pipe/valve system. In the absence of field/experimental studies, the application of finite element analysis (FEA) seems necessary to predict deflection and stresses and to avoid potential downtime associated with disruption of service. This paper described the FEA of a large-diameter pipe–valve system, with different backfills under gravity, overburden, and internal pressure loads. The effects of modeling different components of the system (e.g., flanges, bearing housing, gate disc, etc.) were described and investigated. The goal of this study was to provide insight into the design and installation of direct-bury pipe–valve systems and evaluate current installation methods in the absence of guidelines. In addition, the level of modeling details required for FEA to yield accurate results was discussed.

Basal conditions of Denman Glacier from glacier hydrology and ice dynamics modeling

Abstract. Basal sliding in Antarctic glaciers is often modeled using a friction law that relates basal shear stresses to the effective pressure. As few ice sheet models are dynamically coupled to subglacial hydrology models, variability in subglacial hydrology associated with the effective pressure is often implicitly captured in the basal friction coefficient – an unknown parameter in the basal friction law. We investigate the impact of using effective pressures calculated from the Glacier Drainage System (GlaDS) model on basal friction coefficients calculated using inverse methods in the Ice-sheet and Sea-level System Model (ISSM) at Denman Glacier, East Antarctica, for the Schoof and Budd friction laws. For the Schoof friction law, a positive correlation emerges between the GlaDS effective pressure and basal friction coefficient in regions of fast ice flow. Using GlaDS effective pressures generally leads to smoother basal friction coefficients and basal shear stresses, and larger differences between the simulated and observed ice surface velocities compared with using an effective pressure equal to the ice overburden pressure plus the gravitational potential energy of the water. Compared with the Budd friction law, the Schoof friction law offers improved capabilities in capturing the spatial variations associated with known physics of the subglacial hydrology. Our results indicate that ice sheet model representation of basal sliding is more realistic when using direct outputs from a subglacial hydrology model, demonstrating the importance of coupling between ice sheet and subglacial hydrological systems. However, using our outputs we have also developed an empirical parameterization of effective pressure that improves the application of the Schoof friction law without requiring explicit hydrological modeling.

Fracture toughness tests of shale outcrops: Effects of confining pressure

Brittle rock fracture is a core concept in oil and gas and other rock mechanics projects. However, the understanding of fracture behavior under mechanical well-bottom conditions remains insufficient. This article aims to analyze experimental results for the critical stress intensity factor (KIC) of outcrops from Vaca Muerta carbonate-rich shale rocks, tested under a range of crack depths and confining pressures (0–70 MPa). Fracture toughness (KIC) is determined in multi-notched 1.5″ plugs using a novel experimental set up, in which the crack-driving-force KI and the confinement pressure are both applied by hydraulic systems. Finite fracture-mechanics-based models are used to calculate KI. Our experimental results show that tests carried out at well-bottom pressures lead to apparent rock toughness doubling those for tests at atmospheric pressure.Stress analysis demonstrates that the size of the tensile stressed zone ahead of the crack tip tends to decrease as confining pressure increases. Additionally, compressive deviatoric stresses are developed ahead of the tensile zone, with their magnitude being dependent on the level of confinement. Moreover, triaxial stress states induced by confinement could lead to microcracking ahead of the crack tip. The mechanisms of crack closure and deviatoric stress-induced microcrack initiation are combined to assess a plausible mechanism for rock toughness enhancement under confined conditions. It is concluded that increasing triaxial pressure confinement allows to accurately model the mechanical response of shale rocks under reservoir conditions.

Geoacoustic model based on physical property characterization of the southeastern shelf sediments (Korea Strait) of Korea

Deep-core samples collected from the southeastern shelf (Korea Strait) of Korea were used to characterize physical properties of the area with respect to burial depth. Laboratory analyses of the acoustic and physical properties of the samples were conducted and shear-wave velocity, velocity ratio, and Poisson’s ratio were calculated. Geoacoustic and physical property data at the two sites were in correlation with the lithological units alternating between sandy mud and muddy sand at sedimentary depth. Geoacoustic units (GAUs) identified from sites 19ESDP-104 and 21ESDP-203 were divided into 6 and 15 units with sedimentary depth, respectively, and geoacoustic models were developed for the sedimentary layers around these sites. This suggests that the physical properties are not only controlled by lithology and compaction caused by dewatering with burial depth but also by different sedimentary environments after deposition. Based on the velocity ratio and velocity anisotropy (particularly at site 21ESDP-203), clay particles within the sediment layers had a greater effect on compaction owing to the overburden pressure with burial depth.

Experimental of the confinement effect on the compression strength of concrete

In this paper, the impact of confinement pressure on the compressive strength of concrete was carried out various levels of confinement on two concrete types, B20 and B25. As a result, the influence of confinement pressure on the compressive strength of concrete is quantified for the specific research problem at hand. The research findings provide recommendations for the application of the confinement effect concept in the design of reinforced concrete structures in Vietnam.

Nonlinear data-driven zero-thickness joint element for concrete dam shear keys

Shear keys have been designed into the contraction joints of many concrete gravity dams to provide interlocking and shear transfer between adjacent monoliths. Their shear transferring capacity is usually estimated as the friction and cohesion that can be mobilized across a 2D plane located at the base of the keys. Once the shear stress in the contraction joint exceeds its estimated shear capacity, the shear keys are typically considered failed, and the joint may subsequently only transfer shear through concrete-concrete friction. This behaviour is often simulated in practice by placing tiebreak contact formulations along contraction joints. However, the nonlinear response and potential residual strength of the keys are usually ignored. In this paper, a novel nonlinear zero-thickness joint element named Macro Shear Key Joint Element (MSKJE) is proposed to simulate the response of a detailed shear key while capturing its nonlinear behaviour. A detailed geometrical numerical model of shear keys using a concrete Continuous Surface Cap Model (CSCM) is first validated against experimental data. Then, a parametric analysis of the number of keys in the joint, confinement pressure, initial opening and lateral boundary condition is performed using the validated detailed shear key numerical models. The stress-displacement responses obtained from the parametric analysis are used to calibrate the proposed MSKJE. At the local shear key scale, the MSKJE models present a very similar stress-displacement response compared to the detailed geometrical key models in terms of stiffness, maximum shear capacity, residual strength and nonlinear softening. At the global dam scale, the MSKJE presents a somewhat similar behaviour to the conventional tiebreak formulations. However, the advantages of the MSKJE over the tiebreak contact include its capability to simulate the nonlinear softening of shear keys, and residual sliding displacement, and, once calibrated, it is not necessary to estimate the shear keys capacity a priori.

Mechanisms and Operational Strategies of Multi-Lateral Steam-Assisted Gravity Drainage (SAGD) for Heterogeneous Reservoirs

As the SAGD steam chamber and production performance in heavy oil reservoirs under fluvial sedimentation environment are heavily impacted by reservoir heterogeneity, an innovative strategy was proposed in this study coupling rock dilation and multi-lateral wells in SAGD projects to break the mud barriers and achieve uniform steam chamber growth. True tri-axial experiments and numerical simulation were designed to validate the feasibility of this strategy, based on which the branches of the SAGD well pairs were designed and the operational parameters were optimized for different geologic heterogeneity conditions. The tri-axial experiment results indicate that the rock formations in the heavy oil reservoirs of the F oilfield exhibit significant shear dilation effects under low confinement pressure conditions, with a volumetric dilation capacity of up to 7%. The branches should be deployed in an interleaved manner, with a horizontal displacement of 20 m and a vertical displacement of 6 m. The optimal results are achieved when the branches intersect the interbeds, allowing for enhanced steam chamber conformance and enlarged volume. Dilation zones of 3–8 m can be created above the steam-injection horizontal wells and around the branches in the reservoir during the dilation of SAGD steam chambers. The maximum volume of dilation fluid used for hydraulic dilation is suggested to be less than 2000 m3. This strategy has been validated as being successful in the pilot SAGD well pair in the F oilfield, China, with the SAGD preheating time reduced by 50% and an incremental oil rate of 4.5 tones/day, indicating encouraging potentials in similar heavy oil reservoirs.

Study Of Various Types Of Pvd And Ph.D. Joints On Vertical Drainage Performance

An important factor for increasing the effectiveness of the PVD (Prefabricated Vertical Drains) is the value of sufficient discharge capacity so that the PVD can work optimally. The use of PVD is usually accompanied by the use of PHD (Prefabricated Horizontal Drains) as horizontal drainage and combined with preloading. In the field, the connection is made by winding the PVD to the PHD and then tying it with cable ties. The connection system can cause deformation at the top of the PVD thereby reducing the effectiveness of the PVD discharge capacity. This study aims to find the optimal PVD-PHD connection system for the discharge capacity which is affected by confinement pressure, overburden pressure, and hydraulic gradient with a PVD-PHD connection system discharge capacity tester. The test specimens used were PVD (5mm thick; 100mm wide) and Ph.D. (20 mm thick; wide: 100 mm, 200mm, and 300mm). 4 types of connection systems have been tried, namely connections A1 and A2 where the PHD is connected in a horizontal position, and connections B1 and B2 where the PHD is connected in a vertical position. Of the four connection systems B1, the connection system has the largest discharge capacity value and a significant increase in PHD width from 100 mm to 300 mm with an increase of 7.694% at 50 kPa overburden pressure and 1.0 hydraulic gradient the highest compared to other connection types

Numerical study on compressive mechanical characteristics of filled jointed rock under confining pressure based on PFC

Confining pressure is an important factor affecting the strength and deformation characteristics of rock mass, it is of great significance to study the mechanical and deformation characteristics of jointed rock mass under confining pressure for the construction of deep underground engineering and the prevention of geological disasters. In order to study the mechanical and deformation characteristics of filled jointed rock under confining pressure, based on the laboratory experiment results of static uniaxial compression of filled jointed rock samples, the Particle Flow Code is used to conduct the numerical simulation. The strength characteristics, failure characteristics and micro-cracks development characteristics of filled jointed rock under different confining pressure levels, different joint inclination angles and different sample sizes are analyzed. The results show that the peak stress and peak strain increase with the increase of confining pressure level, and there is a strong linear relationship between peak stress and confining pressure level. The peak stress and initiation stress decrease first and then increase with the increase of joint inclination angle. With the increase of confining pressure level, the change law of initiation stress of filled jointed rock under different joint inclination angles is different. The confining pressure will prolong the development process of micro-cracks in filled jointed rock, which will make the distribution of micro-cracks more dispersed and the total number of micro-cracks increase. The failure mode changes from splitting failure to shear failure with the increase of confining pressure level. The change of joint inclination angle will seriously affect the failure mode and micro-cracks development characteristics of filled jointed rock.

Feasibility for High-Temperature Graphitization of Deformed Meager Coal.

The flattening of nanoscale pores is a crucial step in the process of anthracite graphitization, which primarily occurs due to lithostatic pressure or tectonic stress. Anthracite, a highly metamorphosed form of carbonaceous material, is a primary component of artificial graphite, but its availability is limited in China. Therefore, it is necessary to investigate the graphitization of low metamorphic coal. Note that the pores of deformed coal are substantially compressed, and the evolution of macromolecular structure is accelerated owing to strong tectonic extrusion and shearing action. Herein, the feasibility analysis of coal-based graphite at temperatures of 2200 and 2600 °C was investigated using mylonitized meager coal (YA-M; Ro,max = 2.34%), which exhibits the most intense deformation intensity, and it was compared with undeformed coal (YA-U; Ro,max = 1.87%) from the same coal seam. Results indicate that YA-U primarily consists of cylindrical pores with an average pore diameter of 10 nm, while YA-M exhibits extensive fine bottleneck and parallel plate pores with an average pore size of ∼4 nm. Additionally, interlayer spacing decreases from 0.3592 nm in YA-U to 0.3575 nm in YA-M, while crystallite height increases from 1.9633 nm in YA-U to 2.1424 nm in YA-M. The undeformed coal YA-U exhibits short and disordered aromatic fringes with a localized turbostratic structure that hinders graphitization. In contrast, YA-M exhibits more and longer localized fringe pairs arranged in parallel with improved structural ordering, indicating a favorable environment for graphitization. Ductile deformation narrows the pore diameter distribution, causing deformation pores on the mylonitic granules' surface to close, shrink directionally, collapse, or connect, resulting in obvious flattening characteristics of nanoscale. Based on these results, the graphitization degree of the graphitized products YA-GM-2200 and YA-GM-2600 was 80.23% and 88.37%, respectively, with interlayer spacings of 0.3371 and 0.3364 nm, indicating that these products have entered the semigraphite (medium graphitization) and graphite range, respectively. In contrast, the graphitization degree of YA-GU-2200 and YA-GU-2600 was only 51.16% and 56.98%, respectively, indicating that they have only progressed from anthracite to meta-anthracite (the preliminary graphitization stage). These findings confirm the view that the shear stress generated by the geological age flattens the pores of mylonitized meager coal and optimizes the aromatic lattice arrangement and orientation, facilitating graphitization and lowering the graphitization temperature (from 2800 to 2200 °C). Deformed coal is soft and can be easily crushed into powdered form using the human hand, making it less suitable for transportation. Additionally, owing to the unfavorable separation of minerals and lower hydrocarbon-generating potential, deformed coal application potential and price are consistently lower than those of undeformed coal. However, deformed coal is widely distributed in various coal measures in China, making these research findings valuable in promoting the high value-added utilization of low-rank deformed coal resources and expanding the carbon source of coal-based graphite materials.

Quantifying the effect of rock armour scour protection on eigenfrequencies of a monopile supported OHVS

It is common practice to install scour protection systems during the construction of offshore wind farms. This is because the guidelines allow for a more optimal structural design, as the potential effects of scouring may be ignored. However, multiple sources, as well as measurements, suggest that there is an additional gain in stiffness which is not being taken into account by current guidelines. Scour protection has the ability to provide extra foundational stiffness to the monopile, which results in higher natural eigenfrequencies and mode shapes. In this paper the focus will be on an offshore high voltage station (OHVS) located in the North Sea and the impact of a rubble riprap scour protection on the foundational stiffness of the structure. Furthermore, a proposal is made on how to take these effects into account when calculating for design. This site was equipped with a mobile measurement system containing a multi-axial accelerometer, which was active in a period both before and after installation of the scour protection rock armour layer. In case of similar environmental circumstances, the eigenfrequencies of the OHVS were noticeably higher after the scour protection was installed, thus substantiating the claim of increased foundational stiffness. To perform numerical studies on the impact of the scour protection system on structural dynamics, an OHVS support structure finite element model was built. As no formally agreed upon formulation to include scour protection exists, two possible methods are proposed. One is adding a global accretion layer to model the scour protection as an additional soil layer. The other calculates an overburden pressure due to the scour protection weight, which translates to the stiffening of the upper soil layers of the foundation. Both methods have their own strengths and weaknesses, but more research on different locations and offshore structures is needed to formalize the definitive description for modelling scour protection.

Dynamic behavior of geopolymer stabilized kaolin clay under long-term cyclic loading

Geopolymer has emerged as an alternate cementitious material with a low carbon footprint, as they improve the engineering properties of soft soil without causing a detrimental effect on the environment. This study focuses on developing a combination of geopolymer materials that can fully replace the utilization of traditional binders to stabilize soft soil foundations in geotechnical engineering. Soft kaolin clay was stabilized using ground-granulated blast furnace slag (S) and dolomite (D) as a precursor (P) with combined content of 20% and NaOH: Na2SiO3 ratio of 25:75 as a liquid alkali activator (L) in the geopolymerization process. A series of Unconfined Compressive Strength (UCS) tests were conducted to analyze the influence of various parameters, such as the ratio of S and D, water content of kaolin clay, curing period, and L/P ratio, to select the optimum mix proportions based on strength improvement. This paper also discusses the behavior of geopolymer-stabilized kaolin clay under long-term cyclic loading in stress-controlled conditions. The effect of the S:D ratio, cyclic stress ratio (CSR), curing time, frequency, and confining pressure (CP) on the dynamic shear modulus (G), damping ratio (λ), and degradation index (G/Gmax) were analyzed. The results indicate that the S:D ratio of 16:4 with an L/P ratio of 1, NaOH: Na2SiO3 ratio of 25:75, and water content of liquid limit (LL) cured for 28 days gives the highest UCS value of 1.47 MPa. The initial dynamic shear modulus (Gmax) increases by 60% as the S:D ratio changes from 20:0 to 16:4. The cyclic performance of geopolymer-stabilized kaolin clay in terms of accumulative strain as a function of the number of loading cycles was also enhanced. The findings of the study provide guidance to a better understanding of the long-term dynamic behavior and application of geopolymer-stabilized kaolin clay as a greener approach to ground improvement.

Frictional properties of the décollement in the shallow Nankai Trough: Constraints from friction experiments simulating in-situ conditions and implications for the seismogenic zone

Along subduction zone megathrust faults, the transition from smectite to illite with depth occurs at temperatures of ∼150°C and has long been hypothesized to be one of the dominant controls on the updip limit of the seismogenic zone. Despite this, there have been few experimental studies performed at in-situ conditions to test the frictional stability behavior of subduction zone faults across the smectite-illite transition. In this study, we simulated in-situ temperature-pressure-mineral conditions along the shallow plate boundary in the Nankai Trough, and conducted friction experiments to further constrain the role of smectite-illite transition in seismogenesis in subduction zones. We found that the friction coefficient of the simulated sediment increased with the progress of illitization and followed the Reuss average of friction coefficients of the mineral phases. The obtained friction coefficients and the Coulomb wedge model inferred that the pore pressure conditions around the plate boundary may be over-pressured but not as high as lithostatic pressure. Higher friction coefficients of sediment due to diagenetic processes would be required to sustain pore pressure close to lithostatic along the plate boundary. The velocity dependence of friction coefficient (a−b) was always positive (i.e., velocity strengthening, not seismogenic) except for slip rates less than ∼10 μm/s at 171°C where the smectite-illite transition has almost completed. Therefore, shallow slow earthquakes that occur beneath the outer prism in the Nankai Trough cannot be explained only by the frictional instability of unconsolidated sediment. Since the smectite-illite transition mostly completes at ∼30 km landward from the trough axis, the transition from velocity strengthening to velocity weakening observed at 171°C is caused by temperature-dependent frictional behavior of illite-rich material. Hence, the updip limit of the seismogenic zone in subduction zones is mainly controlled by the frictional properties of illite-rich material, not by the smectite-illite transition.