Deep Mixing Research Articles

Curiouser and curiouser: the peculiar chemical composition of the Li- and Na-rich star in ω Centauri

We present a multi-instrument spectroscopic analysis of the unique Li- and Na-rich giant star #25664 in ω Centauri, using spectra acquired with FLAMES-GIRAFFE, X-shooter, UVES, and HARPS. Li and Na abundances have been derived from the UVES spectrum using transitions weakly sensitive to non-local thermodynamic equilibrium and assumed isotopic ratios. This new analysis confirms the surprising Li and Na abundances of this star (A(Li)NLTE = +2.71 ± 0.07 dex, [Na/Fe]NLTE = +1.00 ± 0.05 dex). Additionally, we provide new pieces of evidence for its chemical characterisation. The 12C∕13C isotopic ratio (15 ± 2) shows that this star has not yet undergone the extra-mixing episode usually associated with the red giant branch bump. Therefore, we can rule out the scenario of efficient deep extra mixing during the red giant branch phase envisaged to explain the high Li and Na abundances. Also, the star exhibits high abundances of both C and N ([C/Fe] = +0.45 ± 0.16 dex and [N/Fe] = +0.99 ± 0.20 dex), which is not compatible with the typical C-N anti-correlation observed in globular cluster stars. We found evidence of a radial velocity variability in #25664, suggesting that the star could be part of a binary system, likely having accreted material from a more massive companion when the latter was evolving in the asymptotic giant branch (AGB) phase. Viable candidates for the donor star are AGB stars with 3–4 M⊙ and super-AGB stars (~7–8 M⊙), both of which are able to produce Li- and Na-rich material. Alternatively, the star could have formed from the pure ejecta of a super-AGB star before the dilution with primordial gas occurred.

Seismic Performance of a Deep Soil Mixing Grid: A Magnitude 7.1 Load Test at the Port of Alaska

The Port of Alaska in Anchorage (Port) has embarked on a multiphase modernization program that includes the development of a new petroleum and cement terminal (PCT) berth. The berth consists of a p...

Occurrence, distribution, and prediction of iron and manganese in groundwater of opencast mines: an example from Inner Mongolia, China.

Based on the geological and hydrologic data, the generation, enrichment, and transportation of iron (Fe) and manganese (Mn) ions in Yimin open-pit mining area were investigated using water quality analysis of groundwater samples, Statistical Product and Service Solutions (SPSS) principal component analysis, and geostatistics analysis by Geographic Information System (GIS). The water quality test results showed that the groundwater of this area was mainly the Na-Ca-HCO3 type, and the content of Fe and Mn ions significantly exceeded the national standard. The groundwater in the mining area was strongly affected by human activity, deep groundwater mixing effect, and water-rock interaction associated with mining activities. It was also found that Fe and Mn in the research area had a large variation in spatial distribution. On the whole, Fe and Mn ion concentrations found in the groundwater to the north of the research area were higher than those in the south area, with the mining area as the dividing line. The removing of non-mineral layer and the draining of gushing water in the mining area resulted in the formation of a water funnel and the strengthening of an effect between each aquifer. The change of hydrogeological and redox conditions in the mining area promoted the dissolution of Fe and Mn in soil and stratum and migration in groundwater.

Investigation on load-carrying capacity of geogrid-encased deep cement mixing piles

This article discusses the feasibility of a geogrid-encased deep cement mixing (EDCM) pile for enhancing the load-carrying capacity (Qult) of a conventional deep cement mixing (DCM) pile, based on the results of experimental and numerical investigations. Firstly, physical model tests were conducted to observe the load–settlement response and load-transfer mechanism. Subsequently, a series of three-dimensional (3D) finite-element analyses was performed to investigate the effect of the stiffness and length of the geogrid encasement. The results indicated that the geogrid encasement effectively improves the Qult of the DCM piles by a factor of two. With the additional confinement provided by the geogrid encasement, the geogrid can also provide a greater contribution to the loading transfer. The increase in Qult became more significant with a lower strength of the DCM. For a particular amount of geogrid, the EDCM pile with a thicker geogrid provided a higher Qult than that with a longer geogrid. The optimum length of the geogrid-EDCM pile was found to be approximately twice that of the pile diameter. Finally, a chart for estimating the efficient effect of geogrid encasements on the Qult of EDCM piles was proposed.

Ice‐Covered Lakes of Tibetan Plateau as Solar Heat Collectors

AbstractThe Qinghai‐Tibet Plateau possesses the largest alpine lake system, which plays a crucial role in the land‐atmosphere interaction. We report first observations on the thermal and radiation regime under ice of the largest freshwater lake of the Plateau. The results reveal that freshwater lakes on the Tibetan Plateau fully mix under ice. Due to strong solar heating, water temperatures increase above the maximum density value 1–2 months before the ice break, forming stable thermal stratification with subsurface temperatures >6°C. The resulting heat flow from water to ice makes a crucial contribution to ice cover melt. After the ice breakup, the accumulated heat is released into the atmosphere during 1–2 days, increasing lake‐atmosphere heat fluxes up to 500 W m−2. The direct biogeochemical consequences of the deep convective mixing are aeration of the deep lake waters and upward supply of nutrients to the upper photic layer.

Influence of sand incorporation on unconfined compression strength of cement-based stabilized soft clay

Cement-based stabilization by deep mixing or grouting is a popular method used to enhance soft clay ground because clay gradation has a significant effect on its unconfined compression strength. However, the functional relationship between the gradation and the strength is currently unclear, which limits the assessment and cognition for the strength formation of cement-based stabilized soft clay. Strength experiments controlling the sand ratio were conducted, in addition to the literature review, to clarify the sand incorporation effect on the strength. An effective cement ratio (the mass ratio of cement to involved clay) was proposed to evaluate the strength of stabilized clay with sand incorporation. In order to combat data dispersion and the labour-intensive nature of these experiments, meso-mechanical numerical simulations were carried out by the finite element method. This served to enrich the data and clarify the mesoscopic mechanism. The numerical result indicates that the strength of stabilized clay at low sand incorporation ratio is mainly controlled by the strength of the cement-clay-water matrix, thus verifying the rationality of studying the effective cement ratio. Subsequently, the threshold sand ratio in the cement-clay-water matrix was discussed to optimize the stabilization of soft clay.

Numerical study of the deformation performance and failure mechanisms of TDM pile-supported embankments

The T-shaped deep cement mixing (TDM) pile is a technique newly used to support embankments built on soft clays. However, comprehensive studies considering the cost effectiveness have not yet been performed, and the TDM pile bending moment behavior under embankment loading is not fully understood. Hence, this paper aims to evaluate the performance of a TDM pile-supported embankment (TPSE) over soft foundation subsoils using 3D numerical modeling based on a case history of a conventional deep cement mixing (DCM) pile-supported embankment (DPSE). The performances of pile-embankment systems are investigated and discussed through several parameters, including the embankment center settlement, lateral movement at embankment toe, deflection and bending moment of the pile at the embankment shoulder, and embankment stability. First, to capture the thickness effect of the uppermost fill material layer, numerical analyses with different fill material thicknesses are examined. Subsequently, extensive parametric studies of TPSE are carried out to assess the TDM pile cap size and length influences. The results of the parametric studies are analyzed and compared to those obtained from DPSE systems with equivalent embankment loading and volume between the TDM and DCM piles. Finally, a preliminary guideline for evaluating the overall improvement performance of TPSE is proposed to determine the best TDM pile cap size for each pile length in comparison with the DPSE case. Some guidelines are also defined, and small TDM piles with thicker caps are suggested to improve pile-embankment systems.

Mixing regimes in the equatorial crater lakes of western Uganda

In order to understand the ecological functioning of lake ecosystems, little is more crucial than knowledge of their mixing regime. However in tropical regions, this information is scarce for all except the largest lakes, and often inferred from occasional observation of physical water-column properties. In this study, a total of 276 water-column profiles of temperature, conductivity and dissolved oxygen collected between 2000 and 2017 in 60 volcanic crater lakes in equatorial western Uganda are used to classify these lakes according to mixing regime. Most study lakes are small (on average 29 ha), but together they cover a spectacularly large gradient in maximum depth (Zmax) between 1.4 m and > 220 m, and consequently they display a wide range of mixing regimes. We identified four lakes as continuous warm polymictic, 12 as discontinuous warm polymictic, and 11 other lakes mix completely once or twice each year. We classify these 11 lakes as equatorial dimictic, because they experience deep mixing during both dry seasons separated by long periods of stable stratification, while the season of complete mixing may differ between years and among neighbouring lakes. The 33 remaining lakes do not usually mix completely and are thus meromictic. Relative depth (Zr) is the main factor controlling the mixing regime of these lakes. The presence of a shallow permanent chemocline reduces mixing depth, and the smallest meromictic lakes (< 3 ha) tend to occasionally mix deeper than predicted by Zr. Based on these variables, predictions can be made about the mixing dynamics of Ugandan crater lakes not covered by our dataset. Previous suggestions that the mixing depth of tropical crater lakes is strongly influenced by the wind shelter provided by a high crater rim or by water-column transparency could not be confirmed.

SOIL-CEMENT PILES FIBER REINFORCED

Problems and insufficient research of the issue of reinforcement of soil-cement piles are revealed. The use of fiber for reinforcing piles made by the deep soil mixing method is proposed. As a result, it is expected to increase the strength of soil cement, which will positively affect the bearing capacity of soil-cement piles on the material. Given the advantages for applications in the studies adopted steel fiber. The program of tests with the application of two-factor matrices of planning of experiment is made. The obtained results showed the degree of influence of fiber reinforcement on the strength of soil cement. Practical results have been obtained that can be used for implementation in the process of installation of soil-cement piles.

Deep-mixing and deep-cooling events in Lake Garda: Simulation and mechanisms

A calibrated three-dimensional numerical model (Delft3D) and in-situ observations are used to study the relation between deep-water temperature and deep mixing in Lake Garda (Italy). A model-observation comparison indicates that the model is able to adequately capture turbulent kinetic energy production in the surface layer and its vertical propagation during unstratified conditions. From the modeling results several processes are identified to affect the deep-water temperature in Lake Garda. The first process is thermocline tilting due to strong and persistent winds, leading to a temporary disappearance of stratification followed by vertical mixing. The second process is turbulent cooling, which acts when vertical temperature gradients are nearly absent over the whole depth and arises as a combination of buoyancy-induced turbulence production due to surface cooling and turbulence production by strong winds. A third process is differential cooling, which causes cold water to move from the shallow parts of the lake to deeper parts along the sloping bottom. Two of these processes (thermocline tilting and turbulent cooling) cause deep-mixing events, while deep-cooling events are mainly caused by turbulent cooling and differential cooling. Detailed observations of turbulence quantities and lake temperature, available at the deepest point of Lake Garda for the year 2018, indicate that differential cooling was responsible for the deep-water cooling at that location. Long-term simulations of deep-water temperature and deep mixing appear to be very sensitive to the applied wind forcing. This sensitivity is one of the main challenges in making projections of future occurrences of episodic deep mixing and deep cooling under climate change.

Laboratory Investigation on the Effect of Microsilica Additive on the Mechanical Behavior of Deep Soil Mixing Columns in Saline Dry Sand

Since loose and salty subgrades consider as problematic barriers while constructing new transportation infrastructures such as railway tracks and roads are required, the current study aims to find a solution to stabilize these kinds of subgrades using the deep soil mixing (DSM) technique and micro silica additive. In the present study a series of experimental DSM columns were executed in a salty sand-filled chamber utilizing a laboratory scale DSM apparatuses. In the first step, by adding three salt percentages of 5, 10 and 20 into the original sand, four different sandy subgrades with a relative density of 70% were prepared. Considering three percentages of 10, 15 and 20 for micro silica additive, the water-to-cement ratio of 1, salt percentages of 0, 5, 10 and 20 totally 150 sand-cement columns were constructed in the lab environment. In continuation, unconfined compression strength (UCS) and elasticity modulus of all capped DSM columns have been determined and interpreted using scanning electron microscope (SEM) images at three ages of 7,14 and 28 days. The results indicated that increasing the salinity of subgrade soil from 0 to 20% resulted in a falling UCS and Young module by 28 and 21% for 28-days specimens. Furthermore, as a solution, adding micro silica in cement-water grout up to 15% resulted in enhancing mechanical characteristics of the DSM columns. So that adding 15% microsilica caused a 21 and 42% increase in UCS and elasticity modulus of 28-days samples respectively, executed in subgrade with 20% salt.

A Review on Deep mixing method for soil improvement

Deep mixing method, the ideal choice that can be used to treat problems arising from the presence of soft clay soil as a basis for constructing a specific structure. Soft clay soil covers large areas in some countries around the world, making it difficult to find suitable places for construction. Soft clay soil which has a high moisture content that makes it with small shear resistance and high settlement capability is not suitable as a bearing layer under the foundations of the facilities, Therefore, the need arose to either the use of a certain type of foundation, such as deep foundations Which are more complicated compared to the second option, or the use a specific treatment technique to improve the properties of the soft clay soil and make it suitable for the construction with the using of a certain type of foundations other than deep foundations. One of the most suitable treatment techniques for soft clay soils in terms of structural purpose, cost and time is the deep mixing process. Deep soil mixing is a complex process in terms of factors that affect the improved soil quality and the processes that causing the improvement. This process has been covered in a lot of books and published researches in many of its aspects, but there is some of that relates to this process has not been significantly highlighted such as improved soil permanence over time, which causes a poor understanding of the behavior of improved soil after the improvement process. In this research paper, everything related to the deep mixing process will be touch upon, starting with the historical development of this process, its beginnings to its types, classifications and applications, and the accompanying chemical and physical processes, in addition to a literary review for what related to this process.

Comparative analysis of the dynamic stability of a multistorey building with different base arrangements

The article presents the results of studies by the numerical method of pile and slab foundation with a reinforced base by the method of deep soil mixing (DSM) under the influence of seismic loads using the LIRA-SAPR software package, which implements the finite element method. The parameters of the computer model are presented, which make it possible to obtain the calculation results. A comparative analysis of the calculation results is carried out. In the course of the study, it was revealed that the use of a slab foundation with a reinforced base by the deep soil mixing method (DSM) is more effective than a pile foundation under seismic loads.

Group II Oxide Grains: How Massive Are Their AGB Star Progenitors?

Presolar grains and their isotopic compositions provide valuable constraints to AGB star nucleosynthesis. However, there is a sample of O- and Al-rich dust, known as group 2 oxide grains, whose origin is difficult to address. On the one hand, the 17O/16O isotopic ratios shown by those grains are similar to the ones observed in low-mass red giant stars. On the other hand, their large 18O depletion and 26Al enrichment are challenging to account for. Two different classes of AGB stars have been proposed as progenitors of this kind of stellar dust: intermediate mass AGBs with hot bottom burning, or low mass AGBs where deep mixing is at play. Our models of low-mass AGB stars with a bottom-up deep mixing are shown to be likely progenitors of group 2 grains, reproducing together the 17O/16O, 18O/16O and 26Al/27Al values found in those grains and being less sensitive to nuclear physics inputs than our intermediate-mass models with hot bottom burning.

Sewage treatment sludge biochar activated blast furnace slag as a low carbon binder for soft soil stabilisation

Portland cement forms the basis of most binders used in deep dry soil mixing, significantly improving the shear strength and compressibility properties of soils. However, due to the high environmental and socio-economic impacts of cement production, there is great interest in developing alternative low-carbon binders for soil stabilisation. One of the most desirable routes involves the use of industrial by-products, such as ground granulated blast furnace slag, whose pozzolanic properties require activation by alkali agents. This paper assesses the feasibility of using sewage treatment sludge biochar as a low-carbon 100% waste-based alternative to traditional alkali agents. Two biochar:slag ratios (0.5:0.5 and 0.67:0.33) were added to an artificial soil at dosages of 7.5 and 10% by dry weight and cured for up to 56 days. The engineering performance of these stabilised soil mixtures was assessed by performing a suite of compressive strength, pH, water content, mineralogical and microstructural analyses. Results were compared with those of untreated and CEM-II stabilised alluvium, along with data published in the literature. Biochar was observed to successfully activate the pozzolanic properties of the slag, whereby the studied mixtures resulted in 28-day strengths that met European soil stabilisation standards requirements. Binder mixtures with higher biochar concentrations achieved greater strengths. The best performing mixture had a biochar-slag ratio of 0.67:0.33 and dosage of 10%, which produced strengths up to 1243 kPa. This study suggests that the biochar-slag binder has encouraging prospects for replacing Portland cements in soil stabilisation, reducing the carbon footprint of the construction sector and improving the circular economy.

Anoxic in situ production of bacterial GMGTs in the water column and surficial bottom sediments of a meromictic tropical crater lake: Implications for lake paleothermometry

Branched glycerol monoalkyl glycerol tetraethers (brGMGTs) are membrane-spanning lipids which were initially identified in marine settings and have more recently been found in peats and lake sediments. In the latter settings, their abundance relative to that of the branched glycerol dialkyl glycerol tetraethers (brGDGTs) appears to increase under warmer climate conditions. Furthermore, their relative distribution in bottom sediments from an altitudinal transect of East African lakes has been linked to local mean annual air temperature (MAAT). To shed light on their sources and identify potential environmental factors driving their production in lakes, we investigated their occurrence and distribution in the water column, surficial lake-bottom sediments, and catchment soils of Lake Chala, a permanently stratified (meromictic) tropical crater lake in East Africa. Data from the water column comprise both suspended particulate matter (SPM) collected monthly at eight depth intervals between 0 and 80 m for 17 consecutive months, and settling particles collected monthly at a depth of 35 m over a period of 53 months. In catchment soils, brGMGTs are either relatively scarce or absent. In the water column, their occurrence is dominated by four of the seven known brGMGTs and mostly limited to the anoxic zone. Whereas the spatial distribution and concentration of brGMGTs in SPM were strongly influenced by the seasonal cycle of strong water-column stratification alternating with deep mixing, this trend is not consistent on the multi-year timescale of the settling-particles record. Moreover, their relative distribution in both SPM and settling particles did not respond to temperature variation during the studied interval. In contrast to the water column, surficial lake-bottom sediments contain the full suite of known brGMGTs in a distinct and spatially uniform fractional abundance, strongly suggesting that the majority of brGMGT production takes place there. BrGMGT-inferred MAAT for the Chala area reasonably estimates measured MAAT and conforms to the East African lake brGMGT calibration. Our results have important implications for the use of brGMGTs as proxy for MAAT, in that outside of the equatorial zone the presumably greater offset between bottom-water temperature and MAAT may create a bias towards cold-season air temperature. Regardless, the more singular source of brGMGTs in lake sediments is an advantage for paleotemperature reconstruction compared to brGDGTs, which several studies now indicate to have a mixed aquatic and terrestrial source of which the relative contribution cannot easily be disentangled or assumed to have been constant through time.

Deep mixed ocean volume in the Labrador Sea in HighResMIP models

Simulations from seven global coupled climate models performed at high and standard resolution as part of the high resolution model intercomparison project (HighResMIP) are analyzed to study deep ocean mixing in the Labrador Sea and the impact of increased horizontal resolution. The representation of convection varies strongly among models. Compared to observations from ARGO-floats and the EN4 data set, most models substantially overestimate deep convection in the Labrador Sea. In four out of five models, all four using the NEMO-ocean model, increasing the ocean resolution from 1° to 1/4° leads to increased deep mixing in the Labrador Sea. Increasing the atmospheric resolution has a smaller effect than increasing the ocean resolution. Simulated convection in the Labrador Sea is mainly governed by the release of heat from the ocean to the atmosphere and by the vertical stratification of the water masses in the Labrador Sea in late autumn. Models with stronger sub-polar gyre circulation have generally higher surface salinity in the Labrador Sea and a deeper convection. While the high-resolution models show more realistic ocean stratification in the Labrador Sea than the standard resolution models, they generally overestimate the convection. The results indicate that the representation of sub-grid scale mixing processes might be imperfect in the models and contribute to the biases in deep convection. Since in more than half of the models, the Labrador Sea convection is important for the Atlantic Meridional Overturning Circulation (AMOC), this raises questions about the future behavior of the AMOC in the models.

Low-Level Atmospheric Flow at the Central North Coast of Brazil

The planetary-boundary-layer (PBL) flow and above is investigated for the central north coast of Brazil, an equatorial region spanning from 8° to 2°S. The daily PBL flow is controlled by vertical entrainment of horizontal momentum from a southerly large-scale flow associated with the Hadley cell, and by a mesoscale pressure gradient force (PGF) created by the differential heating between land and ocean. Near the coast, the flow is from the north-east quadrant comprising a small rotation, probably caused by a weak mesoscale PGF and a weak Coriolis force. Inland it is north-easterly in the morning, but deep mixing during the afternoon brings down momentum from above causing it to become south-easterly. The mesoscale PGF executes a daily 360° rotation at most of the stations. In the afternoon it points to land due to continuous heating of the land, and a sea breeze develops in the presence of the background flow. Once convection dies out, the transfer of horizontal momentum is reduced, and the marine-air layer can flow faster into the continent as a nocturnal jet. As the stable boundary layer grows thicker, this flow tends to be eliminated at the surface. By morning, the mesoscale PGF points north, forcing the inland flow to become south-easterly, while on the coast flow becomes almost easterly. This scenario repeats during dry and wet seasons and can be understood as a consequence of the south–north propagation of an atmospheric circulation resembling a helix with its rotation axis oriented parallel to the shoreline.

The subsurface biological structure of Southern Ocean eddies revealed by BGC-Argo floats

Mesoscale eddies play several key roles in the Southern Ocean by redistributing momentum, potential vorticity, heat and salt. However, the role of eddies in Southern Ocean biogeochemistry has received less attention. Previous studies have been based on satellites or models. In this study, Southern Ocean eddies were matched with BGC-Argo float profiles to investigate the subsurface biological structure of Southern Ocean eddies. We focused on four frontal regions in the Indian sector of the Southern Ocean where data density is greater than in the other basins. The seasonality and regional variability of subsurface chlorophyll and backscatter structures were revealed by calculating anomalies in eddies relative to outside of eddies, in each season and frontal region. Chlorophyll anomalies in the mixed layer were mostly positive for both cyclones and anticyclones in all regions in spring, summer and autumn. Anomalies in winter were close to zero. Eddy pumping was the most likely mechanism sustaining high chlorophyll in cyclones. Eddy-induced Ekman pumping, in combination with deep vertical mixing was likely the driver of enhanced chlorophyll in anticyclones. Backscatter anomalies were mostly consistent with chlorophyll anomalies. Chlorophyll to backscatter ratios (chlorophyll:bbp) were also calculated to help understand the role of light acclimation in the phytoplankton. There were strong subsurface maxima for in-eddy chlorophyll:bbp in spring and summer. Such structures were weak in autumn and winter, but chlorophyll:bbp in the upper 30 m in autumn and winter was mostly higher than spring and summer in the same region. Photo-acclimation to low light at depth was evident for both cyclones and anticyclones in spring and summer in the euphotic zone, while in autumn and winter it was observed throughout the upper water column. The majority of positive chlorophyll anomalies were due to both an increase of biomass and photo-acclimation while few cases were due to photo-acclimation only.

Internal mixing of rotating stars inferred from dipole gravity modes

During most of their life, stars fuse hydrogen into helium in their cores. The mixing of chemical elements in the radiative envelope of stars with a convective core is able to replenish the core with extra fuel. If effective, such deep mixing allows stars to live longer and change their evolutionary path. Yet localized observations to constrain internal mixing are absent so far. Gravity modes probe the deep stellar interior near the convective core and allow us to calibrate internal mixing processes. Here we provide core-to-surface mixing profiles inferred from observed dipole gravity modes in 26 rotating stars with masses between 3 and 10 solar masses. We find a wide range of internal mixing levels across the sample. Stellar models with stratified mixing profiles in the envelope reveal the best asteroseismic performance. Our results provide observational guidance for three-dimensional hydrodynamical simulations of transport processes in the deep interiors of stars.