Stable Growth Stage Research Articles

Experimental study on nonlinear development process of oil-filled equipment fire under external fire source

Abstract The transformer is the key oil-filled equipment in the power system, and its fire behavior seriously affects the safe operation of the power grid. In this paper, in order to analyze the fire development process and combustion behavior of oil-filled equipment, a mesoscale model of transformer equipment was constructed, and fire simulation experiments of transformer equipment under the action of external ignition sources were conducted. The flame temperature, flame height, heat release rate, oil temperature, and pressure were measured. The experimental results show that the oil-filled equipment fire presents the characteristics of nonlinear development. The fire can be divided into three stages: ignition stage, stable growth stage, and combustion mutation stage. The transformer oil near the wall is pyrolyzed by the external heat source, and the combustible gas and transformer oil form a gas-liquid two-phase flow, which is the main reason for the nonlinear development of oil-filled equipment fire. The experimental results are of great significance for the safe operation and fire control of power system oil-filled equipment.

Research on the impact mechanism of changes in the production of medical solid waste in China before and after COVID-19.

The changes of medical solid waste (MSW) output in recent years have had a significant impact on the spread of the virus. There is a high-risk transmission of MSW in various stages such as storage, transportation, and treatment during the COVID-19. To cope with the risks brought by the epidemic, normalized prevention consumes a large amount of protective clothing, medical masks, goggles, packaging bags, and other related medical supplies. There is a significant uncertainty in the amount of MSW output that poses a risk of COVID-19 infection in the event of an emergency, which increases the difficulty of collecting and handling epidemic prevention MSW. The analysis of MSW data from 2000 to 2022 found a stable growth trend before 2019. However, the MSW data was a sudden increase trend from 2020 to 2022, and the COVID-19 in China was characterized by an initial stage, an outbreak stage, and a stable growth stage. The range of MSW output during the epidemic was (1.19-1.75) × 106 t a-1. The amount of MSW was approximately 1.19 × 106 t a-1 during the normalized epidemic period, and its treatment cost was as high as 3.57 × 109 yuan (RMB)·a-1. The distribution of MSW output was uneven due to factors such as climate conditions, population data, and local economy. This study has important reference value for epidemic medical material reserves and MSW treatment.

Investigation on the flow and induced-noise mechanism during the transient start-up process of a mixed-flow pump

PurposeThis study aims to investigate the noise-inducing characteristics during the start-up process of a mixed-flow pump and the impact of different start-up schemes on pump noise.Design/methodology/approachThis study conducted numerical simulations on the mixed-flow pump under different start-up schemes and investigated the flow characteristics and noise distribution under these schemes.FindingsThe results reveal that the dipole noise is mainly caused by pressure fluctuations, while the quadrupole noise is mainly generated by the generation, development and breakdown of vortices. Additionally, the noise evolution characteristics during the start-up process of the mixed-flow pump can be divided into the initial stage, stable growth stage, impulse stage and stable operation stage.Originality/valueThe findings of this study can provide a theoretical basis for the selection of start-up schemes for mixed-flow pumps, reducing flow noise and improving the operational stability of mixed-flow pumps.

Mechanism of NCNTs Growth on Foamed Nickel and Thus-Prepared PS Hydrogenation High-Performance Carrier NCNTs@FN.

Traditional heterogeneous catalysts are affected in the catalytic hydrogenation of PS by the scale effect, viscosity effect, adhesion effect, and conformational effect, resulting in poor activity and stability. Monolithic Pd-CNTs@FN catalysts could eliminate or weaken the impact of these negative effects. We grew nitrogen-doped carbon nanotubes (NCNTs) on monolithic-foamed nickel (FN) and investigate their growth mechanism. Meanwhile, the feasibility of using the NCNTs@FN carrier for PS hydrogenation reaction was also verified. The growth of NCNTs on FN can be divided into 3 stages: initial growth stage, stable growth stage, and supersaturation stage. Finally, a three-layer structure of NCNT layer, dense carbon layer, and FN skeleton is formed. Two types of structures, nickel-doped carbon nanotubes (NiCNTs) and C-Ni alloy, are formed by combining C and Ni, while four nitrogen-doped structures, NPD, NPR, NG, and NO, are formed by C and N. The prepared carrier exhibited an extremely outstanding specific surface area (2.829 × 106 cm2/g) and strength (no NCNTs falling off after 24 h 500 rpm agitation), as well as high catalytic activity for PS hydrogenation after loaded with Pd (2.13 ± 0.95 nm), with a TOF of up to 27.6 gPS/(gPd•h). After 8 repetitions of the catalyst, there was no significant decrease in activity. This proves the excellent performance of Pd-NCNTs@FN in polymer hydrogenation reactions, laying a solid foundation for further research on the mechanism of NCNTs promoting PS hydrogenation and regulating the growth of NCNTs.

Plastic and damage energy dissipation characteristics and damage evolution of Beishan granite under triaxial cyclic loading

The damage and failure of rock under triaxial stress is an irreversible process of energy dissipation, which mainly includes plastic energy dissipation and damage energy dissipation. However, it is difficult and rare to separate plastic energy dissipation and damage energy dissipation and to conduct relevant research on this basis. In this work, a loading control method with a combination of load (0.5 kN/s) and circumferential deformation (0.01 mm/min) was used, and triaxial cyclic loading and unloading tests under five different confining pressures were performed on granite from the Beishan site area of China's high-level radioactive waste repository. The results indicate that the angle between the main fracture surface and the longitudinal axis increases with the increase of confining pressure. According to the characteristics of the stress–strain curve and energy dissipation, the test process can be divided into five stages: the initial compaction and elastic stage, the stable crack growth stage, the unstable crack growth stage, the postpeak unstable fracture stage and the residual stress stage. Importantly, a separate determination method of plastic energy dissipation and damage energy dissipation was proposed, and the evolution characteristics of plastic energy dissipation and damage energy dissipation were revealed. The ratio of plastic energy dissipation to input energy decreases slightly in the initial compaction and elastic stage, stabilizes at a low level in the stable and unstable crack growth stages, increases significantly in the postpeak unstable fracture stage and remains at a high level in the residual stress stage. Meanwhile, the ratio of damage energy dissipation to input energy increases slightly before the unstable crack growth stage, increases abruptly at the end of the unstable crack growth stage and the initial part of the postpeak unstable fracture stage, then declines rapidly, and finally remains at a low level. Furthermore, the qualitative and quantitative research results of damage evolution show that the plastic shear strain and damage variable established by damage energy dissipation can qualitatively and quantitatively describe the damage evolution of Beishan granite respectively. Finally, the verification results of Beishan granite and two other rock types show that the separation method for calculating plastic energy dissipation and damage energy dissipation proposed in this work has good adaptability, and the qualitative damage development analysis based on plastic shear strain can be verified and supplemented with the quantitative damage development analysis based on energy theory.

Effect of electrolyte concentration on bubble evolution and mass transfer characteristics on surface of photoelectrode

In the photoelectrochemical water splitting reaction system, bubbles will cover the reaction area on the photoelectrode surface, affecting the reaction impedance and gas-liquid mass transfer. A laser irradiation system is built and it is coupled with an electrochemical workstation and high-speed microscopic imaging system. The evolution behavior and mass transfer characteristics of single O<sub>2</sub> bubble on the TiO<sub>2</sub> photoelectrode are studied at different electrolyte concentrations (Na<sub>2</sub>SO<sub>4</sub>, 0.1–2.0 mol/L). With the increase of electrolyte concentration from 0.1 mol/L to 2.0 mol/L, the solution resistance and bubble additional resistance decrease, and the overpotential in the stable growth stage of bubble decreases from 0.113 V to –0.089 V. The bubble will cause the fluctuation of overpotential in the nucleation, growth and detachment stages, which is consistent with the impedance change caused by the change of dissolved oxygen concentration in the liquid phase. By analyzing the correlation between gas evolution efficiency and bubble coverage, it is found that the increase of electrolyte concentration will lead the bubble coverage and gas evolution efficiency to decrease simultaneously. By calculating the Sherwood dimensionless number, the results show that the total convective mass transfer coefficient increases with the electrolyte concentration increasing. Single-phase natural convection plays a dominant role in the process of gas product transfer, and its mass transfer coefficient is one order of magnitude larger than that of bubble-induced convection. In summary, by adjusting the electrolyte concentration, the bubble on the gas evolution photoelectrode surface can be effectively removed and the mass transfer of the system can be optimized, which is of great significance in improving the efficiency of photoelectrochemical water splitting.

Damage evolution and strength prediction model of soda residue modified cemented tailings backfill under uniaxial compression

Recycling of solid waste materials reduces filling costs and reduces environmental pollution. This paper analyzed the availability of soda residue modified cemented tailings backfill (SRMCTB), and carried out uniaxial compression experiments to investigate the mechanical properties, energy evolution and damage characteristics of SRMCTB. The results show that the addition of soda residue effectively improves the compressive properties of SRMCTB. The uniaxial compressive strength and elastic modulus of SRMCTB increase and then decrease, and the elastic energy storage limit and total energy storage limit of SRMCTB increase and then decrease with increasing soda residue content. The damage constitutive models considering the compaction stage of SRMCTB were established and the damage evolution curves were plotted. Combined with the damage curves, the damage of SRMCTB can be divided into damage-free stage, damage stable development stage and damage rapid growth stage. The addition of soda residue has an effect on the stable development stage of the damage. When the soda residue content increased, the rate of variation of damage development stages first increases and then decreases. The proportioning parameters of SRMCTB were optimized in combination with the response surface methodology. It shows that when the soda residue content is at 5.45 wt%, the mass concentration is 76 wt% and the tailings gradation is 2:5, SRMCTB has the highest uniaxial compressive strength. A prediction model for the uniaxial compressive strength of SRMCTB was proposed and validated. The average error between the experimental and theoretical values is 8.30%, indicating that the model has a strong applicability and degree of extension.

Experiment and dynamic simulation of hydrogen recovery from hydrogen-containing gas mixtures via hydrate-membrane coupling method

The recovery of hydrogen (H2) form H2-containing gas mixtures, such as petrochemical refinery tail gas, plays a important role in energy conservation and cost reduction. The hydrate-membrane coupling separation process has been demonstrated as a more efficient and energy-saving approach for H2 recovery. This study aims to achieve the synergistic matching of hydration and permeation, so as to explore the underlying mechanisms of coupling and facilitate subsequent experimental studies. In this work, an effective method was developed to calculate the gas consumption rate during the stable growth stage of hydrates. Furthermore, a dynamic matching model of the hydrate-membrane coupling mechanism was employed to simulate the variations in the separation process using infinitesimal methods. The simulations indicate that the H2 concentration of product gas and the feed gas treatment capacity of the coupling method outperform those of hydrate separation and membrane separation alone. Overall, this study provides valuable insights into an efficient and energy-saving method for H2 recovery from H2-containing gas mixtures, utilizing the hydrate-membrane coupling method. These findings hold promising prospects for practical application in the future.

Probability estimation method for fatigue crack growth rate based on a ∼ t data from service

PurposeThe study aims to provide a basis for the effective use of safety-related information data and a quantitative assessment way for the occurrence probability of the safety risk such as the fatigue fracture of the key components.Design/methodology/approachThe fatigue crack growth rate is of dispersion, which is often used to accurately describe with probability density. In view of the external dispersion caused by the load, a simple and applicable probability expression of fatigue crack growth rate is adopted based on the fatigue growth theory. Considering the isolation among the pairs of crack length a and crack formation time t (a∼t data) obtained from same kind of structural parts, a statistical analysis approach of t distribution is proposed, which divides the crack length in several segments. Furthermore, according to the compatibility criterion of crack growth, that is, there is statistical development correspondence among a∼t data, the probability model of crack growth rate is established.FindingsThe results show that the crack growth rate in the stable growth stage can be approximately expressed by the crack growth control curve da/dt = Q•a, and the probability density of the crack growth parameter Q represents the external dispersion; t follows two-parameter Weibull distribution in certain a values.Originality/valueThe probability density f(Q) can be estimated by using the probability model of crack growth rate, and a calculation example shows that the estimation method is effective and practical.

Gurson-Tvergraad-Needleman model-based damage analyses of stainless steel springs at high temperature

In this paper, the Gurson-Tvergraad-Needleman (GTN) model is used to study the damage of stainless steel springs at high temperatures. The basic material and GTN damage parameters are determined from the stress-strain curves of tensile tests. Then the damage evolution of springs at 300 °C to 900 °C is analyzed based on finite element simulations. The results show that below 600 °C, the change of volume fraction of holes in the spring, along with the increase of load, has an obvious low growth stage, followed by a rapid growth stage. But above 600 °C, there is only a stable and rapid growth stage. The fracture positions predicted by finite element damage analyses agree with the experiments for round bars.

Experimental study on the interface fatigue between track slab and self-compacting concrete for CRTS III slab track

With the rapid development of high-speed railways, the CRTS III slab track has become one of the most important track structures. The interface between the precast track slab and cast-in-place self-compacting concrete is easily damaged under fatigue loads. However, experimental research in this field is still relatively lacking. By performing static and fatigue splitting tensile tests, the study first obtained interface failure characteristics between the track slab and self-compacting concrete. Most of the interface failures were mortar failures and mortar-aggregate bond failures, and only a few were aggregate failures. Then, the evolution law of interface strain was analyzed. The interface strain was large in the middle and small on both sides. The change of interface strain with fatigue life ratio was divided into three stages: rapid growth, stable growth, and unstable growth stages. Moreover, the S-N curve of the interface was obtained. Finally, based on the residual strain, an interface fatigue damage model was established to reveal the evolution law of damage variables with fatigue life ratio under different stress levels. This study makes up for the insufficiency of current interface damage research on CRTS III slab track and provides the reference for the subsequent numerical simulation and engineering application.

LONG-TERM LIGHT DEPRIVATION AFFECTS DIGESTIVE FUNCTION IN RATS DURING ONTOGENY

The effects of light deprivation on age-related changes in body weight, food and water intake, as well as the activity of digestive enzymes in pancreatic tissues of male rats was studied. Animals were divided into three groups: the first was in standard light conditions (12 h light/12 h dark, control, LD), the second was kept under conditions of long-term light deprivation from the moment of birth (LD/DD), and the third – from the prenatal period (DD/DD). Prolonged keeping of rats under conditions of light deprivation led to disruption of the age-associated dynamics of the studied parameters, at the same time, the detected changes in response to a specific light condition differed depending on the stage of ontogenesis at which its exposure began. Thus, body mass (age 3 and 18 months) and food and water intake (12 months), amylase and lipase activities (18 months) were lower, and protease activities after the end of the stable growth stage (12, 18 and 24 months) were higher in LD/DD-rats than in LD-rats. Significant changes in the studied parameters were observed under DD/DD conditions in 12‑month-old and 18-month-old rats – body mass was the largest, and food and water intake were the lowest compared to LD and LD/DD. In addition, the maximum amylase activity (1 and 6 months) among the studied groups and the higher values of total proteolytic activity during the period of stable growth (6 and 12 months) were found in DD/DD-rats compared to controls. Thus, our results indicate that light deprivation disrupts the ontogenetic development program of the digestive system of mammals.

Residual life prediction of compressor impeller with microcrack damage

Taking compressor impeller with pressure ratio less than 3 as research object, a prediction method of residual life of compressor impeller was proposed. Considering the action of “inertial” load centrifugal force, finite element method (FEM) technology and sub-model technology were used to comprehensively analyze the mechanical quantities, such as blade stress, and the hot spot stress location of compressor impeller was determined. Crack orientation was determined according to maximum principal stress criterion. The linear elastic fracture mechanics (LEFM) based on FEM and Forman-Newman-de Koning (FNK) model were used to investigate the whole process of crack growth, including stable growth and instability growth stages. The crack growth law and residual life of impeller at different rotate speeds were given. The results show that with the increase of rotate speed, the stress intensity factor (SIF) of initial crack front increases in a quadratic power law, the critical length decreases linearly, and the residual life decreases nonlinearly. The crack grows along the height of blade and then fractures after a deflection of about 90 degrees to the outer edge of blade. The obvious deflection of crack is the sign of crack transition from stable growth stage to instability growth stage. The life of crack in the instability growth stage can be neglected compared with that in the stable growth stage. The analysis method proposed in this article can provide a basis for the damage tolerance evaluation of compressor impeller, and can also provide a reference for maintenance work.

Quantifying the effects of gap on the molten pool and porosity formation in laser butt welding

To obtain a better joint quality in butt welding of aluminum, the gap filling process and the quantification of the gap effects on the molten pool characteristic and the bubble formation were realized by a three-dimensional thermal-mechanistic-fluid coupled model, with the consideration of heat transfer, fluid flow, phase change and recoil pressure. The model was validated by the synchrotron-radiation result. The competition between the solidification and melting at the bottom of the molten pool was uncovered to determine the gap filling process and the molten pool morphology. Gap increased the heat loss, and the molten pool tip was elongated due to gap filling. Four phenomena appeared in sequence in the initial stage of butt welding: I. Gap filling; II. Frozen; III. Remelt; IV. Bubble formation. The result also demonstrated that the gap would disturb the molten pool. In the initial stable growth stage of the molten pool, the larger the gap width, the greater the molten pool depth. The sharp change of keyhole depth was due to the necking formation, while the small fluctuation of keyhole depth with larger gap values resulted from the perturbation by the gap. Bubble formation depends on the degree of the fluid flow and the gap filling due to the unique fluid dropping down phenomenon of butt welding with gap. A continuous melt pool cannot be formed when the gap width beyond 20 μm, which is detrimental to the welding quality. These findings are of great significance for guiding the optimization of butt-welding process, such as reducing the roughness of the butt interface or increasing the clamping force to reduce the butt gap.

Investigation into the Time-Dependent Crack Propagation Rate of Concrete.

Mass concrete structures under long-term loads are susceptible to time-dependent fractures, which pose a threat to their structural integrity and safety. In order to study the crack growth rate of concrete materials under long-term constant load, the data were processed according to the calculation method of fatigue crack growth rate. The relationship between the crack growth rate and strength factor in the stable growth stage was obtained using the Paris formula. The experimental data and theoretical analysis show that the time-dependent fracture curve CMOR(t)-t of the standard three-point bending beam specimens could be divided into three stages. The relationship between the crack propagation rate da/dt(t) in the second stage and the intensity factor K(t) could be well described by the Paris formula. The life of crack growth of a standard three-point curved beam is inversely proportional to the level of constant load. These conclusions can provide data support for further studies on crack extension life under long-term constant load.

Microstructure and Fatigue Performance of Ti6Al4V Produced by Laser Powder Bed Fusion after Post-Heat Treatment

With the development of additive manufacturing (AM), the Ti-6Al-4V alloy manufactured by laser powder bed fusion (LPBF) is becoming more widely studied. Fatigue fracture is the main failure mode of such components. During LPBF processing, porosity defects are unavoidable, which hinders the exploration of the relationship between fatigue performance and microstructure. In this study, a laser remelting method was used to reduce porosity defects inside the Ti-6Al-4V alloy. Three annealing treatments (AT) and three solution-aging treatments (SAT) were used to study the effect of the two-phase zone (α + β) microstructure on fatigue life and fatigue crack growth behavior. Fatigue life and crack growth rate (CGR) curves were obtained, and fatigue fracture surface and crack growth fracture surface were analyzed. The results show that microstructure influences fatigue life but has little effect on CGR. Compared with the as-built specimen, the fatigue life of the AT and SAT specimens increased significantly at 850℃ by 101 and 63.7 times, respectively. The thickness of the α lath and the location of crack nucleation together affect the fatigue life. In the stable growth stage, the layered microstructure of α colonies is the most resistant to crack growth.

Mechanical properties, damage evolution and energy dissipation of cemented tailings backfill under impact loading

In order to explore the influence of average strain rate(ASR) on mechanical properties, damage evolution and energy dissipation of cemented tailings backfill(CTB), the dynamic test of CTB was carried out in the laboratory. This study shows that dynamic peak stress and initial crack stress of CTB increase exponentially with the increase of ASR, indicating that the characteristic stress of CTB has a significant strain rate effect. The dynamic peak strain and initial crack strain of CTB also increase exponentially with the increase of ASR, and the CTB with lower cement content is more likely to produce deformation and failure. The increase of ASR can improve the energy indexes of CTB at the peak stress point, and the increase of cement content can also improve the energy parameters of CTB. The energy storage limit of CTB increases exponentially with the increase of ASR. In addition, a dynamic statistical damage constitutive model of CTB was established considering ASR, and the accuracy and reliability were verified. There are three stages for the damage failure of CTB: rapid growth stage of damage, slow growth stage of damage and stable growth stage of damage. In addition, the high strain rate has a certain retarding effect on the development of cracks inside the CTB, thus inhibiting the rapid growth of damage value. Under impact loading, the CTB samples are mainly axial macroscopic main crack tensile failure and crushing failure. When the average strain rate is higher than the critical strain rate, the failure pattern of CTB is mainly crushing failure. When the average strain rate is basically the same, the damage degree of CTB with higher cement content is smaller.

Influence of subatmospheric pressure on bubble evolution on the TiO2 photoelectrode surface.

The increase of reaction resistance caused by bubble nucleation and long-time growth on the surface of the photoelectrode is an important factor that leads to the low efficiency of photoelectrochemical water splitting. In this study, we adopted an electrochemical workstation synchronous with a high-speed microscopic camera system to achieve in situ observation of oxygen bubble behavior on the surface of TiO2 and to study the internal relationship between the geometric parameters of oxygen bubbles and photocurrent fluctuations under different pressures and laser powers. The results indicate that with the decrease of pressure, the photocurrent decreases gradually and the bubble departure diameter increases gradually. In addition, the nucleation waiting stage and the growth stage of bubbles are both shortened. However, the difference between the average photocurrents corresponding to the moment of bubble nucleation and the stable growth stage hardly changes with the pressure. The production rate of gas mass reaches a peak near 80 kPa. In addition, a force balance model suitable for different pressures is constructed. It is found that as the pressure decreases from 97 kPa to 40 kPa, the proportion of the thermal Marangoni force in the Marangoni force decreases from 29.4% to 21.3%, while the proportion of the concentration Marangoni force increases from 70.6% to 78.7%, indicating that the concentration Marangoni force is the main factor affecting the bubble departure diameter under subatmospheric pressure conditions.

A review on pitting corrosion and environmentally assisted cracking on duplex stainless steel

Duplex stainless steel is widely used in the petrochemical, maritime, and food industries. However, duplex stainless steel has the problem of corrosion failures during use. This topic has not been comprehensively and academically reviewed. These factors motivate the authors to review the developments in the corrosion research of duplex stainless steel. The review found that the primary reasons for the failure of duplex stainless steels are pitting corrosion and chloride-induced stress corrosion cracking. After being submerged in water, the evolution of the passive film on the duplex stainless steel can be loosely classified into three stages: nucleation, rapid growth, and stable growth stages. Instead of dramatic rupture, the passive film rupture process is a continuous metal oxidation process. Environmental factors scarcely affect the double-layer structure of the passive film, but they affect the film's overall thickness, oxide ratio, and defect concentration. The six mechanisms of alloying elements on pitting corrosion are summarized as stabilization, ineffective, soluble precipitates, soluble inclusions, insoluble inclusions, and wrapping mechanisms. In environments containing chlorides, ferrite undergoes pitting corrosion more easily than austenite. However, the pitting corrosion resistance reverses when sufficiently large deformation is used. The mechanisms of pitting corrosion induced by precipitates include the Cr-depletion, microgalvanic, and high-stress field theories. Chloride-induced cracks always initiate in the corrosion pits and blunt when encountering austenite. Phase boundaries are both strong hydrogen traps and rapid hydrogen diffusion pathways during hydrogen-induced stress cracking.

Application of the Richards Model in Settlement Prediction of Loess-Filled Foundations

The Richards equation has high plasticity and compatibility. Compared with other curve equations, it has higher applicability and accuracy in describing S-shaped curves. It can describe the changes in the initial growth stage, rapid growth stage, and stable growth stage of organisms well and reflect the slow-fast-slow change characteristics of growth. In this paper, the Richards model is introduced into the settlement deformation prediction of loess-filled foundation. Taking a high-loess fill site in northern Shaanxi Province as an example, continuous settlement deformation monitoring is carried out for the filling during the project operation period, and the settlement characteristics of a loess-filled foundation are studied using the monitoring results. It is considered that the settlement deformation curve of a loess-filled foundation has similar characteristics; that is, its deformation increases slowly in the early stage, rapidly in the middle stage, and is basically stable at a fixed value in the later stage. Its cumulative deformation time history curve is a smooth S-shaped curve, and the cumulative settlement increases with time and tends to converge. Analysis shows that the time history curves of cumulative settlement, the settlement deformation rate, and the acceleration of loess foundation filling are highly consistent with the Richards model. Compared with the modified exponential model and hyperbolic model, the predicted results are closer to the measured results; that is, the Richards growth model can be used to predict the settlement deformation of a fill foundation.