Width Of Pit Research Articles

Analytical Solution for the Steady Seepage Field of a Foundation Pit in an Anisotropic Layer

The soil piping and drift sand phenomenon is one of the catastrophic failure forms in foundation pit excavations in coastal buildings. Presently, there is a deficiency in the theoretical research regarding the seepage fields around foundation pits, primarily due to the complexity of theoretical solutions given the difficulty in accurately describing the distribution of the groundwater’s hydraulic head in a seepage field. This study proposes an explicit analytical solution for the steady-state seepage field surrounding a foundation pit under anisotropic conditions. A numerical model, constructed with FLAC3D 7.0 software, was utilized to validate the solution presented in this study. The effects of the foundation pit’s width, the distance between the retaining wall and the impervious layer, and anisotropic seepage conditions on the total head are studied through parameter research. The study shows that the flow behavior of a foundation pit is sensitive to parameters such as the anisotropy of the soil layer and the width of the foundation pit. Further, the study also analyzes the influence of the above parameters on the exit gradient and proposes a simplified algorithm for the exit hydraulic gradient at the base of a foundation pit, which can control the error within 5%. This method makes a certain contribution to improving seepage calculations for foundation pits and is applicable to the seepage problem of anisotropic soil layers.

Experimental study on stress and deformation characteristics of foundation pit considering excavation width using 3D printing technology

A sophisticated model of the foundation pit support structure was developed via a model test that incorporated 3D printing technology. A meticulously scaled-down simulation of foundation pit excavation was conducted, utilizing the excavation width of the foundation pit as the sole variable, to evaluate and compare the impact of various foundation pit widths on the force and deformation characteristics of the foundation pit following layer-by-layer excavation. The findings indicate that the stress and deformation characteristics of the retaining structure shift from the “cantilever” mode to a composite mode of “internally convex” or “concave-convex” as the pit is excavated and internal support is installed. The change in pit width has minimal influence on the mechanical response law of the support structure during excavation. Nonetheless, the alteration in the pit width had a direct effect on both the internal force and deformation of the support structure, as well as the change in surface settlement value. In the experiment, wider pits had monitoring values that increased multiple times compared to narrower pits. The width of the foundation pit has a considerable impact on the mechanical response and stability of the pit, which is evident in the relatively lower lateral earth pressure on the outside of the retaining structure in narrower pits. This results in less stress and deformation of the pit structure. Moreover, the decrease in the passive zone of the pit contributes to better stability, indicating that the lower lateral earth pressure on the outside of the retaining structure is an innate reason for the excellent stability of narrower pits.

Determination of Passive Earth Pressure on a Cantilever Retaining Wall in a Narrow Foundation Pit Based on Logarithmic Spiral Sliding Surface

The distribution of passive earth pressure on a cantilever flexible retaining wall in a narrow foundation pit is significantly affected by the adjacent retaining wall and the width of the foundation pit. The deformation mode of the cantilever retaining wall rotating around the displacement zero point makes the soil layer below the displacement zero point retain a nonlimit passive state. The functional relationship between soil mechanical parameters and horizontal displacement in the nonlimit state region, considering the influence of displacement, is established. A new combined failure mode of the soil mass in the passive zone of the narrow foundation pit is proposed, which is the logarithmic spiral fracture surface in the nonlimit state region and the broken line fracture surface in the limit-state region. On this basis, a passive earth pressure calculation model of a narrow foundation pit is proposed and solved by differential layer method. The solution of the passive earth pressure coefficient strictly meets the static equilibrium conditions in vertical and horizontal directions as well as the moment equilibrium conditions. A numerical example is used to analyze the influence of the width of the foundation pit. The smaller the narrow foundation pit width, the greater the passive earth pressure in the lower soil layer and it even greatly exceeds the Coulomb earth pressure distribution. The earth pressure distribution acting on the lower part of the wall near the zero point of displacement decreases sharply and bends. The passive earth pressure distribution is calculated by the theoretical method and compared with the laboratory model test results as well as the engineering test results. It is found that the proper selection of the initial soil mechanical parameters affects significantly the magnitude and distribution of lateral pressure in the nonlimit state region. When the internal friction angle of soil φ0 is small, the lateral pressure in the lower nonlimit state area is relatively small. When both φ0 and δ0 are small, the pressure distribution is closer to the test results and the existing theoretical calculation results.

Influence Law of Foundation Pit Excavation on Stress of Surrounding Tunnel Bolt

Due to the existence of a tunnel next to the foundation pit, the soil surrounding the foundation pit deforms into the foundation pit due to the excavation unloading during the process of foundation pit excavation, which causes the deformation of the tunnel. The supporting structure of the tunnel can ensure the deformation stability of the tunnel, so it is necessary to study the stress of the tunnel bolt supporting structure caused by the excavation of the foundation pit. In this paper, the numerical simulation method was adopted to study the influence of the distance between the foundation pit and tunnel and the width and depth of foundation pit excavation on the stress of the tunnel bolt, and the following results were obtained: (1) When the distance between the tunnel and the foundation pit changes, the axial force of the bolt changes accordingly. The axial force of the left bolt first increases and then decreases, and the largest axial force is located in the middle of the bolt. (2) With the increase in the excavation width of the foundation pit, the stress of the left bolt does not change much. For the left bolt, with the increase in excavation depth, the stress of the left bolt does not increase monotonically but shows a trend of increasing first and then decreasing. When the excavation depth exceeds a certain value, the influence of excavation depth on lateral soil displacement gradually decreases. With the increase in excavation depth, the axial force of the top bolt decreases first and then increases.

Study on Key Parameters for Jet Impacting Pulverized Coal Deposited in Coal-Bed Methane Wells

Cleaning out the pulverized coal deposited at the bottom of a coalbed methane (CBM) well is key to achieving continuous CBM drainage and prolonging the workover period. In this study, Fluent is used in conjunction with the standard k-ε model and the Eulerian-Eulerian model to simulate and analyse jet erosion of deposited pulverized coal particles. The depth and width of the stable erosion pit that is formed by jet-impacting deposited pulverized coal under different conditions are determined and provide a theoretical basis for the cleanout of pulverized coal in the bottom of a CBM well. In this paper, the three parameters of the jet target distance, nozzle diameter and nozzle outlet flow velocity are selected to perform an orthogonal simulation. The change trends in the depth and width of the scouring pit with time are determined. The results show that jet impacting of deposited pulverized coal can be categorised into four stages, periods of rapid growth, stability, jet swing and dynamic stability. A sensitivity analysis shows that the nozzle outlet flow velocity has the strongest influence on the depth of the scouring pit among the selected parameters. The depth of the jet impact pit can reach the maximum depth at t = 3 s, while the width of the impact pit can reach the maximum after t = 7 s. This can provide key design parameters for CBM well pulverized coal impacting operation. It is of great significance for capacity damage control during CBM well workover operation.

Interactions of Foundation Pit on the Underlying Adjacent Existing Underground Structures

The influence of foundation pit excavation on deformation and stability of the supporting structure of underlying adjacent existing tunnel is a comprehensive geotechnical engineering problem, including unloading effect of foundation pit excavation and the coupling effect of deformation of the tunnel surrounding rock and supporting structure. Using numerical calculation methods, this paper changes the different excavation depths and widths of foundation pit and discusses the evolution law about force and deformation of tunnel supporting structure, and the changing relationship of the axial force of tunnel anchor with the distance between foundation pit and tunnel is obtained and compares the influence of different excavation way of foundation pit on the axial force of anchor rod near the existing tunnel. It is found that with the increase of distance between foundation pit and tunnel, the axial force of anchor rod at the top of tunnel under the foundation pit entirely symbolizes a trend of increases first and then decreases, the axial force of the leftmost anchor rod changes from pressure to tension, then decreases gradually. When the foundation pit depth increases, the axial force of the anchor rod at the top of the tunnel increases, the axial force of the leftmost anchor rod decreases, and the width of foundation pit absolutely has little effect on the axial force of the leftmost and the top tunnel.

Effect of laser cleaning on mechanical properties of laser lap welded joint of SUS310S stainless steel and 6061 aluminum alloy

Laser cleaning was successfully applied to laser lap welding of SUS310S stainless steel and 6061 aluminum alloy. The length and width of the fish scale pit tended to increase and decrease with increasing the scanning speed from 3000 mm/s to 7000 mm/s after laser cleaning. While, the depth of the fish scale pit was higher at the scanning speed of 5000 mm/s than the other. In the welded joint, except for metallurgical bonding, a relatively strong mechanical bonding occurred in the interface for the specimens cleaned at the scanning speeds of 3000 mm/s and 5000 mm/s during laser cleaning. The fish scale pit on the cleaned surface of stainless steel could improve the spread out of the melting aluminum alloy, causing the appearance of the relatively strong mechanical bonding. The maximum tensile shear force achieved 1948 N and increased 54% compared to the one without laser cleaning. The application of laser cleaning with the suitable parameters can effectively enhance the mechanical properties of the laser lap welded joints.

Design, construction and analysis for super-wide, deep and large foundation pit

Both developing and developed countries are facing a series of difficulties and challenges in the process of urbanization. In recent years, in order to alleviate the problem of urban congestion, underground space has developed rapidly, and the excavation of foundation pit is the most important step in the development of underground space. This paper takes the foundation pit of the tunnel under construction in Suzhou as a research object. The design width of the foundation pit reaches 61.5 m and the depth reaches 18 m, so it belongs to the super wide and deep foundation pit. Numerical analysis is performed by finite element software to calculate the deformation of the foundation pit. The research shows that the main problem to be solved is the deformation of the foundation pit, and the deformation of side wall of foundation pit tunnel is the most obvious. The maximum deformation of the side wall of the main tunnel and the auxiliary tunnel reached the maximum at 15 m. The maximum deformation of the main tunnel is about 1.3 cm, and that of the auxiliary tunnel is about 0.9 cm. Through targeted design and construction, the mechanical performance of the foundation pit retaining structure is optimized, and the stability of the foundation pit is strengthened. The reasonable retaining structure can ensure the good construction quality. The design and construction of the project can provide reference for related engineering construction.

Wall Displacement and Ground-Surface Settlement Caused by Pit-in-Pit Foundation Pit in Soft Clays

The number of pit-in-pit foundation pit is increasing quickly because of the continuous utilization of underground spaces in urban areas. Based on the co-construction project of Shanghai Museum of Natural History foundation pit and Shanghai Metro Line 13 foundation pit in Shanghai, China, the deformation characteristics of pit-in-pit foundation pit are researched by field observation and centrifugal model tests. The lateral wall displacement of inner foundation pit includes global deformation caused by the outer foundation pit excavation and deflection caused by the excavation of itself. The effect of inner foundation pit excavation on the lateral wall displacement of outer foundation pit and ground-surface settlement is smaller. The two factors affecting the deformation characteristics of pit-in-pit foundation pit, the distance between inner and outer foundation pits (D) and the excavation width of inner foundation pit (Win), are analyzed by centrifugal model tests. The result shows that the maximum lateral wall displacements of inner and outer foundation pits decrease nearly linearly with the increase of D, but increase with the increase of Win.

Analysis on corrosion fatigue cracking mechanism of 17-4PH blade of low pressure rotor of steam turbine

The causes of many blade fractures of a steam turbine in a power plant were studied by means of macro analysis, mechanical tests, metallographic examination, SEM and X-ray fluorescence spectrum analysis (XRF). The results show that the blade cracks due to corrosion fatigue. Cl −,K +react with the turbine blades in the steam environment in physical, chemical and electrochemical ways, causing local spot corrosion on the blades, forming corrosion pits. In addition, the steam condensated has an erosion effect on the blades, both of which form a corrosion fatigue source. The autocatalytic process of block cell is formed when Cl −, K + react with the turbine blades. Under the action of combined load, the current density i of activation dissolution of metal can be expressed as a function of complex stress state.Crack growth has an important relationship with stress, depth and width of corrosion pits. The blade cracks are determined by threshold nominal stress range for crack elongationΔσth, which depends on the depth and width of corrosion pit. The combined alternating stress directly promotes the crack propagation until the fracture fail under the external conditions of alternating stress formed by tensile force, bending force, torsion force, and exciting force.

Base Instability Triggered by Hydraulic Uplift of Pit-in-Pit Braced Excavations in Soft Clay Overlying a Confined Aquifer

The aim of this study is to investigate the coupling effects of re-excavation and hydraulic uplift on base instability of pit-in-pit (PIP) braced excavations. The numerical model of PIP braced excavation in Shanghai soft clay overlying a confined aquifer was established by upper-bound finite element limit analysis (UBFELA) method. The effects of the sensitive design parameters (i.e., the artesian pressure, thickness and undrained shear strength of the aquitard and excavation width of inner pit) on failure mechanisms and upper-bound safety factor (FS) against hydraulic uplift were analyzed. The results show that the value of FS increases with an increase in the thickness and undrained shear strength of the aquitard, but decreases with increasing the artesian pressure and excavation width of inner pit. The failure modes can be typically classified into three categories: circular slip surface in outer pit (M1), hydraulic uplift combined with circular slip surface in entire PIP system (M2), and basal hydraulic uplift in inner pit (M3); then the corresponding critical artesian pressure is determined. Finally, the average value of critical artesian pressure used to distinguish the three types of failure modes is recommended as the design value against hydraulic uplift in the PIP system, and validity is verified by the comparison with the current design methods. The proposed stability design by UBFELA contributes to ensure the serviceability and performance of PIP system.

Experimental study on enhancement of defect detection and defect size estimation in deep groove ball bearing

Rolling elements bearings are widely used in rotating equipment and machines to support load and to reduce friction. The presence of micron sized defects on the mating surfaces of the bearing components can lead to failure through passage of time. The large size defects on bearing elements can be detected/identified by time domain and frequency domain analysis of its vibration signals. However, it becomes difficult to detect local bearing defects at their initial stage either due to their smaller size or presence of noise. In the present experimental study, detection of local defects like crack and pits on bearing races have been carried out. Vibrations generated by healthy bearing and bearing having circular or rectangular defect on either race of bearing have been analyzed using MATLAB software. Signal to noise ratio of vibration signal has been enhanced through self- adaptive noise cancellation. Moreover, width of rectangular defects and diameter of circular defects have been estimated through ‘db5’ wavelet. The estimated defect sizes have been compared and validated through measured actual crack width or pit diameter. Accuracy of results proves that wavelet analysis of time domain signal can be used with confidence to estimate the width of fatigue crack and pit of the bearing races.

Experimental Investigation of the Formation of Rockburst Pits in Circular Tunnel Based on CT Scanning

The rockburst simulation experiments of granite samples with a circular hole under biaxial loading were conducted, and the samples were scanned by computed tomography (CT) at the end of experiment. Through a series of CT images, the failure characteristics of the circular hole wall were analysed to determine the types of rockburst pits, and the detailed formation process of different types of rockburst pits was studied in combination with the crack characteristics around them. The experimental results indicate that there are mainly two types of rockburst pits, namely, pan‐shaped and V‐shaped, which can occur symmetrically or asymmetrically on the left and right sidewall of the circular hole. The formation of rockburst pits is related to the cracks parallel and perpendicular to the principal stress. Cracks parallel to the principal stress can determine the depth of the rockburst pit and affect the type of the rockburst pit, and cracks perpendicular to the principal stress determine the width of the rockburst pit. There is a correlation between the formation process of pan and V‐shaped rockburst pits. During the formation of a V‐shaped rockburst pit, several rockbursts occur, and each rockburst forms a pan‐shaped rockburst pit. In the process of developing from the tunnel wall to the deep rock, the width of the pan‐shaped rockburst pit gradually decreases and a V‐shaped rockburst with a stepped upper and lower boundary is formed.

Finite element analysis on mechanical behavior of semi-exposed pipeline subjected to debris flows

Before the debris flow occurs, the continuous rainfall may cause the pipeline to be partially exposed (semi-exposed pipeline). The semi-exposed pipeline will be under great threat when the debris flow comes. To study the mechanical behavior of semi-exposed pipeline subjected to the debris flow, a finite element model of interaction between soil and semi-exposed pipeline under the impact of debris flow was established for the practical working conditions of semi-exposed pipeline. Effects of relevant parameters were analyzed, including the velocity of debris flow, impact angle of debris flow, massive stone, and parameters of corrosion pit (i.e. the depth, length, and width of corrosion pit). The results show that the velocity and the impact angle of debris flow and massive stone in the debris flow have a significant influence on the stress and deformation of the pipeline, but the exposed length of pipeline has little impact. Non-pressure pipeline can withstand the greater velocity of debris flow. In addition, parameters of corrosion pit (depth, length, and width) all have some impact on pipeline stress. Based on the result of multiple regression analysis, the effects of relevant parameters of corrosion pit on the maximum Von Mises stress of pipeline are ranked as follows: corrosion depth (A) > corrosion width(B) > corrosion length (L). People can propose an effective protection strategy for the pipeline subjected to debris flow based on the research results.

Detection and characterisation of optic nerve and retinal changes in primary congenital glaucoma using hand-held optical coherence tomography

ObjectiveTo investigate (1) the feasibility of scanning the optic nerve (ON) and central retina with hand-held optical coherence tomography (HH-OCT) without sedation or anaesthesia in primary congenital glaucoma (PCG), (2)...

Surface subsidence of pit-in-pit foundation in sand–cobble stratum in Beijing area

A ‘pit in pit’ is a common problem in the excavation of foundation pits and yet this issue has not been researched systematically. The variability of its form not only complicates the condition of the surrounding stress and displacement fields, but also brings difficulty to design and construction. This paper analyses the surface subsidence features of pit-in-pit foundations based on the typical sand–cobble stratum in the Beijing area by applying finite-difference software. According to a comparison of three factors that affect the surface subsidence the most, a conclusion can be drawn that the depth of the inner pit is greater than the edge distance (from the inner pit to the wall of the outer pit), which is greater than the width of the inner pit. The application of stratum compensation theory to pit-in-pit engineering is discussed to analyse the relationship between the surface deformation and lateral displacement of retaining walls. The cause of inaccuracy is also discussed, although this was found to be within acceptable limits. The research findings provide guidance for engineering inspections and control during construction in pit-in-pit foundations in sand–cobble and other similar strata.

Study on Initiation and Growth of Fatigue Crack of Pre-corroded 7B04 Aluminium Alloy Specimen

The purpose is to study the fatigue crack initiation and propagation behavior of aluminum alloy under pre corrosive damage. The high strength aluminum alloy 7B04-T74 with main bearing structure of aircraft was used to prepare single edge notched specimens for pre corrosion and fatigue tests, and the effects of corrosion damage on fatigue crack initiation and propagation are analyzed. It is found that the fatigue crack sprout at the bottom of a single corrosion pit at a low corrosion period, and the length of the crack is equivalent to the width of the corrosion pit, about 0.047mm, and the crack propagates in a straight line. At high corrosion stage, cracks occur at multiple corrosion pits, and the crack propagation paths become tortuous, resulting in glyph cracks.

Finite Element Method Simulations to Study Factors Affecting Buried Pipeline Subjected to Debris Flow

As the debris flow caused by sustained rainfall would cause destructive damage to buried pipeline, the safety of buried pipeline under impact of debris flow draws increasing attention. This paper focuses on the mechanical and deformed behavior of buried pipeline subjected to the debris flow. The effects of relevant parameters are investigated, including the velocity and impact angle of debris flow, massive stone, diameter to thickness ratio of pipeline (D/T), and parameters of corrosion pit (i.e., the depth, length, and width of corrosion pit). A finite model of soil and buried pipeline under the impact of debris flow is established. Multiple regression analysis is implemented to evaluate these influence parameters. The results show that: (1) the velocity and the impact angle of debris flow have a great influence on the pipeline; (2) the massive stone in the debris flow has little effect on the buried pipeline; (3) the internal pressure of the pipeline has an inhibitory effect on the deformation of the pipeline, which can enhance the ultimate bearing velocity of pipeline; (4) D/T determines the ultimate bearing velocity of pipeline. Moreover, the effects of the parameters of corrosion pit on the maximum von Mises stress are analyzed by multiple regression and ranked as follows: corrosion depth (A) > corrosion length (L) > corrosion width (B). The result may provide effective guidance for the prevention of pipeline against debris flow in mountain area.

Wall Deflection and Ground Surface Settlement due to Excavation Width and Foundation Pit Classification

Foundation pits have obvious spatial effect, and the lateral wall deflection, ground movement and basal heave stability are closely related with the excavation width. A database of 92 case histories with extensive excavation widths in soft soil in China are collected and analyzed. The maximum lateral wall deflections and maximum ground surface settlements measured are smaller than the case histories there before. With the increase of excavation width, the maximum lateral wall deflections and the maximum ground surface settlements increase firstly and then tend to stable. The database indicates that the type of retaining structure has little relevance to the excavation width of the foundation pit. There are two types of basal heave according to the excavation width, single-peak and double-peak. Three types of foundation pit considering excavation width, named wide foundation pit, normal foundation pit and narrow foundation pit, are proposed based on the slip circle basal heave analysis approach. The classification method is consistent with the measured data and the simulated results.

Ultimate Strength of Pit Corrosion Damnification on Pressure-Resistant Shells of Underwater Glider

The ultimate strength of the pressure-resistant shells is degraded due to corrosion pit on the surface of the shells. The underwater glider is prone to pit corrosion damage after working in the water for a long time. This study is aimed at development of an assessing formula for ultimate strength of pressure-resistant shells with pit damage. Firstly, a parameterized geometry model of the pit is determined under the assumption that the pits are elliptical. Secondly, a finite element numerical simulation model is established and the numerical simulation results are analyzed to find that the effect of pit damage on the pressure-resistant shell is obvious. Thirdly, the influences of some parameters (relative length, relative width, and relative depth) of pit on the ultimate strength are studied. The regular curve of the influence of geometric parameters on ultimate strength is drawn. Lastly, the ultimate strength assessment formula of pressure-resistant shells was obtained from the data by nonlinear FEM based on the regression function of multiple nonlinear regression analysis by nonlinear regression analysis function regress which can provide the foundation to assess the ultimate strength of damaged pressure-resistant shells.