Pipe jacking is a commonly used trenchless technology to install pipelines especially in congested urban areas or river crossings. However, the estimation of the jacking force is often heavily ...dependent on empirical calculations. The jacking force needs to be greater than the combined frictional resistance and face resistance. This investigation proposes to use a modified Protodyakonov’s arch model to compute the face resistance. A series of direct shear tests is performed to provide data of interface friction coefficient between different types of soil and pipe. The influence of slurry lubricant is also considered. A two-dimensional plane strain numerical model is conducted, where the surrounding soil is simulated as discrete particles and the lining is simplified as a single big particle. The novel modeling technique enables the evaluation of the normal force acting on the pipe. The friction resistance is then determined by multiplying the interface friction coefficient by the normal force. A ‘wavy’ shaped pipeline model is proposed to define an angular deviation influence factor to scale up the calculated jacking force due to pipe misalignment. In the end, comparison between calculated and field measured jacking force is conducted for three different drives in a pipe jacking project to illustrate the effectiveness of the proposed analysis framework.
Microbially induced carbonate precipitation (MICP) has been utilized as a new method to improve loess soil strength. In this study, we investigated the influence of the main parameters on the shear ...strength of MICP-treated loess specimens. Initially, culture media with different formulas and pH values were examined to identify the most efficient medium for loess soil. To explore the shear behavior of MICP-treated loess under general stress levels, unconfined compressive strength (UCS) tests and triaxial tests relevant to the compression strength and vertical loads were performed on MICP-treated loess with different calcium sources, cementation concentrations, and curing periods. Subsequently, calcium chloride was selected as the optimal calcium source based on the ultimate strength of the MICP-treated loess. The effective cementation concentration in the loess soil was between 1.0 and 1.25 M. The ultimate strength of the MICP-treated loess was 3.6 times of the untreated loess. The stress-strain curves indicate that a higher cementing effect can be expected with an increase in the curing period. The formation process of calcium carbonate and the micromorphology of the MICP-treated loess samples were examined using scanning electron microscopy. In this study, we present an environmentally friendly technique for improving loess soil strength.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Saline soil is characterized by high concentrations of soluble salts, which can pose hazards such as frost heaving, uneven thaw settlement, and concrete erosion, especially subjected to freeze-thaw ...(F-T) cycles in the seasonal frozen and frost regions. This study aims at investigating the cement and micro-silica treatments effect on the changes in microstructure, strength property and durability of compacted saline soil, and evaluating the suitability of micro-silica as potential industrial waste materials for altering the mechanical behaviors of saline soil in comparison with the use of cement as traditional binder. Unconfined compression tests, oedometer tests, and freeze-thaw cycle tests were conducted on the untreated and treated saline soil stabilized by cement alone, micro-silica alone, and mixtures binders with cement-micro-silica (CMS). Furthermore, the microscopic characteristics were analyzed by scanning electron microscope and X-ray diffraction test. Tested results indicates that the cement is committed to improving the strength of saline soil, and the micro-silica contributes to improve the F-T cycle durability. The performance of CMS-treated saline soil with unconfined compressive strength of 1.763 MPa and coefficient of collapsibility of 0.056 is superior to that of cemented and micro-silica-treated saline soil alone. Besides, the stabilization with CMS achieved promising results with 3% cement and 4% micro-silica admixture, achieving the purpose of recycling micro-silica and exhibiting the excellent durability with strength loss of 50% due to F-T cycles. The formation of cementitious compounds originated from pozzolanic reactions between cement and micro-silica can effectively improve the behaviors of saline soil.
When heat energy is transferred to concrete through energy conversion by an electric heating system, the overall thermal performance of the early-age concrete is affected by the local high ...temperature of the concrete. This, in turn, affects the accuracy of the prediction and calculation of concrete deformation. Concrete thermal physical parameter models considering the mutual influence of heat and humidity were established to clarify the influence of an electric heat tracing system on the heat transfer performance of concrete at an early age, based on the concepts of equivalent age and hydration degree. Additionally, the COMSOL numerical simulation software was used for realizing the numerical solution of concrete heat transfer. The research indicates that the degree of hydration is affected by the heating provided by the electric heat tracing system. When curing for 40 h, the degree of hydration approaches 0.82 and remains unchanged, which indicates that the hydration of cement was almost complete in less than 2 days. The specific heat value of concrete in the early stage was significantly affected by electric heat tracing. This value of concrete at a high initial temperature was larger than that at a low temperature, while the specific heat of each point in the later stage tended to be the same. The thermal conductivity was significantly affected by the local electrical heating. The higher the temperature was, the lower the thermal conductivity was, which remained stable for two days under the influence of temperature. The key contribution of this research is to provide a coupling model for concrete curing. The findings from the study also provide industry practitioners with a comprehensive guide regarding the specific applications of the electric heating system in early-age concrete curing.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
As the composite pile, the precast concrete piles reinforced with cement-treated soil (PCCS) is formed by driving the precast cement (PC) pile into the deep mixing (DM) column, which has been ...successfully and widely utilized to support buildings and embankments over soft soil. To increase the pile spacing and give full play to the economic merits of the PCCS, a reinforcement scheme, which involves the combined use of rigid piles and flexible columns, employing penetrated PCCSs and floating DM columns is proposed and utilized for soft soil ground treatment. However, there is a lack of feasible method for consolidation behaviors of this combined composite foundation (CCF) reinforced with penetrated PCCSs and floating DM columns under flexible loads. This paper developed an analytical solution to predict the average consolidation degree of this CCF based on a cylinder consolidation model and double-layer ground consolidation theory. The excess pore pressure and average consolidation degree were calculated by considering the composite pile penetration into the cushion. The analytical method agrees well with results obtained by numerical analysis. Additionally, a parametric study was conducted systematically to analyze the effect of key influence factors on the average consolidation degree of this CCF. The results indicate that the consolidation rate of this CCF can be much faster than that of the natural ground. The consolidation rate strongly depends on the compressive modulus and area replacement ratio of PCCSs. The increasing inner core-outer core modulus ratio and the inner core-subsoil modulus ratio increase the consolidation rate of this CCF. In addition, the consolidation rate increases with the gravel cushion-subsoil modulus ratio, while it decreases with the loading period.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The existing Beijing Pingguoyuan Subway Station was extended through a extension project. The excavation for the extension was located directly above the existing station. Complex interactions exist ...between the existing structure and the retaining pile wall of the excavation. Based on this project, three-dimensional finite element models were established to investigate the mechanical characteristics of the embedded and non-embedded retaining pile walls. A parametric analysis was performed for both types of pile walls. The stress and deformation characteristics of the retaining pile walls and existing structures were analyzed. The results show that when the bottom of the non-embedded retaining pile walls are connected to the existing structure, the uplift of the existing structure is essentially constant; however, the maximum displacement of the pile is increased by approximately 2.7 times, and the bending moment of the pile is reduced to 57.1% of the connection condition. As the distance between the embedded retaining pile wall and the existing station increases, the uplift of the existing station increases linearly, whereas the soil between the pile and the station exhibits a non-linear increasing trend. The displacement of the embedded retaining pile wall increases as the inner force decreases. When the distance is greater than 4.7 m, the displacement and force of the pile remains essentially unchanged. The effect of the pile embedded depth on the force and deformation of the pile is mainly observed in the lower part of the pile. As the embedded depth increases, the maximum displacement decreases by approximately 16.9%, the maximum bending moment decreases, and the maximum negative bending moment increases. The key contribution of this research is to provide a prediction method for the mechanical behaviors of a expansion project. The findings from the study also provide industry practitioners with a comprehensive guide regarding the specific applications of the construction technology of a deep excavation structure overlying an existing subway station.
With the development of urban underground space and increased infrastructure functions, both the scale of engineering construction and engineering difficulties have increased globally. In the ...construction of structures in soft strata, especially in coastal areas, the limited bearing capacity of the foundations poses a significant challenge. The composite pile technologies employing an organic combination of the rigid pile andthe flexible column can enable efficient soft ground treatment. In light of prominent global environmental issues, low-carbon energy-saving curing technologies have been rapidly developed for application in geotechnical engineering. This paper discusses progress in research on the mechanical properties of the efficient and low-carbon pile technologies, including the stiffened deep mixing (SDM) column, squeezed branch pile, pre-bored grouting plated nodular (PGPN) pile, precast cement pile reinforced by cemented soil with a variable section (PCCV), and carbonized composite pile (CCP). In addition, it reviews the technical characteristics and recent progress of feasible low-carbon energy-efficient curing technologies. The paper also proposes future directions for theoretical research and technological development of low-carbon pile technologies. The key contribution of this review is to provide insights into efficient and low-carbon pile technologies. In addition, the findings from the study of the pile technologies used in extra-thick soft strata also provide industry practitioners with a comprehensive guide regarding the specific applications and mechanical performance of the pile technologies, which can serve as a stepping stone to facilitate the technological development of the underground space industry.
This study presents a novel construction pre-supporting system for large underground space excavation with shallow depth, Steel Tube Slab system (STS), in which adjacent steel pipes are connected by ...a couple of flanges, bolts and concrete for flexural capacity and lateral stiffness of the whole structure. The STS method is employed for the first time for the construction of the ultra-shallow buried and large span subway station in China, during which ground settlement and structural deformation are monitored. A numerical model for the subway station is established with reliability verified by monitored data comparison from numerical results and investigation on the effect of large span underground excavation on surrounding soil surrounding soil and existing buildings in soft soils. Unlike traditional methods, the STS method can effectively control and reduce the ground settlement during construction, thereby rendering it ideally suited for application in soft soils.
To effectively mobilize concrete strength, a newly developed composite pile, the so-called “Precast Concrete Piles Reinforced with Cement-treated soil (PCCS)”, has been used to improve the bearing ...capacity of soft ground. However, there are limited studies on the consolidation behavior of the PCCS in soft ground. This paper proposes an analytical solution for the consolidation of the composite ground reinforced by PCCSs based on a modified equal strain assumption. This composite ground is simplified as a double-layered consolidation ground according to the geometrical configuration of the PCCS. Subsequently, the analytical expression of the average consolidation degree is derived by considering the pile pierced into the gravel cushion and the additional stress that varies with time and depth. The accuracy of the proposed solution for consolidation is compared and validated by numerical analysis and current calculation methods. Furthermore, a parametric analysis is conducted to systematically investigate the influence factors of replacement ratio, core length ratio, modulus ratio of inner core-soil, modulus ratio of inner-outer core, modulus ratio of cushion-soil, permeability coefficient ratio and loading period on the consolidation behaviors of PCCSs reinforced ground. The proposed solution provides a theoretical guideline available for calculating the long-term settlements of such composite piled foundation.
Cyclic loads resulting from environmental factors such as wind, waves, trains, and construction activities pose a potential hazard to the stability of piled structures. These loads can cause the ...performance degradation of piles, including cumulative settlement and weakening of bearing capacity. This paper presents a semi-analytical solution for evaluating the axial cyclic behavior of jacked piles in structured clays. The solution employs enhanced nonlinear load-transfer models that effectively capture the stress-strain hysteresis and interfacial cyclic degradation characteristics. The key factors influencing pile behaviors are incorporated within the analytical approach, encompassing pile installation, subsequent soil equalization, and the static and cyclic loading encountered during service. The validity of the current solution is established through model pile tests conducted in artificially structured clays under static and cyclic loading conditions. Extensive parameter studies are performed using the proven theoretical approach to emphasize the influence of soil structure and cyclic loading on pile behavior. The results demonstrate a favorable agreement between the theoretical and measured values in static and cyclic pile response analyses. Compared with the response of jacked piles in reconstituted clays, the pile response in structured clays with a cement content of Aw = 4%, including penetration resistance, final ultimate bearing capacity, and the convergence speed of permanent settlement, has been substantially improved. The parameter studies indicate that soil structure and cyclic loading patterns significantly influence the axial cyclic performance of jacked piles in natural clays.
•A semi-analytical solution was presented to evaluate the axial cyclic behavior of jacked piles in structured clays.•Two load-transfer models were formulated to describe the stress-strain hysteresis and cyclic degradation features.•The effects of pile installation, soil equalization, and cyclic loading were incorporated into this analytical framework.•A series of model tests were performed in artificially structured clays to validate the present solution.
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