Search Results (8)

Conventional trenchless pipeline rehabilitation technologies are primarily designed for circular pipelines, with limited applicability to box culvert structures. Even when adapted, these methods often lead to significant reductions in the effective cross-sectional area and fail to enhance the structural load-bearing capacity due to geometric incompatibilities. To overcome these limitations, this study proposes a novel construction approach that employs prefabricated high-strength thin concrete segment liners for the reinforcement of underground box culverts. The feasibility of this method was validated through full-scale (1:1) experimental construction in a purpose-built test culvert, demonstrating rapid and efficient installation. A static stacking load test was subsequently conducted on the reinforced upper section of the culvert. Results indicate that the proposed reinforcement method effectively restores structural integrity and satisfies load-bearing and serviceability requirements, even after removal of the original roof slab. Additionally, a finite element analysis was performed to simulate the stacking load test conditions. The simulation revealed that variations in the mechanical properties of the grout between the existing structure and the new lining had minimal impact on the internal force distribution and deformation behavior of the prefabricated segments. The top segment consistently exhibited semi-rigid fixation behavior. This study offers a promising strategy for the rehabilitation of urban underground box culverts, achieving structural performance recovery while minimizing traffic disruption and enhancing construction efficiency. Full article

Controlled low-strength materials (CLSMs) have been developed using various byproducts for backfilling or void-filling around pipelines or culvert boxes. However, these CLSMs have encountered issues related to their inadequate placement around underground facilities, despite satisfying the performance requirements, especially flowability, recommended by the American Concrete Institute (ACI) 229 committee. In this study, a new CLSM is developed to ensure a significantly higher flowability, lower segregation, and faster installation compared with previously developed CLSMs. This is achieved through a series of laboratory tests. To enhance the flowability and prevent segregation, a calcium-sulfoaluminate-based binder and fly ash are used in combination with two types of additives. The measured flowability of the new CLSM is 700 mm, while its compressive strength and bleeding satisfy the general criteria specified by the ACI 229R-13. In addition, the performance of the developed CLSM is compared with that of predeveloped CLSMs. The new CLSM was not only shown to exhibit the highest flowability, but also to satisfy the specified requirements for compressive strength and bleeding. Overall, it is anticipated that the developed CLSM can significantly reduce the costs related to the disposal of old pavements, the installation of new pavements, and other construction expenses compared to the costs related to the conventional method, even though the expenses for the backfill materials could increase due to the higher production costs of CLSMs than soil. In addition, there is a need to investigate its field applicability in order to evaluate the precise costs, maintenance, and long-term stabilities after installation. Full article

Under weak geological conditions, soil deformation and surface settlement are the key factors affecting the success of shallow-buried rectangular excavation. To investigate this issue, an underpass of Zhongxiao East Road in Taipei City was used as a case study. The surface settlement and lateral deformation of an underground diaphragm wall caused by the excavation of a rectangular pedestrian underpass using the pipe-roof preconstruction method (PPM) were investigated by 3D finite element analysis. The numerical analysis results showed that the constructed numerical analysis model had considerable accuracy. The use of PPM combined with a box culvert structure to form a pedestrian underpass could effectively control the surface displacement above the box culvert. Under the condition of the same sectional area, the smaller the width of the pipe-roof structure, the more the impact on the ground surface was reduced. The maximum positive bending moment and maximum negative bending moment on the pipe roof produced by excavation at each stage were roughly inversely related to the height per the width of the cross-section of the pipe diaphragm structure. The results showed that the pipe-roof structure was suitable for underground excavation with shallow-buried depth in the soft soil of the Taipei Basin. Moreover, the shallow-buried box culvert was more sensitive to the subsidence caused by construction than the deep-buried box culvert. Full article

Crack control for slabs and beams in current design practices in Korea are based on the Frosch’s model, which is adapted in ACI 318. It is more difficult to have consistent quality control in underground construction sites, such as the RC box culverts used for electric power distribution built below the ground level. There are more discrepancies between the as-built dimensions and the design dimensions provided in drawings in these structures. Due to this variability in construction error, the crack widths measured in such structures have higher potential to have more differences than the calculated values. Although crack control is a serviceability concern, if the owner chooses to have a target crack width that needs better control, crack width estimations can be improved by considering such construction variability. The probability-based crack width model suggested in this study will allow minimizing the discrepancies between the measured and calculated crack widths and provide reliable estimations of crack widths. Typical size of slabs and beams ranging between 300 mm (12 in.) to 500 mm (20 in.) used in underground RC box culverts in Korea were tested under the four-point bending test program. The thicker specimens had smaller bar spacings which created more cracks with smaller crack widths. However, with smaller crack widths generated in these specimens, there were more errors between the measurements and calculated values. From site investigations in Korea, the thickness of slabs in underground box culverts varied the most among all parameters. As a result, the bottom concrete covers had the highest variability. Bottom concrete covers and bar spacings are the two most important parameters in concrete crack control. A probability-based crack width estimation model for flexural members was developed in this study to consider this construction variability. Monte Carlo simulations were performed to evaluate the probabilistic characteristics of the design surface crack widths with a target width of either 0.3 mm (12 mils) or 0.5 mm (20 mils). The probabilistic models of design variables included in the crack width estimation model were generated based on field-collected information from construction sites in Korea. Because the surface crack widths in RC flexural members are sensitive to the construction errors of concrete cover depths, and since there are differences between the assumed and actual stress distribution closer to the reinforcing bars, the probability of having surface cracks of 0.3 mm width (12 mils) is found to be quite high, such as 89% at the positive moment region (mid-span, bottom surface) of the top slab in RC box culverts and 45% for the negative moment region (support area, top surface) of the top slab with current design practice. In order to ensure crack widths to be smaller than the design target width, probability-based crack width factors are recommended in this study to improve estimations depending on the selected target reliability levels. Full article

Underground space is being utilized due to the saturation of surface ground. The box culvert, as a representative infrastructure that has moved underground, is installed to protect such fixtures as electricity and gas. Because buried box culverts are necessarily affected by soil, it is important to study the earth pressure according to soil type. Herein, the horizontal earth pressure of the buried box culvert was analyzed. Accordingly, a precisely simulated centrifuge model test was performed. Additionally, the coefficient of earth pressure was analyzed. The results had significant variability because, in the existing theory, the horizontal earth pressure acting on the side of the box culvert was only calculated using the coefficient of earth pressure and the friction angle of the soil. Therefore, a correction factor was deemed necessary for calculating the horizontal earth pressure acting on the side of the box culvert. Full article

Due to the lack of surface space, most structures are heading underground. The box culvert is underground infrastructure and serves to protect the buried structure from the underground environments, but it has a different characteristic from other structures in that the inner space is empty. Therefore, in this study, the vertical earth pressure which is the most significant effective stress acting on a box culvert was measured by conducting a geotechnical centrifuge model test. A box culvert was installed following the embankment installation method, and the vertical earth pressure acting on it was measured considering the cover depth, gravitational acceleration, and loading and unloading conditions. The soil pressure measured was greater than the existing theoretical value under high cover depth and the unloading condition, which is considered as the variability of many soils or the residual stress acting under the loading condition. Finally, a goodness-of-fit test was conducted as a part of variability analysis. The measured earth pressure was found to be considerably larger than the existing theoretical value, and the variability was large as well. This means the existing theoretical equation is under-designed, which should be reflected in future designs. Full article

This study was performed to evaluate the probabilistic characteristics of the flexural strength of reinforced concrete (RC) flexural members adopted for underground box culverts. These probabilistic models were developed to be adopted for the development of limit state load combination formats for underground RC box culverts. The probabilistic models of uncertainties inherent in the basic design variables were developed to evaluate flexural strength using field material test data as well as field survey data collected from various domestic construction sites of underground box culverts in Korea. The basic design variables include concrete strength, steel rebar strength, and section dimensions, such as slab thickness and rebar locations. Some design variables are assumed to have inherent construction error characteristics, which may be different from those inherent in the RC members for buildings and bridges. The bias models on flexural strength were evaluated based on the experimental results of four-point flexural tests on one-way RC slabs, which were fabricated following the general practice adopted in the local underground box culvert construction process. Based on the probabilistic models of basic design variables, as well as the bias models of flexural strength, Monte Carlo simulations were performed to examine the probabilistic characteristics of both ultimate flexural strength and yield moment strength of RC slab members. Some sensitivity analyses were performed to confirm the soundness of various probability models and the assumptions adopted in the development procedure. The proposed procedure may be applied to develop probabilistic resistance models for structural members, in which the construction error characteristics are assumed to be different from other practices. Full article

Tunnels, culverts, and subway stations are the main parts of an integrated infrastructure system. Most of them are constructed by the cut-and-cover method at shallow depths (mainly lower than 30 m) of soil deposits, where large-scale seismic ground deformation can occur with lower stiffness and strength of the soil. Therefore, the transverse racking deformation (one of the major seismic ground deformation) due to soil shear deformations should be included in the seismic design of underground structures using cost- and time-efficient methods that can achieve robustness of design and are easily understood by engineers. This paper aims to develop a simplified but comprehensive approach relating to vulnerability assessment in the form of fragility curves on a shallow two-story reinforced concrete underground box structure constructed in a highly-weathered soil. In addition, a comparison of the results of earthquakes per peak ground acceleration (PGA) is conducted to determine the effective and appropriate number for cost- and time-benefit analysis. The ground response acceleration method for buried structures (GRAMBS) is used to analyze the behavior of the structure subjected to transverse seismic loading under quasi-static conditions. Furthermore, the damage states that indicate the exceedance level of the structural strength capacity are described by the results of nonlinear static analyses (or so-called pushover analyses). The Latin hypercube sampling technique is employed to consider the uncertainties associated with the material properties and concrete cover owing to the variation in construction conditions. Finally, a large number of artificial ground shakings satisfying the design spectrum are generated in order to develop the seismic fragility curves based on the defined damage states. It is worth noting that the number of ground motions per PGA, which is equal to or larger than 20, is a reasonable value to perform a structural analysis that produces satisfactory fragility curves. Full article