Predictable and Unpredictable Uncertainties Delaying the Completion of the Kabatas-Mahmutbey Metro Construction (Istanbul / Turkey)
Ozcelik and F. Tuzlu
ABSTRACT: The design and construction of tunnels is among the most specialized fields in underground engineering. There are various uncertainties during tunnel excavation. Predictable and unpredictable uncertainties are important sources of risk in tunnel engineering. The effect of uncertainty on risk assessment and decision-making is therefore provided priority, particularly for tunnel projects where predictable uncertainty is often the primary cause of risk. First phase of the Kabatas-Mahmutbey tunnel excavation, some collapses occurred due to uncertainties in different parts of the tunnel route. Regardless, Kabatas-Mahmutbey metro line, which is 24.5 km long and consists of 19 stations, was planned to be operational in the first quarter of 2020. For this purpose, electrical and mechanical tests were successfully continuing within the scope of the project. In addition, the signalling works of the metro line were near to the end. All technical uncertainties and risks were thought to be circumvented. In the last phase of the project, Coronavirus Disease 2019 (COVID-19) was encountered as unpredictable uncertainty. Unfortunately, due to COVID-19, the work in the project has been postponed to a later date. With the commissioning of the metro project, which will breathe the traffic of Istanbul upon commissioning, it is foreseen to carry 500 thousand passengers a day.
KEYWORDS: Uncertainty, Tunnel, Construction, 3D Modelling, COVID-19.
Gellan Gum for Strengthening Bentonite-Sand Slurry
T.P.A. Tran, T. Katsumi, and T.N. Tran
ABSTRACT: The strengthening ability associated with adding a gellan gum as an additive to bentonite slurry for use in coating soil surfaces was studied. Bentonite powder was mixed with hot gellan gum hydrogel (i.e., at 200 °C) to different gellan gum concentrations (i.e., 0, 3, 4, 5, 6, and 7% of the mass of distilled water). A series of unconfined compressive strength tests were conducted on gellan gum-bentonite mixtures subjected to various thermal curing conditions. The shrinkage behavior was observed during the thermal curing process. The research also studied the sand size particle effect on the strength and volume change of the dried gellan gum-modified bentonite. The test results showed the soil strengthening effect of adding gellan gum to bentonite slurry which was then added it to bentonite/silica sand mixtures.
KEYWORDS: Gellan gum, Bentonite, Silica sand, Unconfined compressive strength, Thermal curing process.
Comparative Study on Strength and Permeability of Siliceous Sand Treated by MICP and Cement Grouting
Hai-lei Kou, Wang-xiang Hou, Peng-peng Ni, and Jia He
ABSTRACT: Cement and other chemical materials are widely used as additives in soil improvement. Microbially induced calcite precipitation technology has been used in soil improvement as the advantage of green and environmental protection. In this study, the potential of using microbially induced calcite precipitation (MICP) technology replaces cement for treating siliceous sand is presented. A series of laboratory tests were carried out to assess the unconfined compressive strength (UCS) and permeability of microbial-reinforced sand and cement-reinforced sand. The results indicated that, for the experimental siliceous sand with a small particle size (0.63-1.25 mm), the average UCS of the microbial-reinforced sand is significantly higher than that of the cement-reinforced sand under the condition of curing for 7 days, and the quality of cement-reinforced sand is affected by the water-cement ratio. The permeability coefficient of microbial-treated sand is also significantly reduced, which are 0.0007 times and 0.05 times that of pure sand and cement-reinforced (w/c = 2:1), respectively. The porosity reduction of the sample after microbial grouting is up to 13.6%, which is also significantly higher than cement-reinforced. The microstructure study shows that calcite crystals can not only be widely attached to the surface of sand particles, but also better penetrate into the voids between sand particles, formed more effective ways of connection and make more effective bonding. It explains why microbial-reinforced sand has higher unconfined shear strength and lower permeability coefficient than cement-reinforced sand.
KEYWORDS: Siliceous sands, MICP, Cement grouting, Unconfined compressive strength, Coefficient of permeability.
Predicting the UCS of Lime‒Stabilized Clayey Soils
B. Sari-Ahmed, Ghrici, and K. Harichane
ABSTRACT: This study was made in order to develop a statistical model for predicting the unconfined compressive strength (UCS) of lime-stabilized clayey soils. The obtained results showed that the developed model (UCS‒Model) is very efficient and can be used as a reliable tool for predicting the UCS of lime‒stabilized clayey soils. Indeed, both the (F-test) and (t-test) showed that the significance value of UCS‒Model was found to be less than 0.05 which indicates that the lime content and curing time significantly contribute to the constructed model and lead to a better prediction of UCS. In addition, the comparison study between predicted values and experimental data indicated that the UCS model can be reasonably applied to explain the effect of lime content below 10%, and curing time between 0 and 90 days on UCS of clayey soils and to design new mixtures without making an experimental study.
KEYWORDS Clayey soils, Lime, Unconfined compressive strength (UCS), Curing time, Statistical model.
Deterministic and Probabilistic Approach of Seismic Slope Stability Analysis – A State-of-The-Art Review
Partha Pratim Boruah and Arunav Chakraborty
ABSTRACT: For a slope to be safe, it must always have a factor of safety (FOS) against sliding greater than one. If the slope simply experiences elastic deformation, then only the above condition is applicable. However, in the event of a severe seismic motion, the slope deformation may overtake the elastic limit, resulting in permanent displacements. In such a scenario, the FOS considering static output is irrelevant and have to consider seismicity. As a result, the relevance of the stability of the soil slope under seismic loading conditions is growing in the field of geotechnical engineering. A lot of studies have investigated the stability of a slope under static and seismic stress conditions. This paper reviewed various studies where the seismic loading consideration is exits. The purpose of this paper is to know the existing soil slope stability techniques which utilize several dynamic methods which includes finite elements approach, limit equilibrium approach, strength reduction approach, discrete element approach, Newmark’s approaches etc. The deterministic approach and the probabilistic approach are used to conduct the literature review.
KEYWORDS: Seismic analysis, Dynamic analysis, Factor of safety, Slope stability.
