Taeseo Ku

Cement-treated ground often possesses significant spatial variation in strength and stiffness. Studies on the detection and stiffness measurement of weak zones in cement-treated ground using seismic geophysical methods remain limited to date. The work examines the feasibility of using seismic travel time tomography to detect and measure the stiffness of weak zones in cement-treated ground, through 1-g modelling of a cross-hole setup with weak zones of prescribed sizes and stiffnesses. Using bender elements as transmitters and receivers, shear wave velocity profiles across the weak zones are mapped. Sizes, locations, and stiffnesses of the weak inclusions are also inferred from shear wave travel times using GeoTomCG. The results indicate that although the sizes and locations of weak zones can be reliably detected using first-arrival time-picking, the stiffnesses is significantly over-estimated. The latter is due to the arrival of the diffracted waves around the weak zone masking the arrival of the transmitted waves. A modified time-picking method, using the wavelet transform, is developed and is shown to give more reliable stiffness measurements of the weak zone, but not the shape. Combining direct and wavelet time-picking allows the sizes, shapes, locations, and stiffnesses of the weak zones to be more reliably resolved.

In densely populated countries, underground construction and land reclamation could be possible options to solve the demand for land space, thus securing sustainable long-term development of the nation. For example, in Singapore, land reclamation has been widely conducted using excavated materials from underground development. The excavated materials are commonly marine clays that contain sandy soils. To improve the mechanical properties of these soft soils, cement-treated soil stabilization is popularly adopted. In fact, many researchers have investigated the properties of pure cemented clay or pure cemented sand using conventional design parameters such as water content (water/solids) and cement content (cement/dry soil). However, can these terminologies be still used to accurately examine the role of sand in cemented sandy clay mixtures? Through unconfined compression testing, it is herein shown that the use of existing mix design approaches in the literature cannot properly explain the variation of strength with sand content for cemented sandy clay mixtures. A new mix design approach is thus proposed in this study, which ensures that the role of sand in a cemented clay matrix can be quantified.

Understanding the variation of geological interfaces plays a crucial role in the analysis and design of infrastructure systems. Generally, there are two classes of techniques for predicting geological interfaces, for example, interpolation/regression-based techniques and machine-learning-based techniques. In this paper, a Multi-scale Meta-learning Model (M³) methodology is proposed. The new methodology improves the current state-of-the-art techniques by fusing two levels of information: (i) generic characteristics of the sampling locations, for example, coordinates, and (ii) location-specific characteristics, for example, local-scale predictions. The implementation starts from using an array of classic interpolation/regression-based techniques as base learners to provide first-level predictions at a local scale. These predictions are then combined with generic characteristics to train a meta-learner following the stacking ensemble learning framework. In this manner, the location-specific information from the base learners can be simultaneously considered with the generic information in the training process. The variation of rockhead elevation is predicted using the M³ methodology and a comprehensive borehole dataset in Singapore. A detailed comparative study involving several existing methods is also carried out to rigorously validate the M³ methodology. The results show that the M³ methodology achieves 20% improvement in the model performance compared to existing methods, indicating its promising potential in geotechnical site characterization.