Search

Peat is known as problematic ground with low bearing capacity and extensively high compressibility. Bio-cementation or commonly known as microbial-induced calcite precipitation (MICP) has been recently introduced as a ground improvement alternative for peat under waterlogged condition. Using isolated bacteria strains P19 and P21 from tropical peat, it is found that unconfined compression strength (UCS) increases with bacteria concentration at a reducing rate. A maximum unconfined compressive strength of 82.05 kPa was measured with bacteria strain P21 at 108 CFU/mL. For the range of cementation reagent varying from 0.1 to 4.0 mol/kg, the largest strength improvement occurred at 1 mol/kg and 2 mol/kg using indigenous bacteria and bacteria strain P21, respectively, for peat with sand content of 25%. At 4.0 mol/kg, the cementation reagent has detrimental effect to MICP resulting in significant reduction in strength. Due to MICP, the UCS of peat increases with sand content. Calcium carbonate precipitation results in a reduction of permeability and an increment of strength of peat–sand mixture under a submerged condition up to 28 days.

Tunnelling is one of the major construction methods to sustain the increasing demand on development in cities. Although many studies have been carried out to investigate the active failure mechanism of tunnel face in sand, the study of passive failure of tunnel face is relatively rare and most of studies are analytical solutions based on the upper bound theorem. In this paper, centrifuge model tests and three-dimensional finite element analyses have been conducted to study the passive failure mechanisms of tunnel face in sand for tunnels located at cover over diameter (C/D) ratios equal to 2.2 and 4.3. Passive failure pressures of tunnel face as well as ground surface displacements were investigated in centrifuge tests. From both centrifuge and numerical investigations, it is found that for a tunnel located at C/D ratio equals to 2.2, the soil in front of the tunnel face is displaced by the advancing tunnel face while the soil further away from the tunnel face is forced upwards to the ground surface. A funnel-type failure mechanism is observed and the extent of the failure mechanism is narrower than a five-block failure mechanism commonly assumed in an existing upper bound solution. However, the calculated passive failure pressure by the upper bound solution is fairly consistent with the measured face pressure. It is observed that the funnel-type failure mechanism induces surface heaves. Both observed longitudinal and transverse heaves are well described by Gaussian distributions. For a tunnel located at C/D ratio equals to 4.3, the displacements of soil are confined around the vicinity of an advancing tunnel face and a localised failure mechanism associated with ground settlement is observed and computed. There is a large discrepancy between the localised failure mechanism and the five-block failure mechanism. The calculated failure face pressure is higher than the corresponding measured value by 50%. However, reasonable consistency can be found between measured and computed passive face pressures.