Chaminda Gallage

Effects of wetting and drying on the unsaturated shear strength of a silty sand under low suction

Shear strength properties of unsaturated soils are important in evaluating the stability of geotechnical structures such as natural slopes, embankments, retaining walls, excavations, and footings. This importance is more in countries, where intense rainfalls cause the instability of natural and man-made slopes. An unsaturated soil behaves differently at wetting and drying and this difference in behavior is referred to as hysteresis. Hysteresis is often exhibited in soil-water characteristic curves of unsaturated soils. The hysteresis in the shear strength behavior of unsaturated soil under wetting and drying has not been fully investigated. Shear strength tests on unsaturated soils have usually been conducted following the drying process. For example, in a multi-stage shear test, the soil is subjected to increasing matric suctions at subsequent stages. Very few studies have been conducted on the unsaturated shear strength parameters on wetting process by which slopes become more unstable. Re-compacted soil specimens of silt-sand were tested in a modified triaxial apparatus to examine the effects of wetting and drying on shear strength. Identical soil specimens were tested at wetting and drying under controlled air and water pressures. The study was conducted for low suction range, (i.e. 0–50 kPa). The soil at the drying had lower shear strength as compared to the soil at the wetting under the same matric suction. Furthermore, the study was extended to investigate the effects of suction and the hysteresis of SWCC on shear strength parameters such as effective friction angle and the apparent cohesion, c. Results exhibited no significant effects of suction and the hysteresis of SWCC on the effective friction angle. It was observed that the apparent cohesion at the wetting was higher than that was obtained at the drying under the same suction. The c increases as the suction increase with the decreasing rate at wetting.

Direct Shear Testing on Unsaturated Silty Soils to Investigate the Effects of Drying and Wetting on Shear Strength Parameters at Low Suction

AbstractA modified conventional direct shear device was used to measure unsaturated shear strength of two silty soils at low suction values (∼0–50  kPa) that were achieved by following drying and wetting paths of soil-water characteristic curves (SWCCs). The results revealed that the internal friction angle of the soils was not significantly affected by either the suction or the drying-wetting SWCCs. The apparent cohesion of soil increased with a decreasing rate as suction increased. Shear stress–shear displacement curves obtained from soil specimens subjected to the same net normal stress and different suction values showed a higher initial stiffness and a greater peak stress as suction increased. A soil in wetting exhibited slightly higher peak shear stress and more contractive volume change behavior than that of soil in drying at the same net normal stress and suction.

Performance of Buried Tunnels Subjected to Surface Blast Incorporating Fluid-Structure Interaction

This paper uses finite element techniques to investigate the performance of buried tunnels subjected to surface blasts incorporating fully coupled Fluid Structure Interaction and appropriate material models which simulate strain rate effects. Modelling techniques are first validated against existing experimental results and then used to treat the blast induced shock wave propagation and tunnel response in dry and saturated sands. Results show that the tunnel buried in saturated sand responds earlier than that in dry sand. Tunnel deformations decrease with distance from explosive in both sands, as expected. In the vicinity of the explosive, the tunnel buried in saturated sand suffered permanent deformation in both axial and circumferential directions, whereas the tunnel buried in dry sand recovered from most of the axial deformation. Overall, response of the tunnel in saturated sand is more severe for a given blast event and shows the detrimental effect of pore water on the blast response of buried tunnels. The validated modelling techniques developed in this paper can be used to investigate the blast response of tunnels buried in dry and saturated sands.

Response of segmented bored transit tunnels to surface blast

Increasing threat of terrorism highlights the importance of enhancing the resilience of underground tunnels to all hazards. This paper develops, applies and compares the Arbitrary Lagrangian Eulerian (ALE) and Smooth Particle Hydrodynamics (SPH) techniques to treat the response of buried tunnels to surface explosions. The results and outcomes of the two techniques were compared, along with results from existing test data. The comparison shows that the ALE technique is a better method for describing the tunnel response for above ground explosion with regards to modeling accuracy and computational efficiency. The ALE technique was then applied to treat the blast response of different types of segmented bored tunnels buried in dry sand. Results indicate that the most used modern ring type segmented tunnels were more flexible for in-plane response, however, they suffered permanent drifts between the rings. Hexagonal segmented tunnels responded with negligible drifts in the longitudinal direction, but the magnitudes of in-plane drifts were large and hence hazardous for the tunnel. Interlocking segmented tunnels suffered from permanent drifts in both the longitudinal and transverse directions. Multi-surface radial joints in both the hexagonal and interlocking segments affected the flexibility of the tunnel in the transverse direction. The findings offer significant new information in the behavior of segmented bored tunnels to guide their future implementation in civil engineering applications.

Significance of Drying Periods on Nitrate Removal in Experimental Biofilters

Nitrogen is an important nutrient that can impact the quality of aquatic environments when present in high concentration. Even though low concentration levels of ammonium-nitrogen have been observed in laboratory studies in bioretention basins, poor removal or even the production of nitrate-nitrogen within the filter is often recorded in such studies. Ten Perspex biofilter columns of 94 mm (internal diameter) were packed with a filter layer, transition layer and a gravel layer. While the filter layer was packed to a height of 800 mm, transition and gravel layers were packed to a composite height of 220 mm and operated with simulated stormwater in the laboratory. The filter layer contained 8% organic material by weight. A free board of 350 mm provided detention storage and head to facilitate infiltration. The columns were operated with different antecedent dry days (0 d to 21 d) and constant inflow concentration at a feed rate of 100 mL/min. Samples were collected from the outflow at different time intervals, between 2 min and 150 min from the start of outflow, and were tested for nitrate-nitrogen and total organic carbon. Washoff of organic carbon from the filter layer was observed to occur for 30 min of outflow. This indicated washoff of organic carbon from the filter itself. At the same time, a very low concentration of nitrate-nitrogen was recorded at the beginning of the outflow, indicating the effective removal of nitrate-nitrogen. We conclude that the removal of nitrate-nitrogen is insignificant during the wetting phase of a rainfall event and the process of denitrification is more pronounced during the drying phase of a rainfall event. Thus intermittent wetting and drying is crucial for the removal of nitrate-nitrogen in bioretention basins.

On Early Detection and Warning against Rainfall-Induced Landslides (M129)

Traditional approaches to prevent rainfall-induced landslides consist of such stabilization of unstable slopes as installation of retaining walls as well as ground anchors. Although having been useful in mitigation of large slope failures, those traditional measures are not very helpful in mitigation of small slope failures which are less significant in scale but numerous in numbers. It is proposed in the present text for people to install slope instability detectors which find precursors of an imminent slope failure and issue warnings so that people may be able to evacuate themselves prior to fatal slope failures. To achieve this goal, model tests as well as laboratory triaxial tests have been conducted in order to understand the behavior of soil prior to failure. Moreover, numerical analyses on ground water percolation and decrease of factor of safety in the course of rainfall were conducted on a sandy slope in order to support findings from model tests. As a whole, a small instrument is proposed for a use of people which can detect minor displacement and change of moisture content prior to failure in a slope and issue warning through internet.

Effects of Recycled Asphalt Pavement on the Stiffness and Fatigue Performance of Multigrade Bitumen Asphalt

Recycled asphalt pavement (RAP) has received considerable interest in recent years for economic, environmental, and social reasons, and it is necessary to discover what effect the inclusion of RAP into asphalt layers has on the fatigue life of a pavement. Multigrade bitumen is less sensitive to temperature compared to conventional bitumen therefore it is expected multigrade bitumen asphalt to perform much better than conventional asphalt. It is important to investigate the performance of RAP added multigrade bitumen asphalt as an innovative pavement material. Therefore, in this study, the effects of multigrade bitumen types and the amount of RAP content (e.g.: 0% and 15%) on the fatigue life of this innovative pavement material were investigated in the laboratory using a four-point beam fatigue testing apparatus under controlled temperature. The results suggested that Multigrade bitumen asphalt with and without RAP has higher fatigue life compared to the conventional bitumen asphalt. Further, it was revealed that the type of multigrade bitumen type has significant effects on the fatigue performance of multigrade bitumen asphalt. It was observed that the effect of the RAP on the fatigue performance of multigrade bitumen varies with the type of multigrade bitumen. The study was further expanded to develop a master curve for a given mixture and the master curve can be used to obtain flexural modulus of the mixture at a given temperature and at a given loading frequency. The results show that multigrade bitumen and RAP inclusion can have beneficial effects on pavement design thickness.

Estimation of Poisson's ratio and variation of tensile yield strength of composite clay balls used in pebble matrix filtration

Clay balls can be used as alternatives to natural pebbles in pebble matrix filtration, a device for drinking water treatment. These clay balls are subjected to stresses due to self-weight and overburden in water saturated conditions. Although there are empirical relationships for evaluating tensile yield strength (Ts) of clay balls using Poisson’s ratio (µ), diameter (d) of clay balls, and failure polar force (Fs), so far for such calculations the value of Poisson’s ratio (µ) was taken from studies based on clay bricks. However, during ball preparation if clay is mixed with other raw materials from industry wastes such as saw dust or alum sludge in order to enhance the pollutant removal properties of the filter media, then the Poisson’s ratio (µ) of composite balls would be quite different to that of clay bricks. This paper describes a novel method for estimating Poisson’s ratio (µ) of composite clay balls by measuring vertical deformation using linear variable displacement transducers (LVDTs) in uniaxial compressive strength (UCS) apparatus and lateral deformation using particle image velocimetry (PIV).

Effects of reclaimed asphalt materials on geotechnical characteristics of recycled concrete aggregates as a pavement material

The use of recycled concrete aggregates (RCAs) as an unbound pavement material is a perfect alternative to effectively manage the construction and demolition (C&D) wastes. However, the presence of constituents in RCA is led for inconsistent properties. The reclaimed asphalt pavement (RAP) material is one of the major constituents that can be mixed at the recycling process of demolished concrete. Therefore, this study aims to investigate the effects of RAP as a constituent in RCA, on its characteristic properties as an unbound pavement material. To achieve the objectives of the study, five RCA samples were produced by mixing different percentages of RAP by weight. A series of standard laboratory tests were conducted on each sample. The results were then compared with the properties of standard granular pavement materials specified by Queensland Department of Transport and Main Roads (QDTMR), Australia. The analysis depicted that the presence of RAP as a constituent in RCA up to 15% was not significantly affected on the physical properties of RCA as an unbound pavement material.

Estimation of Poisson’s Ratio and Variation of Tensile Yield Strength of Composite Clay Balls Used in Pebble Matrix Filtration

AbstractClay balls can be used as an alternative to natural pebbles in pebble matrix filtration, a method for drinking water treatment. These clay balls are subjected to stresses due to self-weight...