Accurate estimation of soil permeability is crucial in many geotechnical applications. Empirical and theoretical equations based on soil particle size distribution (PSD) offer a fast and cheap way for preliminary estimation of permeability in granular soils, however the results obtained from various formulas available in the literature often show significant discrepancies. While several comparative studies on this topic have been published, no definite conclusions can be drawn on the performance of the predictive equations in comparison with in-situ permeability measurements. Many formulas require porosity or void ratio as input parameter, which is difficult to obtain for granular soil in-situ. In this study we applied 30 predictive equations to estimate permeability of sandy soil in an outwash plain deposit. The equations were divided into 5 groups, based on their structure and the required input parameters. Empirical formulas were used to estimate the expected in-situ porosity range. The obtained permeability values were compared to the results of in-situ permeameter measurements and pumping tests. Significant differences in the results and in their sensitivity to porosity were found between the 5 groups of methods. In general, simple equations which do not include porosity were in better agreement with measurements than the other groups.

Groundwater mapping plays an important role in effective water resource management, sustainable development, and environmental protection. In the tropical Brunei Darussalam (north Borneo Island), groundwater mapping is yet to be provided. The aim of the study is to provide groundwater mapping of Brunei, particularly for the urban and coastal areas of the Brunei-Muara capital district. The study uses a GIS interpolation technique to generate a groundwater contour map based on groundwater data from 572 sacrificial boreholes located in the study area. Remote sensing data and published maps from secondary sources were digitised in ArcGIS software to produce thematic layers for further hydrological evaluations. Results showed that groundwater levels in the study area are generally high and shallow, ranging from 0 to 18 m below ground level with a mean value of 2.9 m. According to the evaluation of geo-thematic layers and groundwater contours, groundwater flows towards the South China Sea in the coastal areas and towards the Brunei River further inland. Hydraulic gradients towards the South China Sea also vary between 0.004 and 0.08. Thus, assuming surface aquifer thickness in the weathered zone between 10 to 20 m, hydraulic conductivities ranges from 1∙10⁻⁵ to 1∙10⁻⁴ m∙s⁻¹, a submarine groundwater discharge (SGD) flux between 4.7∙10⁻⁷ to 4.0∙10⁻⁴ m3∙s⁻¹ per unit width can be estimated for the shallow aquifer. This study provides valuable insights into the groundwater system dynamics so important, which are critical for its future utilisation and protection, aiming to contribute to the national water security in Brunei Darussalam.