A Physical Study of the Effect of Groundwater Salinity on the Compressibility of the Semarang-DemakAquitard, Java Island

Abstract

Semarang-Demak and other cities along the coast of North Java are vulnerable to land subsidence. The presence of saline groundwater in the coastal region is thought to affect the high subsidence rate, in this case the compressibility of the aquitard layer. We aimed to analyze the effects of groundwater salinity on the compression characteristics of the Semarang-Demak clay using physical analysis. Methods included the determination of groundwater salinity, clay mineralogy and fabrics, and consolidation tests under various salinity conditions. The Semarang-Demak clay is dominated by smectite of high activity, and saline clay exists at the depth of 10 to 35 m. Consolidation tests revealed that the increase insalinity increases the average consolidation rate and hydraulic conductivity up to 42% and 37.5%, respectively. Clay fabric analysis showed that the groundwater salinity modified the interconnectivity of pores by changing the fabric into parallel alignments, facilitating faster porewater dissipation, hence the clay is more readily compressed. These findings are useful for explaining the mechanism of the fast-subsiding coastal plains of North Java.

Natural Compaction of Semarang-Demak Alluvial Plain and Its Relationship to the Present Land Subsidence

“…the variety of groundwater condition from fresh to brackish/saline as the result of depositional process. Measurement electrical conductivity in the aquifers and aquitards of the Semarang-Demak alluvial plain Sarah et al, 2018) showed the variety of groundwater quality as shown in the subsurface profiles in Figure 2a and 2b. Both figures show that the electrical conductivity of Damar Formation is low (660 μS/cm), and so is the Garang delta on top of the Damar Formation (EC of 650 -756 μS/cm).…”

“…The engineering properties of the Semarang and Demak clays are influenced by the groundwater salinity, giving different compressive properties and hydraulic conductivity for freshwater and saline clays. The fresh to saline groundwater varies over depth, the material properties were applied according to its groundwater quality as had been outlined by Sarah et al (2018). The material properties used in the numerical and analytical modeling were determined from laboratory test results as summarized in Table 1.…”

Destabilizing the science of soils: Geoscientists as spokespersons for land subsidence in Semarang, Indonesia

“…The first scientists writing about land subsidence were interested in the displacement of the Semarang shoreline (erosion) and tidal flooding. Much of their analysis was based on the work of van Bemmelen, whose observations about unconsolidated sediments were – and still are – mobilized to explain the processes of sedimentation and erosion that are at the core of land subsidence in Semarang (Abidin et al, 2013; Sarah et al, 2018, 2020). Five hundred years ago, the shoreline was up to 5 km inwards from where it is now.…”

“…A reader is likely to come across descriptions that are very similar to this one when doing a basic search engine inquiry about land subsidence. Geological definitions characterize subsidence as the ‘downward displacement of the land surface relative to certain reference surface’ (Abidin et al, 2015: 2) stemming from the ‘subsurface movement of earth materials’ (Sarah et al, 2018: 1). While the definition is relatively straightforward, the excerpt from the USGS website reveals that studying land subsidence is not easy: it is gradual, widespread, and often requires sophisticated technologies.…”

Land Value in a Disaster-Prone Urbanized Coastal Area: A Case Study from Semarang City, Indonesia

“…The problems are varied, interrelated, and complex [10]. Land subsidence, for example, in combination with other coastal-area threats, such as sea-level rise, storm surge, coastal erosion, damage caused by mangrove ecosystems, and being upstream of a river, has multiplied the risks of the coastal areas [9][10][11][12][13][14].…”