Engineering properties of the carbonate sediments along the Abu Dhabi coast, United Arab Emirates

Abstract

The sedimentology of the lagoons and island of Abu Dhabi was studied in detail by Douglas Shearman and his colleagues during the 1960s. Since that time, the island has undergone extensive development and a considerable database on the ground conditions in the area exists from site investigations. Despite this, there are few published data on the engineering characteristics of these sediments; this paper addresses this shortfall. A brief description of the environments of deposition and the sediments of the foreshore, tidal delta and the lagoons of the Abu Dhabi coast is presented, and the effects of both the origin and the diagenesis of these sediments on their engineering properties is examined. Results from a selection of site investigations are summarised to present characteristic values for the various sediments. Physical property data reported include: lithology and grain-size profiles; moisture contents; bulk and dry density; porosity; estimated degrees of saturation; unconfined compressive strength; standard penetration test (SPT) values; soil and groundwater chemistry (chloride and sulphate contents); and plasticity charts (plasticity index versus liquid limit). The ground conditions are also reviewed to identify significant issues that need to be addressed for the design and construction of engineering projects in Abu Dhabi.The data show that there is a contrast between the strength and compressibility and salt content of the granular, often cemented deposits of the higher energy beach and nearshore environments, and the soft or very loose silts and fine sands of the lagoons and sabkha. A decrease in the strength of the underlying calcarenites with depth to about 10 m below ground level is thought to reflect the effect of diagenetic alteration of the carbonates from aragonite to high-magnesium calcite.

Mineralogical and Micro-structural Investigation into the Mechanical Behaviour of a Soft Calcareous Mudstone

“…Water salinity is high and average water depth low, producing a system capable of precipitating carbonates and evaporites rapidly within a short geological timescale. This area is being heavily developed with ambitious construction projects, and yet serious engineering concerns ranging from corrosive and aggressive groundwater, dissolution of evaporites/associated subsidence (often triggered by anthropogenic activities such as ground-source heating/cooling) and piling through Caprock continue to complicate design (Epps 2011). The calcareous mudstone studied was formed in a low energy, hyper-saline environment, with a modern analogy being the area's lagoonal terraces.…”

“…The young soft rocks that make up the subsurface of this part of the Middle East are well studied from a geological standpoint. However, research into their complex mechanical behaviour is less common (Cook 1999;Epps 2011). The geotechnical properties of the soft rocks encountered around the Abu Dhabi coastline are often poorly understood and their unique physical characteristics cause problems when attempting to sample and test them, making the application of many standard laboratory testing methodologies inappropriate (Wright et al 1999;Kanji 2014).…”

“…In lithologies such as these, the time gap between the formation of clasts and their bonding materials can be small, or even occur contemporaneously. Weak cementitious bridges may precipitate within voids far more rapidly than, for example, the formation of iron oxide or alumina bonds in terrestrial and freshwater sediments, introducing a degree of structure that needs to be taken into account when attempting to characterise their behaviour (Aversa and Evangelista 1998;Epps 2011;Truong et al 2012;Fereidooni 2016). Sediments formed in hyper-saline waters are noted to be particularly sensitive to changing physico-chemical environments, which can further complicate the process of carrying out routine geotechnical laboratory tests (which typically demand full saturation), or rock testing procedures (often requiring samples to be dry (Burland and Lord 1970;Hawkins 1998)).…”