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Superficial deposits
Published in A.C. McLean, C. D. Gribble, Geology for Civil Engineers, 2017
In many parts of the world, superficial deposits are formed in situ by the weathering of rocks into soils. The land surface indicates the distribution of such soils only in so far as it still reflects the distribution of parent rocks by erosional landforms that are older than the soils. Such residual soils are uncommon in Britain, where most superficial deposits have been transported by ice, rivers, wind or gravity (acting on deposits on a slope) before deposition. The local environment and the mode of deposition produce a characteristic landform, which may be used to infer the probable nature of the deposits forming it. Transported superficial deposits, the natural agents that moved them, and several of the land-forms related to them, have already been discussed (Section 3.4), so only a few examples are described here.
Ground water
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
Infiltration most readily occurs through open fractures such as joints in exposed rock (Fig. 8.15) and the gaping cracks that may develop in zones of tensile strain flanking areas of mining subsidence (Figs 9.28 and 13.13) an around landslides (Fig. 14.7). The superficial deposits that mantle most areas (Fig. 3.1) also permit infiltration through their pores. Deposits of gravel and sand, and screes are able to infiltrate water without difficulty whereas clay-rich mantles retard the ingress of water and characteristically remain wet after periods of rainfall. Vegetation protects the delicate porous structure of many superficial deposits, especially the crumb-structure of top-soil, and ground covered by vegetation has more uniform infiltration than bare ground. In the absence of such protection the impact of raindrops during periods of intense rainfall may disturb the soil structure and render the surface impermeable.
Evaluation of the Local Site Effects and Their Implication to the Seismic Risk of the UNESCO World Heritage Site Old City of Dubrovnik (Croatia)
Published in Journal of Earthquake Engineering, 2023
Davor Stanko, Tvrtko Korbar, Snježana Markušić
Regardless of whether the bedrock is composed of limestone or dolomite, the bedrock is relatively geomechanically firm and stable, except in fault zones where the carbonates are heavily fractured. The embayment was partly filled by Quaternary clay in the lower part and by sand in the upper part of the natural superficial deposits. The rest of the former accommodation space, that was once covered by a shallow sea, is filled by anthropogenic deposits into flattened central part of the Old City. Especially vulnerable in terms of local site effects is the central filled and flattened part, because of the expected amplification, especially since unconsolidated superficial deposits in areas with complex geological structures act as the main driver of the negative influence on the local site effects. Given that the mapped faults are possibly active, direct damages on the buildings are also possible along the fault lines in case of a co-seismic surface displacements along the faults.
Optimised multivariate Gaussians for probabilistic subsurface characterisation
Published in Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards, 2019
Mohammad B. Abdulla, Rita L. Sousa, Herbert Einstein, Sara Awadalla
Masdar City, the site considered in this case study, is located to the immediate south-west of Abu Dhabi International Airport, about 2 km inland from the coastline and at a relatively low elevation (about + 4.5 m NADD). The general area has a history of temporal rises in groundwater level and localised flooding, and the groundwater is known to lie at a shallow level below the ground surface. The site is underlain by superficial deposits, comprising from top to bottom, sweet sand fill, dune sand, Sabkha (gypsiferous sandy silts and silty clays), marine sands, and then bedrock, comprising siltstone, mudstone, sandstone, and gypsum layers. The coastal Sabkha is the original terrain at the site. The Sabkha layer is very soft and/or very loose, with low density and strength, and high compressibility. As usual in the region, the Sabkha plain has been artificially raised by filling with locally available natural deposits. The land raising brings the new surface above the influence of the capillary action (rise) and thus permits agricultural development in less saline conditions. At Masdar, the site was raised with “sweet sand”, obtained from inland desert dune sand (Fugro 2008; MacDonald 2010). The primary sources of information for the Masdar city site are from boreholes and exploratory pits, specifically twenty-one boreholes and twenty-two exploratory pits. The boreholes and the pits were distributed throughout the site as seen in Figure 4. Sand fill and Sabkha are the common materials in the trial pits; In the boreholes at Masdar city, Sabkha is only observed in the top layers down to a depth of 5 m, with an average layer thickness of about 1.5 m (Fugro 2008; MacDonald 2010). As previously mentioned, in this paper and for illustration purposes, we simplified the problem to two geologies, Sabkha and Sand. The sand layers include sweet sand fill and dune and marine sand, as the focus of the work is to determine the location of the problematic sabkha layers.