ECE 2311_Site Investigation.pdf - ECE 2311 Soil Mechanics...

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ECE 2311: Soil Mechanics II Site Investigations i 8 SITE INVESTIGATIONS .......................................................................................... 1 8.1 Methods of investigation .................................................................................... 1 8.1.1 Trial pits ......................................................................................................... 1 8.1.2 Hand and portable augers ................................................................................ 2 8.2 Sampling ............................................................................................................ 2 8.2.1 Open-drive sampler ........................................................................................ 3 8.2.2 Thin-walled sampler ....................................................................................... 3 8.3 In-situ testing ...................................................................................................... 4 8.3.1 Plate bearing test ............................................................................................. 4 8.3.2 The standard penetration test (SPT) ................................................................ 5 8.3.3 The cone penetration test (CPT) ...................................................................... 7 8.3.4 The vane shear test ......................................................................................... 9
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ECE 2311: Soil Mechanics II Site Investigations 1 8 SITE INVESTIGATIONS A proper ground investigation is an essential preliminary to the execution of a civil engineering project. Sufficient information must be gathered in order to come to a safe and economical foundation and construction design. The principal objects of the investigation are: To determine the thickness and sequence of the various soil strata and level of bedrock; To obtain representative soil samples of the strata and bedrock for identification and classification of relevant soil parameters such as grain size distribution and consistency limits (classification), shear strength parameters, permeability and consolidation coefficients. Further, information about sulphate and organic mater content might be useful with respect to attack of concrete; To obtain information about groundwater movements. In case of a soil investigation for foundation design, the relevant information must be obtained from the influence area of the foundation, the so-called pressure bulb. However, prior to fieldwork it is very important to get an idea of the conditions on site by studying geological maps and memoirs, if available. The significance of this part of the investigation has more than once been overlooked, leading to unnecessary calamities. If no geological information is available, the use of aerial photographs may help in identification of features of geological significance. Before the start of the fieldwork, an inspection of the site on foot must be carried out. Riverbanks, excavations, quarries, road and railway cuttings may yield valuable information about the present soils and rock. The costs of site investigations are dependent on the location and extend of the site, the ground conditions, and the type of project (low cost housing or executive residences). These costs are generally within 0.1 and 2% of the total building costs. A reduction of the site investigation for financial reasons can therefore seldom be justified. 8.1 Methods of investigation A few low-cost methods of investigation will be discussed. For more advanced and costly methods, such as the drilling and boring methods it is referred to the literature. 8.1.1 Trial pits The excavation of trial pits is a simple and reliable method of investigation of the soil conditions on site, but is limited to a maximum depth of 4 to 5 m. The soil can be dug out by hand or by a mechanical excavator. In Kenya, as in most developing countries, the first option is in most cases more economical because of low labour costs. The sides of the pit must be supported, unless they slope down at a safe angle or are stepped. If the pit extends below the existing water table, it should be dewatered, resulting in increasing costs.
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