Authors: Mehdizadeh, A; Fakharian, K


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Mehdizadeh, A & Fakharian, K 2015, 'Field instrumentation of a preloading project with prefabricated vertical drains', in PM Dight (ed.), FMGM 2015: Proceedings of the Ninth Symposium on Field Measurements in Geomechanics, Australian Centre for Geomechanics, Perth, pp. 453-464,

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Excessive consolidation settlements are always considered major threats for construction on soft soils. There are different methods to improve the soft cohesive sublayers. Preloading with vertical drains as one of the cheapest methods was selected for a ground improvement project in southwest of Iran near the Persian Gulf. The construction steps included soil instrumentation such as vibrating wire and Casagrande piezometers, magnetic extensometer, surface settlement monument, inclinometer, installation of prefabricated vertical drains (PVDs) and placement of embankment as a surcharge to accelerate the consolidation time for construction of two water tanks with a total volume of 100,000 m3. The length and spacing of PVDs for both water tanks were 15 and 1.5 m, respectively. This paper presents and compares instrumentation results in different soil layers for both water tanks and investigates the effects of loading rate and its consequences. In addition, back-analyses were carried out using the hyperbolic and Asaoka methods, on the basis of which some recommendations are provided for the instrument installation in soil layers and result interpretations. The findings demonstrate that the instrumentation is an indispensable part of preloading projects, owing to difficulties of evaluating the smear zone effects, design parameters and unpredicted problems during the project progress. Service time of PVDs and instruments, especially piezometers, must be considered for required time of consolidation and the Asaoka method predicts maximum settlements better than the hyperbolic method for this project.

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