Modelling helical screw piles in soft clay and design implications

Abstract

Helical screw piles are a popular solution for relatively low-capacity, removable or recyclable foundations supporting road and rail signage or similar light structures. When specifying a helical screw pile, a designer must choose the active length and the helical plate spacing ratio, which are governed by the number, spacing and size of the individual helices. This paper presents an investigation using transparent synthetic soil and particle image velocimetry to observe the failure of helical screw piles with helical plate spacing ratios of 1 . 5-3 and active lengths up to three times the diameter. For the geometries and properties examined, capacity is shown to be a function of active length and the dominant failure mechanism is characterised by the formation of a cylindrical failure surface. A simple analytical model is developed and used to assess the impact of different design methodologies on immediate displacements under loading. A traditional 'permissible stress' method is shown to be conservative, whereas modern 'partial factor' methods are more economical and lead to greater immediate displacements for a given design load.Designers using modern 'partial factor' approaches, such as Eurocode 7, might benefit from specifying a helical plate spacing ratio of less than 1 . 5 to maximise the stiffness of the response to axial loading and minimise the immediate displacements experienced upon application of working loads.

Quantification of Optical Clarity of Transparent Soil Using the Modulation Transfer Function

“…Early experiments in transparent soil adopted back illumination to silhouette embedded target markers to capture the mechanical response of the soil (Gill and Lehane 2001); however, this has been superseded by laser-aided imaging in conjunction with DIC to illuminate soil texture ( Fig. 1) for increased measurement density and resolution Stanier et al 2013). A summary of the development of transparent soil and recent works is reported by Iskander (2010).…”

“…In both the latter works, the model geometry reported was considerably larger than any previously achieved using precipitated amorphous silica; for example, McKelvey (2002) tests were conducted in a 415 mm diameter chamber, suggesting that fumed silica soils offered superior optical quality than precipitated amorphous silica counterparts. Recent works using a variant of this fumed silica material coupled with DIC laser aided measurement include examination of helical screw piles (Stanier 2011;Stanier et al 2013), stone column ground improvement (Kelly 2013), soil plugging behavior during press-in piling of tubular piles (Black 2012a;Forlati and Black 2014), sample disturbance effects (Black 2012b). …”

Centrifuge Modelling With Transparent Soil and Laser Aided Imaging

“…1). Recent works using this approach relate to model piles (Iskander et al 2002a), shallow foundations (Liu et al 2002;Iskander and Lui 2010), tunnel induced settlements (Ahmed and Iskander 2010), helical screw piles (Stanier et al 2013), stone column group behavior (Kelly 2013), soil plugging in tubular piles (Black 2012a;Forlati and Black 2014), and sample disturbance effects during tube sample recovery (Black 2012b).…”

Transparent Soil to Model Thermal Processes: An Energy Pile Example

“…Gill (1999) used back illumination to silhouette embedded target markers; however, that has been superseded by modern laser aided imaging in conjunction with digital image correlation techniques (for example Sadek et al 2003 andHird et al 2008). Works using this approach include examination of failure mechanics of helical screw piles (Stanier et al 2013), stone column groups (Kelly 2013), and tunnel induced settlements (Ahmed and Iskander 2010) soil plugging behavior during press-in piling of tubular piles (Black 2012;Forlati and Black 2014). The success of transparent soil modeling has long been considered reliant on producing a soil surrogate that offers the highest optical clarity, with low transparency being considered detrimental to the modeling technique (Black and Take 2015).…”