Effect of footing shape on penetration in sand overlying clay

Abstract

This paper reports on a series of centrifuge model tests investigating the effect of shape on the penetration resistance of spudcan and conical footings on sand overlying clay. The effect of footing shape and geometry on single-layer soil has been studied intensely. However, there is still limited understanding for conical footings on sand over clay. In the present study, digital images were captured during penetration of various shapes of half-footing held against a transparent window of a strongbox. The images were then analysed using particle image velocimetry techniques. Experimental evidence has shown that, irrespective of the conical angle of the underside within the range of 7–21°, when the footing penetrates through sand into an underlying clay layer: (a) accumulated radial and deviatoric shear strains along the future failure surface counteract each other, resulting in similar peak resistance in the sand layer and (b) a trapped sand plug of constant height is pushed into the underlying clay layer. These observations serve to justify the previously proposed methods for predicting the full penetration resistance profile on sand overlying clay, which is required to predict the potential for, and severity of, punch-through failure.

Predicting jack-up spudcan installation in sand overlying stiff clay

“…A sand plug is clearly trapped underneath the spudcan, mobilising higher bearing capacity of the soil compared with the single clay case. The measurements from these numerical analyses indicate the height of the sand plug is ~0.9Hs, consistent with the conclusion in Teh (2007) and Hu et al (2016). This manifests that the height of the sand plug was not diminished by interaction with the stiffer underlying clay.…”

“…For instance, the punch-through of a spudcan through a layer of sand into underlying clay has been intensively investigated in a geotechnical centrifuge by Craig and Chua (1990), Teh (2007), Lee (2009), Teh et al (2010), Lee et al (2013a), Hossain et al (2016) and Hu et al (2014aHu et al ( , 2016. Table 1 summarises the testing database, with the shear strength of the clay at the sand-clay interface sum ranging from 7.2 to 25.8 kPa and the shear strength gradient in the range of 0 -2.1 kPa/m.…”

“…Recently, a new analytical penetration resistance-depth profile prediction method (hereinafter called the Hu et al method) was proposed (Lee et al 2013b;Hu et al 2014bHu et al , 2015a. It was calibrated using a testing and numerical database with soft clay strength of 10 -40 kPa at the sand-clay interface and strength gradient of 1.5 -2.5 kPa/m (Lee et al 2013a;Hu et al 2014aHu et al , 2015aHu et al , 2016. In the prediction of the peak penetration resistance in the upper sand layer, the stress level and dilatant response, as well as the embedment depth, are taken into account using a discretised silo approach.…”

Foundation punch-through in clay with sand: analytical modelling

“…This empirical power law is also adopted here: Equations 6 & 7 provide a slightly higher value of DF for a spudcan. The higher DF value for a spudcan reflects the probable generation of increased lateral stresses during qpeak mobilisation due to the conical underside, as illustrated in Lee et al (2013b), which was also demonstrated by the higher accumulated radial strains underneath the spudcan by Hu et al (2015b).…”

“…the friction angle changing with ambient stress level). In addition, image analysis of multi-layered punchthrough centrifuge experiments (see Teh et al 2008;Hossain, 2014;Hu et al 2015b;Ullah et al 2016) indicate that some of the overlying soil becomes entrapped beneath the spudcan foundation during penetration, leading in some instances to increased penetration resistance but a less severe punch-through event. The simple models recommended by SNAME and ISO do not account for these entrapped layers of soil, causing the models to significantly under-predict the penetration resistance during punch-through, and often overpredict the risk associated with uncontrolled leg penetration (Ullah et al 2015).…”