Canadian Geotechnical Journal Centrifuge modelling of uplift response of suction caisson groups in soft clay Journal: Canadian Geotechnical Journal Manuscript ID cgj-2018-0838.R2 Manuscript Type: Article Date Submitted by the 12-Oct-2019 Author: Complete List of Authors: Zhu, Bin; Zhejiang University, College of Civil Engineering and Architecture Dai, Jialin; Zhejiang University, College of Civil Engineering and Architecture Kong, Deqiong;Draft Zhejiang University, College of Civil Engineering and Architecture Feng, Lingyun; Zhejiang University, College of Civil Engineering and Architecture Chen, Yun Min; Zhejiang University, College of Civil Engineering and Architecture Keyword: suction caisson, soft clay, shadowing effect, cyclic loading Is the invited manuscript for consideration in a Special Not applicable (regular submission) Issue? : https://mc06.manuscriptcentral.com/cgj-pubs Page 1 of 43 Canadian Geotechnical Journal Centrifuge modelling of uplift response of suction caisson groups in soft clay Bin Zhu Professor, College of Civil Engineering and Architecture, Zhejiang University, China Jia-lin Dai PhD student, College of Civil Engineering and Architecture, Zhejiang University, China Draft De-qiong Kong (corresponding author) Professor, College of Civil Engineering and Architecture, Zhejiang University, China Ling-yun Feng PhD student, College of Civil Engineering and Architecture, Zhejiang University, China Yun-min Chen Professor, College of Civil Engineering and Architecture, Zhejiang University, China 1 https://mc06.manuscriptcentral.com/cgj-pubs Canadian Geotechnical Journal Page 2 of 43 ABSTRACT This paper describes a programme of centrifuge model tests on the uplift behaviour of suction caisson foundations. The parameters considered were the loading rate, caisson diameter (D), soil strength profile and type of footing (i.e. mono-caisson and tetra-caissons group). The loading responses were examined in terms of total uplift resistance, suction beneath the caisson lid and the vertical displacements of the caisson and at the soil surface. There exists a critical uplift displacement, approximately 0.02D and 0.01D for the mono-caisson and the tetra-caissons group respectively, at which a turning point can be identified in the load-displacement curve. This was found to be attributed to the adhesion on the caisson-soil interface reaching a peak response and then dropping. Of interest is that the tetra-caissonsDraft group exhibits much greater normalised uplift resistance than the mono-caisson before reaching an uplift displacement of about 0.02D, suggesting superiority of the former in term of serviceability. However, a reversed trend was observed at greater displacement, and accordingly an empirical model was derived to quantify the shadowing effect of caisson groups. Regarding the cyclic response, several cycles of large- amplitude loading are sufficient to reduce the ultimate bearing capacity of caisson(s) to below the self-weight of the inner soil plug(s), indicating a transition of failure mechanism. Keywords: suction caisson; soft clay; shadowing effect; cyclic loading 2 https://mc06.manuscriptcentral.com/cgj-pubs Page 3 of 43 Canadian Geotechnical Journal INTRODUCTION In the past few decades, suction caissons have been widely used in offshore engineering as foundations of wind turbines, oil and gas platforms, etc., due to the low cost in construction and the possibility of reuse (Bye et al. 1995; Byrne and Houlsby, 2002). Recently, they have also been considered as an alternative for large-diameter pile foundations in swamp regions that are associated with river networks and their range of application has been widened to include power transmission towers (henceforth as PTTs) built there (Kai et al. 2002; Qin et al. 2014). In the context of Zhejiang Province, China, a large number of PTTs locate in swamp regions where the transmission lines account for 45% of the total line mileage. Tetrapod jacket foundations, with each footing being built on a monopod suctionDraft caisson or a group of caissons, have shown very promising potential to be applied to these PTTs (STDZP, 2014; ZEPDI and IGEZU, 2015). Similar to offshore wind turbines, PTTs are typically very tall structures and are vulnerable to overturning moments induced by extreme wind loads (Richards, 2010). The overturning moment on a jacket structure is primarily sustained by the “tension-compression” response of its upwind and downwind footings (Houlsby et al. 2005), and proper understanding of the uplift response of the upwind footing is of particular importance in the design (Rattley et al. 2008; Senders 2008; Kim et al. 2014b). A number of small-scale laboratory tests have been carried out to look into this issue in soft soil (e.g. Finn and Byrne 1972; Wang et al. 1977; Fuglsang and Steensen 1991). Deng and Carter (2002) assumed three different failure modes for fully drained, partially drained and fully undrained conditions, and proposed a model to assess the uplift bearing capacity of caissons, considering the influences of aspect ratio, loading rates and over- consolidation ratios. Following that, the uplift bearing capacity of a caisson under undrained 3 https://mc06.manuscriptcentral.com/cgj-pubs Canadian Geotechnical Journal Page 4 of 43 conditions was commonly determined as the sum of the resistance on its outer skirt and the reverse end bearing capacity at its bottom, with the caisson and the inner soil plug being treated as a monolithic unit (Byrne and Cassidy 2002; Yun and Bransby 2007; Zhan and Liu 2010). Centrifuge tests have been carried out as well (e.g. Lehane et al. 2014; Du et al. 2016). In particular, Mana et al. (2013) proposed an empirical model to assess the relationship between the reverse end bearing capacity coefficient and the aspect ratio of caisson, and pointed out that the gap between the outer skirt and the soil that developed during uplifting would result in a significant decrease in the suction mobilised beneath the caisson lid. There are, however, fewer studies on the cyclic response. Among them, Byrne et al. (2006) investigated the influence of ambient pressureDraft and loading rate and found that the former has significantly less impact than the latter on the uplift behaviour. Chen and Randolph (2007a) carried out centrifuge tests under both sustained and cyclic vertical loadings and found that the cyclic bearing capacity was 72%-86% of the monotonic one. Wallace and Rutherford (2016) reported that the uplift resistance was not reduced under small-amplitude cyclic loading and, for a caisson with an aspect ratio of one, the uplift resistance significantly reduces if the cyclic amplitude was greater than 0.25% of caisson diameter. The research described above is all confined to mono-caissons, and that on caisson groups is still limited. Andersen et al. (1993) reported field trials on a group of 2 × 2 adjacent caissons with spacing of zero under monotonic and cyclic loading in a lightly over-consolidated soft soil, and found that the ultimate bearing capacity can be reduced by 18%-34% under several dozens of loading cycles. Gourvenec and Jensen (2009) analysed the ultimate uplift capacity of double- 4 https://mc06.manuscriptcentral.com/cgj-pubs Page 5 of 43 Canadian Geotechnical Journal caisson groups with spacing varying from 0 to 7 caisson diameters in ABAQUS. The results showed negligible shadowing effect for caisson groups with spacing greater than one caisson diameter. Similar work was carried out by Kim et al. (2014a) on a tripod-caissons group with spacing 0.3 to 4 caisson diameters and found that the shadowing effect was greater than 0.95 for spacing beyond 0.3 times caisson diameter. This effect was reported by Kim et al. (2015) to be independent of the aspect ratio of caissons through centrifuge and numerical modelling. In summary, studies on the uplift response of caisson groups with small spacing are still limited, and thus are insufficient to guide the design of caisson groups used as a leg of jacket foundations. This paper presents a series of centrifuge tests to shed some light on this issue. The uplift response of both mono-caissons and tetra-caissonsDraft groups is examined in terms of the total uplift resistance, the suction beneath caisson lid and the vertical displacements of the caisson and the surficial soil. The primary parameters considered are the loading rate, the caisson diameter, the skirt thickness and the over-consolidation ratio of soil. CENTRIFUGE MODEL TESTS The centrifuge model tests were carried out in a beam geotechnical centrifuge at Zhejiang University, which has a maximum payload of 400 g·ton and an effective arm radius of 4.5 m. The strong box used for the presented tests is 1.2 m in length, 0.95 m in width and 1.0 m in depth. More details of this facility can be referred to Chen et al. (2010). Soil sample The soil used for all tests was kaolin clay and its properties are listed in Table 1. More details of this soil can be referred to Hu et al. (2011) and Xie et al. (2012). For the soil preparation, dry kaolin 5 https://mc06.manuscriptcentral.com/cgj-pubs Canadian Geotechnical Journal Page 6 of 43 powder was mixed in a vacuum mixer at a water content of 160%, two times its liquid limit, to obtain the slurry. The slurry was then poured into the strongbox, on the bottom of which a 50 mm thick coarse sand layer was laid in advance. The water level was kept 0.2 m above the sand surface before the slurry was poured in order to guarantee full saturation. Three boxes of over-consolidated clay were prepared. The preparation process consists of four steps: (1) consolidate the slurry under 100g for 8 hours and then stop the centrifuge; (2) spread geotextiles upon the clay and add a 0.15 m thick Fujian standard sand on them; (3) consolidate the clay again through gradually increasing the acceleration to 100g and last for 8 hours; (4) remove the geotextiles and sand layer prior to the tests.