A McNamara

Physical Modelling in Geotechnics

Centrifuge modelling has been considered as an effective means for investigating the energy and geotechnical performance of thermo-active geo-structures. A major challenge to correctly model (i) soilstructure heat transfer and (ii) thermo-mechanical behaviour of the geo-structure in a centrifuge is to design a system that could deliver sufficient heat energy (i.e. in terms of flowrate and temperature) under enhanced gravity conditions. This paper reports a new and robust heating/cooling system developed for these purposes and evaluates its performance. The proof heating tests performed up to 50-g suggest that when an appropriate pipe configuration is designed, the heating system is capable of producing a water flowrate up to 13.5 ml/s, which is sufficient to generate a turbulent flow regime within the water circulation pipe, hence maximising the convective heat transfer mechanism. The heating system has been successfully applied to deliver a controllable amount of heat energy, simultaneously, to multiple thermo-a ctive piles in a row for warming up the surrounding soil. With proper thermal insulation of the pipework of the system, temperature loss between the target value at the pipe inlet and the one registered at the entrance of the model structure could be less than 2 ̊C. An idea for extending the system to lower the temperature below ambient is also presented. Author Accepted Manuscript version of Vitali, D, Leung, A, Zhao, R & Knappett, J 2018, A new heating-cooling system for centrifuge testing of thermo-active geo-structures. in A McNamara, S Divall & R Goodey (eds), Physical Modelling in Geotechnics : Proceedings of the 9th International Conference on Physical Modelling in Geotechnics (ICPMG 2018), July 17-20, 2018, London, United Kingdom. vol. 1, Taylor & Francis, pp. 475-480, 9th International Conference on Physical Modelling in Geoetchnics, London, United Kingdom, 17/07/18. https:// doi.org/10.1201/9780429438646 ing heat losses. Thus, this system requires two slip rings for fluid circulation, which could limit inflight activities for smallto medium-size centrifuge facilities that have only a limited number of slip rings. Normally, inflight activities such as hydraulic jacking, earthquake simulation, application of rainfall would require at least one slip ring for operation. The system developed in Hong Kong, on the contrary, avoids the use of multiple slip rings by introducing two independent components: one for the heating of the glycol-water solution and one for its cooling. The heating is achieved by using an electric heating element that is submerged in an aluminium tank filled with the circulating fluid. The cooling, on the other hand, is obtained through the Peltier effect to bring the fluid temperature below the ambient. The two components are arranged on the centrifuge together with a pump that allows the fluid to be delivered to the model foundations (Shi et al. 2016). The system requires a considerable amount of space to mount the two components onto a centrifuge and is therefore less suitable to be adopted by smallto medium-size beam centrifuges. This paper details the development of a new heating/cooling system that can be mounted at the geotechnical beam centrifuge facility at the University of Dundee. Due to the limit of space, only the heating component of the system is discussed here. The working principle of the system is elaborated and its performance under different gravity levels is evaluated.There are soil-structure interaction problems for which it is important to model both the relative soil-structure stiffness and strength. Examples from the earthquake engineering field include the design of resilient rocking-isolated foundations and the seismic stabilisation of slopes using piling. In both cases the aim is to ensure a preferred failure mode happens first in the soil instead of the structure i.e. controlled bearing failure of the foundation or soil yielding around piles. A recently developed model reinforced concrete for centrifuge testing can simulate stiffness and strength simultaneously, but suffers from variability in the material properties, as does the full-scale material. This paper presents a series of element tests on the variability of model reinforced concrete elements representative of large square monolithic bridge piers and slender square piles. Coefficients of variation for various material and element properties have been determined and shown to be similar to typical values for full-scale reinforced concrete elements obtained from the literature. It is also demonstrated that curing time beyond 28 days does not substantially affect strength and variability and that models of different absolute volume can be produced without inducing detrimental size effects. The results are used to discuss the selection of mean design strengths for model structural elements in centrifuge experiments using a quantitative statistical approach where there are competing structural and soil failure modes. Author Accepted Manuscript version of Knappett, J, Brown, M, Shields, L, Al-Defae, A & Loli, M 2018, Variability of small scale model reinforced concrete and implications for geotechnical centrifuge testing. in A McNamara, S Divall, R Goodey, N Taylor, S Stallebrass & J Panchal (eds), Physical Modelling in Geotechnics: Proceedings of the 9th International Conference on Physical Modelling in Geotechnics (ICPMG 2018), July 17-20, 2018, London, United Kingdom. 1 edn, CRC Press-Taylor & Francis Group, pp. 241-246, 9th Int. Conf. on Physical Modelling in Geotechnics 2018 , London, United Kingdom, 17/07/18. https://doi.org/10.1201/9780429438646 will exhibit greater variability in key material properties (e.g. bending stiffness, EI and moment capacity, Mult) than equivalent ‘elastic’ models (made typically out of aluminium alloys, steel or plastics). If this variability is similar to that of field reinforced concrete, this would potentially represent another way in which the model RC is a closer analogue of field concrete. This paper will address this issue of variability by presenting test data of both the variability in fundamental mechanical properties of the individual material components (e.g. compressive strength and tensile strength of the model concrete; yield strength of the model reinforcement) and of full reinforced concrete structural elements. This will be compared with extensive data from the literature for field reinforced concrete. The elements tested will be based on those used in recent geotechnical centrifuge testing programmes, and the results will be used to discuss the implications of variability on model design, using the example of a reinforced concrete bridge pier on a foundation designed to provide rocking-isolation under seismic actions.This paper presents some initial physical modelling of root-soil interaction for trees under lateral loading, such as from wind or debris flows. 3-D printing of interconnected systems of ABS plastic root analogues, which are mechanically representative of tree roots, is used to produce idealised root architectures with varying root depth, spread and root area ratio (RAR, i.e. amount of roots). These models are installed within a granular soil with a wooden model trunk attached and laterally loaded to measure the push-over behaviour. From this data, the moment capacity and rotational stiffness of the model root systems are evaluated. The results suggest that (i) the effect of the central tap root on strength and stiffness is low, compared to the lateral and sinker roots, but otherwise proportional to RAR; and (ii) that development of analytical predictive models may be able to adapt existing procedures for conventional geotechnical systems, such as pile groups. Author Accepted Manuscript version of Zhang, X, Knappett, J, Leung, A & Liang, T 2018, Physical modelling of soil-structure interaction of tree root systems under lateral loads. in A McNamara, S Divall & R Goodey (eds), Physical Modelling in Geotechnics: Proceedings of the 9th International Conference on Physical Modelling in Geotechnics (ICPMG 2018), July 17-20, 2018, London, United Kingdom. vol. 1, Taylor & Francis, pp. 481-486, 9th International Conference on Physical Modelling in Geoetchnics, London, United Kingdom, 17/07/18. https://doi.org/10.1201/9780429438646 4.5%, and this is believed to cover a wide range of species (Mao et al., 2012). The thick main stem of a tree, which is also called the trunk, transfers lateral loading directly to the root system via the stump. According to research conducted by Danjon et al., (2008), the diameter of the stump has been observed in the field to be approximately 250 mm for some tree species in slopes. This value was adopted in this study, resulting in a stump 25 mm in diameter at 1:10 scale. To simplify the design of the tree model and subsequent analysis, a circular trunk was designed having the same diameter, constant with height, such that DBH was 25 mm. The length of the trunk was 750 mm, thereby representing a tree 7.5 m tall. For the selected DBH, the ZRT dimensions of the short and wide root system would exceed the threshold of the available 3-D printer. In order to make the models printable, the ZRT was set to twice the DBH, for both the deep and shallow model systems.

Physical modelling of lime stabilisation in soft soils around deep excavations

The availability of space above ground decreases as cities expand, causing a demand for very deep underground structures so developments must mitigate the risk of damaging adjacent buildings. This is especially critical in soft clays where ground movements are considerable and can extend far beyond the excavation site. This paper investigates the efficacy of a shallow lime stabilised clay layer on reducing heave and the settlement profile behind an embedded retaining wall. Centrifuge modelling at 160 g was used to observe surface and subsurface soil movements of a 12 m deep excavation (H) supported by a retaining wall of 8.8 m embedment at prototype scale. Since this research focussed on measures used to minimise heave the model comprised a high stiffness, fully supported ‘rigid wall’ to eliminate ground movements attributed to wall deformation. A direct comparison between a reference test, with no improvements and a test comprising H/2 thick 5% lime stabilised layer indicated that the lime treatment increased the excavation stability by a factor of three.