GEOLAB Project CTP-ISSR: monotonic and cyclic Centrifuge Tests on Piles in sand proving Innovative Solutions to enhance Structural Resilience

The existing structures are facing continuous changes in the loading conditions during their life span. These changes can be caused by weather and geo-hazards events and affect the structural response which is often already weakened by aging.

A clear understanding of the interaction between the superstructure, the foundation and the surrounding soil is one of the crucial aspects to deal with to adapt the design and maintenance to enhance the resilience of new and existing Critical Infrastructures (CI) often used way beyond their life expectancy. Besides the loading components already considered in engineering practice, CI endure loads having cyclic nature acting continuously during service life (e.g., wind) which are often disregarded in foundation design and may trigger collapse mechanisms. Indeed, they are usually modelled as equivalent-static actions thus neglecting the accumulation of generalized permanent displacements of piled foundations in terms of settlement, sliding and rotation. This lack in design approach can be attributed to very limited research contributions dealing with cyclic loads. 

The CTP-ISSR (monotonic and cyclic Centrifuge Tests on Piles in sand proving Innovative Solutions to enhance Structural Resilience) project aims at investigating via centrifuge tests the behaviour of piled foundations under different loading types. Two series of centrifuge experiments on annular shaped pile groups and isolated piles embedded in Hostun sand were carried out at an increased gravity of 50 g in the Turner Beam Centrifuge at Schofield Centre, University of Cambridge. 

To simulate prototype reinforced concrete piles, model piles, made up of cement and metal wires, were manufactured in laboratory by means of an ad-hoc mould and manual mortar pouring. Such modelling is necessary to replicate the strong dependency of pile cross-sectional moment capacity on the axial force. Piles were installed in an 850 mm steel tub filled with manually poured Hostun sand prepared with a low-to-medium relative density. 

The first test included 2 single piles ad 2 groups of 8 piles connected by a circular rigid cap clear from the soil. The model foundations were subjected to monotonic vertical loads or vertical eccentric cyclic loads of different amplitudes and frequencies. In a similar fashion, the second test included 3 single piles ad 2 pile groups. In this case, the model foundations were subjected to monotonic vertical or horizontal loads or to cyclic horizontal loads of different amplitudes and frequencies. The response of the foundation system, in terms of loads and displacements, was monitored through loads cells, Linear Variable Differential Transformers (LVDTs) and Micro-Electro-Mechanical-Systems (MEMS). A miniaturized Cone Penetration Test (mini-CPT) was used to characterize the soil before the test execution. The experimental campaign was accomplished in 10 working days (including, among the others, model preparation, execution, data acquisition and dismantle). 

The results of the experiments will serve as benchmarks for the development of a non-linear macroelement for piled foundation. Adopting this innovative approach allows the strengthening of CI resilience by adapting the traditional design and maintenance to properly consider changing loading conditions in a simple yet reliable manner. 

Researchers and practitioners will be the beneficiaries of the outcome of this research, expecting the following advantages: 
- Innovation in design, with the possibility to properly consider cyclic loading;
- Improvement in identifying possible failure mechanisms and the required maintenance to enhance reliability and resilience of existing CI.