Measurement and modelling of soil-structure interaction for open caisson shafts
Keywords: multi-axis force sensor
underground construction
interface
optical fibre
numerical modelling
physical modelling
lubrication
reinforced concrete
bearing capacity
caissons
open caisson shafts
construction
force sensor
fibre optic
calibration
soil mechanics
geotechnical engineering
machine learning
friction
soil-structure interaction
shafts
caisson shafts
SSI
sensor technology

Monolithic open caissons are a common method of constructing underground shafts, for applications including storage tanks, pumping stations, deep foundations and tunnelling. The caisson construction process uses concurrent casting of massive reinforced concrete walls and excavation of soil within the structure, causing the caisson to ‘sink’ into the ground under its self-weight. Achieving controlled sinking relies on accurate estimation of the resistance generated by the interaction between the caisson and the surrounding soil. The lack of dedicated research and design methods means there is significant uncertainty in determining the frictional and bearing resistances during design, particularly in cohesionless soils.
This thesis aims to explore the soil-structure interaction during caisson sinking in sands, through numerical modelling and experimental testing. The outputs of recent caisson field monitoring were limited by critical failures of electrical sensors. To facilitate measurement of the caisson-soil contact stresses, the development of more robust sensor technology is necessarily pursued. A new framework for multi-axis force sensing is demonstrated, to enable optical strain measurement in the form of FBGs to be utilised. This is applied to create a sensor for detailed measurements of combined normal and frictional interface stresses in underground construction applications. Numerical simulation and physical testing build confidence in the new design, as well as exploring a novel approach for measuring effective normal stress.
Friction at the caisson external surface is explored through laboratory-scale testing in dense sand, using a bespoke large-displacement interface shear apparatus. The new optical sensors are employed to obtain localised contact stress measurements and particle image velocimetry provides observations of the soil displacements. The results give insight into the mechanisms associated with different interface conditions, including over-cut creation and lubrication.
The bearing capacity at the base of the caisson wall is explored through numerical modelling. An extensive parametric study using finite element limit analysis considers the influence of salient caisson parameters, including the taper angle and cutting shoe. A complimentary targeted study with finite element analysis is used to assess the influence of soil non-associativity and the development of horizontal reaction. Detailed observations of the soil failure mechanisms provide new insight into the bearing behaviour in sands. The numerical results are compared against recent laboratory-scale experimental tests and used to develop simplified closed-form expressions for bearing capacity estimation in routine design.