plug at the bottom of the shaft, which was also toed into the side walls using a shear key type construction. While this concrete plug provides resistance in the long term, base heave was a significant concern immediately after excavating to the final depth, before the plug could be constructed and the concrete achieve its maximum compressive strength. A 2D numerical model of the shaft was developed that considered the soil layer response and the stiffness of the structural elements forming the shaft walls. The model was calibrated using oedometer tests, triaxial results and falling head permeability experiments. Three separate analyses were considered: (i) an undrained type soil response, (ii) a drained soil response and (iii) staged construction considering all of the consolidation phases. In the first analysis, which was relevant to the short-term condition, the soil was considered to behave undrained. This analysis showed that immediately after excavating to the base of the shaft, the unloading of the soil created negative pore pressures below the base. These negative pore pressures can also be thought of as “soil suctions,” and provide added stability to the soil below the shaft, which ensured the shaft was stable immediately after the excavation. The second analysis considered the soil to act as fully drained, and, therefore, represented the soil condition following dissipation of the excess negative pore pressures. This analysis showed the shaft to be completely unstable with the soil plug heaving upwards and the shaft collapsing as the failure surface intersected the gravel aquifer, resulting in a catastrophic failure. The final analysis, which was much more complex, considered the time taken to build the shaft and modelled the partially drained consolidation during the construc- tion process. Once the final dig was com- plete, the analysis then considered the time dependent change in the pore pressure regime in the soil as the material transitioned from undrained to drained conditions. This analysis predicted the factor of safety as a function of time following excavation to the target base elevation. The development of the failure mechanism was apparent and the time to failure was shown to be in the order of four months. This type of analysis allowed 72 • DEEP FOUNDATIONS • JAN/FEB 2017 Seepage analysis of the proposed flood defence system Case Study - Flood Defence Embankments The Office of Public Works is responsible for identifying urban areas in Ireland that are at risk of periodic flooding, and, in one such location, the local town and county councils had developed a flood defence scheme to protect life and property from both severe high tides and river flooding due to extreme rainfall events. GDG was commissioned to complete the design of the flood defences along a 500 m (1,640 ft) long cul-de-sac to the north of a tidal stretch of river and adjacent to a golf course that has been identified for future development. The defences that were required in this area were a combination of flood walls to contain the river and a flood relief channel to relieve flood waters that may threaten a nearby low-lying residential area. The proposed design required a reinforced concrete gravity retaining wall to complete the flood defences. A detailed hydrogeological analysis and numerical seepage study was undertaken. GDG also provided the designs for the relief channel to alleviate flooding, for a road access to the golf course, for the relocation of services and for a pedestrian access from the nearby road to an underpass at the adjacent rail line. interfaces — they are provided by engineers with relevant experience and a will to dedicate time to critically think about challenging problems. Real value is provided when these traditional engi- neering ideals are combined with the most up-to-date numerical modelling capa- bilities. A tradesman relies on their tools to complete a job to a high standard and so does a geotechnical engineer; however, the final finish is driven by the engineer and not the tools available to them. While numerical model l ing sof tware has improved dramatically, the final solution adopted for construction should be one that recognises (a) the underlying limitations of the software, (b) the level of calibration, (c) simplifications in the soil behaviour and geometry that are modelled, and (d) the experience of the user in analysing the problem at hand. Engineering should always be done by engineers and not computers! Paul Doherty BE, Ph.D., C.Eng., is the managing director of GDG, a specialist geotechnical engineering consultancy in Dublin, Ireland, providing innovative geotechnical solutions across a broad range of civil engineering sectors. John O’Donovan Ph.D., C.Eng., MICE, a senior engineer leads the urban construction sector at GDG specialising in the design of founda- tions and basement structures and in the assessment of ground movement on existing buildings. a pragmatic construction programme to be developed that relied on a relatively quick construction of the base plug. A suite of monitoring points was also established on the shaft walls and base to allow the heave to be assessed and compared with the predic- tions to provide confidence in the residual safety of the shaft at any moment in time. Conclusions Despite the advancement of computing power and the improved efficiency of numerical modelling software, we must remain diligent in our search for robust, reliable and efficient engineering solutions. These solutions are not driven by increased computing power or user-friendly software
