Computational Engineering & Analysis Deliver Effective Project Solutions

Tech Insights

Computational engineering enables the analysis of large amounts of data to understand, predict, and solve complex problems. Implementing computational engineering and analysis in underground development can provide innovative and cost-effective solutions for design optimization, construction risk mitigation, and added reliability.

Modeling Tools and Their Uses

• Two-dimensional (2D) and three-dimensional (3D) numerical modeling capabilities for complex linear and nonlinear, static, and time-dependent problems in geotechnical, structural, and hydrogeologic engineering using the state-of-the art technologies and computer programs
• Modeling and analysis for design and construction of underground structures, including small to large tunnels and caverns, with specialization in:
  • Construction sequence and excavation methods
  • Design of temporary and permanent structures for tunnels and caverns
  • Design of temporary and permanent supports for deep excavations
  • Seismic response of structures (pseudo-static and dynamic soil-structure interaction [SSI] analyses) of tunnels, shafts, and at intersections with different configurations
  • Assessment of impacts of underground construction on
  • adjacent existing structures
  • Assessment of impacts of future developments on existing underground structures
  • Groundwater flow and coupled fluid-mechanical interaction analysis
  • Thermal and coupled thermal-mechanical analysis

CASE STUDY

Analyzing Alternative Location for East Ventilation Shafts and Adit

The Atlanta Plane Train Tunnel West Extension is being constructed at Hartsfield-Jackson Atlanta International Airport using the Sequential Excavation Method (SEM). The tunnel has shallow ground cover and passes underneath the SkyTrain and MARTA stations and airport terminal buildings. It includes a twin tube Bifurcation with a span of over 45 feet (13.7 m) and ground cover of 12 feet (3.7 m). The tunneling is being undertaken beneath the world’s busiest airport, while impacts to its operations are minimized. One of the key design and construction challenges has been to develop effective solutions for addressing potential risks and uncertainties associated with the construction of the East Ventilation Shafts and Adit, which also have mixed-face conditions.

The location of the East Ventilation Shafts and Adit is constrained by the ventilation system requirements. The early design concept called for constructing these shafts underneath the terminal buildings, which would pose significant challenges for construction and controlling ground movements adjacent to the building foundations. An alternative location to the west of the existing ventilation plenum was subsequently proposed. This avoids the impact to the terminal building structures; however, it places the East Ventilation Shafts and Adit in close proximity to the Bifurcation and rock pillar nose with only a 5-foot (1.5 m) separation of rock at the interface between the North Shaft and the North Tunnel shoulder.

An assessment of the viability of this location was performed based on 3D numerical analysis using FLAC3D. The analysis looked into the effects of the excavation sequence of the Bifurcation and East Ventilation structures as well as the stress redistributions occurring following these excavation stages. The analysis also examined the pillar stability, especially at the area of the narrow nose, where there is a width of about 6 feet (1.8 m). The results of this 3D analysis indicated that this alternative location was acceptable from the perspectives of ground movement and overall stability of these combined and was, therefore, carried forward in the detailed design and construction. The numerical modeling in this case played a critical role in helping validate the feasibility of this alternative design solution, eliminate significant construction risks associated with the original solution, and ultimately achieve savings in construction schedule and costs. The results from the numerical modeling also provided the project team including both the designer and the contractor with the expected ground response, which was used to establish the requirements of geotechnical instrumentation and monitoring and necessary contingency measures during construction. Actual observed ground behavior and successful completion of the bifurcation and ventilation shafts offered a validation of this numerical modeling.