Designing our Underground Space

This paper suggests that a simple micro-scale approach combined with a macro-scale approach to determine equivalent design parameters provides effective numerical modeling of pre-reinforced zones in a tunnel. This new technique enables the determination of the equivalent stiffness of pre-reinforced zones with a series or parallel combination of ground, grout bulb, and steel pipe stiffness. For verification, the authors compared the 3D numerical results from the suggested approach with the results of a precisely-simulated model which has the best precision. The comparison suggests that two cases achieve the best approximation to the solution: one case is related to the series-parallel stiffness system (SPSS) in which a bulb and steel are coupled in parallel and then connected to the ground in series; the other case is related to the series stiffness system (SSS) in which only a bulb and steel are coupled in series. The SPSS is recommended for calculating the stiffness of pre-reinforced zones because the SSS is inconvenient and time-consuming. The SPSS provides a slightly bigger vertical displacement at the crown of a tunnel in weathered rock than in other cases, and it produces almost identical results to a precisely-simulated model of horizontal displacement at the tunnel springline and at the ground surface settlement. The SPSS suggested in this paper effectively represents the behavior mechanism of pre-reinforced zones when compared to a precisely-simulated model.

This paper focuses on the contribution of drilled piers in making an awesome project spectacular. Drilled piers and shallow foundations combined to eliminate temporary retention systems to support the Texas Medical Center's tallest building at thirty-two stories. Reduced development costs amounted to over $1 million when consideration is given to increased useable basement space, eliminating cantilevered floors for one full bay to the property line, and eliminating interior perimeter columns to effectively minimize the depth of basement excavation. The unique foundation features of the building and parking garage are highlighted along with the structural concepts for pier design. Drilled piers formed the temporary and permanent basement wall along with the sole support for thirty-two stories of exterior column loads of the reinforced concrete tower. The drilled pier design considered installation only by the Slurry Displacement Method in general accordance with the ACI 336.1-89 & 01 specifications, and modified for the site-specific design. Pier design details are furnished along with the concrete mix designs and construction engineering results of the completed piers to the base of the basement excavation. The Design Geotechnical Engineer investigated, designed, construction engineered, inspected, and monitored.

This study attempts to evaluate the possible tunnel influence parameters and their rating in soft rock and gravel formations. A total of seven main influence parameters were considered to be sensitive to the stability of tunnel excavation, including depth of tunnel, class of rock material, scale of rock cementation, stability of excavation face, groundwater inflow quantity, turbidity of groundwater, and the effect of ground softening due to groundwater. The data related to these parameters were recorded and rated by the site engineer from three representative tunnels, then an appropriate weighting of each parameter was applied from statistical analysis. By comparing the rock rating with the selected support types and monitoring results, the influence parameters and their weightings were determined. In addition, the soft rock tunnel classifications and their corresponding support types are also proposed in this paper.

Trench Remixing and Deep wall method (TRD) developed in Japan was recently used to construct two demonstration test cells at a site in Long Beach, California. Seawater intrusion occurs through a shallow aquifer of 30 meters in depth, which fouls the fresh water supply aquifers farther inland. The physical cutoff wall concept calls for a soil mix wall constructed into the underlying aquifer, and spanning the 3.5-kilometer length of the gap. During design, hydrogeologic evaluation indicated variability in the aquitard beneath the shallow aquifer; therefore, test cells to depths of 20 and 24 meters were utilized to assess seepage under the wall with respect to wall depth. Testing of 5 different soil-mixes indicated a slag-cement-sepiolite-soil mix as optimal for field application that could achieve a permeability of 1 x 10^–6 cm/sec or less in the saline environment. Use of the TRD method to construct the test cells proved to be effective at producing a homogeneous soil-sepiolite mix wall to a depth of 24 meters. The resulting permeability of the wall as demonstrated by laboratory and field tests is less than 1 x 10^–6 cm/sec. During testing, seepage past the wall into the test cells was predominantly through the aquitard sediments beneath the test cell walls with the deeper wall more effective at cutting off the underflow.