As part of an expansion to the Memorial Hermann Heart and Vascular Institute's (MHHVI) facilities on the Texas Medical Center campus in Houston, Texas, Hayward Baker, Inc. was selected to construct a new below ground parking structure immediately adjacent to an existing structure. The new facility required the excavation of approximately 42,000 sq. ft. to a depth ranging from 18 to 30 ft. below ground surface. Our client, Hayward Baker, was hired by the owner under a design-build approach to provide a temporary excavation support system for this project. Geosyntec was selected by Hayward Baker to prepare and certify the support system design. Our client's goals were to (i) design a system that did not require alteration of the parking facility design or construction methodology and (ii) install the excavation support system without disrupting hospital operations or adjacent utility systems.
Geosyntec's Scope of Services
Geosyntec developed the detailed excavation design that was protective of the adjacent structures and cost-effective. Based on our analysis of the proposed parking structure elements, the excavation was best supported by an anchored soldier beam and lagging wall. Specific design challenges included: (i) the vicinity of the excavation wall to a primary vault for controls for a portion of Houston's public transit system; (ii) the on wall line interference of the corner of an existing three story hospital annex; (iii) the excavation geometry required for the soldier beam and lagging board connections; and (iv) the requirement that anchor heads not be exposed beyond the edge of the lagging. Depending on the location of the anchored wall and depth of excavation, our design specified the installation of one or two tiers of grouted prestressed ground anchors. Our optimization of the excavation design prior to construction included 100-ft long anchors angled at 30 degrees below the horizontal as opposed to shorter anchors inclined at 20 degrees below the horizontal. The implementation of our design revealed a number of unforeseen obstacles involving installation issues and field conditions. As a result, Geosyntec was called upon to provide construction phase-engineering services throughout the construction process. Examples include:
- Installation of 25 soldier beams in concrete-backfilled holes approximately 4 ft. higher than called for in our design. Our solution was to add a second lower anchor within 5 ft. of the bottom of the excavation to limit cantilever movements below the upper anchor.
- Potentially, a roadway and utilities could have been impacted by a 10-ft high steep slope from the top of the soldier beam and lagging wall to the existing grade. Our solution was to specify a 10-ft high soil nail wall to sit above the anchored wall to support this portion of the excavation.
- During construction, concerns were raised as to whether movements behind a 42-in diameter drilled shaft wall could be limited to the contract-specified value of 0.5 in. After performing LPILE analyses and reviewing historical movement data for excavations in Houston, our geotechnical engineers concluded that anticipated lateral movements of this system could be as much as 1 in. Our solution was to design an anchoring system for the drilled shafts to limit movements and a reinforced concrete wale beam to transfer anchor loads.
Geosyntec designed an excavation program that avoided the significant number of obstructions (foundations, utilities, etc.) existing outside of the excavation footprint. Hayward Baker has indicated that, based on the amount of obstructions encountered, the use of shallower anchors would have resulted in significant delays. Furthermore, we were able to keep Hayward Baker on track by the timely delivery of solutions to address field conditions and installation issues.