Micropile Underpinning at Hines Maple Terrace
The owners of the Maple Terrace Building in the Uptown neighborhood of Dallas, TX wanted to renovate, expand, and convert its seven-story building from apartments to office space. Their property sits in an area that is known as one of the finest live-work-play communities in the city.
The project included a nine-story building expansion with a three-level basement constructed immediately adjacent to a new 22-story residential tower with a two-level basement. GEI was contacted to assist with the critical foundation support for the existing structures.
Due to the proximity of the two buildings and additional building projects, the existing building, founded on shallow spread footings, required support not only to protect the existing structure but to facilitate the project’s needed excavation.
Our team used micropile underpinning to help transfer the loads from the existing building to the rock below the bottom of the excavated elevation. Underpinning supports a building’s foundation from the underside of the foundation.
During the pre-bid phase with limited information available, GEI designed an underpinning plan for the existing building’s footings using common industry methods. The design assumed the existing footings would be suitable for re-use as a pile cap supported by micropiles.
However, after the project was awarded, a field investigation – performed to understand the dimensions and conditions of the existing footings – revealed that the concrete strength of the existing footings was much lower than anticipated. Additionally, the footings featured different shapes and sizes, and several were thinner than expected. This prohibited the use of the underpinning approach recommended in the pre-bid design phase.
GEI used the information from the field investigation to quickly re-design the underpinning system without impacting the overall construction schedule.
Our team revised the design to connect the micropiles to the building’s grade beams, bypassing and abandoning the existing footing. GEI developed a flexible design to accommodate the varying footing and grade beam geometries.
The connection included a load transfer frame using tensioned tierods, which clamped onto the existing structure, and steel beams to transfer the load into the micropiles.
The specific dimensions and layout of the underpinning used at each footing varied based on the footing geometry and unique building geometry. In some cases, the underpinning connection needed to fit around building corners and the building’s columns and pilasters.
The underpinning design allowed the building to remain stable during excavation for the building expansion and the new tower basement, and for the next steps in construction to continue without delay.
The owner, their project team, and the client were satisfied with the successful completion of the underpinning work. The building’s owner has started selling the units in the new tower, adding to the buzz in the growing Uptown community.
