Urban Center at Liberty Park

Liberty Park is a mixed-use development nestled in the forested hills along I-459 and Liberty Parkway in Vestavia, Alabama. Beginning in the early 1900's this property was extensively undermined with room and pillar coal mining operations. The mining operations had continued through the late 1950's. A network of abandoned underground mines lies beneath this property. At some locations, two seams were mined resulting in two levels of underground mines with some of the mines extending below the Cahaba River.

Torchmark (formerly Liberty National Insurance Company) purchased the property in the early 1980's. While it was generally known that the property had underground coal mines, the client was unaware of the extensive nature of relatively shallow depth of the underground mines.

BHATE Geosciences was retained after the property was purchased by Torchmark to conduct a detailed evaluation to define the extent of the mines and prepare a development plan with a mine subsidence risk assessment. Our initial evaluation included review of historical mining maps and a field survey of the mine shaft openings and surface subsidence features. Deep mine probes were conducted with underground examination using borehole cameras to evaluate the condition of the mine void existing supports. The evaluation and information gathered was used to develop cost effective remediation plans. The structures in this development are supported on a variety of foundation systems as follows:

* Spread Footings after excavation and backfilling of shallow mine voids and replacement with engineered fill. Mine voids were less than 40' deep. Example: Boy Scouts- Alabama Headquarters

* Complete Mine Void Filling for Subsidence Mitigation: The entire mine void in the affected zone was filled by creating barrier walls and flush grouting. Example: Federal Reserve Bank of Atlanta- Birmingham Branch. The Federal Reserve Bank preferred complete mine filling to prevent potential future issues of unauthorized mine access under the structure.

* Grout Pillars with Drilled Pile Foundations: Buildings 1000 and 1200 are 6 stories each with a total buidling area of 326,000 SF. Mine depths ranged from 60' to 90' below the ground surface. Grout pillars were used for subsidence stabilization for vertical and horizontal movements and for slab on grade support with foundations extended through the mine cavity using a 10" diameter pipe pile through the pillars for a load capacity of 120 tons.

* Grout Pillar Subsidence Stabilization and Shallow Footing Foundations: Building 1400 and 1500 are supported on stabilized mine shafts to mitigate subsidence. A variable pattern of grout pillars constructed from ground surface was used depending on the mine depth. Shallower mine depth required more closely spaced group pillars. Mine depth ranged from about 100' to 160'

*WINNER* 1994 ABC Excellence in Construction Award