Foundation Solution Using Helical Piers
August 27th, 2008 | by |Written by Paul Mauer, President, Levelift Systems, Inc.
Construction is currently progressing on the multi-million dollar expansion of the Baltimore Washington Medical Center. Formally known as North Arundel Hospital, the facility opened in 1967 as a 107-bed hospital serving the local community. The current renovations, including a new Inpatient Tower, will more than double the size and capacity of the original facility.
Columns for the Inpatient Tower were located adjacent to existing column footings. The original design called for the existing column spread footings to be incorporated in to a reinforced concrete mat that would support the existing columns as well as the proposed columns. Existing concrete spread footings were to be enlarged to form a large reinforced concrete mat encompassing three existing footings and support for two of the new tower columns. Because of the depth of the existing footings, the concrete mat would step up from a maximum depth of 9 feet below the existing floor slab (adjacent to the Cafeteria) to a shallow depth of 1.5 feet below the slab at the other end. Construction of the mat required an excavation of approximately 10 feet deep and nearly 20 feet on either side of the proposed footings.
Two major problems associated with the removal of such a massive amount of the existing floor slab. First, the footings were located deep within the existing structure, making spoil removal difficult and expensive. Second, with the required lay-back area, the hospital would be forced to close the existing Cafeteria forcing the administration to provide other forms of food service for employees and patients. The decision was made that the cafeteria must remain open therefore, the general contractor on the project reviewed alternatives to the proposed spread footings.
To avoid over-loading the existing footings, new column loads had to be transferred from floor level to at least a depth equal to the existing footings. Helical piers were an obvious solution given they create no spoils when installed; the shafts are segmented, allowing installation in the existing structure with a floor-to-floor height of approximately 10 feet; soil borings indicated firm bearing material approximately 15-20 feet below the existing footings; access to the project area was blocked for larger equipment and compressors due to the interior space.
The column loads to be supported presented the biggest challenge associated with this project. Drawings called for new column loadings of 150,000 lbs. compression.
Given the limited access, the helical piers were designed to be installed with a hand-held drive head with the power pack situated on the low roof adjacent to the footing locations. Therefore, the ultimate capacity of the piers was limited to 60,000 lbs. based on 6,000 ft-lbs. drive head assigned to the project.
The design solution for the footings was to provide many helical piers with a working capacity of 30,000 lbs. each. The new column load was added to the existing column load to create one new larger load to be supported. The required footing extension was sized based on the 30,000 lbs. capacities of the helical piers combined with the soil bearing beneath the existing footing. The new section was doweled to the existing footing to provide a combined response to the new column loading and the existing column loading. The location of the new column and the fact the existing footing was located 8 inches below the existing floor slab, dictated that some form of a combined footing was necessary for this column. The number and spacing of the helical piers was calculated to provide a similar anticipated settlement as the existing spread footing to avoid any potential differential movement.
Existing footings in the second new tower column were located 44 inches below the existing floor slab. This additional depth allowed the integration of a reinforced concrete grade beam to act as a transfer beam for the new column load. Support pads were designed with helical piers at the bearing portions of the transfer beam creating a sort of ?saddle bag? shaped foundation. The project was completed on time and within budget.