Building Underpinning

 

By
Gerard Hillenbrand, P.E.

 Everything You Always Wanted to Know about Underpinning

(But Were Afraid To Ask)

The May 21st, 2007 dinner meeting hosted by New York chapter at Patrick Conway’s restaurant on short block away from Grand Central Terminal had a record turnout. Once again, many representatives of our city’s architectural community including a substantial contingent of women experts and careerists joined a large group of engineering practitioners. The intriguing subject featured the expert team of Joe Tortorella, P.E., and Rick Ellman, P.E., who are developing a local reputation as the reigning experts on this critical aspect of public safety in our city’s construction industry. This expert team was supplemented by Dan Eschenasy, P.E., of the Department of Emergency Operations, who lent his support for the views and procedures presented by the guest speakers, and was available to answer the inevitable questions related to N.Y. City’s building code and its rules and proposed revisions.

The speakers began their presentation with vivid slide pictures of recent building collapses which resulted in dangers to public safety, costly delays and repairs, occasion injuries and deaths, as well as the resulting deluge of lawsuits, public recriminations and loss of confidence in the professionalism of the construction industry. The pictures showed a series of collapses in Brooklyn (Frequently showed a series of collapses in Brooklyn (Frequency of one to two per month) where attached and semi-attached buildings ranging in height from one to six stories are common. The failures occur when a building lot is excavated for a new structure, and the excavation weakens the foundation or structural support of an adjacent building resulting in the collapse of the adjacent building in a rapid and unpredictable scenario. In almost all cases the excavation causes a loss of the adjacent building’s foundation support and this loss is attributed to inadequate underpinning on the foundation in question. The remedy, of course, is the correct design and construction of the underpinning in the first place. But after a collapse the department of buildings will issue a stop work order and require that all affected adjacent buildings be vacated, thus protecting the public safety until remedial action is approved and in place.

The speakers then proceeded to define the correct design of the various underpinning techniques – Mr. Tortorella from the Structural Engineering perspective and Mr. Ellman from the Geo-technical or Foundations perspective. First of all, the developer of the new building should select an architect with experience and a track record in building constructions of this type and this Architect should involve structural and Geo-Technical Engineers as early as possible in the design process. The developer and Architect should also enlist the cooperation of the owners of the adjacent buildings and keep them informed of progress and any discovered problems. The choice of a construction contractor, usually decided by competitive bidding, should also be decided as early as possible in the process, with suitable experience and expertise as important factors as well as price. The Engineers should then supervise test borings and digs to complete all aspects of the site investigation including recommending the number and location of the various borings. Continual monitoring of this process is essential, as well as an investigation of the construction history of adjacent properties. Test borings provide analysis of the supporting soils, effects of vibrations contributing to soil settlement, the ground water level at the site, and the running sand conditions that is, the tendency of wet soil to move and then collapse.

The quality of the adjacent structure’s foundation also must be investigated. Discovery of weak adjacent soil conditions will necessitate underpinning the foundation and possibly consolidation and/or de-watering of adjacent soils. Pile driving at the construction site can cause supporting soil settlement and a tlt in the adjacent structure. Also nearby subway traffic may contribute to vibration sensitivity at the site and its environment. The need for underpinning in these circumstances is best determined by excavating test pits at strategic locations adjacent to the foundation in question. The test pits themselves must be correctly designed and reinforced with such techniques as Jack Piles, Bracket Piles and Arched Supporting walls over the pit excavation. The pit itself may have to be reinforced with piers and/or concrete walls and may require additional support from added tapered and forged steel wedges as well as pre-loading from hydraulic jacks. Even with all these precautions, cracks in the adjacent foundation are hard to avoid. These cracks require the use of jet grouting to provide a connection to solid ground. In more serious cases, slurry walls may be necessary and even direct connection to solid rock may be required.

The speakers next emphasized that there is really no substitute for proper design of the underpinning before construction begins. Detailed designs must be submitted to the Department of Buildings to establish an accurate engineering record. Engineers have the duty to educate the contractors diligently on the details, means and methods specifying the underpinning, the surrounding soil conditions, and the need for special bracing and load transfer. Above all, engineers must stay involved throughout the construction process, remembering to fully comply with Rule 52 before the start of excavations.

Despite all these precautions, breakdowns at the construction site sometimes occur and remedial repairs are required. Remedial techniques include the use of bracket wall ties, lateral bracing and reinforcement, and connecting straps, as well as steel shims for load transfer. Al remedial construction should be approved and monitored by the engineers of record and certified by the Building Department, complete with correct drawings defining the actions taken. Special care must be taken to extend underpinning beyond the corners of existing foundations, using crack gauges to define the severity of field problems, continuing vibration monitoring and timely inspections of all remedial efforts. Unexpected field problems may even require hiring special engineering consultants. During these periods’ contractors and engineers should be mindful of the need to keep the owners of neighboring buildings completely informed as to discover problems and the proposed solutions.

This meeting concluded with the usual vigorous question and answer period which also touched on the handling of lawsuits and the need for adequate and extensive insurance coverage for all eventualities the speakers recommended developing and action plan for handling these problems before the start of construction.

The meeting was co-sponsored by the Metropolitan Section of the AMSE and all attendees were eligible to earn 1 Professional Development Hour of credit toward the continuing education requirements mandated by the N.Y. State Education Department.

 Posted by at 2:48 pm