FOUNDATION AND RETAINING WALL FAILURES: GATHERING THE CLUES Prepared by Scott J. DiFiore, P.E.

When foundations and retaining walls fail, owners, insurance adjusters, and engineers work together to identify causation to develop appropriate rehabilitation options, costs, and eventually implement repairs. Reference documents, site features, and distress characteristics all provide important clues. Recognizing or gathering these clues quickly is key to resolving these situations efficiently and effectively.

REFERENCE MATERIAL • Drawings and Reports: Project drawings or reports, if available, will often be in the hands of the building owner or building department. These valuable resources can help the investigative team identify the structural system, materials, utility information, prior structure use, renovations, or other relevant site information. • Subsurface Investigation Reports: Prior test borings or test pits will describe soil and groundwater conditions and possibly the pre-construction recommendations for foundations or retaining walls. Subsurface conditions give hints of acceptable bearing stratum and help determine whether the structure is bearing on shallow or deep foundations. Measurements for the groundwater table are typically contained in these reports and may include the fluctuation with time. • Weather Records: Weather records, available from the National Oceanic and Atmospheric Administration weather stations, track precipitation, temperature, and wind speeds. Understanding weather trends leading up to the time of failure can help determine if they were contributors.

SITE FEATURES • Water Features: Natural water features such as ponds, rivers, or marshes are strong indicators of potential unsuitable compressible soils at the site and also of a high groundwater table. • Depressions and Stockpiles: Any localized depressions could be evidence of buried debris with open voids. Stockpiles with debris and irregular materials could indicate a similar condition, if the stockpile was formed with material excavated from the site. • Topography and Drainage: Water can be detrimental to foundations and retaining walls. It adds lateral pressure to walls and adversely affects bearing capacity. Topography, in conjunction with site drainage features, such as downspouts, illustrates the direction of water runoff. • Utilities: Visible utility covers help locate utility lines. Organizations, such as Dig Safe in New England, will locate public underground utilities upon request. Broken utilities can collapse and/or leak, which can affect the structures they are immediately below or adjacent to.

IMAGES: Slab settlement adjacent to foundation wall (top); Cracks on facade indicative of differential foundation settlement (middle); Bulging and overhanging in gabion retaining wall (bottom) SIMPSON GUMPERTZ & HEGER September 2012 FOUNDATION AND RETAINING WALL FAILURES: GATHERING THE CLUES (continued)

DISTRESS CHARACTERISTICS • Voids or Erosion: Voids between soil and structure can indicate settlement or erosion. Small excavations below the basement slab or shallow footings can help expose such conditions. • Rotation: Retaining walls are installed to be vertical, or even with a negative batter (slight tilt into the slope). If the top of the wall overhangs the bottom, there could be stability concerns. Similarly, for buildings, out-of-level beams, and out-of-plumb columns or walls can be indicative of differential foundation settlement. • Cracks: Cracks in slabs, walls, columns, or beams illustrate structural behavior and could be a result of normal shrinkage or temperature fluctuation. However, cracks can also be indicative of structural distress. Crack thickness may vary over its length, which can help define the type of distress. • Efflorescence and Staining: Efflorescence is evidence of moisture migration and staining on foundation or retaining walls can indicate prior high water levels and seepage, which could have contributed to damage.

By recognizing the signs of failure, owners and insurance adjusters can play a crucial role in identifying clues and gathering information in order to help their engineering teams quickly solve the problems at hand.

IMAGES: Staining on lally column indicative of past flooding (top); Void beneath concrete slab-on-grade illustrating soil settlement (bottom)

Scott J. DiFiore is a geotechnical and structural engineer with over twenty years of experience in the design, investigation, and rehabilitation of foundations and retaining walls. Scott is a member of the Geotechnical Advisory Committee for the Massachusetts Board of Building Regulations and Standards (BBRS) and serves on the Code Advisory Committee for the Deep Foundations Institute (DFI). He currently teaches the SGH Professional Engineering review course on Foundations and Retaining Structures and previously taught Engineering Mechanics at Tufts University. E-mail Scott at [email protected].

Simpson Gumpertz & Heger (SGH) is a national engineering firm that designs, investigates, and rehabilitates structures and building enclosures. Our award-winning work encompasses building, transportation, nuclear, water/wastewater, and science/defense projects throughout the United States and in more than thirty other countries.

For more information, please visit www.sgh.com

SIMPSON GUMPERTZ & HEGER September 2012