Presented by:  Dov Leshchinsky, Ph.D., P.E.
University of Delaware   

Presented January 17, 2008

The Geotechnical Group held its 33rd Annual Martin S. Kapp Dinner Lecture at the Il Campanello Restaurant in Manhattan on January 17, 2008. The lecture took place after a buffet dinner attended by a full house of more than 100 attendees. It focused on the design and construction of mechanically stabilized earth (MSE) walls and the lessons learned from three specific case history failures, each due to unique conditions.

Dr. Leshchinsky began the lecture by describing several MSE walls that had failed due to a build-up of water pressure behind the walls and the progressive block failures that ensued. Such shallow failures were described as common and often result in loss of fascia blocks without compromising the stability of the geo-reinforced soil.

Dr. Leshchinsky then presented three case histories to illustrate more severe failures in design and construction methods.  They included a geogrid reinforced wall adjacent to a Shop ‘N Save Supermarket, a 54-foot high wall with geotextile reinforcement at a Walgreen’s Store development outside Atlanta, Georgia, and a seven-mile long Airport Runway Seawall in Sydney, Australia.

The Shop ‘N Save retaining wall was designed and constructed to increase the land area of a formerly filled property for the purpose of constructing the supermarket and parking areas. Within a year of construction, cracks of up to 1.5” were noted in the walls of the supermarket structure. Parametric studies were conducted utilizing various analysis methods and computer models. The analyses revealed that the critical slip plane of the slope was beyond the limits of the installed geo-grid and that the safety factor ranged between 1.2 and 1.3. The designer had also assumed that the backfill would act as a ϕ-c material with a high cohesion value. If cohesion was zero, the factor of safety reduced to 1.1. Although the safety factors were greater than unity, it was Dr. Leshchinsky’s experience that ground movements increased as the safety factor approaches unity and a greater amount of the available shear strength of the soil is mobilized. Therefore, he concluded that the slope would continue to move and recommended that the wall be stabilized with anchors. Dr. Leshchinsky attributed this failure to a failure in communication and overlap between the several different entities (civil, geotechnical and wall design engineers) responsible for wall design and concluded that as much as possible a wall design should be performed by one entity.

The Walgreen’s retaining wall was similarly intended to increase parking area for the new store. The retaining wall had a gabion facing with geotextile reinforcement and included vertical drainage risers that were installed within the reinforced soil, contrary to the design. During construction it was noted that the amount of fill placed did not sum to the required fill level. There was also little construction oversight to enforce compaction requirements. It was concluded that the lateral connections between the vertical risers probably sheared due to settlement caused by poor soil compaction during construction. The broken risers then fed surface water runoff directly into the reinforced soil leading to ground loss and continued settlement of the backfill. The wall was repaired using soil anchors. Dr. Leshchinsky concluded by stressing the importance of proper backfill compaction and warned against the installation of drainage within the reinforced soil.

The third case involved a design-build seawall constructed for the Kingsford Smith Airport, Sydney, Australia. The wall was constructed of precast double T-wall panels that were back filled with dredged sand. The joints between the panels were protected by non-woven geotextile that were spot glued into place prior to filling and compaction. Sinkholes were observed to develop near the joints of the panels. The case was brought to litigation. During litigation, it was found that the contractor had increased soil lifts used during backfill compaction from the specified 12” to 24”. The inadequate compaction led to settlement and drag down forces on the geotextile fabric opposite the joint. The spot glue used to secure the geotextile fabric did not hold up to the dragging down during compaction. The gaps were left unprotected and backfill washed out due to wave action. Judgment was found against the Contractor who fixed the problem by jet grouting behind the T-wall.

Dr. Leshchinsky concluded the lecture by reiterating the importance of performing a complete design of retaining walls and the importance of transferring that design to the contractor so that it is constructed to meet the intent of the design. Following a number of questions from the audience, Dr. Leshchinsky was presented a Golden Apple from Geotechnical Group Chairman Terry Holman, Ph.D., P.E. in appreciation for the lecture.