Earth Retention System Failure


A water-management district was conducting major improvements at one of its treatment plants that required significant excavation to install new sewer mains. The deep open-cut excavation was braced by an earth-retention system composed of numerous soldier piles and steel-bracing elements. During construction, the earth retention system failed, resulting in collapse of the sloped embankment. To prevent further collapse, the excavation contractor filled the trench with gravel, effectively burying the evidence.

Effective forensic methods for recovering and documenting the evidence were critical to determining root cause, meeting loss-recovery requirements, and avoiding future problems with reconstruction. The water reclamation district retained CTLGroup to determine the root cause of the failure as expeditiously as possible. The scope of the investigation included numerous field examinations, monitoring the collapse site throughout demolition and recovery of evidence, studying the materials and structural components recovered, and performing structural analysis to ascertain the reasons for the failure. Time was of the essence in the recovery process, as the collapse occurred in the vicinity of a commuter rail line, which was undermined and shut down as a safety precaution.

Stabilizing the area and rebuilding the soil embankment was ongoing 24 hours per day, so site conditions were constantly changing. Safe documentation of conditions was hampered by the presence of earthmovers and heavy equipment everywhere. To safely monitor the artifact retrieval progress without having to position personnel on site 24/7, the forensic team installed remote surveillance cameras in multiple locations and deployed engineers to the scene as demolition activities were approaching areas of interest. This approach permitted an efficient site investigation process so that the contractor and plant could minimize lost time on the project and the commuter train service could be restored as quickly as possible.

The earth-retention system was composed of several similar-looking components. To accurately reconstruct the failure, the forensic team labeled the recovered items in-situ with unique identifiers based on the design drawings prior to cutting and removing them to an off-site laydown yard for hands-on inspection. This was critical to understanding the root cause of the failure and which party was responsible for the damages incurred by the water reclamation district and the commuter rail service agency. Design and fabrication deficiencies of the bracing elements spanning between the earth retention system soldier pile walls were keys to the failure. The well-documented investigation ensured the owners had hard facts about causation to permit recovery of damages from their loss.

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