Dearborn & Fort: Final Geotechnical Engineers Causal Report


A general summary of the report conclusions and recommendations is provided below:
1. Unexpected and significant ground movement that resulted in two major heave areas and damage to existing buildings, pavement, and utility structures occurred at the southeast corner of the intersection Fort Street and Dearborn Street in Detroit, Michigan during the evening of Saturday, September 11, 2021.

2. Based on video footage from security cameras, the majority of the ground movement 
happened between about 7:00pm and 7:35pm on September 11, 2021.

3. From observations collected during our initial site visit on September 13, 2021, and our 
knowledge of the subsurface characteristics of this area of Detroit, we suspected that the primary cause of the ground movement was related to the presence of a nearby stockpile of metallic scrap and soft ground conditions. This report primarily addresses the geotechnical aspects of site investigation and analysis that were performed post-incident to determine the most likely cause of the ground upheaval.

4. Central to the two major heave areas is the corner of a property owned by Fort Iron and Metal. Since the mid-1980s, this corner of the parcel near Fort Street and Dearborn Street has been used for scrap storage. At the time of the incident on September 11, 2021, a large stockpile of mill scale was situated in this exact area of the parcel. The post-incident topographic survey indicated that the peak elevation of the stockpile was almost 610 feet, or about 25 feet above surrounding street grades. Based on the video camera footage, it appears that the top elevation of the stockpile was lowered during the failure event, indicating that the top of the mill scale pile was likely situated at a maximum elevation greater than 610 feet. Based on a sample of mill scale collected from the stockpile and our laboratory testing, the mill scale has a unit weight of about 230 pcf. This value is in the range of about two times the unit weight of most soils.

5. In the 100-day period before the incident, 15.8 inches of rainfall was recorded at the 
Detroit/Pontiac weather station. This is about 150% of the normal rainfall received during 
this timeframe.

6. Somat performed two soil borings between September 15 and September 18, 2021. Soil samples were collected for numerous laboratory tests, and in-situ vane shear tests were performed to determine the shear strength of the clay soils encountered in the borings. Additionally, an inclinometer was installed in one of the boreholes to measure lateral subsurface displacement in the soils.

7. The results of our field investigation and laboratory testing indicated a subsurface profile typical for this area of Detroit, including zones of very soft to soft clay soils encountered between about 20 to 55 feet below existing grades, or within elevation range 565 to 535 feet.

8. Section 5 of this report provides a hypothesis and rationale for the most likely causes of the incident. We have considered several reasonable, but wide-ranging possibilities 
including mine subsidence, artesian groundwater, methane gas, utility rupture, seismic 
activity, excessive rainfall, soil shear failure, and time-dependent consolidation settlement of the clay soils underlying the site.

9. Ultimately, it is our professional engineering opinion that the weight of the stockpiled mill scale material exceeded the shear strength capacity of the underlying clay soils to cause the heaved areas and disruption to the existing structures adjacent to the Fort Iron site. Other factors such as consolidation settlement may have contributed to the incident and/or to the timing of the incident, but the primary cause of the soil failure was the load imposed by the mill scale stockpile.

The summary presented above is general in nature and should not be considered apart from the entire text of the report with all the qualifications and considerations mentioned therein. Details of our findings are discussed in the following sections and in the appendices of this report.

Link to Dearborn & Fort Report