U.S. Department of Labor
Occupational Safety and Health Administration
Cover photo was provided by
St. Croix, U.S. Virgin Islands
This report was prepared by
Scott Jin, Ph.D., P.E.
Office of Engineering Services
Directorate of Construction
On August 22, 2006, around 11:00 a.m., a more than 40 foot high, steep excavated slope suddenly collapsed. The collapsed earth deposits buried a nearby excavator and killed the operator inside the cab. The excavator was cleaning the base of the slope when the incident occurred.
Personnel from the Puerto Rico Area Office (PRAO) of the Occupational Safety and Health Administration (OSHA) arrived at the scene within hours of the incident. The OSHA investigation began soon after the incident and included interviewing witnesses, taking photographs and collecting soil samples for classification tests. In the course of the investigation, the same personnel from PRAO made additional site visits to interview the owner and contractors, take additional photographs and collect additional contractual and technical information.
On August 24, 2006, OSHA Regional Administrator, Region II, requested the Directorate of Construction, OSHA National office, Washington, DC, to provide engineering assistance in assessing the collapse and in determining the cause of the incident. Note that we did not have an opportunity to visit the incident site and take any measurements.
We reviewed the geology and the soil survey around the construction site. We examined earth deposits from the failed surface and the overall shape of the failure based on the photographs. In addition, we calculated the slope and height of the excavation from the topographic survey after the incident. Based on the above, we concluded that:
The following is a chronological list of information received from the Puerto Rico Area Office (PRAO) during the investigation.
On August 25, 2006, DOC received 11 photographs of the collapse. These photographs were taken by the general contractor, GEC, LLC, on the day of the incident.
On August 30, 2006, DOC received 20 more photographs of the collapse. These photographs were taken by the OSHA compliance office on August 22-23, 2006. DOC also received the grading and drainage plan of the construction site (Drawing No. C-106 and C-107), the police report and a newspaper article describing the incident. After reviewing the above information, DOC submitted a request for additional information to the area office.
On October 16, 2006, DOC met with the area director of the OSHA area office in Hunt Valley, MD, to discuss the incident, and received the following requested information:
On January 19, 2007, DOC received the demolition specifications and the operation & maintenance manual of the damaged excavator, Komatsu PC220LC.
The project under construction was a 40,000 square foot retail and office complex with two levels of parking. It is named East End Plaza, located at the east end of St. Thomas, U.S. Virgin Islands (USVI). The two parking levels and the plaza of the project were being constructed in steps along the slope of a mountain near its toe as shown in Figures 1 and 2.
The East End Plaza, LLC, owns the project. Springline Architects, Harris Civil Engineers and Garland Wilson were the architect, civil engineer and structural engineer of the project, respectively. GEC, LLC and Apex Construction Company, Inc. were the general contractor and excavation subcontractor of the project, respectively. Except for Harris Civil Engineers from Winter Park, FL and GEC, LLC from St. Croix, USVI, all the others are local firms in St. Thomas, USVI.
The project was in the first phase of construction, i.e., excavation and site stabilization, when the incident occurred. At that time, the excavation of the parking areas into the mountain slope was almost completed. The retaining wall between the upper and lower levels of the parking area, near the middle of the project site, had been constructed as shown in Figure 4.
On the morning of August 22, 2006, the employee/operator of the excavator (Komatsu PC220LC) used a hydraulic breaker to loosen the earth deposits from the mountainside. He also used the same breaker to loosen the base of the excavated slope to prepare the upper level of the parking area.
Around 11:00 a.m., the over 40 foot high, steep excavated slope suddenly collapsed. The collapsed earth crushed the cab side of the excavator and then buried it entirely (Figures 5 and 6). As a result of the collapse, the employee inside the cab was killed. At the time of the incident, the excavator was breaking the base of the excavated slope.
From the vicinity map of the contract drawing, G-1.01, the project site is identified at the east end of St. Thomas Island near Redhook Bay. The geological map of this location identifies the bedrock as Water Island Formation (Kw). The rocks near the ground surface are subject to weathering, such as hydrothermal alternation, silicification, oxidation and sulfidization. As a result, the rock mass has produced numerous fractures with brightly colored rocks visible at Red Hook of St. Thomas. In addition, due to heavy weathering, the strength of the rock blocks has also weakened to soil-like deposits.
Based on the soil maps, the project site is within the SrF soil unit. SrF stands for Southgate-Rock, located on the side slope of a volcanic mountain. The outcrop complex has a 40 to 60 percent slope and a 40 percent rock outcrop area. However, based on Figure 3, the slope of the original ground surface at the project site is estimated to be 40 to 45 percent and the rock outcrop is estimated to be less than 1 percent. Thus, the depth of weathering at the project site is much deeper than the soil map indicates.
Soil borings were not performed for the north excavation. The only site specific geotechnical information was based on seven test pits performed by R & R Caribbean (RRC) for Springline Architects (SLA) during the design stage. The test pits were located in the vicinity of the proposed retaining wall and building (Figure 3). The test pits were dug with a backhoe to approximately 12 to 15 feet in depth. There were no logs or reports for the test pits. However, the RRC indicated to SLA that:
"The soils located at the test pits were relatively easy to excavate and that no major rocks were encountered. The soil strata matched that of neighboring excavation cuts where no major rocks were encountered."
The above statement is consistent with the less than 1% of rock outcrop at the project site. In addition, since the site is near the toe of the volcanic mountain, the depth of weathering should be relatively uniform around 15 feet deep.
Based on the original topography of the site before the excavation (Figure 3) and the corresponding configuration after the collapse (Figure 4), six cross sections around the collapsed area were developed. The location of the sections is shown in Figure 4. The first two sections (Figures 7 and 8) were to the north of the collapsed area. From these two figures, the following information was identified:
Figures 9 through 12 were developed using the above information. The upper portion of the cut slope in Figure 9 appeared to be intact. The earth deposits on the base of the slope appeared to be pushed over from the south during the rescue operation. This section could represent the north end of the collapse. Figures 10 and 11 were sections in the middle of the collapse. As indicated, the average thickness of the collapsed earth deposits was 8 feet.
The upper portion of the cut slope in Figure 12 appeared to be intact. The earth deposits at the base of the slope appeared to be pushed over from the north. This section could represent the south end of the collapse. Based on Figures 9 through 12, the average height of the excavation was 43 feet and the width of the collapse was 60 feet. As a result, the volume of collapsed materials is estimated as:
60 ft wide x 8 ft thick x 43 ft high = 20,640 ft3 = 764 yd3.
Note that the above earth mass was excavated at a 1/4 (H) to 1(V) steep slope and collapsed. Thus, the earth mass could not be regarded as stable rock by OSHA definition (29 CFR Part 1926, Subpart P, Section 650(b), Paragraph Stable Rock).
Photographs of the excavated and collapsed surfaces were examined from north to south. Figure 13 represents the excavated surface north of the collapse. As discussed, the exposed surface appeared to be heavily fractured rocks with indentations and over breaks caused by the hydraulic breaker during the excavation. Figures 14 and 15 represent the lower portion of the collapsed surface at its north end. They indicate similar heavily fractured rocks with two large cavities caused either by the excavation over breaks or the collapse.
Figures 16 through 18 represent the upper portion of the collapse from north to south. The earth deposits in the upper tension zone varied from residual soil to weathered rock. Weathered rock might appear to be rock but it does not have the strength of rock, as is evident from the weathered rock being crushed to small pieces at the collapsed area.
It is interesting to examine Figures 17 and 19. In Figure 17, taken on September 1, 2006, a large weathered rock block could be seen in a seemingly stable position in the upper middle portion of the figure. But Figure 19, taken 25 days later on September 26, 2006, shows that the weathered rock block had fallen, indicating its instability.
Figures 19 and 20 show numerous brightly colored fractures indicating that the rock mass was heavily weathered. Figure 20 also represents the type of rock pieces at the center of the collapse.
Figures 21 and 22 represent the identifiable rock slabs in the excavated surface south of the collapsed area. However, this rock structure only occurred in less than 10% of the 500 foot long excavation.
The figure on the cover of this report presents the overall shape of the collapse. As shown, the collapse was in a near circular or spoon shape. Based on references presented in Figures 23 and 24, a collapse of this shape would mostly occur in overburden soil, weathered rock, waste rock or heavily fractured rock. Therefore, the earth deposits in the collapsed area could only be residual soil, weathered rock or heavily fractured rock. They were more soil-like than rock-like deposits.
Based on the three OSHA laboratory tests, samples of earth deposits collected at the incident site were classified as granular (sandy gravel) materials with no cohesion, corresponding to OSHA Type C soil. However, because of the presence of larger pieces of earth deposits at the site, the earth mass could perhaps be classified as OSHA Type B soil.
Based on the above descriptions, analysis and discussion, we reached the following conclusions:
*Accessibility Assistance: Contact OSHA's Directorate of Construction at (202) 693-2020 for assistance accessing PDF materials.
All other documents, that are not PDF materials or formatted for the web, are available as Microsoft Office® formats and videos and are noted accordingly. If additional assistance is needed with reading, reviewing or accessing these documents or any figures and illustrations, please also contact OSHA's Directorate of Construction at (202) 693-2020.
**eBooks - EPUB is the most common format for e-Books. If you use a Sony Reader, a Nook, or an iPad you can download the EPUB file format. If you use a Kindle, you can download the MOBI file format.Back to Top
The Department of Labor does not endorse, takes no responsibility for, and exercises no control over the linked organization or its views, or contents, nor does it vouch for the accuracy or accessibility of the information contained on the destination server. The Department of Labor also cannot authorize the use of copyrighted materials contained in linked Web sites. Users must request such authorization from the sponsor of the linked Web site. Thank you for visiting our site. Please click the button below to continue.