The results of the site investigation suggested that the mode of failure was due to weathering and degradation of thin shale layers within sandstone. These shale layers weathered over time due to groundwater infiltration, and produced a slide surface for the bedrock above. Based on the tension cracks observed at the site, it was estimated that the size of the sliding mass (inter- bedded sandstone and shale block) was approximately 400 ft (122 m) wide by 150 ft (46 m) upslope by 20 ft (6 m) thick. forecasts during the slide repair. In addition, video coverage of the site often included footage from drones, which provided unique views of and insight into the tension crack pathways that would not have been possible otherwise. The views from the drones provided better understanding of the failure mechanism as well as continuous updates to the public. Social media also included “Facebook updates,” and, in general, social networks coverage brought significant attention and advertisement for the shopping mall and its stores. There were also a fair number of Slide surface observed during the site investigation The EADS Group surveyed points along the slope two times a week to estimate the rate and direction of movement. It was determined that the sliding block was moving at a rate of approximately 2 in/day (5 cm/day). Based on the survey, locations above the tension cracks indicated no movement further upslope. Based on the rate of movement, the size of the failure, and the inherent risk for the shopping plaza, the high voltage electric lines at the base of the slope were de-energized. A few stores within the shopping plaza were required to operate on gas generators due to the de-energized electric lines. The Role of the Media The shopping plaza, an underground gas line and high voltage electric lines at the base of the existing failure quickly made headlines on the media. Media coverage of the site provided benefits and disadvan- tages to the project. There were many valuable contributions from the media during the site investigation and stabilization work. Continuous news reports provided updates and weather 84 • DEEP FOUNDATIONS • MAR/APR 2018 negatives from the news reports. Many of the reports contained information that was often not accurate or was overly dramatic, which created unnecessary anxiety for the public about the safety of the surrounding area. These reports often included potential causes of the failure and proposed mitigation treatments that were inaccurate. The misleading nature of this information and the spotlight that these reports projected on the project added another aspect of stress during the overall stabilization efforts. A few examples include statements such as: “cracks opening from 1 ft to 15 ft (0.3 to 4.6 m) wide in one week,” or referring to the slide as an “expanding fault with potential to collapse,” which is not technically correct. Some inaccurate reports also speculated that if the electric posts were hit by the slide the “whole area will be electrified,” and the “hill side would break apart.” Other reports indicated that the “police” were very concerned, when, in reality, the cooperation with the local authorities was remarkable. Landslide Remediation Design Following the site investigation, the engineering team developed a landslide treatment in an expeditious manner. In the end, two alternates were selected as the most feasible options. These options were then relayed to the site owner and local contractors to determine the best option for the site. The first option consisted of a permanent rock buttress at the base of the original rock cut. The buttress was to be constructed with borrow rock with dimensions determined from stability analyses of the slope. To stabilize the upper Drone coverage of the failure (provided by PA SkyOps) portion of the slide and to prevent cracking further upslope, the existing tension cracks were to be backfilled with rock or grouted. Although stability analysis for this proposed alternate indicated that it was feasible, site constraints, such as the space taken by the permanent buttress, made this option less attractive. Due to the location of the rock cut with respect to the existing shopping center, a pe rmanent rock but t r e s s would significantly reduce access for trucks delivering goods to the commercial buildings. In addition, there were concerns with how the existing gas line, the electric lines and drainage at the toe of the slope would be affected by the permanent rock buttress. The permanent rock buttress was ul t imately ruled out when i t was determined that construction would be delayed as much as two weeks due to quarrying and transporting the rock to the site. Given the rate of movement of the slide and the growing operating expenses to the shopping plaza for the gas powered generators, this option was eliminated. The second alternate consisted of removing the existing failure mass and using a portion of the rock from this zone as a temporary buttress to prevent future failures. Since the existing material contained degradable shales, this buttress was not considered a permanent solution. Instead, the failed rock mass could be left in place until shear pins were installed above the existing slide surface. The shear pins would be used to prevent additional tension cracks from forming after the removal of the existing failure mass. With the upper slope stabilized, the temporary
