In preparation for the construction of a new education annex and primary health care clinic inside Folsom State Prison, SAGE was hired to perform a geological and structural reconnaissance of the site and develop geohazard mitigation designs to be included in the project’s construction documents.
The proposed footprint of the expansion was located within an existing open recreation yard with outdoor showers on the west side and covered restroom facilities on the east side. Slopes along the west, northern, and southwest boundaries of the recreation yard consisted of dry, hand stacked rock walls, which were in close proximity to the proposed facility’s footprint. The non-engineered walls were thought to be potentially susceptible to vibration and ultimately a cause for concern for short-term construction safety, as well as a long-term seismic hazard for the new building.
SAGE’s evaluation consisted of a historical document review, as well as a geologic and structural reconnaissance of the site utilizing rope access techniques. As a result of the facility’s access limitations within the active prison yard, equipment such as a man lift or other scaffolding required a significant and lengthy approval process. Due to these constraints, SAGE proposed utilizing rope access techniques as a method to save our client time and money. The rope access inspection was supplemented with 3D LiDAR imagery (captured by a third party) to serve as the basis for evaluation and design.
SAGE’s evaluation determined that the walls and slopes have generally performed well with a few minor slides noted over the history of the facility. The walls and slopes would likely continue to perform under static conditions. However, we concluded the rock walls may be susceptible to failures due to disturbance and vibrations (short-term) from construction of the new facilities. Beyond the construction period, there was additional concern of failure during future earthquake-induced ground shaking events (long-term). Of particular concern were the primary 19-foot-tall rock walls around the recreation yard, as well as the rock walls at the top of the slope above the recreation/shower area.
Various alternatives were considered for the rock wall stability improvements. These alternatives included demolition, grading, and replacement of the existing rock walls; rock fall netting to contain and limit rock fall potential; shotcrete flash coating over existing; and structural buttress wall overlays. Ultimately, a combination of rock fall netting, shotcrete flash coats, and buttress wall overlays were employed for various wall components. The selection criteria was based on proximity to the new facility structures, construction zones, and current general condition.
Two smaller, independent stacked rock walls were located well above the recreation yard and set back from the new facilities. Projected rock fall trajectories did not appear to pose a direct threat to the facilities; however, construction safety was still a notable concern. The designs for these two walls were limited to rock fall netting at one location, and shotcrete flash coating for the other. Each of these repairs were cost-effective solutions and minimized demolition, earthwork, and labor.
For the primary tall stacked rock walls around the yard perimeter, a more robust structural design was required and consisted of reinforced concrete buttress walls directly anchored to the large blocks to shore up the existing stacked rock walls. Analysis considered the approach of fully encasing the existing walls to create a pseudo-monolithic behavior of the walls to better withstand seismic and vibration forces. Of key consideration in the design was drainage. Previously, ground water was allowed to freely pass through the ungrouted stacked walls. To emulate the same behavior, and reduce potential loading, an engineered drainage panel system was included in the design to relieve pour pressures behind the new walls. Netting was considered for this location, but the block size and wall height exceeded the desired parameters for netting. Demolition and full replacement was also considered, but a buttress wall was selected as a cost-effective measure that would permit the existing walls to remain in place and ultimately reduced demolition, earthwork, and temporary shoring.
Upon selection of the preferred alternative and structural analysis for the structural features, SAGE prepared engineering design drawings and construction documents for mitigating the identified concerns. The design documents were included in an addendum to the large facility expansion construction contract. SAGE continued to provide engineering and geologic support for the project by performing construction observation and consultation services related to construction field questions, civil utility conflicts, and differing field conditions.