Case Study: Retirement Community Transformer Vault Stabilization

Problem

A retirement community hired an engineer to update utilities for their facility, while keeping a transformer in use for power. During excavation for the project, a void was discovered beneath the concrete pad which had significantly compromised the soils.

This void was likely caused by a 10,000-gallon water leak event from several years prior. The compromised soils raised concerns about the transformer and duct bank being at risk during the project due to soil subsidence during excavation. The site contained essential utilities, including a transformer, HVAC units, and a generator, all of which needed to remain operational throughout the project. Given the presence of live electrical lines through the duct bank, further excavation posed significant safety risks.

Solution

To address the soil instability while ensuring the safety of the utilities, Utility Services Group (USG) implemented a multi-step approach.

Subsurface utility locates were conducted for thorough utility mapping, identifying conduit systems and preventing accidental contact with live wires running through the duct bank. USG then implemented polyurethane foam injection to reestablish soil connectivity and soil density at multiple depths. The polyurethane was injected beneath the compromised duct bank to provide structural support and mitigate soil subsidence. Throughout the process, a laser level was used to monitor the slab for “bumps,” which is when the polyurethane comes in contact with the slab and creates a lifting movement. Since the objective of the project was to stabilize the slab and soils, when a “bump” was observed, USG would stop injection to prevent lift. A camera inspection also assessed void conditions before and during the injection process.

Results

The polyurethane injection process allowed stabilization without power disruptions to the retirement community.

The subsurface conditions were strengthened, mitigating settlement risks and reinforcing the soils supporting the duct bank and the transformer. This solution reduced overall project risks and costs. The stabilized ground enabled safe excavation for the utility upgrades without compromising the structures. The project was completed in three days, providing a safer and less invasive alternative to conventional shoring methods.
By employing advanced polyurethane injection techniques, USG successfully reinforced the transformer vault area, saving time, money, and avoiding disruption to the retirement community and preventing delays to the project.